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The cortex and consciousnessEdit
The cerebral cortex consists of a set of specialised areas that process different aspects of sensation and motor control. There are about ten times as many nerve fibres going from the cortex to the thalamus as there are from the thalamus to the cortex (Destexhe 2000).
Histologically the cerebral cortex is a layer of greyish neurons overlying a huge mass of white nerve fibres, the cerebral medulla. The cortex consists of six main layers. The upper layers receive input from the relays in the thalamus such as the lateral geniculate, from the thalamus in general and from other areas of cortex plus a few specialised inputs from other locations. The lower layers give rise to output fibres that largely connect with the thalamus and other areas of cortex although particular specialised processors in the cortex may also have direct connections elsewhere such as to motor nuclei.
The cerebral cortex has many functions and is divided up into numerous separate processors. The most important function of the cortex from the point of view of consciousness studies is that it creates models. These models are most easily experienced when there is a lack of sensory input such as in dreaming, day dreaming, lucid dreaming or experiencing imaginary speech (thinking). In ordinary waking life the modelling processes create a model of the world around us and within us based on sense data and associated data. This model consists of overlapping sounds, images, smells etc. and is a combination of perceptual fields from all the senses.
There is considerable evidence that the parts of the brain that deal with imagining (modelling) things are also the parts that deal with perception (ie: modelling the world). The overlap between imagination and normal perception is not complete because, as Tong(2003), in a review of visual consciousness, put it: "Internally generated experiences share some, but not all, of the phenomenal properties of actual perception". There is also considerable overlap between the areas used for imaginary speech (thought) and actual speech, areas dealing with the control of sensation and of the tongue etc. being used in actual speech but not in imagined speech (Fu et al 2002). Kreiman et al (2000) investigated the activity of single neurons in humans and also found that the brain activity evoked by visual imagination overlapped that which occurs upon direct stimulation by the same image.
Our conscious experience consists of the output of the cortical modelling processes. The cerebral cortex itself appears to be non-conscious. The evidence for the non-conscious nature of the cerebral cortex is reinforced by lesion studies that show that up to 60% of the cerebral cortex can be removed without abolishing consciousness. Either hemisphere can be removed or much of the front or back of the cerebral cortex can be cut off yet consciousness persists. The cerebral cortex is often assumed to be the "seat" of consciousness because this collection of organs is relatively large in humans but the truth seems to be that the cortex is a collection of processors that provide an input to experience. There is also a substantial amount of neurophysiological evidence that the cortex is non-conscious.
Libet et al (1967) found that there could be cerebral cortical activity in response to weak stimulation of the skin without any conscious awareness of the stimulus. This work provides a neurophysiological basis for subliminal (non-conscious) perception and also shows that large areas of the cerebral cortex can be active without conscious experience. The insensitivity of experience to cortical activity has been further confirmed by Libet et al (1979). They electrically stimulated the cerebral cortex of conscious patients and discovered that the stimulus must be continued for about 0.5 seconds for subjects to report a conscious experience of the stimulation. What is the cortex doing in the 0.5 seconds between the start of stimulation and the report of awareness of the stimulation? It is probably synchronising its various processors and creating a waking dream, a structured set of events that accounts for the activity.
The 'Attentional Blink' (Raymond et al 1992) is also consistent with the concept of the cerebral cortex being a device that creates models. In the 'Attentional Blink' the identification of an object impairs the identification of a second object that is presented within 0.5 seconds of the first. Raymond et al used a stream of letters (11 letters per second) and the identification of a first letter impaired the identification of a subsequent 'probe' letter in the stream. If the probe letter followed the first letter within about 180 msecs it could easily be identified, suggesting that chunks of about 180 msecs of data stream are modelled together. Christmann & Leuthold (2004) have theorised that the 'Attentional Blink' involves perceptual and central components of visual processing. This is supported by the fMRI studies of Marois et al (2004) who presented subjects with faces mounted on scenes of places. The scenes of places often went undetected by subjects but they activated regions of the medial temporal cortex involved in high-level scene representations, the parahippocampal place area (PPA). When the scenes of places were detected by the subjects there was activity in the frontal cortex and the PPA activity was increased. These experiments are consistent with the idea of a cerebral cortex that is a multiprocessor system that creates consistent models of the environment for presentation to some other part of the brain.
