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Who Am I? Locating the neural correlate of the self

Who Am I? Locating the neural correlate of the self Volume 4 † Number 2 † June 2011 10.1093/biohorizons/hzr018 Advance Access publication 4 May 2011 ......................................................................................................................................................................................................................................... Review Sean Webber* 9 Thorncombe Close, Muscliffe, Bournemouth, Dorset BH9 3QL, UK. * Corresponding author: Tel: þ44 7938121012. Email: afromcpie@hotmail.com Supervisor: Stas Glazewski, Keele University, Life Sciences. ........................................................................................................................................................................................................................................ Exploration into the domain of consciousness and ‘self’ originated within the realm of philosophical thought. However, neuroscientific research facilitates the transition from conceptualization to empiricism, allowing scientists to locate the underlying neural mechanisms behind this phenomenon. Binding the multiplicity of conscious modalities, including the sense of ownership over one’s experiences, agency over actions and first-person perspective relating to memory, emotion, spatial and environmental awareness, involves a specific integrative mechanism. It is suggested that the predominant candidate for this faculty lies with synchronous firing between distal assem- blies of neurones. However, each cell assembly relates to a specific cognitive capacity, the majority of which is circumstantially recruited as and when necessary, and remains transient in nature. The pervasive and underlying aspect of the conscious self comes from the sensation of ownership over phenomenal experience. This remains omnipresent during waking consciousness and can be correlated with activity within the medial prefrontal cortex. This paper reviews evidence from fMRI and PET data, along with investigations involving lesions, neurological dysfunction and meditation providing a map of cooperative neurological regions associated with the various categories of the conscious self. These regions have been located predominantly within the parietal and prefrontal cortices. Key words: medial prefrontal cortex, consciousness, ownership, self. Submitted September 2010; accepted March 2011 ........................................................................................................................................................................................................................................ experience. The mechanisms behind this integration have Introduction been the source of numerous investigations when trying to Addressing the question of consciousness and the illusive solve the binding problem. As described by Revonsuo as sense of ‘self ’ requires the combination of a number of ‘consciousness-related binding’, which is distinctly separate domains, including philosophy, psychology and neuro- from the integration of purely sensory information, defined science, in order to help provide a multidimensional model as ‘stimulus-related binding’, consciousness here refers that offers a suitable explanation. Naturalistic philosophy specifically to the subjective perspective adopted from an dictates that conscious awareness stems from neural pro- egocentric frame of reference. cesses, and likewise a sense of ‘self ’ must correlate with Proposed as a prerequisite for the capacity for integration, identifiable neural activity. Translated within the field of Vogeley and Fink suggest that it is necessary to adopt a first- neuroscience, it should be possible to localize the neural pro- person perspective, with this perspective integrating the cesses relating to the capacity of the ‘self ’ and discern ‘world model’ with the subject, forming a ‘self-model’. whether this can be attributed to a specific region of neural Described by Gallagher as the ‘minimal’ self, this includes anatomy and/or synchronicity between cooperating regions both a sense of ownership over an experience and a sense within the brain. of agency over decisions and actions. The combination of The mind automatically attends to an array of phenom- both external (e.g. visual, auditory, olfactory, etc.) and enal experiences, including motivation, memory, emotion internal (e.g. proprioception) sensory information is inte- and a multitude of sensory modalities, coherently represent- grated into a unified experience, over which various aspects ing an apparent objective reality from a subjective perspec- of an egocentric perspective have dominion. tive. Despite variance in the structures and geographical At this point it is easy, as many psychologists and philoso- locations of the neural architecture responsible for these pro- phers have been guilty of, to attribute this phenomenon to a cesses, there exists an observably integrated, singular mysterious ghost in the machine. An internal homunculus ......................................................................................................................................................................................................................................... The Author 2011. Published by Oxford University Press. This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http://creativecommons.org/licenses/by-nc/2.5), which permits unrestricted non-commercial use, distri- bution, and reproduction in any medium, provided the original work is properly cited. 165 Review Bioscience Horizons † Volume 4 † Number 2 † June 2011 ......................................................................................................................................................................................................................................... within the brain, centrally oriented to receive input and considering the whole context of a situation. They suggest orchestrate action. However, this explanation proves unsa- binding must occur initially within specific domains (e.g. tisfactorily circular when considering the mechanisms under- visual, temporal, working memory) prior to further inte- lying this homunculus and the origin of the control behind it. gration between these domains. The primary candidate Sometimes disguised under the alias of a ‘central executive’, mechanism suggested for the phenomenon of binding is the this explanation fails to specifically isolate the neural pro- transient and specific synchronization of neuronal activity, cesses involved with generating the conscious self, and as originally proposed by Crick and Koch, utilizing the tem- leans more towards folk psychology than empirical science. poral facet of activity throughout different groups of func- Although both a sense of agency and ownership occur tionally specific neurones (assemblies). 7 8 simultaneously during voluntary action, David et al. empha- Engel et al. cited evidence for both interhemispheric syn- size a distinction between the two. During involuntary limb chronization between assemblies of neurones across the movements, there is a definite lack of the sense of agency hemispheres and intrahemispheric synchronization of usually attributed to conscious movement, however the assemblies at different regions within the same hemisphere, sense of ownership over the experience persists. This distinc- along with temporally significant synchronicity between cells tion is important as it highlights the transient nature of within the frontal cortex, all with a temporal accuracy agency and, in this case, the continuity of ownership pervad- within milliseconds. It was also demonstrated that synchroni- ing throughout this conscious experience. zation of neuronal firing within visual binding ceases if inco- The purpose of this paper is to review the evidence com- herent images are viewed by opposing eyes. Shown nicely in piled from investigations into consciousness and empirically an experiment involving interocular rivalry, in which images locate the neural processes responsible for generating the are presented dichoptically, i.e. images to both eyes cannot egocentric perspective, as defined as a sense of ownership. be matched and unified into a single percept, a shift in Evidence will be discussed from four angles. Firstly, the sen- ocular dominance was discovered with signals from only sation arises from a global synchronicity of neural activity. one eye considered for further processing, ultimately result- Secondly, there are distinct anatomical regions within the ing in a corresponding oculomotor response. This study brain specifically designed to generate this self-perspective. found an increase in synchronicity between neurones Thirdly, diseases and lesions within specific areas of the involved with the dominant eye, and a decrease in those brain can result in symptoms of self-dissociation, providing neurones involved with the suppressed eye. It was concluded evidence for the location of the neural correlate of self. that synchronicity of neuronal firing in early levels of proces- Fourthly, meditation practitioners claim an ability to dissolve sing determined whether a signal enters into conscious per- a sense of self. With the aid of fMRI studies, this provides ception to then undergo further processing, eventually useful insight into the possible neurological mechanisms resulting in an oculomotor response. and anatomy involved with the generation of this The problem faced when trying to apply this model to the perspective. unified self-perspective stems from a necessary lack of syn- chronization between cell assemblies relating to different rep- resentations. The conclusions drawn rest on the premise that Synchronicity synchronization integrates neuronal activity within an assem- To begin with, evidence will be examined to determine how bly of spatially separate neurones, and helps only to segment the