It is generally accepted that when subjects move voluntarily in response to a reaction signal, the cerebral cortex plays a major role in identifying the sensory stimulus and releasing the instructions to move. Indeed, theoretical models have been put forward in psychophysiological studies to symbolize the steps of such cerebral processing (Gratton et al. 1988). In contrast, startle reactions occur via a subcortical reflex mechanism. Sensory inputs activate the reticular formation and the descending reticulo-spinal tract to the spinal cord (Davis et al. 1982). Because of the differences in the length of the circuits, as well as in the amount of sensory processing, the latencies of the startle reaction are much shorter than those of a voluntary reaction. In muscles of the forearm, a startle reaction occurs at less than 80 ms. In contrast, the voluntary reaction time to a visual 'go' signal is of the order of 150 ms. The reaction time to auditory and somatosensory stimuli is shorter but even then rarely less than 100 ms (Brown et al. 1991a;Thompson et al. 1992;Pascual-Leone et al. 1992). Recently, Valls-Sol e et al. (1995) have shown that reaction times can be considerably reduced if a very loud, startling, sound is given at the same time as the visual 'go' signal. The amount of shortening is much greater than that observed in conventional intersensory facilitation (Nickerson, 1973), and presumably represents a specific startle-related effect. The question is what neural mechanisms are responsible for this Journal of Physiology (1999) 1. The reaction time to a visual stimulus shortens significantly when an unexpected acoustic startle is delivered together with the 'go' signal in healthy human subjects. In this paper we have investigated the physiological mechanisms underlying this effect. If the commands for the startle and the voluntary reaction were superimposed at some level in the CNS, then we would expect to see alterations in the configuration of the voluntary response. Conversely, if the circuit activated by the startling stimulus is somehow involved in the execution of voluntary movements, then reaction time would be sped up but the configuration of the motor programme would be preserved. 2. Fourteen healthy male and female volunteers were instructed to react as fast as possible to a visual 'go' signal by flexing or extending their wrist, or rising onto tiptoe from a standing position. These movements generated consistent and characteristic patterns of EMG activation. In random trials, the 'go' signal was accompanied by a very loud acoustic stimulus. This stimulus was sufficient to produce a startle reflex when given unexpectedly on its own. 3. The startling stimulus almost halved the latency of the voluntary response but did not change the configuration of the EMG pattern in either the arm or the leg. In some subjects the reaction times were shorter than the calculated minimum time for processing of sensory information at the cerebral cortex. Most subjects reported that the very rapid responses were produc...

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vHIT provides phenotypic information that differentiates these autosomal ataxias and can serve as a strategy to orient genetic diagnosis. A correlation between VOR and SARA raises the possibility of using VOR gain as a neurophysiologic biomarker for disease severity.

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