The question arises as to whether or not the modulatory effect of collaterals on the sensory information passing through the thalamus simply involves just a gating operation, that is the removal of information. We have already seen that this is not the case as a collateral effect generates the experience of heaviness when holding a suitcase, independent of any incoming sensory impulses (Figure 7). Humphrey suggests that collaterals may also sustain impulse traffic in sensory nerves after the sensory perception has passed. The effect of this would be to experience sensations without there be any continuing effect on sensory receptors, although these receptors would have been involved in the initiation of the experience in the first place. This brings up a question concerning the time over which consciousness of a sensation occurs. The tricky nature of the experience of time in consciousness as compared with objective time, measured by a clock for instance, is well illustrated by the 'cutaneous rabbit' perceptual illusion (Figure 8). In this illusion a series of taps to the wrist is followed by taps to the upper forearm and then to the shoulder, as shown in Figure 9A. Surprisingly this is experienced as a series of taps that are equally spread out along the whole length of the arm, rather than confined to just three positions on the arm, as if an animal (a 'rabbit') had run up the arm. Even more surprising is the result of just giving the series of taps to the wrist in the absence of any taps to the forearm or the shoulder (Figure 9B): in this case the taps are all experienced as confined to the wrist without any of them appearing to be spread out along the arm. Why then in the first experiment did the brain interpret the taps at the wrist as experienced spread out along the arm whereas in the second case they remain confined at the wrist? With reference to Figure 8, a plausible explanation why the first five taps in A were experienced as distributed along the arm whereas the five taps in B were confined to the wrist is that the taps are not perceived simultaneous with the events. In a certain window of time (1 to 2 seconds) the brain determines the most likely spacetime story relating to the taps: in A the preliminary story that all five taps occur at the wrist is wiped out by the later arriving taps so that the final story that enters consciousness is that the taps are spread out equally in a space-time sequence; in B the preliminary story that all five taps occur at the wrist is not wiped out by any later events and so this enters consciousness. The brain then uses the time available before behavour is acted out to arrive at the most reasonable story based on sensations (the taps) and past experience to arrive at an interpretation. This window in time could be delineated by the earliest time at which sensations enter the brain and the latest time at which the experiences might be used to modify behavour.

The actual time at which occurrences are first registered in the brain might not then be the same as the times allocated to them by consciousness. Another example of this is illustrated in Figure 10A, which shows the distribution of dermatomes for skin sensations as in Figure 4. The nerves leading from the dermatomes over the buttocks to the brain clearly involve a much longer pathway than do the nerves from the dermatomes over the neck to the brain. It might be naively expected then that if one was to be touched simultaneously on the buttocks and the neck, according to objective timing, then the experience of being touched on the neck would enter consciousness before that of being touched on the buttocks. But this is not the case, as it depends on the context in which this touching occurs as to whether one has the conscious experience of being touched in one place or the other within a certain window of time The hypothetical graph in Figure 10B illustrates that the time of experiencing being touched on various parts of the body (or on different dermatomes) need not coincide with the objective time of the sequence of touchings. The brain creates the most likely story, using the information that it receives from sensory receptors, the context in which this is gathered, and past experience, before allocating times to particular events. Humphrey suggests that collaterals not only gate incoming sensory activity, for example at the level of the thalamus, but they can also sustain that activity after the sensory receptors are no longer stimulated. This would then give rise to a sensation that is extended in time within consciousness. It gives rise to an important idea in Humphreys' scheme, namely that of the 'sustained sentient loop', in which the issuing of an outgoing command over a collateral can give rise to a sensation that is extended over time in consciousness by the sustained activity of the collateral.

Figure 10. [Click image to enlarge] The complexity of the sense of time in the brain is again illustrated by considering the experiences relating to someone touching you simultaneously on the neck (at sensory skin or dermotome level C4 in A) and on the buttocks (at sensory dermatome level S3 in A). The nerves bringing information to the brain from C4 and S3 are clearly very different in length; as they have about the same rate for conducting impulses it would be expected that information concerning touch at S3 would enter consciousness at a later time than that from touching at C4. However, the actual time at which the occurrences are first registered in the brain is only part of the information that is used to allocate times to them entering consciousness; assumptions regarding the circumstances of this touching will also be used to allocate times. The brain then creates a story before it allocates the time to particular events; it does m simply take the actual time of arrival in the brain of impulses as if there were simply some finishing line in the brain which monitored the time at which the line was crossed by impulses. The graph in B illustrates this process by showing a line of 'events' 1 to 5 that are the experimental time for the objectively timed events of being touched on different sensory dermatomes in the patio-temporal sequence S3 to C2 shown. The series of touches at one fifth of a second intervals from S3 to C2 in the order shown may be experienced as the temporal series 1 to 5, that is as a spatially continuous stroking frum, the buttocks to the head, depending on the story created by the brain, given the circumstances.