Bregman's (1990) auditory continuity illusion is another example of how sensory events are modelled. If a pure tone is followed by broadband noise and the noise followed by the same pure tone it seems as if the tone occurs throughout the period of noise. If the noise is not followed by the pure tone there is no sound of the tone during the period of noise. This effect is similar to the results found by Libet because a delay of several hundred milliseconds between sensory stimulation and conscious experience is needed to account for the apparent rewriting of history after the second tone appears.
The 0.5 second delay required for the cortex to model an event has implications for the role of conscious experience in the control of our lives. If experience is always 0.5 seconds behind the true present instant then how can we be said to control anything? The brain must be acting automatically whilst performing most tasks. The 0.5 second delay also seems to contradict our everyday experience. We certainly feel like we are aware of things in less than 0.5 seconds, for example, the direct stimulation of sense organs seems to be experienced much more rapidly than the delayed experience of cortical stimulation. In fact subjects report that they are conscious of stimuli, such as being touched or seeing flashing lights, within 0.1 to 0.2 seconds of the event. So how can subjects report events within 0.2 seconds even though it seems to take 0.5 seconds for the cortex to generate activity that can be experienced? The simplest explanation is that the reaction occurs automatically within 0.2 seconds and then the conscious experience of this reaction occurs 0.3 seconds later. This gives a total 0.5 seconds delay before conscious experience whilst allowing fast reactions.
Libet et al extended their experiments by stimulating a "relay nucleus" in the thalamus that intercepts signals from the senses before they reach the somatosensory cortex. It was found that when this nucleus was stimulated for 0.5 seconds the subjects reported that the stimulus occurred 0.2 seconds after it had begun. When the nucleus was stimulated for less than 0.5 seconds the subjects did not report any sensation. This supports the concept of a 0.5 second delay whilst the cortex puts a stimulus in context before it is experienced.
These experiments show that our experience is an output of cortical processing rather than the processing itself. If our conscious experience is non-cortical then this raises the possibility that the non-conscious cerebral cortex can perform actions without conscious control. Of course, the cortex does this all the time when we are indulging in skilled or routine behaviour. The ability of the non-conscious cortex is quite remarkable; for instance car drivers sometimes discover that they have driven for several miles without conscious experience of driving, even at the level of having no recollection of the route.
Although it might be accepted that much of our everyday behaviour is automatic is there any behaviour that is definitely initiated by conscious experience? This is probably a pointless question because consciousness is about observation, not action; however, despite this there have been several experiments that have attempted to determine the relationship between consciousness and action.
In 1964 Kornhuber and Deecke performed a series of experiments that measured the electrical activity from the scalp (EEG) during voluntary actions. They averaged many EEG's from subjects who were about to move a finger and discovered that there is an increase in scalp potential before the movement takes place. The increase in potential can start as long as 2 seconds or so before the movement and is known as the "readiness potential" (Bereitschaftspotential). The readiness potential is strange because it seems to contradict our conscious experience; we do not decide to move a hand and then wait 2 seconds before the hand moves. It seems that the non-conscious brain may be taking things into its own hands.
Libet et al (1983) extended the readiness potential experiments by asking subjects to observe a Wundt clock whilst flexing a finger. The Wundt clock had a spot of light that moved around a circle every 2.56 seconds and allowed the subjects to obtain timings that were related to their mental experiences. When the subjects flexed a finger it was found that the readiness potential occurred about 0.5 seconds before the finger moved and the subjects reported they were going to move the finger about 0.2 seconds before the movement. This suggested that a subject's cerebral cortex was preparing for the movement about 0.3 seconds before the subject was conscious of this. Libet's experiments have been reproduced elsewhere (see Keller & Heckhausen 1990). (It is important to note that the subjects in Libet's experiment were asked to wait until they felt the urge to move the finger.) These results are consistent with the idea of the cortex as a modelling system that constructs a consistent model of events to pass on to whatever mediates conscious experience.
Perception, Imagination, Memory and DreamsEdit
Functional Magnetic Resonance Imaging (fMRI) has shown that similar areas of brain are used during perception involving the senses as during imagination (Tong 2003, Kosslyn and Thompson 2003). The phenomenal substrate of the mental images that occur in both modes of brain activity has not yet been found.
