Cortical neural correlates of the binaural masking level difference (BMLD)

Abstract

Single-cell responses to binaural masking level difference (BMLD) stimuli were measured in the primary auditory cortex of Urethane-anesthetised guinea pigs. Firing rate was measured as a function of the presentation level of 500 Hz S0 and STT pure tone signals in the presence of N0 and NTT maskers. The maskers were white noise, low-pass filtered at 5 kHz, with a spectrum level of 23 dB SPL. Responses were similar to those previously reported in the inferior colliculus (IC). At the lowest tone signal levels, the response was dominated by the noise masker, at higher signal levels the firing rate either increased or decreased. Signal detection theory was used to determine detection threshold. Very few neurones yielded measurable detection thresholds for all four stimulus conditions, and there was a wide range in thresholds. However, across the entire population, the lowest thresholds were consistent with human psychophysical BMLDs. Tone and noise delay functions could be used to predict the shape of the firing-rate vs. signal-level function. In summary, like in the IC, the responses were consistent with a cross-correlation model of BMLD with detection facilitated by either a decrease or increase in firing rate.