Figure 11. [Click image to enlarge] Diagrams showing the regions of high neuronal activity in the brain associated with simple and complex motor (muscular) tasks and with the rehearsal of motor tasks without any muscular activity. A shows the region of high excitability in the brain that occurs when the subject is asked to simply flex a single finger against a spring. One area of excitability is confined to the motor cortex that drives the motoneurones of the spinal cord necessary for contracting the muscles responsible for finger flexion; the other area of excitability is the somatosensory cortex that receives the sensory stimuli from sensory receptors in the flexing muscle and in the joints that are moved in the finger. B shows the region of high excitability in the brain that occurs when the subject is asked to perform a more complex motor act, this time involving the placing of a key in a lock and turning it. In this case a new area of excitability is found in the brain in addition to the motorcortex and somatosensory cortex. This new area is the supplementary motor cortex in the midline of the brain as shown. Supplementary motor cortex carries out the selection of suitable neurones in motor cortex to perform the finger movement sequence involved in the more complex motor task. C shows the region of high excitability in the brain that occurs when the subject is asked to carry out a mental rehearsal of the complex motor act in B (with the key) only. In this case the supplementary motor area is excited but not the motor or somatosensory cortex. Note that in this case the subject issues commands associated with the complex motor act but does not allow them to be carried out. These results were obtained by Roland who by monitoring the rate of local blood flow in different regions of the brain with non-invasive techniques, was able to determine the areas of excitability. Active neurones require more oxygen than others and so require a greater blood flow; monitoring this then gives a measure of the areas of high neuronal activity.

The brain can possess neurones which are active and which are not directly involved in either sensation or the issuing of a motor command. By monitoring the rate of local blood flow in different regions of the brain with non-invasive techniques, Roland has been able to determine the areas of neuronal excitability. Active neurons require more oxygen than others and so require a greater blood flow; monitoring this then gives a measure of the areas of high neuronal activity. Figure 11 shows how this technique has been used to determine the distribution of active neurones involved in the intention to perform a motor act. Active neurones are found in the motor cortex if a finger is flexed against a spring as expected; in addition active neurones are found in the somatosensory cortex which is of course receiving kinesthetic information from the muscles being contracted (Figure 11A). However, if a more complex motor act is executed, such as turning a key in a lock, then another set of active neurones is brought into action, in the area of the brain called the supplementary motor cortex (Figure 11B); this area is always active when complex motor activity is taking place. If now the turning of a key in a lock is simply rehearsed mentally, with no motor command being executed, then the supplementary motor cortex possesses active neurones as before but the motor cortex and the somatosensory cortex do not (Figure 11C). This is then an example of the motor system operating in the absence of any motor output at all.

The central idea in Humphreys' scheme is that collaterals, perhaps originally associated with the motor system during evolution, may give rise to a sustained sentient loop without there being any motor act performed. We have seen that the motor system itself, in the case of the supplementary motor cortex, may give rise to activities that do not result in a motor action. The issuing of commands that set up a sentient loop amounts to the experiencing of sensations over time; this is a process that has become modified from the original collateral effects which simply acted on incoming sensory information. Humphreys' ideas concerning the evolution of the 'sustained sentient loop' are summarized in Figure 12. At first there was a simple nerve pathway consisting of a sensory input, which might be related to a noxious stimulus to the skin, resulting in a motor output involving withdrawal from the site of the stimulus. In Humphreys' terminology this amounts to a 'wriggle of rejection'. It is shown in Figure 12A as involving the brain but it would be better represented in vertebrates by a reflex sensory nerve pathway that passes directly from the skin to motoneurones in the spinal cord and from there to the appropriate muscles, as in Figure 6. The next stage in the evolution of the sentient loop involves modification of the incoming sensory signal by a collateral from the outgoing motor signal, as in Figure 12B; examples of this occur in the gating out of components of the signals to do with the action of muscle receptors involved in gamma motoneurone activity by motor collaterals, discussed in relation to Figure 6. With the further evolution of collateralization the motor command could modify and sustain over time the information coming into the brain along a sensory pathway so as to sustain a sensory experience, as shown in Figure l2C; the projection from the motor cortex to the reticular nucleus of the thalamus provides just such as pathway for modifying and sustaining the sensory input arriving from primary afferent fibres, as discussed in relation to Figure 8B. Finally the stage is reached during evolution when collaterals, originally associated with motor commands, are now used to generate sensations independent of any sensory input to the brain, as in Figure 12.

