DIVERSITY IN FREQUENCY-RESPONSE PROPERTIES OF SACCULAR AFFERENTS OF THE TOADFISH, OPSANUS-TAU

The frequency response of primary saccular afferents of toadfish (Opsa nus tau) was studied in the time and frequency domains using the rever se correlation (revcor) method, Stimuli were noise bands with flat acc eleration spectra delivered as whole-body motion. The recorded acceler ation waveform was averaged over epochs preceding and following each s pike. This average, termed the revcor, is an estimate of the response of an equivalent linear filter intervening between body motion and spi ke initiation. The spectrum of the revcor estimates the shape of the e quivalent linear filter. Revcor responses were brief, damped oscillati ons indicative of relatively broadly tuned filters. Filter shapes were generally band-pass and differed in bandwidth, band edge slope, and c haracteristic frequency (74 Hz to 140 Hz). Filter shapes tend to be in dependent of stimulus level. Afferents can be placed into two groups w ith respect to characteristic frequency (74-88 Hz and 140 Hz). Some hi gh-frequency afferents share a secondary peak at the characteristic fr equency of low-frequency afferents, suggesting that an afferent may re ceive differently tuned peripheral inputs. For some afferents having s imilar filter shapes, revcor responses often differ only in polarity, probably reflecting inputs from hair cells oriented in opposite direct ions. The origin of frequency selectivity and its diversity among sacc ular afferents may arise from a combination of hair cell resonance and micromechanical processes. The resulting frequency analysis is the si mplest yet observed among vertebrate animals. During courtship, male t oadfish produce the 'boatwhistle' call, a periodic vocalization having several harmonics of a 130 Hz fundamental frequency. The saccule enco des the waveform of acoustic particle acceleration between < 50 and ab out 250 Hz. Thus, the fundamental frequency component of the boatwhist le is well encoded, but the successive higher harmonics are filtered o ut. The boatwhistle is thus encoded as a time-domain representation of its fundamental frequency or pulse repetition rate.