integrated perspective is generated. It would be prudent related sensory information within awareness. This does not, to initially look at the mechanisms underlying stimulus- however, equate to unifying all modalities globally into the related binding and then investigate any additional features entity of consciousness. The compartmentalization of struc- present in consciousness-related binding. Engel et al. look tured representations from sensory information within tem- specifically at the integration or binding of sensory infor- poral binding does not fully account for the macrocosmic mation relating to sensory awareness as an aspect of con- integration associated with consciousness. However, intro- sciousness. The study highlights several important duced within this study comes the notion of frequency-specific components of the ‘binding problem’. Firstly it addresses synchronicity within the gamma-band frequencies (40 – the distribution of the processes relating to a particular 80 Hz), as evidenced during electroencephalogram (EEG)/ aspect of cognition, emphasizing that a number of com- magnetoencephalogram (MEG) experiments. A correlation ponents are typically involved. Secondly it states how is suggested between gamma-band frequencies exhibited during any given period of sensory awareness, information within waking human brains and conscious awareness. The relating to a multitude of different objects and events is sim- citation of evidence for gamma-band synchronization ultaneously processed, therefore requiring a specificity when between ventral occipital and prefrontal areas, and prefron- distinguishing between these separate mental represen- tal and posterior regions during visuospatial working tations. Regarding consciousness-related binding, there is a memory tasks, supports claims that perhaps gamma-band necessity to suitably amalgamate representations when synchronization plays a role in the integration of ......................................................................................................................................................................................................................................... 166 Bioscience Horizons † Volume 4 † Number 2 † June 2011 Review ......................................................................................................................................................................................................................................... geographically distant regions, and specifically, assemblies the subsequent implementation of that action, has lead to relating to different modalities. The thalamus has been ident- debate over the existence of a single neurological module ified as particularly important in this respect as evidenced by responsible for adopting a first-person perspective, and sub- intralaminar neurones firing within the gamma-band fre- sequently transposing that into a third-person perspective. quency to an array of projections throughout the cortex. This requires a close relationship and proximal neurocircui- Here it has been shown that increased attention to auditory try between the two. This opinion reflects the ‘simulation stimulation brings about greater synchronicity within this theory’ of the self , which conflicts with the ‘Theory’ frequency. stating that each capacity relates to distinctly separate Conceding that a unified, coherent consciousness does not processes. equate to homogeneous synchrony, which presents during Vogeley et al., designed a semantic experiment to address the unconscious period of sleep, Engel and Singer speculate this dispute and determine whether the assignment of a synchronicity as the mechanism which binds specific subsets ‘TOM’ perspective recruited separate neural architecture to of neuronal assemblies, all of which could be mediated by a that involved with the assignment of a ‘SELF’ perspective. hierarchy of binding responsible for the cross-modal syn- They employed a number of conditions, each presenting a chronization. It has also been suggested that propagation story followed with a question to monitor the correct assign- of cross-modal synchronisation can be attributed to ‘coinci- ment of perspective. This was assessed by asking the subject dence detectors’. These are cortical neurones that relay syn- to either infer a behaviour of a third party or participate as chronisation from dominant input assemblies by adjusting one of the characters: (i) a baseline was determined presenting the timing and rhythm of their output. The precise mech- unlinked sentences and asking a question relating to one of anism responsible for generating the hierarchy is yet to be the sentences; (ii) a coherent story was presented and a ques- fully understood, however comparisons are made between tion posed requiring a description of an event, with no par- externally generated sensations and internal influences on ticular assignment of perspective; (iii) an isolated ‘TOM’ neural assemblies by coincidence detectors within the thala- condition was achieved by presenting a story in which the mus, which propagates synchronized output signals through- subject had to correctly assign a third-person perspective out its cortical network. without including themselves as a character; (iv) a combined Synchronisation of distant cell assemblies is an attractive ‘TOM’ and ‘SELF’ condition was achieved by presenting a proposition for integrating a multitude of cognitive processes scenario which required an answer demonstrating the assign- into a single, coherent perspective, and is one which is sup- ment of both a third-person and a first-person perspective and ported by a plethora of experimental evidence. However, (v) an isolated ‘SELF’ condition was achieved by presenting in the search for the ‘subject’ of the perspective, the sensation the subject with a story in which they participated as the of ownership, synchronisation cannot represent the only sub- only character. Functional magnetic resonance imaging strate for consciousness. Rather it serves as an instrument for ( fMRI) provided maps upon which corresponding neurologi- communicating between distant regions of the brain and cal activity could be illustrated. integrating multiple processes into a unified experience. As Results indicated some differentiated patterns of neural with all studies investigating synchronisation, cell assemblies activity for each condition, suggesting variance in the and specific neural anatomy serves as the vital components. neural architecture needed for generating the two perspec- It should therefore be considered that the sensation of own- tives. The third-person perspective related to activity princi- ership might originate from the involvement of a particular pally in the anterior cingulate cortex (Fig. 1a), previously region or regions of the brain, which can be integrated via attributed to processing emotional experience as part of the specific synchronization to generate the global conscious limbic system and more specifically the Papez Circuit. The experience. ‘SELF’ condition generated activity predominantly in the right temporoparietal junction and the medial superior parietal lobes (Fig. 1b). Neural anatomy It has been proposed that the sensation of ownership, as The pervasive sensation of a first-person perspective, as separate from agency, is a pervasive and constant aspect of described by Gallagher as the ‘minimal self ’, has been inex- normal consciousness, and therefore cannot be switched tricably linked to the cognitive capacity for adopting a third- off during waking consciousness. According to this postu- person perspective, as initially described by Premack and late, the ‘TOM’ condition, despite not explicitly pertaining Woodruff as a ‘theory of mind’ (TOM). This involves the to a first-person perspective, should already prescribe ability to model another person’s mental states with an implicitly to this perspective (e.g. the ‘TOM’ condition understanding that the beliefs held by others are internalized asks Person A to adopt a third-person perspective resulting and interpretive, as opposed to exact representations of an in the statement: ‘I think Person B believes ... ’ however 19 20 objective reality. The discovery of mirror neurons, this automatically includes a sense of ownership over the which become active in both observation of an action and opinion expressed). A parallel can be drawn between the ......................................................................................................................................................................................................................................... 