Abstract
Eysenck's claim that sodium amytal shortens and dexedrine lengthens the duration of spiral aftereffect was not borne out in any of four experiments designed to demonstrate it, including a replication of his study. A further replication, different only in the stimulus used, yielded no effect of amytal or dexedrine. Actual measurement of aftereffect rate immediately following the eliciting stimulus and after selected delays showed an exponential decay function for aftereffect rate but did not demonstrate any effect of the two drugs. This repeated failure to demonstrate a change in aftereffect as a result of the administration of drugs known to affect neuron firing thresholds has implications for the understanding of neurophysiology of visual motion perception. It was proposed that motion aftereffect is based on a comparison of the states of two neural systems both of which are equally affected by the drugs.
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