Auditory spatial and object processing in the human planum temporale: No evidence for selectivity

Although it is generally acknowledged that at least two processing streams exist in the primate cortical auditory system, the function of the posterior dorsal stream is a topic of much debate. Recent studies have reported selective activation to auditory spatial change in portions of the human planum temporale (PT) relative to nonspatial stimuli such as pitch changes or complex acoustic patterns. However, previous work has suggested that the PT may be sensitive to another kind of nonspatial variable, namely, the number of auditory objects simultaneously presented in the acoustic signal. The goal of the present fMRI experiment was to assess whether any portion of the PT showed spatial selectivity relative to manipulations of the number of auditory objects presented. Spatially sensitive regions in the PT were defined by comparing activity associated with listening to an auditory object (speech from a single talker) that changed location with one that remained stationary. Activity within these regions was then examined during a nonspatial manipulation: increasing the number of objects (talkers) from one to three. The nonspatial manipulation modulated activity within the "spatial" PT regions. No region within the PT was found to be selective for spatial or object processing. We suggest that previously documented spatial sensitivity in the PT reflects auditory source separation using spatial cues rather than spatial processing per se.