Journal and Proceedings of The Royal Society of New South Wales
Volume 123 Parts 3-4 [Issued December, 1993]
pp.111-124 |
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A Consideration of Humphrey's "Cerebral Sentient Loop" Explanation of Consciousness from "A History of the Mind" by Nicholas Humphrey
MAX BENNETT
The evolution of the nervous system may have started a thousand millions years ago with the sponges or just six hundred and fifty million years ago with polyps like jelly-fish and corals. Sponges are extremely simple multicellular organisms (Figure 1A). A section through a sponge, when stained with silver, shows some cells that connect one side of the body wall of the animal to the other (Figure 1B); it has been claimed that these may be primitive nerve cells. With the evolution of the Colenterates, such as hydra, jelly-fish and corals, the identification of nerve cells and muscles is unequivocal: jelly fish have two layers of cells with a jelly-like substance seperating the two, giving the animal some rigidity (Figure 2A). Nerve nets for the control of swimming, tentacle position and feeding are composed of either bipolar or multipolar neurones (Figure 2B). These nerve nets come together in integrating centres, where a mixture of both neurone types may be found (Figures 2C and 2D); these centres are known as ganglia.
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| Figure 1. [Click image to enlarge] The Porifera or sponges are very simple multicellular parasites. They do not possess a nervous system and it is controversial as to whether they have any neurones. In A is shown the simple motor reactions of the fresh water sponge Ephydrata, with the mouth chimney changing its' form as water is drawn into the body through small pores and passed out through the mouth. In B is shown some of the cell types that stain with silver in the sponge Sycon Raphanus; the outer surface is connected to the inner surface by two cells with long and thin processes that may be nerve cells; the cells on the inner surface are collar cells ( called choanocytes). | Figure 2. [Click image to enlarge] The Colenterates or polyps like hydra, jelly-fish, sea-anemones and corals have a sack-like body with tentacles as shown in A. In B are shown the two different kinds of neurone networks present in Aurells Aurata: one is composed of neurones that possess two axons (bipolar neurones ) and these are promenent in relation to the radial and circular muscles that are exposed in this drawing; the other is composed of neurones with more than two processes (multipolar neurones) and these are shown in relation to the gastric cavity. Both bipolar and multipolar neurones from each nerve net are apposed to each other in collections on neurones called ganglia, as shown in C; here the input to the bipolar cells a2sociated with the muscle is transferred to the multipolar neurones associated with the gastric cavity. This collection of neurones into a ganglion for the purposes of neural integration occurs for the first time in the Coelenterates. |
A very large increase in the complexity of the nervous system occurs with the appearance of the flatworms (Figure 3). In this case the ganglia are fewer in number and concentrated at one end of the animal, which may be distinguished as the head for primitive eyes and mouth are found there (Figure 3) This is the first sign of the head ganglia which are eventually destined to evolve in their most complex form into the brain of Homo Sapiens (Figure 4), which receives sensory information from nerve receptors in different areas of the skin called dermatomes (numbered in Figure 4) as well as from the distance receptors such as the eyes, ears and nose.
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 | Figure 3. [Click image to enlarge] The grouping of large numbers of neurones into an integrating centre or ganglion. which is not symmetrically placed in the animal, first occurs in the Platyhelminthes or flatworms. Shown here is the dorsal nerve plexus of Notoplana Atomata( Polycladida) converging on the head ganglion. The labels refer to the 'gpl' (genital nerve plexus), hdn' (posterior dorsal nerve) and 'tau' (tentacle eyes). | Figure 4. [Click image to enlarge] The grouping of neurones into a head ganglion that provides an integrating centre for the nervous system reaches its' most complex level of evolution in the brain of Homo Sapiens. Shown are the individual areas of skin, each subserving a different set of neurones, that bring information to the brain concerning such sensations as touch, pressure, temperature and pain. The individual areas are labelled C2 to C5 (cervical spinal cord levels 1 to 5), TI to T12 (thoracic spinal cord levels 1 to 12), Ll to L5 (lumbar spinal cord levels 1 to 5) and S I to S4 (sacral spinal cord levels 1 to 4). Nerves enter the spinal cord at each of these levels C2 to S4. |
Nicholas Humphrey, until recently Director of the Unit of Animal Behaviour at Cambridge University, has written a book called " A History of the Mind". Humphrey argues that in the most primitive animals, such as jelly fish, the nerve nets convey information from the body wall concerning sensory phenomenon such as touch towards the ganglia which then issue an outgoing signal for the muscles of the body wall to respond, for example with a contraction giving a wriggle (Figure 5A). Humphrey goes on to suggest that with further evolution of the nervous system the outgoing signal to the muscles of the body wall in response to an incoming sensory signal became modified so as to actually alter the incoming signal as well as to contract the muscles (Figure 5B); this collateral effect, of the outgoing motor nerves altering the signal arriving along the sensory nerves, may even be present in the early evolved nervous system of flatworms. Indeed an even further degree of collateralization can occur in which the motor collateral can give rise to sensory experiences independent of any incoming sensory signals (Figure 5C). Another form of collateralization involves the outgoing motor signal in response to a sensory input modifying the incoming sensory signal, without actually contracting muscles at all (Figure 5D); these collaterals can be used to sustain the sensory experience well after the actual event that gave rise to the initial sensation has passed. This ability to maintain a sensation at will by using the collateralization effect is called 'sustaining the sentient loop'. The final evolution of this process, according to Humphrey, probably only occurs in the higher mammals. It involves the motor output that has been modified to only change an incoming sensory signal now gedrating and sustaining sensory signals itself within the brain in the absence of any sensory input (Figure 5E). The nervous system is in this way able to voluntarily generate sensations and maintain them at will. It is this ability to use the sentient loop that is the highest form of consciousness.
