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Publisher Summary This chapter reviews the psychophysical evidence for the role of spectral cues in sound localization and the manner in which they are combined with the other binaural cues to sound location. In addition, some of the bioacoustic, psychophysical, and neurophysiological studies that have examined the mechanisms of encoding and processing of these cues are reviewed. There is strong evidence that the information used by listeners to resolve the ambiguity in interaural time divergence (ITD) and interaural level divergence (ILD) cues is spectral in nature. The possibility that spectral cues to sound location are specific to the lateral angle of the source has been examined in two studies. Several aspects of spatial cue processing examined neurophysiologically are particularly relevant to the processing of spectral information. The chapter focuses on results obtained using the mammalian auditory system. Recent human psychophysical work has shown that under appropriate conditions the auditory system has a remarkable ability to relearn to use modified spectral cues to a sound location.

The role of the superior colliculus in attending and orienting to sensory stimuli is facilitated by the presence within this midbrain nucleus of superimposed maps of different sensory modalities. We have studied the steps involved in the development of topographically-aligned maps of visual and auditory space in the ferret superior colliculus. Injections of fluorescent beads into the superficial layers showed that the projection from the contralateral retina displays topographic order on the day of birth (PO). Recordings made from these layers at the time of eye opening, approximately 1 month later, revealed the presence of an adult-like map of visual space. In contrast, the auditory space map in the deeper layers emerged gradually over a much longer period of postnatal life. In adult ferrets in which one eye had been deviated laterally just before eye opening, the auditory spatial tuning of single units recorded in the contralateral superior colliculus was shifted by a corresponding amount, so that the registration of the visual and auditory maps was maintained. Chronic application of the NMDA-receptor antagonist MK801 disrupted the normal development of the auditory space map, but had no effect on the visual map in either juvenile or adult animals, or on the auditory map once it had matured. These findings indicate that visual cues may play an instructive role, possibly via a Hebbian mechanism of synaptic plasticity, in the development of appropriately tuned auditory responses, thereby ensuring that the neural representations of both modalities share the same coordinates. Changes observed in the auditory representation following partial lesions of the superficial layers at PO suggest that these layers may provide the source of the visual signals responsible for experience-induced plasticity in auditory spatial tuning.