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Neural mechanism that radomnises behaviourA Neural Mechanism that Randomnises Behaviour R.H.S. Carpenter Physiology Laboratory, University of Cambridge Journal of Consciousness Studies, vol. 6, No. 1, 1999, pp. 13-22 Keywords: saccade, superior colliculus, randomnisation, Libet The abstract starts by pointing out that the time taken to react voluntarily to stimulus is far longer than can be accounted for by known nervous system processing. The strength of response is shown to rise in proportion to the incoming sensory data, until a critical level at which action is taken is reached. However, the rate of rise fluctuates randomly from trial to trial. This claim is based on studies of neurons in the frontal eye field and the time taken between presenting a visual stimulus and making a saccade (an eye movement). The saccade itself is very quick, lasting only 20-30ms, but the system is not designed for speed in other respects. The average gap between presentation of the stimulus and the saccade is 200ms. Normal processing in the nervous system is claimed to account for at most one third of this time. The shortest route from the retinal receptors to the eye muscles passes through the superior colliculus and should take only 60ms. However, the colliculus receives input that comes ultimately from the parietal cortex and the frontal eye fields. The control is inhibitory, otherwise the eyes would be constantly darting towards each and every stimulus. The blanket inhibition has to be lifted for a saccade to be made. The colliculus lacks the information to make useful decisions, because it registers only where things are in space, but not what they are. The biggest problem is seen to be in a series of trials the response time varies over a surprisingly large range. While the average saccadic latency is 200ms, on some 5% of trials the latency is either less than 150ms or more than 300ms. In the first stage of the latency period neurons distinguish between a target stimulus and distractors. This takes about the same period of time, about 70ms, whether the eventual latency period is short or long, so the whole of the variability is concentrated in the latter part of the latency period. The article suggests that this means that the variability is not due to noise in the sensory pathways, but to something introduced by the brain. The randomness of the reaction times is seen as a function of deliberate randomisation by neural processes in the brain. Carpenter says that the underlying process is obscure, although he points out that its is consistent with the Penrose/Hameroff model, and that the delay periods involved are similar to those seen in Libet’s experiments. Carpenter goes on to speculate as to what evolutionary advantage would favour randomisation. He argues that there would be an adaptive advantage in the resulting unpredictability, as opposed to deterministic responses that would be easier for a predator or prey to predict. Carpenter, R.H.S. Oculomotor procrastination in Eye Movements Cognition and Visual Perception (1981) Carpenter, R.H.S. Movement of the Eyes (1988) Carpenter, R.H.S. Human saccadic latency to targets of differing contrast and probability: Evidence for neural randomisation Carpenter, R.H.S. and Williams M.L.L. Neural computation of log likelihood in the control of saccadic eye movements Nature 377 pp59-62 |
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