Excitatory and inhibitory components of the eyeblink responses to startle evoking stimuli, studied in the human subject.

Integrated EMG recordings have been used to study the eyeblink component of the human startle reflex. They have shown that the response, to either an auditory or a painful stimulus, consists of an initial excitation followed by a more prolonged period during which a second stimulus, of the same or different sensory modality, fails to evoke a response, or evokes one which is reduced in amplitude. The period of reduced responsiveness does not follow voluntary or spontaneous eyeblinks. Increasing the duration of the startle evoking stimulus has little effect on the excitatory component of the response, but prolongs the subsequent period of reduced responsiveness. When stimuli of one modality are presented repetitively to the subject, the eyeblink response is habituated; the response to a subsequent testing stimulus, of a different sensory modality, is then smaller than that evoked by the testing stimulus alone, but greater than that evoked by the testing stimulus when it follows a single conditioning stimulus. It is concluded that the excitatory and inhibitory components of the startle reflex are at least partially separable and that stimulus novelty has some significance in eliciting a response.