Ganis et al (2004) used fairly complex perceptual and imagination tasks that activated large areas of the brain, they found an overlap between the brain areas activated during perception and imagery. The principle areas that were different in the two tasks were found in the primary sensory areas of the visual cortex. Other areas in the visual cortex and activity in the rest of the brain showed a remarkable degree of overlap. The authors suggested that the differences in the activity of primary visual cortex may have been due to differences between the perceptual and imaginary stimuli such as speed of onset etc. The hippocampus was not activated.
It is intriguing that, contrary to object imagery, spatial imagery such as predicting when a cross on a screen would fall on an imaginary letter actually seems to inhibit activity in sensory visual cortex (Aleman et al). Both fMRI and blocking with transcranial magnetic stimulation (TMS) showed that the posterior parietal cortex was involved in the spatial imagery.
Imagery involving places and faces activates the place and face areas that are activated during perception (Ishai et al 2000).
The recall and recognition of things also seems to involve very similar brain areas to those used during perception. Wheeler and Buckner (2003) showed that areas involved in perception were also involved in the recall of the perceptual stimuli.
Recall causes activation of areas used in perception but also seems to use areas that may be particularly related to the process of recall iself, such as the left parietal cortex (Konishi et al 2000) (Brodmann's area 40/39). Frontal and parietal regions are involved in the recognition of whether stimuli have been experienced before.
Image generation during sleep seems to differ from that during imagination and recall. In particular it seems to involve a few well defined areas of cortex and considerable activation of the posterior thalamus.
Sleep studies have shown that people dream throughout sleep. However, dreams are more frequent during the REM (rapid eye movement) periods of sleep than the NREM (non-REM) periods. Dreams are reported after 70-95% of awakenings in REM sleep and 5-10% of awakenings in NREM sleep. REM dreams are more visual than NREM dreams which are more 'thoughtlike' (Solms 2000). Thoughtlike events (mentation) are reported after 43% of awakenings from NREM sleep.
Solms (1997) found that patients who had lesions in the parietal-temporo-occipital junction reported a cessation of visual images in dreams. Solms also found that patients with lesions in the white matter inferior to the frontal horns of the lateral ventricles, in the ventromesial quadrant of the frontal lobes, also reported loss of dreaming. Loss of dreaming is also reported by leucotomised patients with frontal ventromesial damage. Damasio et. al. (1985) and Solms (1997) also reported that some patients with damage to the medial prefrontal cortex, the anterior cingulate cortex, and the basal forebrain became confused about what was real life and what was dreaming (waking dreams occurred).
Studies using fMRI show that the sensory occipital lobe (BA 18) and posterior thalamus, especially the lateral geniculate nuclei, are activated in REM sleep, weaker activations of the posterior cingulate, putamen and midbrain were also found (Wehrle et al 2005, Loveblad et al 1999). These findings are consistent with activation of the ponto-geniculo-occipital system (PGO) during REM.
So dreams may be more like primary activations of sensory cortex than imagining or recall. This suggests that dreams have a thalamic origin or are managed via connections from the cortex through the thalamus to the visual cortex.
Hallucinations seem to differ from dreams. In Charles Bonnett Syndrome patients can have clear hallucinations. These, like imaginations, seem to involve areas of the visual cortex that deal with processed data, for instance hallucinations of faces activate the "face area" rather than visual cortical area V1 (ffytche et al 1998).
More about ModelsEdit
Our dreams are clearly models that form a 'dreamworld' but the idea that perception might be like a dream that is updated by sensation is not so obvious. Experience seems to be an active model of the world (virtual reality) based on sense data rather than a simple mapping of retinal and other sensory data. This is demonstrated by visual illusions such as the Ames Room, Spoke Illusion and Muller Lyer illusions shown below:
Notice how the circle is distorted without any distortion in the 'spokes', it is as if the circle has been treated as a separate object by the processes in the brain that rearranged it. In all of these illusions the brain has rearranged large areas of the visual field and has managed the input as a collection of 'objects' that are manipulated separately. Even movement seems to occur in some figures showing that the brain models the position of things:
The creation of a model is also demonstrated by the illusion of movement experienced when we watch the cinema or television. This is due to the cortical modelling that is known as 'short-range apparent motion' rather than flicker fusion or persistence of vision. It is intriguing that, although it has been known for decades that the joining together of static images in our minds is due to modelling activity in the brain the myth that it is due to persistence of vision or flicker fusion is universal. As Anderson and Anderson (1993) noted:
Indeed, in the past decade, psychoanalytic-Marxist film scholars have retained the model implied by persistence of vision: theirs is a passive viewer, a spectator who is "positioned," unwittingly "sutured" into the text, and victimized by excess ideology.
