Neural responses in the inferior colliculus to binaural masking level differences created by inverting the noise in one ear

We have measured the responses of inferior colliculus neurons in the anesth etized guinea pig to signals which in human psychophysical experiments reve al a release of masking as a result of binaural processing (the binaural ma sking level difference: BMLD). More specifically we have used diotic tones at 500 Hz (So) masked by noise that is either identical at the two ears (No ) or inverted in one ear (N pi). This combination of signals and noise mask ers produces a prominent masking release in humans such that the So signal is about 6-12 dB more detectable in the presence of the N pi noise than the No noise. Low-frequency inferior colliculus neurons are sensitive to the i nteraural delay of the masking noise and generally respond most to the comp onents nearest their best frequency. Since most inferior colliculus neurons have peaks in their delay functions close to zero interaural time delay th is means that while No noise is effective in driving the unit, N pi noise i s much less effective. As the level of an So tone was progressively increas ed in the presence of No and N pi noises, the first response could be eithe r an increase or a decrease in the activity due to the noise. However, beca use N pi generated little or no activity itself, the predominant response t o the So tone was an increase in discharge in this condition. Masked thresh olds were defined as the point at which the standard separation D (related to the d' of signal detection theory) = 1 in either direction. BMLDs were m easured in single neurons and in the majority of units were in a direction consistent with the psychophysical observations irrespective of the directi on of the discharge rate change that occurred at threshold. The lowest mask ed thresholds always occurred at or near the signal frequency of 500 Hz. An average value of the single unit BMLD around 500 Hz was 3.6 dB (NoSo vs. N pi So) compared with 6.6 dB for the NoSo versus NoS pi BMLD we had previou sly reported. This lower magnitude is consistent with the hierarchy of huma n psychophysical BMLDs.