Superior-posterior temporal cortex decodes distances to sound sources
Quick and accurate localization of perceptual objects in our environment is a fundamentally important ability where the sense of hearing significantly complements that of vision. For instance, objects that are occluded or out of one’s field of vision (such as a rare bird chirping on branch behind a bird watcher) are efficiently and almost automatically segregated and localized by the auditory system in the three-dimensional space that surrounds oneself. While a number of previous studies have suggested that there are neurons in superior-posterior temporal cortical areas specialized in localization of the directions that sounds emanate from, it has remained less well known wherein and how distance to sound sources are processed in the human brain. Importantly, the most salient sound distance cue, intensity of the sound, is not always a reliable one, as sound intensity can and does vary independently of source distance. Therefore it is feasible to assume that there are also other cues that the auditory system uses to decode distances to sound sources.
A recent ingenious study by Dr. Norbert Kopčo et al. (2012) combined psychophysics, computational modeling, and functional magnetic resonance imaging to probe the neural basis of sound distance processing. The authors presented healthy volunteers with sounds at varying distances (15-100 cm) in a virtual reverberant environment. The behavioral results suggested that direct-to-reverberant ratio is, out of the intensity-independent distance cues, the most reliable one, but that discrimination performance is best explained by utilization of a combination direct-to-reverberant ratio and inter-aural level difference cues. Furthermore, inspection of the functional magnetic resonance data collected during presentation of the sounds at varying distances disclosed planum temporale and posterior superior temporal gyrus contralateral to the direction of stimulation as the auditory system structures underlying the decoding of distances to sound sources.
Reference: Kopčo N, Huang S, Belliveau JW, Raij T, Tengshe C, Ahveninen J. Neuronal representations of distance in human auditory cortex. Proc Natl Acad Sci USA (2012) 109: 11019-11024. http://dx.doi.org/10.1073/pnas.1119496109