Our experience of the cinema is like a dream updated by sensation rather than sensation updated by interpretation. In fact the most compelling evidence for the modelling power of the brain is the existence of dreams; our dreams are often models of worlds that do not exist and involve little or no sensory input yet can involve effects as powerful as any television drama.
Short range apparent motion occurs when the interval between presentations of an object is brief (c. 50-100 msecs). Motion modelling in response to longer intervals is known as long range apparent motion. There is evidence that the modelling in short range apparent motion is enhanced if the moving patterns are similar to moving human forms (such as patterns of dots outlining a person)(Thornton et al 1998). The accuracy of predicting movement can actually improve if the interval between presentations is increased when human forms are used.
Motion modelling can also be seen in visual illusions such as the Waterfall Illusion (motion aftereffect). The waterfall illusion is commonly seen after viewing a sequence of scrolling credits on the television; when the credits stop rolling it appears as if they briefly move in the opposite direction. Tootel et al (1995) have used fMRI to show that this is correlated with activity in the motion modelling area of visual cortex (area MT/V5). The waterfall illusion is also associated with an intriguing aftereffect known as storage of the motion aftereffect. Normal motion aftereffects last for up to about ten seconds after the stimulus, however, if the subjects close their eyes for the normal duration of the aftereffect then reopen them they see the illusion for almost the normal duration. Culham et al (1999) used fMRI to show that activity in area MT/V5 was low during the period when the eyes were closed then increased dramatically when the eyes were opened. This is strongly suggestive of a modelling mechanism outside MT/V5 that has adapted to motion and then models stationary data with movement in the wrong direction.
Visual area MT/V5 is also involved in the separation of moving visual scenes into sprites or objects that move together as a whole within a scene (Muckli et al 2002).
The way that mental models may be the basis of ordinary reasoning was outlined by Johnson-Laird (1980), based on earlier work by Kenneth Craik.
Studies of 'change blindness' and 'inattentional blindness', where subjects fail to spot outrageous changes in their environment, also demonstrate that we experience a model and suggest that the brain must analyse an object to incorporate it fully into the model (See for instance Rensink (2000), Simons & Rensink (2005)).
Blindsight studies illuminate the relationship between the cerebral cortex and our experience. When the visual cortex is removed subjects become almost totally blind. If the visual cortex on one side is removed subjects become relatively blind in the contra-lateral hemifield. One of the most revealing studies of blindsight is Marcel's 1998 paper: " Blindsight and shape perception: deficit of visual consciousness or visual function?".
It is useful when considering blindsight to contemplate for a while the appearance of the world with both eyes closed and then with one eye closed. When both eyes are closed our experience is of a darkish space radiating out from our heads, with one eye closed we tend to ignore the darkish areas that cannot be seen even though they are still present. Marcel notes that patients who have a right blind field still have an underlying visual field on the right side and that this can even contain conscious visual experience. This sounds a bit like the darkish space that we all experience if deprived of visual input on one side. As Marcel says: "A question that naturally arises is whether the loss is a 'total' loss of visual consciousness in the blind field. It is often assumed to be so, especially by those who discuss blindsight without carefully reading the literature or working with the subjects. One can immediately respond negatively to the question.."
The consciousness of the completion of Kanizsa figures in blindsight patients is particularly indicative of the preservation of the field even though the content was largely missing. A Kanizsa figure is shown below:
If we put Marcel's observations together with cortical anatomy and function it seems that the space of our experience is located outside of the cerebral cortex. The cortex generates much of visual and other content but it does not generate the space. A reasonable hypothesis is that the field of brain activity that is the space of our conscious experience is located in the sub-cortical brain. This space is loaded with the output of the cortex.
The Role of the ThalamusEdit
The thalamus is connected to the entire bottom layer of the cerebral cortex. It is the nexus of the various cortical processors as well as a recipient of independent input from most of the rest of the brain.
