DETECTABILITY INDEX MEASURES OF BINAURAL MASKING LEVEL DIFFERENCE ACROSS POPULATIONS OF INFERIOR COLLICULUS NEURONS

In everyday life we continually need to detect signals against a backg round of interfering noise (the ''cocktail party effect''): a task tha t is much easier to accomplish using two ears. The binaural masking le vel difference (BMLD) measures the ability of listeners to use a diffe rence in binaural attributes to segregate sound sources and thus impro ve their discriminability against interfering noises. By computing the detectability of tones from rate-versus-level functions in the presen ce of a suprathreshold noise, we previously demonstrated that individu al low-frequency delay-sensitive neurons in the inferior colliculus ar e able to show BMLDs. Here we consider the responses of a population o f such neurons when the noise level is held constant (as conventionall y in psychophysical paradigms). We have sampled the responses of 121 u nits in the inferior colliculi of five guinea pigs to identical noise and 500 Hz tones at both ears (NoSo) and to identical noise but with t he 500 Hz tone at one ear inverted (NoS pi). The result suggests that the neurons subserving detection of So tones in No (identical noise at the two ears) noise are those neurons with best frequencies (BFs) clo se to 500 Hz that respond to So tones with an increase in their discha rge rate from that attributable to the noise. The detection of the inv erted (S pi) signal is also attributable to neurons with BFs close to 500 Hz. However, among these neurons, the presence of the S pi tone wa s indicated by an increased discharge rate in some neurons and by a de creased discharge rate in others.