167 Review Bioscience Horizons † Volume 4 † Number 2 † June 2011 ......................................................................................................................................................................................................................................... Figure 1. The fMRI images displaying the anatomical locations active during various conditions of the experiment. (a) During the isolated (‘TOM’) con- dition, activity was localized to the anterior cingulate cortex and medial aspect of the superior frontal cortex. (b) During the isolated ‘SELF’ condition, activity was found predominantly in the right temporoparietal junction. (c) Significant activity common to both the ‘TOM’ and ‘SELF’ conditions was found in the right prefrontal cortex. Reprinted from ref. 28, with permission from Elsevier. sensation of agency and third-person perspective, as both and ‘other’ condition. Due to vast incalculably subjective faculties are transient and employed circumstantially, views on well-known ‘others’ (e.g. Charles Darwin) and whilst the sensation of ownership persists. Therefore, rather subject variability regarding friends, it was decided that the than utilizing this study to isolate the neural correlate of ‘mother’ would be a suitably valid ‘other’, allowing the time- the ‘self ’, it is important to consider the interaction scale in which the subjects have known their mothers to between the conditions and locate parallel neurological remain relatively constant. Results highlighted the dorsome- activity, prevalent throughout both. A common region of sig- dial prefrontal cortex as a particularly common area of nificant activity, present during both the ‘TOM’ and the activity for both conditions, indicating an integrated ‘SELF’ conditions, lies within the right prefrontal cortex network associated with self and other cognition. (Fig. 1c). Vogeley and Fink categorize the self-model into specific The self-reference effect (SRE) describes how during contexts and examine fMRI data relating to activity when semantic tasks, words that can be applied to oneself are sig- stimulated by context-specific experiments. These categories nificantly more easily recalled than those of simple semantic include: (i) first-person perspective in space, referring to the judgements or judgements linked with a third-person per- centred subjective perspective of one’s own body in space; spective. This indicates that the incorporation of neural (ii) first-person perspective in action, referring to a sense of architecture responsible for generating the self has a distinct agency over movements and actions; (iii) first-person per- impact on memory. Furthermore, this study claims that the spective in social interaction, which replicates and lends level of familiarity between the subject and the designated support to prior TOM/SELF-based studies; (iv) first-person ‘other’ has a bearing on the SRE. The study examines var- perspective and body representation, including one’s internal iance between the neural anatomy involved with the ‘self ’ model of one’s body and sensation of bodily movements (e.g. ......................................................................................................................................................................................................................................... 168 Bioscience Horizons † Volume 4 † Number 2 † June 2011 Review ......................................................................................................................................................................................................................................... Table 1. Summary of results from Vogeley and Fink First-person First-person perspective First-person perspective in First-person perspective and Self and the environment perspective in space in action social interaction body representation ........................................................................................................................................................................................................................................ Medial parietal cortex Left inferior parietal lobe Medial aspects of the superior Right inferior parietal cortex Anterior and posterior parietal lobe cingulated cortex Right inferior parietal Precentral gyrus Right temporo-parietal Posterior parietal cortex Medial parietal cortex cortex junction Right posterior parietal Superior frontal gyrus Prefrontal cortex cortex Occipito-temporal junction Anterior insula proprioception) and (v) a view of the self-conceptualized in for example, disrupts one’s awareness of bodily movements, relation to an environmental context. The resulting fMRI yet ownership over the remaining sensations continues. It is data shows minimal deviation from a collective of anatom- clear, however, that the experimental data link both the par- ical features exhibiting activity throughout these categories. ietal and prefrontal regions of the brain with various aspects Table 1 summarizes the results from this study, detailing of the first-person perspective. the categories of self and the corresponding regions of Recent evidence suggests that consciousness stems from activity, based on fMRI and lesion data. activity located predominantly within the dorsolateral pre- Although the above-mentioned categories are valid and frontal cortex. This study proposes that conscious aware- important characteristics for generating a normal function- ness, despite appearing to be continuous, actually relies ing, holistic sense of self, they are not omnipresent during upon changes in cognition in which awareness updates in wakeful consciousness. It is important to distinguish which distinct cycles. Based on research by Libet , discovering a aspects are transient so as to negate them from consideration time lag of 350 ms between instigating an effect and it and isolate that which is pervasive. Some aspects of the ‘self ’, actually entering into awareness, it is suggested that this for example, are not present until the early stages of human delay allows for comprehensive neural feedback and sup- 19 28 development. Carpendale and Chandler highlight a dra- ports his claim that conscious awareness is discontinuous. matic shift in perspective relating to theory of mind for chil- The brain, however, generates a seemingly continuous stream dren between the ages of 3 and 7 years old, suggesting this of consciousness. It is proposed that conscious vectors within faculty is not an omnipresent feature. Disorientation due to the dorsolateral prefrontal cortex add the sensation of conti- sensory deprivation can also disrupt one’s first-person per- nuity by detecting changes in stimuli, in much the same spective in space. It seems that these regions are recruited way as motion vectors connect an ever-changing stream of during specific tasks and as such remain transient. For still images to produce the impression of motion. Evidence example, the first-person perspective as a conceptual idea suggesting this region of the brain is important for awareness within an environmental context, described as the ‘self- of change comes from repetitive transcranial magnetic stimu- reference’, spikes activity in the anterior and posterior cingu- lation studies, in which awareness of changes within the late cortex and medial parietal cortex. However, when visual field was impaired when applied to the right dorsolat- higher level processing is required, activity shifts to regions eral prefrontal cortex. This region has also been linked to more capable and designed for a specific task, reducing conscious identification of changes within auditory 27 31 32 activity within these default areas. There is, however, diffi- stimuli and working memory. Supporting evidence culty in assessing the internal state of resting individuals suggests that this region is particularly important within exhibiting this default activity. It is also unclear whether the study of consciousness. Activity within the dorsolateral other categories of the self reside, at least in part, within prefrontal cortex ceases during REM sleep. Interestingly, this region, or whether they reside elsewhere. as confirmed by Vogeley and Fink, during this period of The sensation of the ‘self ’ in relation to the body incorpor- unconsciousness, activity ceases in orbital and inferior parie- ates regions of the parietal cortex as well as the prefrontal tal cortices, suggesting the global sensation of conscious cortex. The position of a subjective perspective in relation awareness stems from both parietal and dorsal aspects of to space also holds some similarity to a sense of ownership the prefrontal regions of the brain. over one’s experience, however these categories are not Carpendale and Chandler’s aforementioned claim that wholly congruent with David’s et al., definition. Paralysis, children remain fixed within a self-centred perspective, ......................................................................................................................................................................................................................................... 169 Review Bioscience Horizons † Volume 4 † Number 2 † June 2011 ......................................................................................................................................................................................................................................... prior to the development of the theory of mind, further sup- ownership over experiences and is best exemplified by a con- ports the importance of this region. It was discovered that dition known as intermanual conflict. Having undergone a children also displayed greater activity within the medial pre- corpus callosotomy, in which the hemispheres of the brain frontal cortex and the anterior cingulate cortex during ‘self ’ have been surgically disconnected, patients often exhibit con- conditions. However Vanderwal et al., illustrated that the flicting actions between the two hands, and the sense of own- dorsomedial prefrontal cortex is not exclusively dedicated ership over one of their hands disappears. In right-handed to the ‘self ’, as the SRE links both ‘self ’ and ‘other’ con- people, with dominant left hemispheres, their sense of owner- 26 38 ditions to this region. It seems that despite an obvious cor- ship remains within their dominant hand. This lends great relation with the self-perspective, this region is not unique to support for the localization of ownership residing within the this faculty. dominant medial prefrontal cortex. The disease schizophrenia has been described as a clinical disorder disrupting the self-model. Depersonalization, as a Disease and lesion data clinical syndrome of schizophrenia, includes disruption of A particularly useful area when locating functional brain the sensation of ownership and involves the sensation that regions comes from examining diseased or damaged brains. thoughts have been inserted along with a loss of agency Not limited by the low temporal resolution of imaging tech- over actions. This has been explained in terms of decreased niques such as fMRI, or the low spatial resolution of EEG/ metabolic activity within the prefrontal cortices and even MEG, brain dissection can relate deficits in cognitive per- total loss of activity within the dorsolateral prefrontal formance to