"All parts of the brain may well be involved in normal conscious processes but the indispensable substratum of consciousness lies outside the cerebral cortex, probably in the diencephalon" Penfield 1937.
"The brain stem-thalamocortical axis supports the state, but not the detailed contents of consciousness, which are produced by cortex" Baars et al 1998.
Readers who believe that there can be no images in the brain should read Perceiving perception and Time and conscious experience before tackling this article.
Conscious experience is a simultaneous set of events, this means that it is a space, more than one thing at an instant being the definition of a spatial arrangement of things. If we are physicalists then events in a space must have a location in the universe. So where are the events that compose conscious experience?
Natural Realists and Direct Realists believe that the events that compose conscious experience are the actual objects in the world that are related to this experience. This idea has been discussed elsewhere (why direct and naive realism are unscientific). I can meditate and produce quite strong lucid dreams so naive realism has never seemed like a reasonable idea to me. I also enjoy art and when I look at the beautiful embroidery on the sleeve of
Directly linked to The Museum Network: Portraits and Portraiture http://www.museumnetworkuk.org/portraits/artworks/wallace/img4.html
Franz Hals' "Laughing Cavalier" I know that what I imagine to be beautiful embroidery is just a couple of masterful brush strokes.
My brain creates the image of the world that I call "experience" and this process is obvious when we look at the way our eyes dart from place to place to supply data about the salient points in a view. Our eyes dart about but the view is steady. Neuroscience has shown that the brain fills in any gaps between the salient points in our experience, for instance if two lights flash in succession it fills in the gap between them in the visual cortex with a moving pattern of nerve activity (See larsen et al 2006). The embroidery on the "Laughing Cavalier" is probably completed by my brain in a similar fashion. So, if the brain goes to all the trouble of "filling in" the missing parts of sensation to make a fulfilling perceptual experience, where is this experience?
Wherever conscious experience might occur it does not require much detail. Notice how, when you look at this page, you only see a few letters in any detail at any moment. Only an area of about 2 cm in diameter is present at high resolution and this would require less than a million pixels. If each pixel needed one neurone and the brain has a hundred thousand million neurones then my experience could reside in about a fifty thousandth part of the brain - it could happen in a volume the size of half a drop of water. (Incidently, it is the time extension of experience that contains the ephemeral higher detail that we cannot quite pin down).
The other important feature of conscious experience is that almost any single part of the brain can be removed without permanently removing consciousness (Bogen 1995). This suggests that wherever the site of conscious experience might reside it is relocatable or multiple. The idea that we might have multiple sites that can host consciousness is vaguely disturbing but not impossible and is compatible with the idea of "dominance" in neural function.
Any site that hosts conscious experience would need to be connected in such a way that the features in the visual field are related to each other in the same way as the neural activity that represents it. The neural activity that represents the letter "t" in the written word "it" would be next to the neural activity that represents the "i". This adjacency of sensory events is normal in the brain and known as 'topological mapping'. Topological maps in the brain tend to be fairly robust, requiring long periods to be re-wired, so the topological veracity of experience suggests that wherever our experience is located it should be the recipient of organised, topological projections from the various specialised processing centres of the cerebral cortex.
The existence of dreams etc. means that experience must be able to receive the output of a flexible modelling system (temporo-frontal cortex?), although this is overridden by sensory input to the cortex.
The spatial properties of experience are linked to the vestibular system (experience rotates if we turn round and round too much and moves with the movement of the head). This means that wherever experience is located must receive an important input from the organs of balance and motion detection in the head.
The fact that people remain conscious after large areas of cerebral cortex are removed and even anencephalic children are regarded as conscious suggests that the site of conscious experience is somewhere between the medulla oblongata and the thalamus inclusive. The most likely location for conscious experience in this zone is the thalamus because it receives a truly vast innervation from the cerebral cortex. (Even neuroscientists often forget that the cerebral cortex is a collection of specialist processors that provide an input to the thalamus - in functional terms the thalamus is at the top of the processing heirarchy, the cortex is only physically on top).
The thalamus is also an important site for long range synchronisation of cortical processes, being implicated in both long range gamma and beta band EEG synchrony. It is important in this role because the cortico-thalamo-cortical pathways are more numerous than the cortico-cortical pathways. That cerebral cortical synchronisation is needed for the occurrence of conscious experience is strong evidence that conscious experience is not directly in the cortex: it confirms the data from subliminal stimulation, general anaesthesia, coma etc that shows that unsynchronised cortical activity is always occurring without any conscious experience even in awake patients. This dependence of consciousness on cortical input to the thalamus is nicely demonstrated by the recovery of thalamo-cortical connectivity during recovery from persistent vegetative state (Laureys et al 2002).
There are areas of the thalamus, such as the intralaminar nuclei (ILN), which cannot be bilaterally removed without causing loss of consciousness through coma, mutism etc. . However, one side of the ILN can be removed and the patient can recover. It would be interesting to know what patients experience after the removal of one side of the ILN, if this were the side that was the putative host for conscious experience. My guess is that conscious experience would be transiently hazy but that patients may respond to external stimuli as if they were functioning well. This prediction is based on the function of conscious experience as a manager of the global state of the brain (see below).
