EFFECTS OF BILATERAL AND UNILATERAL ABLATION OF AUDITORY-CORTEX IN CATS ON THE UNCONDITIONED HEAD ORIENTING RESPONSE TO ACOUSTIC STIMULI

1. Reflexive head orienting responses (ORs) elicited by bursts of wide-band noise were investigated in cats after bilateral or unilateral ablation of the auditory cortex, and the cats' performance was compared with that of control cats. The OR was used as an indication of ability to orient toward the azimuthal direction of a source of sound. 2. To adequately test this ability, a unique combination of stimulus duration and position of the sound source was selected on each trial. Stimulus durations (0.1, 0.3, and 1.5 s) were selected so that the offset of a burst of noise occurred before, during, or after an OR. The stimuli were produced from speakers positioned approximately at the interaural horizontal plane within each quadrant of a cat's auditory field. The ORs were recorded on moving film and analyzed quantitatively. 3. In the control cats, the trajectory of the OR was characterized by a saccadic profile (rapid steplike movement, monophasic velocity, and biphasic acceleration). The accuracy of the OR varied directly with stimulus duration, suggesting that the response was modified by auditory feedback produced by a head movement during the stimulus. Corrective responses executed during long-duration (1.5 s) stimuli reduced residual mean error to <7-degrees in each of the control animals. The mean error in orientation was smaller for sources located in the frontal sound field than for sources located behind the coronal plane the head (>90-degrees). When brief (0.1 s) stimuli were presented behind the head, the cats confused back with front directions on most of the trials. 4. Compared with performance in the control cats, bilateral destruction of the auditory geniculocortical system severely impaired a cat's ability to orient consistently and accurately toward a source of sound. Latencies to the onset of ORs were increased, the magnitudes of ORs were reduced, average error in orienting to a sound source was larger under every stimulus duration-source position combination, relatively few corrective responses were executed, and residual mean error was significantly elevated (bilateral = 28.1-degrees; control = 3.1-degrees). Several animals with bilateral lesions also made vertical errors in orienting to azimuthal sources of sound. 5. However, in the bilateral lesion group, ORs were initiated in the correct right or left direction; and, rather than eliminating accurate responses altogether, the lesions reduced the probability of their occurrence. Furthermore, the saccadic profile of the response was preserved, providing evidence that the motor control system for the OR was not perturbed by the bilateral lesions. 6. In the cats with unilateral lesions, there were no detectable behavioral deficits in orienting to sources of sound located in the sound fields contralateral or ipsilateral to the cortical lesions. Performance in the unilateral lesion group was indistinguishable from performance in the control group. 7. The results suggest that the auditory cortex of one brain hemisphere is necessary for normal acoustical orientation in cats. use other investigators have shown- that unilateral ablation of auditory cortex may impair sound localization in the sound field contralateral to the lesion, the results also suggest that different pathways within the auditory forebrain are involved in sound localization and acoustical orientation, respectively.