Are Interaural Time and Level Differences Represented by Independent or Integrated Codes in the Human Auditory Cortex?

Sound localization is important for orienting and focusing attention and for segregating sounds from different sources in the environment. In humans, horizontal sound localization mainly relies on interaural differences in sound arrival time and sound level. Despite their perceptual importance, the neural processing of interaural time and level differences (ITDs and ILDs) remains poorly understood. Animal studies suggest that, in the brainstem, ITDs and ILDs are processed independently by different specialized circuits. The aim of the current study was to investigate whether, at higher processing levels, they remain independent or are integrated into a common code of sound laterality. For that, we measured late auditory cortical potentials in response to changes in sound lateralization elicited by perceptually matched changes in ITD and/or ILD. The responses to the ITD and ILD changes exhibited significant morphological differences. At the same time, however, they originated from overlapping areas of the cortex and showed clear evidence for functional coupling. These results suggest that the auditory cortex contains an integrated code of sound laterality, but also retains independent information about ITD and ILD cues. This cue-related information might be used to assess how consistent the cues are, and thus, how likely they would have arisen from the same source.