RESPONSES OF NEURONS IN THE INFERIOR COLLICULUS TO BINAURAL MASKING LEVEL DIFFERENCE STIMULI MEASURED BY RATE-VERSUS-LEVEL FUNCTIONS

The psychophysical detection threshold of a low-frequency tone masked by broadband noise is reduced by less than or equal to 15 dB by invers ion of the tone in one ear (called the binaural masking level differen ce: BMLD). The contribution of 120 low-frequency neurons (best frequen cies 168-2,090 Hz) in the inferior colliculus (ICC) of the guinea pig to binaural unmasking of 500-Hz tones masked by broadband noise was ex amined. We measured rate-level functions of the responses to identical signals (So) and noise (No) at the two ears (NoSo) and to identical n oise but with the signal inverted at one ear (NoS pi): the noise was 7 -15 dB suprathreshold. The masked threshold was estimated by the stand ard separation, ''D''. The neural BMLD was estimated as the difference between the masked thresholds for NoSo and NoS pi. The presence of So and S pi tones was indicated by discharge rate increases in 55.3% of neurons. In 36.4% of neurons, the presence of So tones was indicated b y an increase in discharge rate and S pi tones by a decrease. In 6.8% of neurons, both So and S pi tones caused a decrease in discharge rate . In only 1.5% of neurons was So indicated by a decrease and S pi by a n increase in discharge rate. Responses to the binaural configurations were consistent with the neuron's interaural delay sensitivities; 34. 4% of neurons showing increases in discharge rate to both So and S pi tones gave positive BMLDs greater than or equal to 3 dB (S pi tones we re detected at lower levels than So), whereas 37.3% gave negative BMLD s greater than or equal to 3 dB. For neurons in which So signals cause d an increase in the discharge rate and S pi a decrease, 72.7% gave po sitive BMLDs greater than or equal to 3 dB and only 4.5% gave negative BMLDs greater than or equal to 3 dB. The results suggest that the res ponses of single ICC neurons are consistent with the psychophysical BM LDs for NoSo versus NoS pi at 500 Hz, and with current binaural intera ction models based on coincidence detection. The neurons likely to con tribute to the psychophysical BMLD are those with BFs near 500 Hz, but detection of So and S pi tones may depend on different populations of neurons.