The Dual Stream model of speech/language processing holds that there are two functionally distinct computational/neural networks that process speech/language information, one that interfaces sensory/phonological networks with conceptual-semantic systems, and one that interfaces sensory/phonological networks with motor-articulatory systems (Hickok & Poeppel, 2000, 2004, 2007). We have laid out our current best guess as to the neural architecture of these systems in our 2007 paper:
It is worth pointing out that under reasonable assumptions some version of a dual stream model has to be right. If we accept (i) that sensory/phonological representations make contact both with conceptual systems and with motor systems, and (ii) that conceptual systems and motor-speech systems are not the same thing, then it follows that there must be two processing streams, one leading to conceptual systems, the other leading to motor systems. This is not a new idea, of course. It has obvious parallels to research in the primate visual system, and (well before the visual folks came up with the idea) it was a central feature of Wernicke's model of the functional anatomy of language. In other words, not only does the model make sense for speech/language processing, it appears to be a "general principle of sensory system organization" (Hickok & Poeppel, 2007, p. 401) and it has stood the test of time.
So, all that remains is to work out the details of these networks. A new paper in PNAS by Saur et al. may provide some of these details. In an fMRI experiment, they used two tasks, one that they argued tapped the dorsal stream pathway (pseudoword repetition), and the other the ventral stream pathway (sentence comprehension). The details of the use of these tasks leave something to be desired in my view, but they did seem to highlight some differences, so I'm not going to quibble for now. Here's the activation maps (repetition in blue, comprehension in red):
Notice the more ventral involvement along the length of the temporal lobe (STS, MTG, AG) for comprehension relative to repetition, as well as the more posterior involvement in the frontal lobe for repetition.
They then used peaks in these activations as seeds for a tractography analysis using DTI. Here is a summary figure showing the distinction between the two pathways (red = ventral, blue = dorsal).
The authors localize the white matter tract of the dorsal pathway as being part of the arcuate/superior longitudinal fasciculi and the tract of the ventral pathway as part of the extreme capsule (not the uncinate).
I haven't looked closely at the details of the analysis (I would love to hear comments!), but this sort of study seems just the ticket to getting us closer to delineating the functional anatomical details of the speech/language system.
G Hickok, D Poeppel (2000). Towards a functional neuroanatomy of speech perception Trends in Cognitive Sciences, 4 (4), 131-138 DOI: 10.1016/S1364-6613(00)01463-7
G Hickok, D Poeppel (2004). Dorsal and ventral streams: a framework for understanding aspects of the functional anatomy of language Cognition, 92 (1-2), 67-99 DOI: 10.1016/j.cognition.2003.10.011
Gregory Hickok, David Poeppel (2007). The cortical organization of speech processing Nature Reviews Neuroscience, 8 (5), 393-402 DOI: 10.1038/nrn2113
D. Saur, B. W. Kreher, S. Schnell, D. Kummerer, P. Kellmeyer, M.-S. Vry, R. Umarova, M. Musso, V. Glauche, S. Abel, W. Huber, M. Rijntjes, J. Hennig, C. Weiller (2008). Ventral and dorsal pathways for language Proceedings of the National Academy of Sciences, 105 (46), 18035-18040 DOI: 10.1073/pnas.0805234105