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Abstract:
1. We studied the sensitivity of auditory neurons in the barn owl's brain stem to the direction of apparent acoustic motion. Motion stimuli were generated with an array of seven free-field speakers (Fig. 2). Motion-direction sensitivity was determined by comparing the number of spikes evoked by counterclockwise (CCW) motion with the number of spikes evoked by clockwise (CW) motion. A directionality index (DI) was defined to quantify the measurements. The statistical significance of the directional bias was determined by a chi 2 test that used the responses to stationary sounds as the null hypothesis. 2. During the search for acoustic neurons, dichotic stimuli were presented via earphones, and the sensitivity of the units for interaural time difference (ITD), interaural level difference (ILD), and frequency was measured. After a unit had been isolated, its response to moving and stationary free-field stimuli was recorded. Most of the neurons that responded to dichotic stimulation responded also to free-field stimulation. At 61 of the 211 recording sites, the response was motion-direction sensitive. 3. The spontaneous activity of all neurons was low, so that some 95% of the recorded activity was due to an excitation caused by the stimuli. 4. Neurons sensitive to the direction of motion were found in many nuclei of the auditory pathway such as the nuclei of the lateral lemniscus, the subnuclei of the inferior colliculus (IC), and the optic tectum (OT) (Figs. 3 and 5-8, Table 1). 5. In 61% of the motion-direction-sensitive neurons, the response to motion in the preferred direction was equal to the response to stationary sounds, whereas in 75% of the neurons, the response to motion in the null direction was lower than the response to stationary sounds (Table 2, Fig. 6). This observation suggested a null-direction inhibition as one important factor of generating motion-direction sensitivity. 6. Neurons having a high motion-direction sensitivity usually responded phasically, whereas tonically active neurons exhibited a low motion-direction sensitivity (Fig. 9). 7. Velocity tuning was broad (Fig. 7). A shallow peak appeared around 310 degrees/s within the range tested (125-1,200 degrees/s, 33 cells). 8. A silent gap between the bursts from successive speakers caused a decrease in motion-direction sensitivity. This decrease was linear with gap duration and depended on the apparent velocity (Figs. 10-13).
