Physiological correlates of comodulation masking release in the mammalian ventral cochlear nucleus

Comodulation masking release (CMR) enhances the detection of signals embedd ed in wideband, amplitude-modulated maskers. At least part of the CMR is at tributable to across-frequency processing, however, the relative contributi on of different stages in the auditory system to across-frequency processin g is unknown. We have measured the responses of single units from one of th e earliest stages in the ascending auditory pathway, the ventral cochlear n ucleus, where across frequency processing may take place. A sinusoidally am plitude-modulated tone at the best frequency of each unit was used as a mas ker. A pure tone signal was added in the dips of the masker modulation (ref erence condition). Flanking components (FCs) were then added at frequencies remote from the unit best frequency. The FCs were pure tones amplitude mod ulated either in phase (comodulated) or out of phase (codeviant) with the o n-frequency component. Psychophysically, this CMR paradigm reduces within-c hannel cues while producing an advantage of similar to 10 dB for the comodu lated condition in comparison with the reference condition. Some of the rec orded units showed responses consistent with perceptual CMR. The addition o f the comodulated FCs produced a strong reduction in the response to the ma sker modulation, making the signal more salient in the poststimulus time hi stograms. A decision statistic based on d' showed that threshold was reache d at lower signal levels for the comodulated condition than for reference o r codeviant conditions. The neurons that exhibited such a behavior were mai nly transient chopper or primary-like units. The results obtained from a su bpopulation of transient chopper units are consistent with a possible circu it in the cochlear nucleus consisting of a wideband inhibitor contacting a narrowband cell. A computational model was used to confirm the feasibility of such a circuit.