specific regions within the brain. Lesions cortex. Although this evidence does not differentiate found within the right posterior parietal cortex are sympto- between ownership and agency, it does lend support to pre- matic of ‘egocentric disorientation’. This inhibits the sub- vious studies suggesting the involvement of the prefrontal ject’s ability to orient themselves in relation to the external cortex. environment without visual cues. It involves a deficit in com- munication between one’s self in relation to one’s body, and Effects of meditation subsequently one’s conceptual worldview. Vogeley and Fink link the disability anosognosia with damage to the Finally, the intended outcome of many meditative practices right parietal cortex. This condition prevents sufferers from involves the dissolution of a sense of agency and volition. being aware of other disabilities such as blindness, limiting The meditator’s aim is to relinquish conscious control and their first-person perspective in relation to their body rep- remain an objective observer of phenomena. PET was used resentation. These examples show damage resulting in loss to measure cerebral blood flow in meditation practitioners or impairment of aspects of the self-model previously during various stages of meditation, with the aim to high- described as transient. This evidence cannot entirely rule light changes within neurological activity during various out these regions as contributors to the pervasive sense of states of consciousness. Whilst in the meditative state, sub- ownership, as the remaining, undamaged parts may be suffi- jects showed a reduction in blood flow to the dorsolateral cient to maintain this capacity. Nor can we deduce that the prefrontal, anterior cingulate and orbital frontal cortices, prefrontal cortex is exclusively responsible for this omnipre- striatum, thalamus and cerebellum. Citing evidence of sent level of consciousness, as lesions found here do not re-entrant loops within these areas, upon which post- entirely eradicate ‘higher conscious experience’. In cases synaptic potentials are regulated by dopamine, the study where the lateral prefrontal cortex has been completely suggested that this inhibition of volition could be related to destroyed, however, patients present with no indication of the dopaminergic system. C-raclopride was used to any levels of consciousness and remain in a persistent vegeta- measure dopamine binding. Results indicated a 7.9% tive state, suggesting a strong link for its involvement in decrease in the C-raclopride binding potential of dopamine generating a conscious self. within the striatum, correlating approximately with a 65% Consciousness, however, remains an illusive subject to increase in extracellular dopamine. This study, although measure and many studies require a linguistic capacity in not specifically linked with a sense of ownership, highlights order to express conscious awareness. Sevush describes a the reduced activity within cortical pathways associated form of non-verbally expressible consciousness correlated with the various aspects of the sense of self. The study with anatomy separate from the left lateral prefrontal cortex. described two circuits involved with both the striatum and It is suggested that due to direct links with the language frontal cortex (Fig. 2). centres of the brain, specifically Broca’s area, the left lateral Reduced function within circuit (i) results in ‘orbitofrontal prefrontal cortex relates to an ‘extended consciousness’, syndrome’ characterized by poor decision-making and be- whereas bilaterally the medial prefrontal cortex correlates havioural disinhibition such as impulsiveness. Reduced with a ‘core consciousness’. This ‘core consciousness’ function within circuit; (ii) results in ‘anterior cingulate syn- appears to correspond with the pervasive sensation of drome’ characterized by symptoms of apathy and ......................................................................................................................................................................................................................................... 170 Bioscience Horizons † Volume 4 † Number 2 † June 2011 Review ......................................................................................................................................................................................................................................... Figure 2. Image displaying the two circuits incorporating both the striatum and frontal cortex. (a) ‘Originates in the lateral orbital frontal cortex, passes through the ventromedial frontal cortex, ventral striatum, medial and dorsomedial globus pallidus, ventral anterior and medial dorsal thalamus andbackto the orbitofrontal cortex.’ (b) ‘Originates in the anterior cingulate gyrus, passes through the ventral striatum, the rostrolateral globus pallidus, the medial dorsal thalamus and back to the prefrontal cortex.’ disinterest. Both sets of symptoms have striking similarities considerations and a possible need to re-evaluate the criteria with aspects of the meditative state in which the sensation of before confirming death or diagnosing coma. agency diminishes. However, a prevalent aspect of this med- Lesions to the pons or malformation of the basilar artery itative state includes the sensation of ownership over one’s can result in a condition known as cerebromedullospinal dis- experiences described as an impartial observational perspec- connection or locked-in syndrome, in which patients present tive. This, therefore, helps partially disregard the regions with no indication of consciousness, however, upon recov- inactivated during meditation, allowing the focus to reside ery, later describe being fully aware but unable to express on the medial prefrontal cortex, as already associated with themselves. Advances in brain – computer interface tech- the sensation of ownership. nology allow patients to use focused attention to control the movement of a cursor on a screen. This can help improve quality of life by facilitating communication in patients that are ‘locked-in’ or suffer from poor muscular Conclusion control. Both areas of study could greatly help expand our understanding of the neural mechanisms involved with gen- The subjective nature of this research creates a major limit- erating consciousness. ation in terms of interpretation of mental states. The exper- This paper has systematically narrowed down regions imental validity relies upon accurate descriptions from the within the brain associated with the various aspects of con- meditation practitioners concerning their subjective experi- sciousness. Due to the ambiguity surrounding the definition ences, however the evidence does empirically show systematic of ‘consciousness’, it has been necessary to explore a multi- change within the neural chemistry during these states. tude of facets suggested within this field. The ‘self-model’ Further research into anaesthetic awareness would con- has incorporated internal and external sensory information tribute greatly to this ongoing debate. This would allow and sensations of a first-person perspective relating to owner- for the inactivation of much of the transient aspects of con- ship, agency, working memory and various bodily frames of sciousness, and help isolate the neural correlate of the ‘core reference. It has been assumed throughout that the sensation self ’. Locating this ‘core self ’ has serious implications regard- of ownership is an omnipresent phenomenon during waking ing brain death, and subsequent decisions for organ trans- consciousness and that the different ‘transient’ conscious plantation. If activity relating to the ‘core self ’ resides states occur circumstantially, recruiting specific neurological definitively within a particular subset of neurones found in assemblies dedicated to their specific domains. These have the medial prefrontal cortex, physicians can use this infor- included distinct regions of the parietal lobe and prefrontal mation to search for neurological activity in patients present- cortex. It is proposed that these contingent conscious states ing with little or no consciousness. Currently, two medical arise, flavouring global consciousness with perspectives of opinions are required to assess the level of brain activity agency, TOM, spatial and visual awareness, etc., all resting within patients prior to brain death being established, and on a foundation of ownership as the ‘core self ’. organs made available for transplantation. Detection of Consistent with fMRI, PET and lesion data, the pervading activity within this specific region can indicate the presence sense of ownership over one’s perspective and experience has of conscious awareness, raising a number of ethical ......................................................................................................................................................................................................................................... 171 Review Bioscience Horizons † Volume 4 † Number 2 † June 2011 ......................................................................................................................................................................................................................................... corresponded consistently with neural assemblies within the References medial prefrontal cortex, although it cannot be concluded 1. 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Igasaki T, Fujiwara K, Murayama N, Hayashida Y (2009) A new development of EEG-based brain–computer interface: for cursor control with calibration 37. Damasio AR (1998) Investigating the biology of consciousness. Philos of suitable frequency band. Clin Neurophysiol 120: 156–157. Transac Royal Soc 353: 1879–1882. ........................................................................................................................................................................................................................................ ......................................................................................................................................................................................................................................... http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png Bioscience Horizons Oxford University Press

