Habituation of the auditory startle reaction is reduced during preparation for execution of a motor task in normal human subjects.

The generalized acoustic startle reaction is a useful but labile phenomenon that undergoes a relatively rapid habituation with repeated stimuli and can be modified by changes in the environmental context. In the present work, we examined the influence of the experimental set on size, latency and habituation rate of auditory startle reactions in 15 normal volunteers. Series of 5 acoustic startling stimuli were applied on each of the following 4 conditions: (1) at rest in a quiet and dull ambience; (2) at rest but engaged in a busy environment; (3) being prepared to execute a reaction time task; and (4) focussing the attention to an impending visual stimulus. We found that the amplitude of the response to the first stimulus was significantly larger in condition 1, and habituation rate was significantly reduced in condition 3, with respect to all other conditions. The enhancement of the startle reaction in a quiet ambience may result from reduction of the sensory inputs from the environment, which would otherwise act as prepulses and cause a certain degree of inhibition. The reduced habituation rate of the startle reaction with preparation for a reaction time task may be due to a transient block of the cortical inhibitory control on inputs from non-expected sensory afferents. A careful description of the experimental conditions with which the startle reactions are tested is required for meaningful comparison of the results from different laboratories. The possibility to reduce habituation of the startle reaction with preparedness for execution of a motor task has important implications with regard to the applicability of the startle reaction on research and clinical practice.