Sound localization: effects of unilateral lesions in central auditory system.

Abstract

Cats with unilateral lesions of the central auditory system were tested behaviorally for their ability to localize the source of a single click of randomized intensity. Each case was tested in 2 ways: in the first, 7 matched speakers were spaced each 30.degree. of azimuth around a semicircle in front of the cat and centered on its interaural midpoint; in the 2nd test (a less-refined one duplicating past techniques) only 2 of the speakers were used, one 30.degree. to the left, the other 30.degree. to the right of the cat''s midsaggital plane. In each case with a substantial unilateral lesion, regardless of its level along the auditory pathway from cochlea to cortex, the more-refined sound-localization test yielded reliable evidence of a profound and probably permanent perceptual deficit. Lesions at or below the level of the superior olivary complex (i.e., cochlea, trapezoid body or superior olivary complex itself) resulted in sound-localization deficits either bilaterally or in the sound field ipsilateral to the lesion. Lesions above the level of the superior olivary complex (i.e., from lateral lemniscus to auditory cortex) resulted in sound-localization deficits in the sound field contralateral to the lesion. Apparently, like the representations of visual or somatosensory space, auditory space is represented contralaterally at all but the lowest levels of the auditory system; that it is the trapezoid body-superior olivary complex that probably accomplishes this contralateralization of the auditory field; this structural complex serves the auditory system in a manner essentially analogous to the optic chiasma in the visual system or the spinal and bulbar decussations in the somatosensory systems. Because each case with a high-level lesion yielded evidence of only slight or temporary deficits in the 2nd less-refined sound-localization test; the reason that the essential contralaterality of the auditory system was not demonstrated earlier may be due to mistaken persistence in using this simpler but less-sensitive method of testing sound-localization ability.