Who Am I? Locating the neural correlate of the self

Bioscience Horizons , Volume 4 (2) – Jun 4, 2011

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Volume 4 † Number 2 † June 2011 10.1093/biohorizons/hzr018 Advance Access publication 4 May 2011 ......................................................................................................................................................................................................................................... Review Sean Webber* 9 Thorncombe Close, Muscliffe, Bournemouth, Dorset BH9 3QL, UK. * Corresponding author: Tel: þ44 7938121012. Email: afromcpie@hotmail.com Supervisor: Stas Glazewski, Keele University, Life Sciences. ........................................................................................................................................................................................................................................ Exploration into the domain of consciousness and ‘self’ originated within the realm of philosophical thought. However, neuroscientific research facilitates the transition from conceptualization to empiricism, allowing scientists to locate the underlying neural mechanisms behind this phenomenon. Binding the multiplicity of conscious modalities, including the sense of ownership over one’s experiences, agency over actions and first-person perspective relating to memory, emotion, spatial and environmental awareness, involves a specific integrative mechanism. It is suggested that the predominant candidate for this faculty lies with synchronous firing between distal assem- blies of neurones. However, each cell assembly relates to a specific cognitive capacity, the majority of which is circumstantially recruited as and when necessary, and remains transient in nature. The pervasive and underlying aspect of the conscious self comes from the sensation of ownership over phenomenal experience. This remains omnipresent during waking consciousness and can be correlated with activity within the medial prefrontal cortex. This paper reviews evidence from fMRI and PET data, along with investigations involving lesions, neurological dysfunction and meditation providing a map of cooperative neurological regions associated with the various categories of the conscious self. These regions have been located predominantly within the parietal and prefrontal cortices. Key words: medial prefrontal cortex, consciousness, ownership, self. Submitted September 2010; accepted March 2011 ........................................................................................................................................................................................................................................ experience. The mechanisms behind this integration have Introduction been the source of numerous investigations when trying to Addressing the question of consciousness and the illusive solve the binding problem. As described by Revonsuo as sense of ‘self ’ requires the combination of a number of ‘consciousness-related binding’, which is distinctly separate domains, including philosophy, psychology and neuro- from the integration of purely sensory information, defined science, in order to help provide a multidimensional model as ‘stimulus-related binding’, consciousness here refers that offers a suitable explanation. Naturalistic philosophy specifically to the subjective perspective adopted from an dictates that conscious awareness stems from neural pro- egocentric frame of reference. cesses, and likewise a sense of ‘self ’ must correlate with Proposed as a prerequisite for the capacity for integration, identifiable neural activity. Translated within the field of Vogeley and Fink suggest that it is necessary to adopt a first- neuroscience, it should be possible to localize the neural pro- person perspective, with this perspective integrating the cesses relating to the capacity of the ‘self ’ and discern ‘world model’ with the subject, forming a ‘self-model’. whether this can be attributed to a specific region of neural Described by Gallagher as the ‘minimal’ self, this includes anatomy and/or synchronicity between cooperating regions both a sense of ownership over an experience and a sense within the brain. of agency over decisions and actions. The combination of The mind automatically attends to an array of phenom- both external (e.g. visual, auditory, olfactory, etc.) and enal experiences, including motivation, memory, emotion internal (e.g. proprioception) sensory information is inte- and a multitude of sensory modalities, coherently represent- grated into a unified experience, over which various aspects ing an apparent objective reality from a subjective perspec- of an egocentric perspective have dominion. tive. Despite variance in the structures and geographical At this point it is easy, as many psychologists and philoso- locations of the neural architecture responsible for these pro- phers have been guilty of, to attribute this phenomenon to a cesses, there exists an observably integrated, singular mysterious ghost in the machine. An internal homunculus ......................................................................................................................................................................................................................................... The Author 2011. Published by Oxford University Press. This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http://creativecommons.org/licenses/by-nc/2.5), which permits unrestricted non-commercial use, distri- bution, and reproduction in any medium, provided the original work is properly cited. 165 Review Bioscience Horizons † Volume 4 † Number 2 † June 2011 ......................................................................................................................................................................................................................................... within the brain, centrally oriented to receive input and considering the whole context of a situation. They suggest orchestrate action. However, this explanation proves unsa- binding must occur initially within specific domains (e.g. tisfactorily circular when considering the mechanisms under- visual, temporal, working memory) prior to further inte- lying this homunculus and the origin of the control behind it. gration between these domains. The primary candidate Sometimes disguised under the alias of a ‘central executive’, mechanism suggested for the phenomenon of binding is the this explanation fails to specifically isolate the neural pro- transient and specific synchronization of neuronal activity, cesses involved with generating the conscious self, and as originally proposed by Crick and Koch, utilizing the tem- leans more towards folk psychology than empirical science. poral facet of activity throughout different groups of func- Although both a sense of agency and ownership occur tionally specific neurones (assemblies). 7 8 simultaneously during voluntary action, David et al. empha- Engel et al. cited evidence for both interhemispheric syn- size a distinction between the two. During involuntary limb chronization between assemblies of neurones across the movements, there is a definite lack of the sense of agency hemispheres and intrahemispheric synchronization of usually attributed to conscious movement, however the assemblies at different regions within the same hemisphere, sense of ownership over the experience persists. This distinc- along with temporally significant synchronicity between cells tion is important as it highlights the transient nature of within the frontal cortex, all with a temporal accuracy agency and, in this case, the continuity of ownership pervad- within milliseconds. It was also demonstrated that synchroni- ing throughout this conscious experience. zation of neuronal firing within visual binding ceases if inco- The purpose of this paper is to review the evidence com- herent images are viewed by opposing eyes. Shown nicely in piled from investigations into consciousness and empirically an experiment involving interocular rivalry, in which images locate the neural processes responsible for generating the are presented dichoptically, i.e. images to both eyes cannot egocentric perspective, as defined as a sense of ownership. be matched and unified into a single percept, a shift in Evidence will be discussed from four angles. Firstly, the sen- ocular dominance was discovered with signals from only sation arises from a global synchronicity of neural activity. one eye considered for further processing, ultimately result- Secondly, there are distinct anatomical regions within the ing in a corresponding oculomotor response. This study brain specifically designed to generate this self-perspective. found an increase in synchronicity between neurones Thirdly, diseases and lesions within specific areas of the involved with the dominant eye, and a decrease in those brain can result in symptoms of self-dissociation, providing neurones involved with the suppressed eye. It was concluded evidence for the location of the neural correlate of self. that synchronicity of neuronal firing in early levels of proces- Fourthly, meditation practitioners claim an ability to dissolve sing determined whether a signal enters into conscious per- a sense of self. With the aid of fMRI studies, this provides ception to then undergo further processing, eventually useful insight into the possible neurological mechanisms resulting in an oculomotor response. and anatomy involved with the generation of this The problem faced when trying to apply this model to the perspective. unified self-perspective stems from a necessary