Figure 12. [Click imager to enlarge] Evolution of the sentient loop and therefore consciousness as envisioned by Humphrey and superimposed on the primate brain. According to Humphrey to feel a sensation in consciousness is to issue a command or outgoing signal; sensation is then the making of the sensory response. A, shows simple incoming sensory pathways to somatosensory cortex and an associated outgoing motor act initiated by the motor cortex in response to the sensory signal. This may be likened to the 'wriggle of acceptance or rejection' that Humphrey traces back to simple animals like sponges; the wriggle is the motor response to the motor command that is issued in response to the sensory input. B, the next level of sophistication was the evolution of the corollary discharge, by which the motor command in response to the sensory signal is used to modify that signal. We have seen how corollary discharges may modify the information about kinesthetic experience. C, Humphrey's suggests that the corollory discharge associated with a motor command in the context of a particular sensory experience may become modified so that the motor command is not executed and the corollory discharge is then used to sustain in subjective time the sensory experience. This gives the 'after glow' of a sensory stimulus, that is the experience is maintained in subjective time even though it has passed in objective time. D, finally, the motor command can be given without any sensory input from the environment, creating a 'cerebral sensory loop'. To feel a particular sensation is to engage in an appropriate form of sentition (the activity of sensing) and so issue an appropriate outgoing signal from the brain.' It is this process which is consciousness.

D, the cerebral sentient loop is now independent of the environment. The experience of a sensation involves a positive act of issuing an appropriate outgoing signal from the brain. According to Humphrey sensing is not a passive act but involves participating in the act of 'sentition' or the issuing of a command, originally associated during evolution with the motor system only. Since these commands can be issued without any trigger from the environment it is possible to have a rich 'stream of consconsiousness' that is generated from within the brain itself.

Does Humphreys' thesis stand up to critical attention? I have tried to flesh out the ideas in his book by reference to what we know about collateral effects and feedback pathways that modify incoming sensory signals bringing us information about our environment. The idea of 'sentition' whereby the nervous system issues a command that results in a sensory experience and therefore consciousness is a novel one. According to this idea consciousness first appears during evolution with the species that uses motor collaterals to generate or modify sensory inputs to the brain. It is possible that this occurred as early as the evolution of the flat worms if it can be shown that they are able to modify the sensory input to their central head ganglia by means of motor collaterals. Any animal that can issue commands for altering or generating sensory activity, and can by this means make a sensory response, possesses consciousness. The idea does have the great attraction of providing some basis for continuity in the emergence of consciousness rather than just positing it as the special preserve of Honto Sapiens or even of just the mammals. For me its deficiency is that it does not provide a framework that is sufficiently specific to suggest a research plan that allows testing the central hypothesis of the sustained sentient loop as the basis for consciousness. Although consciousness can only be examined by introspection, the non-invasive techniques for examining the neurophysiological concomitants of mental functioning, such as Positron Emission Tomography, may help to clarify the issues. It will be interesting to see if those areas of the brain involved, for example, in forms of cognition that do not involve language, are also active in other mammals than the primates under suitable conditions. The role of collateralization in the evolution of such areas might then be an interesting subject for study.

N.Humphrey (1992) "A History of the Mind" Chatto & Windus.

G.Edelman (1992) "Bright Air, Brilliant Fire" Penguin Press.

D.C.Dennett (1991) "Consciousness Explained" Penguin Press.

J.Searle (1992) "The Rediscovery of the Mind" M.I.T. Press.

C.Blakemore and S.Greenfield (1987) "Mindwaves" Blackwells.

Scientific American (1992) "Mind and Brain" September Issue.

R.Gregory (1988) "The Oxford Companion to the Mind" Oxford.

Ciba Foundation (1993) " Experimental and Theoretical Studies of Consciousness" Wiley.

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