Responses of neurons in the inferior colliculus to dynamic interaural phase cues: Evidence for a mechanism of binaural adaptation

Responses to sound stimuli that humans perceive as moving were obtained for 89 neurons in the inferior colliculus (IC) of urethan-anesthetized guinea pigs. Triangular and sinusoidal interaural phase modulation (IPM), which pr oduced dynamically varying interaural phase disparities (IPDs), was used to present stimuli with different depths, directions, centers, and rates of a pparent motion. Many neurons appeared sensitive to dynamic IPDs, with respo nses at any given IPD depending strongly on the IPDs the stimulus had just passed through. However, it was the temporal pattern of the response, rathe r than the motion cues in the IPM, that determined sensitivity to features such as motion depth, direction, and center locus. IPM restricted only to t he center of the IPD responsive area, evoked lower discharge rates than whe n the stimulus either moved through the IPD responsive area from outside, o r up and down its flanks. When the stimulus was moved through the response area first in one direction and then back in the other, and the same IPDs e voked different responses, the response to the motion away from the center of the IPD responsive area was always lower than the response to the motion toward the center. When the IPD was closer at which the direction of motio n reversed was to the center, the response to the following motion was lowe r. In no case did we find any evidence for neurons that under all condition s preferred one direction of motion to the other. We conclude that response s of IC neurons to IPM stimuli depend not on the history of stimulation, pe r se, but on the history of their response to stimulation, irrespective of the specific motion cues that evoke those responses. These data are consist ent with the involvement of an adaptation mechanism that resides at or abov e the level of binaural integration. We conclude that our data provide no e vidence for specialized motion detection involving dynamic IPD cues in the auditory midbrain of the mammal.