The binaural masking level difference (BMLD) is a phenomenon whereby a signal that is identical at each ear (S0), masked by a noise that is identical at each ear (N0), can be made 12-15 dB more detectable by inverting the waveform of either the tone or noise at one ear (S, N). Single-cell responses to BMLD stimuli were measured in the primary auditory cortex of urethane-anesthetized guinea pigs. Firing rate was measured as a function of signal level of a 500 Hz pure tone masked by low-passed white noise. Responses were similar to those reported in the inferior colliculus. At low signal levels, the response was dominated by the masker. At higher signal levels, firing rate either increased or decreased. Detection thresholds for each neuron were determined using signal detection theory. Few neurons yielded measurable detection thresholds for all stimulus conditions, with a wide range in thresholds. However, across the entire population, the lowest thresholds were consistent with human psychophysical BMLDs. As in the inferior colliculus, the shape of the firing-rate versus signal-level functions depended on the neurons' selectivity for interaural time difference. Our results suggest that, in cortex, BMLD signals are detected from increases or decreases in the firing rate, consistent with predictions of cross-correlation models of binaural processing and that the psychophysical detection threshold is based on the lowest neural thresholds across the population.

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.