RESPONSES OF VENTRAL COCHLEAR NUCLEUS ONSET AND CHOPPER UNITS AS A FUNCTION OF SIGNAL BANDWIDTH

1. The responses of units in the ventral cochlear nucleus in anestheti zed guinea pigs have been measured to best-frequency tones, noise band s geometrically centered around the unit best frequency, and noise ban ds asymmetrically positioned around the best frequency. 2. Each unit i solated was characterized using peristimulus time histograms (PSTHs) t o best frequency tones at 20 and 50 dB suprathreshold, frequency-inten sity response areas and rate-versus-level functions in response to bes t-frequency tones and wideband noise. The data reported here are deriv ed from full analyses of 5 chopper units and 17 onset units. The onset s were divided into onset-I (On(I)), onset-L ( On(L)), and onset-C (On (C)) by the criteria described by Winter and Palmer: the PSTHs of On(I ) units show only an onset response, On(L) units respond with a single spike at onset followed by a low level of sustained activity, and On( C) units have PSTHs with one to four onset peaks and low levels of sus tained discharge. 3. In response to geometrically centered noise bands of constant spectral density, the discharge of chopper units and one On(I) unit increased over a relatively narrow range of bandwidths, cor responding to the equivalent rectangular bandwidth calculated from the ir response area, and then became constant. In contrast, On(L) and On( C) units showed increases in discharge rate with noise bandwidth over very wide ranges of bandwidth. The growth of the discharge rate with n oise bandwidth was approximately linear on double logarithmic axes and therefore could be described by a power function with an exponent of 0.37. This relation held even for noise levels near threshold. 4. When noise bands with constant spectral density (at the input to the earph one) were presented with one edge fixed at the unit's best frequency, the discharge rate of most chopper units and the one On(I) unit increa sed over a narrow range of bandwidths and then became constant. This p attern was observed irrespective of whether the second edge of the noi se was progressively increased above, or decreased below, the best fre quency. For two of the chopper units, in which lateral inhibitory side bands could be demonstrated, increasing the noise bandwidth led first to increases and then to decreases in the discharge rate as the noise energy impinged upon the sideband. The chopper units act like energy d etectors with a filter corresponding to their single tone response are a, but, for some units,with the addition of inhibitory sidebands. 5. F or the On(L) and On(C) units, increasing the noise bandwidth above or below best frequency caused progressive increases in the discharge rat e over wide ranges of bandwidth. These increases oc curred even for lo w noise spectral densities. The growth in discharge rate for these ons et units was well fitted at all spectral density levels by power funct ions: one above best frequency and one below. At levels of the noise 4 0 dB above the unit threshold, the point at which the discharge rate r eached 90% of its maximum was, on average, about 2 octaves below best frequency and 1 octave above. For some onset units, changes in the dis charge rate were seen as the noise bandwidth was varied over about 14 kHz, which is about one-third of the total frequency hearing range of the guinea pig. 6. The data for onset units is consistent with the hyp othesis that onset units in the ventral cochlear nucleus achieve their precision in the temporal domain by integration of the inputs from au ditory nerve fibers with a wide range of best frequencies. The range o f frequency over which onset units integrate frequency matches that of the inhibitory input to dorsal cochlear nucleus neurons, suggesting a possible role as an inhibitory interneuron.