a response elicited during nerve conduction studies, consisting of muscle action potentials evoked from orbicularis oculi muscles after brief electric or mechanical stimuli to the cutaneous area supplied by the ophthalmic branch of the trigeminal nerve. Characteristically, there is an early response (approximately 10 msec after stimulus) ipsilateral to the stimulation site (labeled R1) and bilateral late responses (approximately 30 msec after stimulus; labeled R2); the latter are responsible for the visible twitch of the orbicularis oculi muscles.