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| Figure 5. [Click image to enlarge] Evolution of the 'sentient' loop according to Humphrey. A: during evolution a most elemental form of nervous system consists of sensory neurones bringing in information, concerning for example touch and pressure, to an integrating centre consisting of a large number of interconnected neurones constituting a head ganglion; this then issues motor command to contract an appropriate muscle given the type of sensory information received by the head ganglion. B: the next level of sophistication was reached with the appearance of collateral nerve branches emanating from the outgoing motor nerves and ending in relation to the incoming sensory nerves; in this way the motor command was able for the first time to modify the sensory input to the head ganglion (see Figure 6 for an example of this process). C: these collaterals then became modified in two important ways, one of which is shown here; on issuing a motor command the collateral is able to induce a sensory experience independent of any input to the head ganglion along the sensory nerves themselves (see Figure 7 for an example of this process). D: the other important way in which the collaterals became modified is that they could be used to modify incoming sensory signals independently of any motor signals at all (see Figures 8B and 11 for examples of this process). E: the final level of sophistication involves the appearance during evolution of the 'sentient loop', in which the collateral acts on its own without any motor command being issued or sensory information about the environment being received; the head ganglion or brain can in this way generate its own sensory experiences, and it is this process that constitutes consciousness ( for an example of the brain generating activity in a voluntary way, without motor or sensory activity, see Figure 11). | Figure 6. [Click image to enlarge] The corollary discharge. Diagrammatic representation of the brain and spinal cord showing the possible levels of corollary discharge by which motor output from the cortex acts on incoming kinesthetic signals arising from the sensory neurones. Corollary discharges are obtained from motor commands and they can influence perception either by modifying incoming sensory signals (in this case at the level of the thalamus) or by acting independently of the incoming sensory signals. Kinesthesia is the sensation by which body weight, position, muscle tension and movement are perceived. Corollary discharges can alter the way in which such kinesthetic signals arising from sensory endings in muscles are interpreted. Such sensory endings in muscle spindles may send signals relating to the length and velocity of movement of a particular set of muscles; these spindles will also send signals arising from their being activated by a certain class of motoneurones in the spinal cord called gamma motoneurones. ]be signals due to the gamma activation of the spindles are removed by a corollary discharge, which at the same time allows the signals from the spindles due to the length and velocity changes to be perceived. In the example shown gamma motoneurones are activated from the motor cortex giving rise to spindle receptor discharges; these discharges together with the additional discharges due to the contraction of the muscles are received by the sensory neurones and transmitted through the group of neurones constituting the gracilis and cuneatus to the thalarnus and thence to the somatosensory cortex; here they give rise to the sense of movement of the muscles. However the initial motor discharge of impulses gates out the sensory discharge relating to gamma motoneurones exciting the spindle receptors; this gating may occur at the many regions of interaction between motor and sensory pathways in the brain and are shown here as occurring in the thalamus for definiteness only. |
In order to make these ideas of Humphreys clear it is necessary to look in detail at the functioning of the human nervous system. First of all what are collateral effects and can they operate in such a way as to modify sensory signals? One of the simplest motor acts that engages the brain and the spinal cord is shown in Figure 6: here a sensory stimulus, such as that arising from sensory spindle receptors in muscle cells concerned with indicating the length and velocity of shortening of the muscle, is relayed through the sensory neurones just outside the spinal cord to the nerve cells just inside the cord within an area called the substantia gelatinosa; these signals are then sent to the main relay station for sensory activity propagating between the spinal cord and the brain, in the group of neurones called the nucleus gracilis and nucleus cuneatus; from there the signals are sent to the thalamus in the brain, which is the receiving area for nearly all the sensory input to the overlying mantle of the brain or cortex; finally the thalamus projects the information to that part of the cortex which is called the somatosensory area, concerned with the analysis of information derived from sensory receptors in the limbs. The kind of information in the signals processed by the somatosensory cortex may require that the muscles that gave rise to the sensory input in the first place be contracted. In this case neurones in the part of the cortex concerned with contracting muscles, namely the motor cortex, project a signal to the appropriate motoneurones in the spinal