2 SEPARATE INHIBITORY MECHANISMS SHAPE THE RESPONSES OF DORSAL COCHLEAR NUCLEUS TYPE-IV UNITS TO NARROW-BAND AND WIDE-BAND STIMULI

1. The principal cells of the dorsal cochlear nucleus ( DCN) are mostl y inhibited by best frequency (BF) tones but are mostly excited by bro adband noise (BBN), producing the so-called type IV response character istic. The narrowband inhibitory responses can be explained by the inh ibitory influence of interneurons with type II response characteristic s. However, it is not clear that all the details of the type IV respon ses can be accounted for by this neural circuit. In particular, many t ype IV units are inhibited by band-reject noise(notch noise); type II units tend to be only weakly excited by these stimuli, if at all. In t his paper we study the relationships between the narrowband, inhibitor y and the wideband, excitatory regimes of the type IV responses and pr esent the case for the existence of a second inhibitory source in DCN, called the wideband inhibitor (WBI) below. 2. Type IV units were stud ied using pure tones, noise bands arithmetically centered on BF, notch noise centered on BF, and BBN. We measured the rate-level function (r esponse rate as function of stimulus level) for each stimulus. This pa per is based on the responses of 28 type IV units. 3. Evidence for low -threshold inhibitory input to type IV units is derived from analysis of rate-level functions at sound levels just above threshold. Notch no ise stimuli of the appropriate notch width produce inhibition at thres hold in this regime. When BBN is presented, this inhibition appears to summate with excitation produced by energy in the band of noise cente red on BF, resulting in BBN rate-level functions with decreased slope and maximum firing rate. A range of slopes and maximal firing rates is observed, but these variables are strongly correlated and they are ne gatively correlated with the strength of the inhibition produced by no tch noise; this result supports the conclusion that a single inhibitor y source is responsible for these effects. 4. By contrast, there is a weak (nonsignificant) positive correlation between the strength of the inhibitory effect of notch noise and the slope/maximal firing rate in response to narrowband stimuli, including BF tones. The contrast betw een this positive nonsignificant correlation and the significant negat ive correlation mentioned above suggests that more than one inhibitory effect operates: specifically, the type II input is responsible for i nhibition by narrowband stimuli and a different inhibitory source, the WBI, produces inhibition by notch stimuli. 5. Several lines of eviden ce are given to show that type II units cannot produce the inhibition seen with notch noise stimuli. First, inhibitory effects are seen at n otch widths well beyond estimates of the bandwidth of type II inhibito ry inputs; second, the threshold of inhibition produced by notch noise stimuli is usually well below that of type II units for noise; and th ird, the firing rate of type II units decreases monotonically as the b andwidth of a narrow noise band is widened, whereas the peak response rate of type IV units can increase, decrease, or vary nonmonotonically under the same conditions. 6. Type IV units can be classified along t wo continua, according to their responses to narrowband stimuli on the one hand and to wideband stimuli on the other. Units differ along eac h continuum in the strength of inhibitory effects for narrowband or wi deband stimuli, respectively. There may be an inverse relationship bet ween the two continua, although the relationship is not statistically significant in the data of this paper.