The function of conscious experience is somewhat obscure because it does not seem to do anything in particular. For instance, non-conscious reflex reactions occur in a quarter of a second or less but there is a delay of about half a second before sensory stimuli become part of conscious experience (Libet et al 1979). So conscious experience is not needed to deal with most events. However, when a person is unconscious the brain can do very little to respond to the world. This means that conscious experience is not directly required for responding to most events in the world but is crucial for the overall function of the brain. Conscious experience is therefore most likely to be involved in the global stability and control of brain function.
The Intralaminar Nuclei of the thalamus are possible candidates for conscious experience. The Reticular Nucleus of the thalamus is also a possible candidate although this seems to be divided up into specialised areas like the cerebral cortex. Merker's work with anencephalic children suggests that there is also the possibility of conscious experience residing at the level of the superior colliculus (See Strehler 1991). However, I would guess that Merker's patients had developed this possible mesencephalic site for conscious experience to a higher degree than might be found in people with an intact cerebrum.
Perhaps many areas of the non-cerebro-cortical brain can take over the function of consciousness (ie: stabilising brain function as a whole) but conscious experience will be impoverished if it is not hosted in areas that have been evolved to host cerebro-cortical output.
In summary, I would largely agree with Alex Green's analysis of the location of conscious experience but would consider it to be more flexibly located than Green proposed. I also suspect that the neural representation of conscious experience might be stretched or deformed provided the adjacency relations (what is next to what) are preserved.(ie:the mapping from the brain in 3D to experience in 4D may involve further transformations that restore homotopy). The main reason why no-one is looking for a site for conscious experience is that they do not believe in the existence of time and so adhere to a cosmology that does not permit the idea of conscious experience in the brain (see the other articles on this site). However, if someone wants to look they will find that conscious experience is hosted in a volume of brain that is probably less than the size of a pea that normally receives input from all areas of the cerebral cortex in an organised, topological fashion and which has activity that is delayed by about 0.5 secs relative to the time of sensory stimulation. The function of consciousness is so crucial to the operation of the organism that this site for consciousness will be protected by a secure location and multiple redundancy.
The empirical description of conscious experience does not forbid more esoteric locations such as whole brain electromagnetic fields etc. but the ordered form of experience favours definite locations that have pre-arranged connections with the cortex.
Readers should beware of regarding this analysis as reductionist. According to the analysis given elsewhere in this blog our future accretes onto the time extended form of our experience. We are at least four dimensional and experience is not a passive screen, rather it is a template and an impulsion.
Where to start looking
Modern magnetic resonance imaging has sufficient resolution to pick up activity in a pea sized piece of brain. If it is possible to find a non-cerebro-cortical site that has activity in response to any stimulus that becomes part of conscious experience then that site is a prime contender for being the location of conscious experience. It is possible that the ILN already fulfill this criterion.
Postscript: the Canadian Hogan twins have a link between their thalami and appear to share sensations and even share a common mental space.
An MRI scan of the Hogan twins brain is shown above with the inter-thalamic link clearly demonstrated.
Interesting articles
Arnold Trehub (2007). Space, Self, and the Theater of Consciousness. Consciousness and Cognition 16 (2):310-330.
References
Bogen, J.E. (1995). On the neurophysiology of consciousness: I. An overview. Consciousness and Cognition, 4, 52-62. http://www.its.caltech.edu/~jbogen/text/bogen-95.pdf
Bogen, J.E. (1995). On the neurophysiology of consciousness: Part II. Constraining the semantic problem. Consciousness and Cognition, 4, 137-158. http://www.its.caltech.edu/~jbogen/text/concog95.htm
Larsen, A., Madsen, K.H., Lund, T.E., and Bundesen, C. (2006). Images of Illusory Motion in Primary Visual Cortex. Journal of Cognitive Neuroscience. 2006;18:1174-1180.
Laureys, S. et al. (2002). Brain function in the vegetative state. Acta neurol. belg., 2002, 102, 177-185 http://www.jsmf.org/meetings/2003/nov/LaureysANB2002.pdf
Libet, B., Wright, E. W., Jr., Feinstein, B., and Pearl, D.(1979). Subjective referral of the timing for a conscious sensory experience. Brain, 102, pp. 192-224.
Merker, B. (2007) Consciousness without a cerebral cortex: A challenge for neuroscience
and medicine. Behavioral and Brain Science http://www.bbsonline.org/Preprints/Merker-03062006/Referees/Merker-03062006_preprint.pdf
Strehler, B.L. (1991) Where Is the Self? A Neuroanatomical Theory of Consciousness. Synapse 4:44-91. 1991. http://www3.interscience.wiley.com/journal/109702768/abstract?CRETRY=1&SRETRY=0
Damage to a part of the right parietal cortex results in a loss of awareness in the left part of one's egocentric visual space -- hemi-spatial neglect. This suggests that neuronal mechanisms in the parietal cortex, in addition to the thalamic intralaminar nuclei are needed to sustain conscious experience.
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