cord connected to these muscles. These are of two different kinds, namely alpha motoneurones that are attached to cells in the muscle in question that produce the force; the other kind are the gamma motoneurones that are connected with cells that contract in the sensory receptor apparatus itself; contraction of these cells changes the characteristics of the sensory receptors so that they rapidly send signals to the sensory neurones outside the spinal cord and from there to the alpha motoneurones, leading to the contraction of the bulk of the muscle cells; they also send signals to the sornatosensory cortex via the thalamus along the pathway already described. Two kinds of information are then sent via the sensory neurones to the brain: one of these relates to the signal arising from the sensory receptors in the muscle concerned with the position, tension and movement of the muscle, known collectively as kinesthesia; the other relates to the signal arising from the receptors as a consequence of their being contracted by the gamma motoneurones. This latter signal is gated out before it reaches consciousness by a collateral signal from the motor pathway as shown in Figure 6; the sensory signal concerned with the state of kinesthesia of the muscle is not gated out, but is allowed to reach consciousness. The level in the brain or spinal cord at which this gating procedure is carried out is not known; it is shown to occur at the level of the thalamus in Figure 6 simply for the sake of definiteness. Humphrey is therefore correct in his assertion that modification of sensory signals can occur before they reach consciousness as a consequence of a collateral effect from the motor pathway.
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Figure 7. [Click image to enlarge] The corollary discharge. This figure gives a diagrammatic representation of the brain and spinal cord illustrating a possible output from the motor cortex responsible for the perception of heaviness of a held object. For definiteness this corollary discharge is shown at the level of the basal ganglia. Such discharges can give rise to the sensation of muscular effort as occurs when lifting and supporting an object. In the example shown neurones in the motor cortex (called Betz cells) that project to the motoneurones in the spinal cord are illustrated; Betz cells may be activated to contract muscles involved in lifting the limbs or an object such as a suitcase; when they do this a corollary discharge is sent (at the level of the basal ganglia?) which gives rise to the perception of heaviness of the limb or suitcase, and this is simply related to the extent of the motor discharge that occurs. Subjects that experience a stroke may have to send a larger than normal discharge down the remaining functional Betz cells to achieve the aim of lifting their arms and so experience them as as an enormous burden.
The graph shows the results of an experiment in which the subject has to support a 9 lb. weight with one arm ( the experimental arm) while being asked at intervals to choose what they thought were equal weights to be supported in the same way by the other arm (the control arm). When the experimental arm was allowed to rest between the trials the subject choose weights with the control arm close to the 9 lb weight held by the experimental arm (see 'rest curve'). If however the experimental arm had to support the 9 lb weight continuously, then the subject choose weights with the control arm that were successively greater (see 'fatigued ' curve) than the 9 lb weight indicating the increased sense of heaviness. This arises from the increase in corollary discharge with time as muscles have to receive a greater discharge to support the weight continuously. The perception of heaviness does not arise from sensory signals in the muscle being relayed back to the brain. This graph is due to experimental work of McCloskey, Ebeling and Goodwin carried out in 1974. |
Corollary discharges from the motor pathway can also be used to generate a sensation independent of any incoming sensory signals. They can generate sensations of muscular force or heaviness although they cannot generate sensations of movement. Figure 7A shows how the sensation of the heaviness of an object held in the hand is generated by a collateral effect. The pathway from the motor cortex to the alpha motoneurones is shown to give off a collateral branch at the level of the basal ganglia; this, it is hypothesized, can generate a sensation in the cortex of the degree of heaiviness of the object by firing impulses in proportion to those that are being propagated down the motor pathway. It follows that when a muscle is weakened by fatigue, such as when holding a heavy suitcase, a greater number of impulses are required by the non-fatigued component of the muscle in order for the muscle to continue lifting the suitcase; the collateral then receives a greater number of impulses and so a greater sensation of heaviness is experienced. An experimental example of this is given in Figure 7B[;] here a comparison is made between the extent to which a subject perceives the heaviness of a suitcase held continually in one hand by comparing it with a known weight held in the other for a short period of time. The graphs show that in this matching experiment the known weight chosen to be equivalent to the suitcase gradually increases over time, indicating the increased sensation of heaviness. This sensation is due to the collateral effect.
