The time course of binaural masking in the inferior colliculus of guinea pig does not account for binaural sluggishness.

Psychophysical studies show a slower response to changes in the specifically binaural input than to changes in the monaural input (binaural sluggishness). However, there is disagreement about the time course. Tracking changes in a target yields fast time constants, while detecting a constant target against a varying background yields the slowest. Changes in the binaural properties of a target are tracked up to high rates by cells in the midbrain. Indeed cells respond rapidly to a step change and then the firing rate slowly adapts. These experiments, though, are analogues of psychophysical experiments that give the faster time constants. Sluggishness should be more apparent physiologically in a binaural masking paradigm, detecting a short tone in a noise masker with a step change in masker correlation: the small change in firing rate due to the signal must be detected against the adapting firing rate change caused by the step change in the masker. However, in 40 inferior colliculus cells in the anesthetized guinea pig, in a direct analogue of the psychophysical masking paradigm, measuring thresholds for short tones across a transition in a binaural masker (e.g., from N0S0 to NpiS0) provided little evidence of sluggishness within individual cells despite masking level differences in these cells comparable with previous data. Previous studies of physiological correlates of binaural masking level difference suggested that different psychophysical thresholds arise from different populations of cells. This suggests the hypothesis that sluggishness may result from a change in focus between the different populations of cells signaling threshold in different binaural configurations rather than within the intrinsic properties of the cells themselves.