Neural mechanisms for sound localization

Although the efforts to find a place map of sound direction within the auditory system of mammals has been reinspired by the recent discoveries in owl, progress to date has not been encouraging. Neither the inferior colliculus nor auditory cortex has yielded immediate evidence of such a map, despite ingenious and persistent efforts to find it. Thus, at present, the evidence suggests that a head-referenced map of auditory space is more likely to be found in structures more motor than sensory in function--in the deep layers of the superior colliculus or brainstem tegmentum, for example. Insofar as these structures have been implicated in eye, ear, and head orientation toward a sound source, one might expect that premotor units for orienting would be sensitive to sound direction and thus, collectively, constitute a map of auditory azimuth isomorphic to the map of motor azimuth. However, even for these structures, the possibility for significant variation among mammalian species exists. Because many candidate motor structures (such as the deep superior colliculus) receive input from the cerebral cortex, and because the role of auditory cortex in sound localization seems to vary widely among mammals (38, 51) an equal amount of variation in auditory-motor maps may also exist.