Humphrey is correct then in his suggestion that collateral effects can modify both the kinds of sensations that enter consciousness as well as generate sensations that did not arise from the workings of our sensory receptors. Figure 7 summarizes the situation. Humphrey speculates that early during evolution nerve pathways were layed [sic] down that allowed an animal to respond to say a noxious stimulus to the skin by 'wriggleing' away; in higher vertebrates this simplest pathway might consist of the primary sensory neurones just outside the spinal cord that receive information concerning noxious stimulation projecting to upper motoneurones in the reticular formation wbich then project down to the lower motoneurones and from there to the muscles which are to be contracted to produce "wriggleing" (Figure 8A). At a later stage of evolution mechanisms were put in place that allowed the nervous system to 'gate' out sensory information, using projections from the brain to the sensory gate-way to the cortex , the thalamus, as shown in Figures 8B and 8C. We have already seen how information gathered by primary sensory neurones concerned with muscle receptors can be gated out before it reaches the somatosensory cortex by means of a collateral feedback from the motor cortex at the level of the thalamus (Figure 8B). Such a feedback could occur via the well known pathway from motor cortex to basal ganglia and from there to the reticular nucleus that lies just outside the thalamus; this then projects to the somatosensory cortex (Figure 8B). Sensory information that is gathered by the retina is also 1 gated' as it passes through the thaIamus on the way to the visual cortex, as shown in Figure 8C. The primary visual pathway is from the retina to the thalamus and from there to the visual cortex; neurones exist in the cortex that project back to the thalamus where they can gate the incoming visual information (Figure 8C). There is then evidence for both the modulation of signals arising from the primary sensory neurones as well as from the visual sensory neurones at the level of the thalamus. In this way the brain can determine the sensory information which reaches it.
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| Figure 8. [Click image to enlarge] Diagrammatic representation of the brain and spinal cord showing different kinds of interactions between sensory pathways and motor pathways.
A, illustrates the simplest pathway involving the brain in a motor pathway. Primary sensory neurons relay information concerning kinesthesia, temperature and touch via the spinothalamic tract to the reticular formation of the hindbrain. Here a reflex act is initiated by exciting motorneurons to contract muscle in relation to the sensory stimulus. B, illustrates how kinesthetic gating, referred to in relation to Figure 6, may occur. The motor cortex activates neurons in the basal ganglia which in turn inhibit neurons in the reticular nucleus of the thalamus which normally inhibit neurones in the thalamus that are responsible for conducting the kinesthetic discharge to the somatosensory cortex. Primary motor cortex can then modulate the sensory information that can enter perception through this pathway. C, illustrates how visual information passes from the retina to the thalamus and from there to the visual cortex. This cortex itself contains neurones that project back to the thalamus; these neurones in the cortex can gate the information allowed to pass through the thalamus to the cortex. Both B and C show how the brain itself can modulate the perceptions of the world which it might allow to reach consciousness. |
Figure 9. [Click image to enlarge] The sense of time in the brain as illustrated by the 'cutaneous rabbit' perceptual illusion. Shown are diagrams of arms in which the following experiments were carried out to illustrate the subjective nature of the space-time extent of experience.
A: taps were delivered in the sequence shown (1 to 10 on the left) on the arm at one-tenth of a second apart so that the final tap was given at 1.8 seconds; the first five taps occur at the wrist, the next two on the forearm near the elbow and the last three at the shoulder region (the subject was not allowed to observe these procedures). Surprisingly the subject experienced the second tap as displaced from the wrist and the rest of the taps at equal distances along the length of the arm (at 1 to lO on the right). It is in this sense that the brain interprets the taps as if an animal (rabbit?) had run up the arm. B: five taps were then delivered in the sequence as shown, namely only on the wrist (on the left 1 to 5) and these were experienced as all occurring at the wrist (on the right 1 to 5). The original experiments were performed by Geldard and Sherrick in 1972. |
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.
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| 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.
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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.
Some further reading.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.