Desynchronizing responses to correlated noise: A mechanism for binaural masking level differences at the inferior colliculus

We examined the adequacy of decorrelation of the responses to dichotic nois e as an explanation for the binaural masking level difference (BMLD). The r esponses of 48 low-frequency neurons in the inferior colliculus of anesthet ized guinea pigs were recorded to binaurally presented noise with various d egrees of interaural correlation and to interaurally correlated noise in th e presence of 500-Hz tones in either zero or pi interaural phase. In respon se to fully correlated noise, neurons' responses were modulated with intera ural delay, showing quasiperiodic noise delay functions (NDFs) with a centr al peak and side peaks, separated by intervals roughly equivalent to the pe riod of the neuron's best frequency. For noise with zero interaural correla tion (independent noises presented to each ear), neurons were insensitive t o the interaural delay. Their NDFs were unmodulated, with the majority show ing a level of activity approximately equal to the mean of the peaks and tr oughs of the NDF obtained with fully correlated noise. Partial decorrelatio n of the noise resulted in NDFs that were, in general, intermediate between the fully correlated and fully decorrelated noise. Presenting 500-Hz tones simultaneously with fully correlated noise also had the effect of demodula ting the NDFs. In the case of tones with zero interaural phase, this demodu lation appeared to be a saturation process, raising the discharge at all no ise delays to that at the largest peak in the NDF. In the majority of neuro ns, presenting the tones in pi phase had a similar effect on the NDFs to de correlating the noise; the response was demodulated toward the mean of the peaks and troughs of the NDF. Thus the effect of added tones on the respons es of delay-sensitive inferior colliculus neurons to noise could be account ed for by a desynchronizing effect. This result is entirely consistent with cross-correlation models of the BMLD. However, in some neurons, the effect s of an added tone on the NDF appeared more extreme than the effect of deco rrelating the noise, suggesting the possibility of additional inhibitory in fluences.