A paper recently published in PNAS reports a fascinating example of task-specific perceptual reorganisation in deaf participants that raises interesting questions regarding the involvement of high-level auditory cortex in temporal processing.
The study found that a rhythmic sequence task involving visual stimuli (a flashing disc) evoked activity in a high-level auditory region in deaf participants. The region – called area Te3 – showed stronger responses to temporally patterned sequences of visual flashes compared to visual sequences comprised of isochronous stimulation. In participants with intact hearing however, area Te3 showed rhythm selective responses only to auditory sequences, confirming that this region is typically involved in auditory processing. The authors concluded that auditory sensory depravation led to a reorganisation of the pathways servicing high level auditory cortex, a suggestion supported by connectivity analysis showing increased connectivity between area Te3 and visual area MT/V5 in deaf participants.
Although a striking example of perceptual reorganization, what is interesting about this conclusion is that the authors interpret the results as evidence of task-specific reorganization of high-level cortex. The implication here being that area Te3 is specialized for rhythmic processing in a modality independent manner. To support their argument, the authors note similar evidence of modality independent functional specialization in blind participants who show activation in visual cortex to auditory stimuli.
How could it be that an auditory selective region could come to be visually selective in deaf participants? One answer may lie in the residual hearing reported by the deaf participants. A table in the supplementary materials indicates that all the participants used hearing aids (outside the study) and that most reported their speech perception to be poor-moderate. This is interesting since listeners with low hearing rely more on visual temporal cues from the face to facilitate speech intelligibility. The increased utilization of visual timing cues to improve auditory processing may have led to a strengthening of the structural pathways between higher auditory cortex and visual cortex.
If so this raises questions regarding the degree to which area Te3 should be considered a task-specific region (i.e., modality independent, selective for timing tasks), or an auditory region typically involved in the temporal organisation of speech. Posterior STS is a multi-sensory region and shows many areas that are strongly selective for audio-visual speech perception. To identify the properties of area Te3, a more careful analysis of stimulus specific and task specific responses would need to be carried out within individual participants before any definitive claim can be made regarding the functional properties of this region.