Friday, July 17, 2009

Maps and streams in auditory cortex -- continued...

As promised, here is some continued discussion of some of the claims made by Rauschecker & Scott in their recent Nature Neuroscience paper. Click here for the original entry.

A second claim R&S make is this:

2. "This expanded scheme [their model] closes the loop between speech perception and production and proposes a common computational structure for space processing and speech control in the postero-dorsal auditory stream" p. 722, figure caption.

Just to set the record straight, this perception-production loop has been closed since at least 1874 when Wernicke proposed his own dual stream model in which one stream links auditory speech representations with (widely distributed!) conceptual systems, a "ventral stream", and another links auditory speech representations with motor speech representations, a "dorsal stream". Of course David and I have elaborated extensively on Wernicke's proposal with our version of a dual stream model. So there's nothing really new here in R&S's proposal.

What is different than our (HP) version of a dual stream model is the second part of the statement above tying both auditory space and sensory-motor functions to a common computation network. We have suggested that spatial and sensory-motor functions are subserved by distinct systems within the planum temporale region. The only published data directly relevant to this question comes from an examination of the coordinates of activations associated with spatial hearing tasks versus sensory-motor tasks. Data from some of my own published studies suggests that distinct areas are involved. Here are coordinates for activations associated with spatial hearing (-54, -26, 11) (Smith et al., 2007) compared with those for sensory-motor processes for speech (-51, -46, 16)(Hickok et al., 2003). Note that the foci are 2cm apart in the y direction. More directly, I can tell you that we have now completed a within subject fMRI study that shows distinct activation foci for space vs. sensory-motor processes in the planum temporal. This work has been presented at conferences but has not yet appeared in print.

This, together with anatomical facts regarding the regions involved (see subsequent posts) indicate that there is not a common computational mechanism for spatial and sensory-motor functions of the "dorsal stream".

Hickok, G., Buchsbaum, B., Humphries, C., & Muftuler, T. (2003). Auditory-motor interaction revealed by fMRI: Speech, music, and working memory in area Spt. Journal of Cognitive Neuroscience, 15, 673-682.

Smith, K. R., Saberi, K., & Hickok, G. (2007). An event-related fMRI study of auditory motion perception: no evidence for a specialized cortical system. Brain Res, 1150, 94-99.

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