FREQUENCY EXTENT OF 2-TONE FACILITATION IN ONSET UNITS IN THE VENTRALCOCHLEAR NUCLEUS

1. The frequency threshold curves (FTCs) of 91 single units in the coc hlear nucleus of the anesthetized guinea pig were measured using a con ventional single-tone paradigm and a two-tone paradigm designed to elu cidate the frequency extent of two-tone facilitating in onset units (O n). Units were classified according to existing classification schemes into primary-like (n = 3), chopper (n = 23), and three onset groups: On(I) (n = 12), On(c) (n = 29), and On(L) (n = 24). Histological recon structions show onset units to be widely distributed within the ventra l cochlear nucleus in a manner generally consistent with its tonotopic organization. 2. The FTCs of onset units differed in their minimum th resholds, the steepness of their high- and low-frequency cutoffs, and their sharpness of tuning as quantified by the quality factor at 10 dB (Q(10dB)) above best frequency(BF) threshold values. There was consid erable overlap in the sharpness of tuning between onset units and audi tory nerve fibers, as indicated by the distribution of Q(10dB) values in the octave around 10 Id-It: onset units had Q(10dB) values of 3.56 +/- 1.38 (SD), compared with 6.3 +/- 2.48 for auditory nerve fibers. T he tuning of chopper units was similar to that of auditory nerve fiber s (5.52 +/- 1.46). 3. Seventy-five percent of onset units showed some degree of facilitation (a threshold reduction) when their FTCs were me asured in the presence of BF tones 4 dB below BF threshold. The freque ncy extent of such facilitation was variable, with a maximum of 6 octa ves around the BF. In extreme cases facilitation could be measured whe n the BF tone was as low as 30 dB below BF threshold. 4. In 17% of ons et units, suppressive effects were evident, as shown by noncontiguous frequency regions of facilitation These suppressive effects might be a reflection either of suppression in the auditory nerve input or of a direct inhibitory input to the onset units. The strength of this effec t suggests that inhibition is a likely explanation, consistent with th e finding in previous morphological studies of profuse synapses with p leomorphic vesicles on multipolar cells. 5. FTCs of chopper and primar y-like units measured in the presence of BF tones showed little facili tation. The facilitation that was observed in chopper units was confin ed to a narrow region around BF and disappeared when the facilitatory tone was lowered to 4 dB below BF threshold. 6. These data support the hypothesis that onset units, but not chopper or primary-like units, r eceive excitatory inputs from auditory nerve fibers with a wide range of BFs. However, the frequency range of facilitation and the magnitude of the threshold facilitation varied from unit to unit, suggesting th at the off-BF inputs from auditory nerve fibers are not evenly distrib uted or equally effective in all units.