Conditioning auditory stimuli and the cutaneous eyeblink reflex in humans: differential effects according to oligosynaptic or polysynaptic central pathways.

Electromyographic activity of orbicularis oculi muscles in humans was elicited by percutaneous electrical stimulation of the supraorbital brahcn of the trigeminal nerve. The reflex consists of an early brief ipsilateral R1 and a later prolonged consensual R2. The threshold for R1 was considerably elevated compared to that of R2. In one experiment brief acoustic stimuli, at 70 dB SPL, were presented at various intervals, from 5 to 800 msec, prior to the eliciting stimulus. In a second experiment similar stimuli, with intensities varying from 30 to 70 dB SPL, were given at the fixed lead time of 100 msec. In each experiment the preliminary acoustic stimulus enhanced R1 and depressed R2. Potentiation of R1 developed more rapidly than did depression of R2 and exhibited an early and a late peak, whereas depression had a single intermediate trough. Both effects linearly increased with increases in the intensity of the acoustic prepulse. These restuls are discussed in relation to the neuronal circuits responsible for the expression of the two reflex components.