lack of syn- chronization between cell assemblies relating to different rep- resentations. The conclusions drawn rest on the premise that Synchronicity synchronization integrates neuronal activity within an assem- To begin with, evidence will be examined to determine how bly of spatially separate neurones, and helps only to segment the integrated perspective is generated. It would be prudent related sensory information within awareness. This does not, to initially look at the mechanisms underlying stimulus- however, equate to unifying all modalities globally into the related binding and then investigate any additional features entity of consciousness. The compartmentalization of struc- present in consciousness-related binding. Engel et al. look tured representations from sensory information within tem- specifically at the integration or binding of sensory infor- poral binding does not fully account for the macrocosmic mation relating to sensory awareness as an aspect of con- integration associated with consciousness. However, intro- sciousness. The study highlights several important duced within this study comes the notion of frequency-specific components of the ‘binding problem’. Firstly it addresses synchronicity within the gamma-band frequencies (40 – the distribution of the processes relating to a particular 80 Hz), as evidenced during electroencephalogram (EEG)/ aspect of cognition, emphasizing that a number of com- magnetoencephalogram (MEG) experiments. A correlation ponents are typically involved. Secondly it states how is suggested between gamma-band frequencies exhibited during any given period of sensory awareness, information within waking human brains and conscious awareness. The relating to a multitude of different objects and events is sim- citation of evidence for gamma-band synchronization ultaneously processed, therefore requiring a specificity when between ventral occipital and prefrontal areas, and prefron- distinguishing between these separate mental represen- tal and posterior regions during visuospatial working tations. Regarding consciousness-related binding, there is a memory tasks, supports claims that perhaps gamma-band necessity to suitably amalgamate representations when synchronization plays a role in the integration of ......................................................................................................................................................................................................................................... 166 Bioscience Horizons † Volume 4 † Number 2 † June 2011 Review ......................................................................................................................................................................................................................................... geographically distant regions, and specifically, assemblies the subsequent implementation of that action, has lead to relating to different modalities. The thalamus has been ident- debate over the existence of a single neurological module ified as particularly important in this respect as evidenced by responsible for adopting a first-person perspective, and sub- intralaminar neurones firing within the gamma-band fre- sequently transposing that into a third-person perspective. quency to an array of projections throughout the cortex. This requires a close relationship and proximal neurocircui- Here it has been shown that increased attention to auditory try between the two. This opinion reflects the ‘simulation stimulation brings about greater synchronicity within this theory’ of the self , which conflicts with the ‘Theory’ frequency. stating that each capacity relates to distinctly separate Conceding that a unified, coherent consciousness does not processes. equate to homogeneous synchrony, which presents during Vogeley et al., designed a semantic experiment to address the unconscious period of sleep, Engel and Singer speculate this dispute and determine whether the assignment of a synchronicity as the mechanism which binds specific subsets ‘TOM’ perspective recruited separate neural architecture to of neuronal assemblies, all of which could be mediated by a that involved with the assignment of a ‘SELF’ perspective. hierarchy of binding responsible for the cross-modal syn- They employed a number of conditions, each presenting a chronization. It has also been suggested that propagation story followed with a question to monitor the correct assign- of cross-modal synchronisation can be attributed to ‘coinci- ment of perspective. This was assessed by asking the subject dence detectors’. These are cortical neurones that relay syn- to either infer a behaviour of a third party or participate as chronisation from dominant input assemblies by adjusting one of the characters: (i) a baseline was determined presenting the timing and rhythm of their output. The precise mech- unlinked sentences and asking a question relating to one of anism responsible for generating the hierarchy is yet to be the sentences; (ii) a coherent story was presented and a ques- fully understood, however comparisons are made between tion posed requiring a description of an event, with no par- externally generated sensations and internal influences on ticular assignment of perspective; (iii) an isolated ‘TOM’ neural assemblies by coincidence detectors within the thala- condition was achieved by presenting a story in which the mus, which propagates synchronized output signals through- subject had to correctly assign a third-person perspective out its cortical network. without including themselves as a character; (iv) a combined Synchronisation of distant cell assemblies is an attractive ‘TOM’ and ‘SELF’ condition was achieved by presenting a proposition for integrating a multitude of cognitive processes scenario which required an answer demonstrating the assign- into a single, coherent perspective, and is one which is sup- ment of both a third-person and a first-person perspective and ported by a plethora of experimental evidence. However, (v) an isolated ‘SELF’ condition was achieved by presenting in the search for the ‘subject’ of the perspective, the sensation the subject with a story in which they participated as the of ownership, synchronisation cannot represent the only sub- only character. Functional magnetic resonance imaging strate for consciousness. Rather it serves as an instrument for ( fMRI) provided maps upon which corresponding neurologi- communicating between distant regions of the brain and cal activity could be illustrated. integrating multiple processes into a unified experience. As Results indicated some differentiated patterns of neural with all studies investigating synchronisation, cell assemblies activity for each condition, suggesting variance in the and specific neural anatomy serves as the vital components. neural architecture needed for generating the two perspec- It should therefore be considered that the sensation of own- tives. The third-person perspective related to activity princi- ership might originate from the involvement of a particular pally in the anterior cingulate cortex (Fig. 1a), previously region or regions of the brain, which can be integrated via attributed to processing emotional experience as part of the specific synchronization to generate the global conscious limbic system and more specifically the Papez Circuit. The experience. ‘SELF’ condition generated activity predominantly in the right temporoparietal junction and the medial superior parietal lobes (Fig. 1b). Neural anatomy It has been proposed that the sensation of ownership, as The pervasive sensation of a first-person perspective, as separate from agency, is a pervasive and constant aspect of described by Gallagher as the ‘minimal self ’, has been inex- normal consciousness, and therefore cannot be switched tricably linked to the cognitive capacity for adopting a third- off during waking consciousness. According to this postu- person perspective, as initially described by Premack and late, the ‘TOM’ condition, despite not explicitly pertaining Woodruff as a ‘theory of mind’ (TOM). This involves the to a first-person perspective, should already prescribe ability to model another person’s mental states with an implicitly to this perspective (e.g. the ‘TOM’ condition understanding that the beliefs held by others are internalized asks Person A to adopt a third-person perspective resulting and interpretive, as opposed to exact representations of an in the statement: ‘I think Person B believes ... ’ however 19 20 objective reality. The discovery of mirror neurons, this automatically includes a sense of ownership over the which become active in both observation of an action and opinion expressed). A parallel can be drawn between the ......................................................................................................................................................................................................................................... 