Level dependence of cochlear nucleus onset unit responses and facilitation by second tones or broadband noise.

1. The responses of onset units in the cochlear nucleus of the anesthetized guinea pig have been measured to single tones, two-tone complexes, and broadband noise (BBN; 20-kHz bandwidth). The onset units were subdivided into three groups, onset-I (OnI), onset-L (OnL), and onset-C (OnC), on the basis of a decision tree using their peristimulus time histogram (PSTH) shape and discharge rate in response to suprathreshold best-frequency (BF) tone bursts. 2. PSTHs were constructed from responses either to single tones at a unit's BF or to BBN as a function of level. When sufficient sustained activity could be elicited from the unit, arbitrarily defined as > 100 spikes/s, a coefficient of variation (CV) was calculated; the majority were characterized by a CV that was similar to transient chopper units (0.35 < CV < 0.5). First spike latency decreased monotonically with increasing sound level. For the majority of onset units, the first spike timing was very precise. 3. BF rate-level functions recorded from OnL and OnC units did not show any signs of discharge rate saturation at the highest sound levels we have used (100-115 dB SPL). No systematic relationship was observed between the threshold at BF and the shape of the rate-level function. BBN rate-level functions were typically characterized by higher discharge rates than in response to BF tones. However, for OnI units and a minority of other onset units, there was little difference in the shape of their rate-level functions in response to BF tones or BBN. 4. The threshold of most onset units to BBN was similar to the threshold to a BF tone that had similar overall root-mean-square (RMS) energy. The BBN threshold was, on average, 5.5 dB greater than the BF threshold. This result contrasts with that found in auditory-nerve fibers recorded in the same species, with the use of an identical sound system, where the threshold to BBN was, on average, 19.4 dB higher. The mean threshold difference between BBN and BF tones for a population of chopper units recorded in the same series of experiments was 17.7 dB. The relative thresholds to BBN and BF tones indicated that the bandwidths near the onset units' BF threshold were broader than could be estimated with the use of single tones. Ten units were characterized by bimodal response areas.(ABSTRACT TRUNCATED AT 400 WORDS)