Cortical processing of auditory space: Pathways and plasticity

Abstract

Contrary to popular belief, which places auditory space processing wholly in the brainstem, several lines of evidence suggest that auditory cortex plays an important role in spatial perception. Lesion studies in animals and humans demonstrate severe deficits in sound localization after damage to auditory cortex. Single-unit recording studies find neurons tuned to spatial location in auditory cortical areas. While these neurons exist already in primary auditory cortex, their prevalence and sharpness of spatial tuning increases in nonprimary areas of the caudal belt, as defined in nonhuman primates. The firing of neurons in these latter areas also shows a tighter correlation with the behavioral performance of alert monkeys engaged in sound localization behavior. Caudal belt and parabelt project to posterior parietal cortex and to areas of prefrontal cortex, such as the frontal eye and pinna fields, known to be involved in spatial perception. This has led to the notion that a posterior-dorsal processing stream is intimately involved in aspects of auditory spatial perception. The existence of such an auditory where-stream is also suggested by functional neuroimaging studies in humans in which subjects process stationary or moving sounds in space. Consistently, posterior aspects of the superior temporal cortex and adjoining inferior parietal cortex are activated during these tasks. Thus, while brainstem nuclei perform an important service by computing some of the basic parameters necessary for spatial processing, such as interaural time and intensity differences, these parameters are integrated (together with monaural spectral cues that depend on head and pinnae) at the cortical level. Auditory space perception, including perception of motion in space, is, therefore, ultimately accomplished at the cortical level. Animals and humans that grow up blind use their auditory modality for localization in far space. Areas in parietal and occipital cortex that are ordinarily used for vision become activated by auditory input. This leads to an expansion of auditory areas in the dorsal stream into visual territory and to a simultaneous sharpening of auditory spatial tuning in these neurons. Together, this massive cross-modal reorganization leads to superior performance of blind as compared to sighted individuals in auditory spatial tasks.