A recent Nature Neuroscience review paper by Josef Rauschecker and Sophie Scott is worth a look. On my reading, the paper is an attempt to reconcile or integrate Rauschecker's Dual Stream model, a 'what'/'where' model, with the version of a Dual Stream model that we (Hickok & Poeppel, 2000, 2004, 2007) and others (e.g., Richard Wise, Sophie Scott, et al. 2001) have been promoting, namely a 'what'/sensory-motor model. There's lots of good information and ideas in the paper (many of them previously elaborated) and it is good to see that Rauschecker is now agreeing with us that the dorsal stream supports sensory-motor integration. However, the effort to reconcile/integrate the 'where' and sensory-motor functions of the dorsal stream fall short in my view as do some of the more general claims by these authors.
Here are some of the claims made by R&S that I believe are not supported by existing evidence:
1. "Speech perception and production are left-lateralized in the human brain" p. 720
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.
3. "The postero-dorsal stream interfaces with premotor areas and pivots around inferior parietal cortex, where a quick sketch of sensory event information is compared with a predictive efference copy" p. 722, figure caption.
4. "The postero-medial planum temporale area ... is an auditory area important in the act of articulation" p. 721. [emphasis added to indicate where the statement is incorrect]
So let's take these in turn, one post at a time.
1. "Speech perception and production are left-lateralized in the human brain"
I won't argue with the claim that speech production is left-lateralized, but I will argue with the speech perception claim. Evidence from chronic lesion, acute stroke, Wada, and split brain studies have shown that damage to the left hemisphere produces only relatively mild effects on the perception of speech sounds (see Hickok et al. 2008 & Rogalsky et al. 2008 for recent data and discussion). Here, for example, is the brain lesion in a recent case we (C. Rogalsky, T. Love, S. Andersen, H. Damasio, G. Hickok) had the opportunity to study (watch for our forthcoming paper which describes several cases).
This patient performed at 100% accuracy in a four alternative forced choice (4AFC) word-to-picture matching comprehension test where all foils where phonological and at least one differed from the target by only one feature. Performance on a discrimination (same-different judgment) task involving pairs of words that differed by only one feature (e.g., goat-boat) was > 95% correct and non-word discrimination performed at 88% correct. This level of performance is not expected in a patient with no left superior temporal lobe given R&S's claim for left-lateralization.
Rauschecker, J., & Scott, S. (2009). Maps and streams in the auditory cortex: nonhuman primates illuminate human speech processing Nature Neuroscience, 12 (6), 718-724 DOI: 10.1038/nn.2331
Hickok, G., Okada, K., Barr, W., Pa, J., Rogalsky, C., Donnelly, K., Barde, L., & Grant, A. (2008). Bilateral capacity for speech sound processing in auditory comprehension: evidence from Wada procedures. Brain Lang, 107(3), 179-184.
Hickok, G., & Poeppel, D. (2000). Towards a functional neuroanatomy of speech perception. Trends in Cognitive Sciences, 4, 131-138.
Hickok, G., & Poeppel, D. (2004). Dorsal and ventral streams: A framework for understanding aspects of the functional anatomy of language. Cognition, 92, 67-99.
Hickok, G., & Poeppel, D. (2007). The cortical organization of speech processing. Nat Rev Neurosci, 8(5), 393-402.
Rogalsky, C., Pitz, E., Hillis, A. E., & Hickok, G. (2008). Auditory word comprehension impairment in acute stroke: relative contribution of phonemic versus semantic factors. Brain Lang, 107(2), 167-169.
Wise, R. J. S., Scott, S. K., Blank, S. C., Mummery, C. J., Murphy, K., & Warburton, E. A. (2001). Separate neural sub-systems within "Wernicke's area". Brain, 124, 83-95.