167 Review Bioscience Horizons † Volume 4 † Number 2 † June 2011 ......................................................................................................................................................................................................................................... Figure 1. The fMRI images displaying the anatomical locations active during various conditions of the experiment. (a) During the isolated (‘TOM’) con- dition, activity was localized to the anterior cingulate cortex and medial aspect of the superior frontal cortex. (b) During the isolated ‘SELF’ condition, activity was found predominantly in the right temporoparietal junction. (c) Significant activity common to both the ‘TOM’ and ‘SELF’ conditions was found in the right prefrontal cortex. Reprinted from ref. 28, with permission from Elsevier. sensation of agency and third-person perspective, as both and ‘other’ condition. Due to vast incalculably subjective faculties are transient and employed circumstantially, views on well-known ‘others’ (e.g. Charles Darwin) and whilst the sensation of ownership persists. Therefore, rather subject variability regarding friends, it was decided that the than utilizing this study to isolate the neural correlate of ‘mother’ would be a suitably valid ‘other’, allowing the time- the ‘self ’, it is important to consider the interaction scale in which the subjects have known their mothers to between the conditions and locate parallel neurological remain relatively constant. Results highlighted the dorsome- activity, prevalent throughout both. A common region of sig- dial prefrontal cortex as a particularly common area of nificant activity, present during both the ‘TOM’ and the activity for both conditions, indicating an integrated ‘SELF’ conditions, lies within the right prefrontal cortex network associated with self and other cognition. (Fig. 1c). Vogeley and Fink categorize the self-model into specific The self-reference effect (SRE) describes how during contexts and examine fMRI data relating to activity when semantic tasks, words that can be applied to oneself are sig- stimulated by context-specific experiments. These categories nificantly more easily recalled than those of simple semantic include: (i) first-person perspective in space, referring to the judgements or judgements linked with a third-person per- centred subjective perspective of one’s own body in space; spective. This indicates that the incorporation of neural (ii) first-person perspective in action, referring to a sense of architecture responsible for generating the self has a distinct agency over movements and actions; (iii) first-person per- impact on memory. Furthermore, this study claims that the spective in social interaction, which replicates and lends level of familiarity between the subject and the designated support to prior TOM/SELF-based studies; (iv) first-person ‘other’ has a bearing on the SRE. The study examines var- perspective and body representation, including one’s internal iance between the neural anatomy involved with the ‘self ’ model of one’s body and sensation of bodily movements (e.g. ......................................................................................................................................................................................................................................... 168 Bioscience Horizons † Volume 4 † Number 2 † June 2011 Review ......................................................................................................................................................................................................................................... Table 1. Summary of results from Vogeley and Fink First-person First-person perspective First-person perspective in First-person perspective and Self and the environment perspective in space in action social interaction body representation ........................................................................................................................................................................................................................................ Medial parietal cortex Left inferior parietal lobe Medial aspects of the superior Right inferior parietal cortex Anterior and posterior parietal lobe cingulated cortex Right inferior parietal Precentral gyrus Right temporo-parietal Posterior parietal cortex Medial parietal cortex cortex junction Right posterior parietal Superior frontal gyrus Prefrontal cortex cortex Occipito-temporal junction Anterior insula proprioception) and (v) a view of the self-conceptualized in for example, disrupts one’s awareness of bodily movements, relation to an environmental context. The resulting fMRI yet ownership over the remaining sensations continues. It is data shows minimal deviation from a collective of anatom- clear, however, that the experimental data link both the par- ical features exhibiting activity throughout these categories. ietal and prefrontal regions of the brain with various aspects Table 1 summarizes the results from this study, detailing of the first-person perspective. the categories of self and the corresponding regions of Recent evidence suggests that consciousness stems from activity, based on fMRI and lesion data. activity located predominantly within the dorsolateral pre- Although the above-mentioned categories are valid and frontal cortex. This study proposes that conscious aware- important characteristics for generating a normal function- ness, despite appearing to be continuous, actually relies ing, holistic sense of self, they are not omnipresent during upon changes in cognition in which awareness updates in wakeful consciousness. It is important to distinguish which distinct cycles. Based on research by Libet , discovering a aspects are transient so as to negate them from consideration time lag of 350 ms between instigating an effect and it and isolate that which is pervasive. Some aspects of the ‘self ’, actually entering into awareness, it is suggested that this for example, are not present until the early stages of human delay allows for comprehensive neural feedback and sup- 19 28 development. Carpendale and Chandler highlight a dra- ports his claim that conscious awareness is discontinuous. matic shift in perspective relating to theory of mind for chil- The brain, however, generates a seemingly continuous stream dren between the ages of 3 and 7 years old, suggesting this of consciousness. It is proposed that conscious vectors within faculty is not an omnipresent feature. Disorientation due to the dorsolateral prefrontal cortex add the sensation of conti- sensory deprivation can also disrupt one’s first-person per- nuity by detecting changes in stimuli, in much the same spective in space. It seems that these regions are recruited way as motion vectors connect an ever-changing stream of during specific tasks and as such remain transient. For still images to produce the impression of motion. Evidence example, the first-person perspective as a conceptual idea suggesting this region of the brain is important for awareness within an environmental context, described as the ‘self- of change comes from repetitive transcranial magnetic stimu- reference’, spikes activity in the anterior and posterior cingu- lation studies, in which awareness of changes within the late cortex and medial parietal cortex. However, when visual field was impaired when applied to the right dorsolat- higher level processing is required, activity shifts to regions eral prefrontal cortex. This region has also been linked to more capable and designed for a specific task, reducing conscious identification of changes within auditory 27 31 32 activity within these default areas. There is, however, diffi- stimuli and working memory. Supporting evidence culty in assessing the internal state of resting individuals suggests that this region is particularly important within exhibiting this default activity. It is also unclear whether the study of consciousness. Activity within the dorsolateral other categories of the self reside, at least in part, within prefrontal cortex ceases during REM sleep. Interestingly, this region, or whether they reside elsewhere. as confirmed by Vogeley and Fink, during this period of The sensation of the ‘self ’ in relation to the body incorpor- unconsciousness, activity ceases in orbital and inferior parie- ates regions of the parietal cortex as well as the prefrontal tal cortices, suggesting the global sensation of conscious cortex. The position of a subjective perspective in relation awareness stems from both parietal and dorsal aspects of to space also holds some similarity to a sense of ownership the prefrontal regions of the brain. over one’s experience, however these categories are not Carpendale and Chandler’s aforementioned claim that wholly congruent with David’s et al., definition. Paralysis, children remain fixed within a self-centred perspective, ......................................................................................................................................................................................................................................... 169 Review Bioscience Horizons † Volume 4 † Number 2 † June 2011 ......................................................................................................................................................................................................................................... prior to the development of the theory of mind, further sup- ownership over experiences and is best exemplified by a con- ports the importance of this region. It was discovered that dition known as intermanual conflict. Having undergone a children also displayed greater activity within the medial pre- corpus callosotomy, in which the hemispheres of the brain frontal cortex and the anterior cingulate cortex during ‘self ’ have been surgically disconnected, patients often exhibit con- conditions. However Vanderwal et al., illustrated that the flicting actions between the two hands, and the sense of own- dorsomedial prefrontal cortex is not exclusively dedicated ership over one of their hands disappears. In right-handed to the ‘self ’, as the SRE links both ‘self ’ and ‘other’ con- people, with dominant left hemispheres, their sense of owner- 26 38 ditions to this region. It seems that despite an obvious cor- ship remains within their dominant hand. This lends great relation with the self-perspective, this region is not unique to support for the localization of ownership residing within the this faculty. dominant medial prefrontal cortex. The disease schizophrenia has been described as a clinical disorder disrupting the self-model. Depersonalization, as a Disease and lesion data clinical syndrome of schizophrenia, includes disruption of A particularly useful area when locating functional brain the sensation of ownership and involves the sensation that regions comes from examining diseased or damaged brains. thoughts have been inserted along with a loss of agency Not limited by the low