Responses to Acoustic Stimuli from Single Units in the Eighth Nerve of the Bullfrog

Abstract

Single unit recordings from the eighth nerve of the bullfrog reveal two strikingly different kinds of auditory units. Both kinds of units exhibit sharply frequency dependent sensitivity (tuning curves). “Simple” units are maximally sensitive to tone bursts of frequency between 1000 and 1500 cps. Their frequency sensitivity and their capacity to “follow” pulse stimuli are well-matched to the spectral composition and pitch period, respectively, of the bullfrog's croak. Simple units cannot be inhibited by acoustic stimuli. “Complex” units are inhibited by acoustic signals in the frequency range from 300 to 1000 cps. Some complex units are excited by acoustic stimuli, some are excited by both acoustic and vibratory stimuli, and some are excited by vibratory stimuli alone. Complex units that can be excited by sound are most sensitive to frequencies between 200 and 700 cps. Simple and complex units probably derive from separate sense organs (the basilar and amphibian papillae, respectively) within the otic capsule. There is evidence that all of these responses are from primary neurons; moreover, it has been shown that the inhibition is not under efferent control. Extremely sensitive units of both types are found with thresholds comparable to those of human listeners. Responses to tone bursts, noise bursts, click trains, and frog croaks are compared. The roles of spectral and temporal pattern of the stimulus in determining the response of a unit are explored.