The thalamus is subdivided into numerous small and medium sized nuclei that between them receive inputs from every process in the nervous system (the white fibres in the illustration above largely penetrate the thalamus). The thalamic nuclei are interconnected which means that any of them could, potentially host activity from anywhere in the body or brain. Although the founders of neurology such as Hughlings Jackson and Penfield & Jasper located conscious experience in the diencephalon, including the thalamus, this is no longer the conventional wisdom. The small size of the thalamic nuclei means that they cannot support the processes that are assumed to compose access consciousness, however, even some of the smallest thalamic nuclei host millions of synapses so size would not be an obstacle if the thalamus contains the substrate of phenomenal consciousness. Indeed, the diencephalon and the thalamus in particular can be shown to be excellent candidates for a possible location of phenomenal experience.
The Intralaminar Nuclei of the thalamus. The white space above and to the left of RN is the third ventricle. MD=mediodorsal nucleus. CM=Centromedian nucleus, RN=red nucleus (not part of thalamus) The black areas are stained white fibres. Picture from: http://www.neurophys.wisc.edu/ University of Wisconsin and Michigan State Comparative Mammalian Brain Collections. Preparation of image has been funded by the National Science Foundation, as well as by the National Institutes of Health. May only be used with these acknowledgements.
If the thalamus contains a location for conscious experience then lesions should abolish this experience. Unlike the cerebral hemispheres, lesions of the thalamus do indeed seem to abolish consciousness. The area that is most sensitive to lesions contains the Intralaminar Nuclei, especially the Parafascicular and Centromedian Nuclei. If these are damaged bilaterally patients suffer death, coma, akinetic mutism, hypersomnia, dementia and other equally serious impairments of consciousness that depend upon the size and placement of the lesions (Bogen 1995, Schiff & Plum 1999). In cases of fatal familial insomnia, in which patients exhibit many of these symptoms, there is marked neuron loss in the Intralaminar Nuclei (Budka 1998). The symptoms of bilateral damage to the ILN are often so severe that it is possible that the patients cease to be conscious and are being coordinated by automatic cortical processes.
Laureys et al (2002) investigated recovery from 'persistent vegetative state' (wakefulness without awareness). They found that overall cortical metabolism remained almost constant during recovery but that the metabolism in the prefrontal and association cortices became correlated with thalamic ILN and precuneus activity. Again confirming that thalamo-cortico-thalamic activity is required for consciousness and that cortical activity by itself is not conscious. Yamamoto et al (2005) investigated persistent vegetative state and found that deep brain stimulation (25Hz) of the centromedian-parafascicular complex (19 cases) or mesencephalic reticular formation (2 cases) resulted in 8 of the patients emerging from persistent vegetative state.
As Bogen(1995) demonstrates, the ILN receive inputs, either directly or indirectly, from every part of the CNS but what do they do?
Interest in the thalamus has recently been revived by the theories of Newman & Baars (1993), Baars, Newman, & Taylor1998) and Crick & Koch (1990). In Baars, Newman and Taylors' (1998) theory it is suggested that "The brain stem-thalamocortical axis supports the state, but not the detailed contents of consciousness, which are produced by cortex". They also propose that the "nucleus reticularis thalami" (Thalamic Reticular Nucleus, TRN), which is a thin sheet of neurons that covers the thalamus, is involved in a selective attention system. This concept is reinforced by the way that point stimulation of the TRN causes focal activity in the overlying cortex (MacDonald et al 1998) and the way the TRN is organised topographically (ie: has activity that is like an electrical image of receptor fields).
The thalamus is ideally placed for integrating brain activity, if tiny parts of the thalamus are removed consciousness is abolished and the thalamus is involved in attention and the global integration of cortical activity. Any impartial judge might pronounce that the site of conscious experience has been found, possibly in the ILN of the thalamus, but no one can say how it works.
General Anaesthesia and the ThalamusEdit
General anaesthesia should result in a profound depression of activity in the ILN if these are indeed the sites of the conscious state. White & Alkire (2003) administered halothane or isoflurane to volunteers and used positron emission tomography (PET) to monitor brain activity. They found severe depression of activity in the thalamus. The depression appeared to be higher in the non-specific nuclei than in the relay nuclei of the thalamus. In other words the anaesthesia is neither turning off the cortex nor turning off the input to the cortex but it is turning off an important part of the thalamus. Fiset et al (1999) have also demonstrated a similar pattern of medial thalamic inactivity and cortical activity in propofol anaesthesia. Suppression of cortical activity is not the cause of unconsciousness; for instance, the anaesthetic agent chloralose leads to increased neural activity in the cortex relative to conscious patients (Cariani 2000).
This material has been imported fom the wikibook "Consciousness"under the GNU Free Documentation License.