Inferior colliculus neuronal responses to masking-level-difference stimuli

Seventy-one inferior colliculus neurons, with best frequencies below 1.5 kHz, were studied in a binaural, forward-masking paradigm in chinchilla. Masker and signal frequencies were presented at neuronal best frequency. Masker level was set 10-15 dB above neuronal threshold and varied to include a range of signal-to-masker ratios and overall intensities. Without the masker, 33 of the neurons preferred an in-phase signal (SO), 29 an out-of-phase (S pi) signal, and the remaining 9 had 'no-preference' (NP), responding equally well to SO and S pi. Complete protocols from 53 of the 71 neurons were obtained with and without maskers over a range of levels. With an in-phase masker (NO), some neurons responded better to dichotic (NOS pi) than to diotic (NOSO) sounds. Generally, they maintained a particular phase preference with and without masker. Some neurons, however, altered phase preference and responsivity when binaural maskers were added to signal. Signal-to-noise ratios between 0 and 30 dB were sufficient to differentiate neuronal responsiveness to NOSO and NOS pi. The results suggest that identical neural mechanisms are not involved in processing unmasked (SO or S pi) and masked binaural sounds (NOSO, NOS pi). Furthermore, changes in neuronal sensitivity favor the NOS pi condition upon addition of noise (NO) to the signal (SO or S pi). We conclude that greater neural activity is generated with stimuli which produce masking-level difference than stimuli that do not.