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.

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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.