temporal resolution of imaging tech- over actions. This has been explained in terms of decreased niques such as fMRI, or the low spatial resolution of EEG/ metabolic activity within the prefrontal cortices and even MEG, brain dissection can relate deficits in cognitive per- total loss of activity within the dorsolateral prefrontal formance to specific regions within the brain. Lesions cortex. Although this evidence does not differentiate found within the right posterior parietal cortex are sympto- between ownership and agency, it does lend support to pre- matic of ‘egocentric disorientation’. This inhibits the sub- vious studies suggesting the involvement of the prefrontal ject’s ability to orient themselves in relation to the external cortex. environment without visual cues. It involves a deficit in com- munication between one’s self in relation to one’s body, and Effects of meditation subsequently one’s conceptual worldview. Vogeley and Fink link the disability anosognosia with damage to the Finally, the intended outcome of many meditative practices right parietal cortex. This condition prevents sufferers from involves the dissolution of a sense of agency and volition. being aware of other disabilities such as blindness, limiting The meditator’s aim is to relinquish conscious control and their first-person perspective in relation to their body rep- remain an objective observer of phenomena. PET was used resentation. These examples show damage resulting in loss to measure cerebral blood flow in meditation practitioners or impairment of aspects of the self-model previously during various stages of meditation, with the aim to high- described as transient. This evidence cannot entirely rule light changes within neurological activity during various out these regions as contributors to the pervasive sense of states of consciousness. Whilst in the meditative state, sub- ownership, as the remaining, undamaged parts may be suffi- jects showed a reduction in blood flow to the dorsolateral cient to maintain this capacity. Nor can we deduce that the prefrontal, anterior cingulate and orbital frontal cortices, prefrontal cortex is exclusively responsible for this omnipre- striatum, thalamus and cerebellum. Citing evidence of sent level of consciousness, as lesions found here do not re-entrant loops within these areas, upon which post- entirely eradicate ‘higher conscious experience’. In cases synaptic potentials are regulated by dopamine, the study where the lateral prefrontal cortex has been completely suggested that this inhibition of volition could be related to destroyed, however, patients present with no indication of the dopaminergic system. C-raclopride was used to any levels of consciousness and remain in a persistent vegeta- measure dopamine binding. Results indicated a 7.9% tive state, suggesting a strong link for its involvement in decrease in the C-raclopride binding potential of dopamine generating a conscious self. within the striatum, correlating approximately with a 65% Consciousness, however, remains an illusive subject to increase in extracellular dopamine. This study, although measure and many studies require a linguistic capacity in not specifically linked with a sense of ownership, highlights order to express conscious awareness. Sevush describes a the reduced activity within cortical pathways associated form of non-verbally expressible consciousness correlated with the various aspects of the sense of self. The study with anatomy separate from the left lateral prefrontal cortex. described two circuits involved with both the striatum and It is suggested that due to direct links with the language frontal cortex (Fig. 2). centres of the brain, specifically Broca’s area, the left lateral Reduced function within circuit (i) results in ‘orbitofrontal prefrontal cortex relates to an ‘extended consciousness’, syndrome’ characterized by poor decision-making and be- whereas bilaterally the medial prefrontal cortex correlates havioural disinhibition such as impulsiveness. Reduced with a ‘core consciousness’. This ‘core consciousness’ function within circuit; (ii) results in ‘anterior cingulate syn- appears to correspond with the pervasive sensation of drome’ characterized by symptoms of apathy and ......................................................................................................................................................................................................................................... 170 Bioscience Horizons † Volume 4 † Number 2 † June 2011 Review ......................................................................................................................................................................................................................................... Figure 2. Image displaying the two circuits incorporating both the striatum and frontal cortex. (a) ‘Originates in the lateral orbital frontal cortex, passes through the ventromedial frontal cortex, ventral striatum, medial and dorsomedial globus pallidus, ventral anterior and medial dorsal thalamus andbackto the orbitofrontal cortex.’ (b) ‘Originates in the anterior cingulate gyrus, passes through the ventral striatum, the rostrolateral globus pallidus, the medial dorsal thalamus and back to the prefrontal cortex.’ disinterest. Both sets of symptoms have striking similarities considerations and a possible need to re-evaluate the criteria with aspects of the meditative state in which the sensation of before confirming death or diagnosing coma. agency diminishes. However, a prevalent aspect of this med- Lesions to the pons or malformation of the basilar artery itative state includes the sensation of ownership over one’s can result in a condition known as cerebromedullospinal dis- experiences described as an impartial observational perspec- connection or locked-in syndrome, in which patients present tive. This, therefore, helps partially disregard the regions with no indication of consciousness, however, upon recov- inactivated during meditation, allowing the focus to reside ery, later describe being fully aware but unable to express on the medial prefrontal cortex, as already associated with themselves. Advances in brain – computer interface tech- the sensation of ownership. nology allow patients to use focused attention to control the movement of a cursor on a screen. This can help improve quality of life by facilitating communication in patients that are ‘locked-in’ or suffer from poor muscular Conclusion control. Both areas of study could greatly help expand our understanding of the neural mechanisms involved with gen- The subjective nature of this research creates a major limit- erating consciousness. ation in terms of interpretation of mental states. The exper- This paper has systematically narrowed down regions imental validity relies upon accurate descriptions from the within the brain associated with the various aspects of con- meditation practitioners concerning their subjective experi- sciousness. Due to the ambiguity surrounding the definition ences, however the evidence does empirically show systematic of ‘consciousness’, it has been necessary to explore a multi- change within the neural chemistry during these states. tude of facets suggested within this field. The ‘self-model’ Further research into anaesthetic awareness would con- has incorporated internal and external sensory information tribute greatly to this ongoing debate. This would allow and sensations of a first-person perspective relating to owner- for the inactivation of much of the transient aspects of con- ship, agency, working memory and various bodily frames of sciousness, and help isolate the neural correlate of the ‘core reference. It has been assumed throughout that the sensation self ’. Locating this ‘core self ’ has serious implications regard- of ownership is an omnipresent phenomenon during waking ing brain death, and subsequent decisions for organ trans- consciousness and that the different ‘transient’ conscious plantation. If activity relating to the ‘core self ’ resides states occur circumstantially, recruiting specific neurological definitively within a particular subset of neurones found in assemblies dedicated to their specific domains. These have the medial prefrontal cortex, physicians can use this infor- included distinct regions of the parietal lobe and prefrontal mation to search for neurological activity in patients present- cortex. It is proposed that these contingent conscious states ing with little or no consciousness. Currently, two medical arise, flavouring global consciousness with perspectives of opinions are required to assess the level of brain activity agency, TOM, spatial and visual awareness, etc., all resting within patients prior to brain death being established, and on a foundation of ownership as the ‘core self ’. organs made available for transplantation. Detection of Consistent with fMRI, PET and lesion data, the pervading activity within this specific region can indicate the presence sense of ownership over one’s perspective and experience has of conscious awareness, raising a number of ethical ......................................................................................................................................................................................................................................... 171 Review Bioscience Horizons † Volume 4 † Number 2 † June 2011 ......................................................................................................................................................................................................................................... corresponded consistently with neural assemblies within the References medial prefrontal cortex, although it cannot be concluded 1. 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Journal

Bioscience HorizonsOxford University Press

Published: Jun 4, 2011

Keywords: medial prefrontal cortex consciousness ownership self

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