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.