I'm not sure that anyone really believes that the planum temporale (PT) is a functional monolith but you wouldn't know it from the literature. People talk about THE planum temporale -- e.g., Griffiths and Warren's The Planum Temporale as a Computational Hub -- as if it were one functional thing. Cytoarchitectonic data tells us that this is highly unlikely. There are multiple fields in the PT and the posterior half is not even considered part of auditory cortex, based on its laminar organization. Now we have fMRI evidence clearly showing at least two functional subdivisions in the PT.
Two functions that have repeatedly been linked to the PT are spatial hearing and auditory-motor integration. Some authors have linked these two abilities computationally into a single mechanism:
This expanded scheme ... proposes a common computational structure for space processing and speech control in the postero-dorsal auditory stream. -Rauschecker & Scott, 2009, p. 722.This raises the question, are the same regions in the PT involved in processing spatial and auditory-motor information? A new study addresses this question directly.
In an fMRI study subjects participated in four auditory conditions: listening to stationary noise, listening to moving noise, listening to pseudowords, and shadowing pseudowords (covert repetition). As with previous studies, contrasting the shadow and listen conditions should activate regions specific to auditory-motor processes, while contrasting the stationary and moving noise conditions should activate regions involved in spatial hearing. Subjects (N = 16) showed greater activation for shadowing in left posterior PT (yellow), area Spt, when the shadow and listen conditions were contrasted. The motion vs. stationary noise contrast revealed greater activation in a more medial and anterior portion of left PT (red).
Seeds from these two contrasts were then used to guide the DTI analysis in an examination of connectivity via streamline tractography, which revealed different patterns of connectivity.
It's not straightforward to infer computational function from mapping data, but given that completely different areas are involved and given that the connectivity patterns appear to be different, perhaps the computational mechanisms for spatial hearing and auditory-motor integration are not shared after all.
In any case, it is now very clear that, as with Broca's area, the PT is not functionally homogeneous.
Isenberg, A., Vaden, K., Saberi, K., Muftuler, L., & Hickok, G. (2011). Functionally distinct regions for spatial processing and sensory motor integration in the planum temporale Human Brain Mapping DOI: 10.1002/hbm.21373
Griffiths, T. D., & Warren, J. D. (2002). The planum temporale as a computational hub. Trends in Neuroscience, 25(7), 348-353.
Rauschecker, J. P., & Scott, S. K. (2009). Maps and streams in the auditory cortex: nonhuman primates illuminate human speech processing. Nature Neuroscience, 12(6), 718-724