AUDITORY MOTION INDUCES DIRECTIONALLY DEPENDENT RECEPTIVE-FIELD SHIFTS IN INFERIOR COLLICULUS NEURONS

This research focused on the response of neurons in the inferior colli culus of the unanesthetized mustached bat, Pteronotus parnelli, to app arent auditory motion. We produced the apparent motion stimulus by bro adcasting pure-tone bursts sequentially from an array of loudspeakers along horizontal, vertical, or oblique trajectories in the frontal hem ifield. Motion direction had an effect on the response of 65% of the u nits sampled. In these cells, motion in opposite directions produced s hifts in receptive field locations, differences in response magnitude, or a combination of the two effects. Receptive fields typically were shifted opposite the direction of motion (i.e., units showed a greater response to moving sounds entering the receptive field than exiting) and shifts were obtained to horizontal, vertical, and oblique motion o rientations. Response latency also shifted as a function of motion dir ection, and stimulus locations eliciting greater spike counts also exh ibited the shortest neural latency. Motion crossing the receptive fiel d boundaries appeared to be both necessary and sufficient to produce r eceptive field shifts. Decreasing the silent interval between successi ve stimuli in the apparent motion sequence increased both the probabil ity of obtaining a directional effect and the magnitude of receptive f ield shifts. We suggest that the observed directional effects might be explained by ''spatial masking,'' where the response of auditory neur ons after stimulation from particularly effective locations in space w ould be diminished. The shift in auditory receptive fields would be ex pected to shift the perceived location of a moving sound and may expla in shifts in localization of moving sources observed in psychophysical studies. Shifts in perceived target location caused by auditory motio n might be exploited by auditory predators such as Pteronotus in a pre dictive tracking strategy to capture moving insect prey.