BINAURAL MASKING LEVEL DIFFERENCES IN THE INFERIOR COLLICULUS OF THE GUINEA-PIG

The binaural masking level difference (BMLD) is a striking and well-do cumented psychophysical effect which relates to the ability to use the phase of low-frequency sounds to dissociate them from masking noise. When identical tones and noise are presented to both ears, detectabili ty is improved by up to 15 dB simply by inverting the phase of either the tone or noise in one ear. Measurements of BMLDs were made in singl e delay-sensitive neurones in the inferior colliculus of the guinea pi g. These have confirmed and extended an earlier report [D. M. Caird, A . R. Palmer, and A. Rees, Hear. Res. 57, 91-106 (1991)] by demonstrati ng that when signals are optimized for the frequency, level, and inter aural delay sensitivities of each neurone, BMLDs can be measured which are in a direction, and of a magnitude, consistent with appropriate p sychophysical observations in human subjects. In addition, BMLDs were found to be consistent with the delay sensitivities of the neurones to the signal and masker, the major determinant of the masked threshold for optimized signals being the activity evoked in the neurone by the masking noise. Within-channel signal-to-noise (S/N) ratios at masked t hreshold for single neurones varied from +20 to -7 dB, depending on th e binaural configuration and the units' delay sensitivities. In single neurones, the size of the BMLD for optimized signals increased with t he level of the noise. The BMLD increased by 5 dB over a 40-dB range o f noise, consistent with psychophysical observations. This came about because as noise level increased, masked threshold for optimized tones increased more slowly in N-pi noise than in N-0 noise. For all binaur al comparisons, both positive (pi signals more detectable, as in the p sychophysics) and negative BMLDs were observed, often in the same neur one, a result entirely consistent with the sensitivity to the interaur al delay of the noise and tone signals. For 500-Hz signals in zero and pi phase masked by identical noise the majority of BMLDs determined w ith the PEST procedure was negative, a result which is taken to indica te that increases in spike rate may not be an appropriate cue for mask ed threshold under these conditions. (C) 1996 Acoustical Society of Am erica.