Representation of Sounds by Responses of Single Units in the Eighth Nerve of the Bullfrog

Abstract

Single-unit recordings from the eighth nerve of the bullfrog have revealed two strikingly different types of response activity. The type of unit most often encountered is maximally sensitive to tone bursts of frequency between 1000 and 1500 cps. This type of unit cannot be inhibited by acoustic stimuli. Its frequency sensitivity and its capacity to follow pulse stimuli are well matched to the spectral composition and pitch period of the bullfrog's croak, respectively. The second type of unit is inhibited by acoustic signals in the frequency range between 300 and 1000 cps. Some units belonging to this group are not excited by acoustic stimuli but respond to vibrational stimuli; some respond to both acoustic and vibrational stimuli; and some respond to acoustic stimuli alone. Units of this type which can be excited by sound are usually most sensitive to frequencies between 200 and 400 cps, but occasionally units most sensitive to frequencies between 700 and 900 cps have been encountered. There is evidence that both types of unit activity are recorded from primary neurons. Responses to clicks, tones, noise bursts, click trains, and frog croaks were studied.