Several observations suggest to me a connection between conduction aphasia and disruption to our proposed sensory-motor integration area Spt.
1. Spt is located in the posterior planum temporale region. The lesion distribution in conduction aphasia seems to be centered on this same location (Baldo, et al. 2008).
2. Spt activity is modulated by word length (Okada, et al., 2003) and frequency, and has been implicated in accessing lexical phonology (Graves et al. 2008). Conduction aphasics commit predominately phonemic errors in their output and these errors are increased by longer, less frequent words.
3. Spt is not speech specific in that tonal/melodic tasks also activate this region (Hickok, et al. 2003). Conduction aphasics appear to have deficits that also affect tonal processing (Strub & Gardner, 1974).
The idea is that this sensory-motor circuit is critical in supporting sensory guidance of speech output, and that such guidance is most critical for phonemically complicated words/phrases and/or for low frequency words or for items with little or no semantic constraint (e.g., non-words, phrases like "no, ifs, ands, or buts"). If a word is short or used frequently, the claim goes, its motor representation can be activated as a chunk rather than programmed syllable by syllable.
One problem, raised by Alfonso Caramazza in the form of a question after a talk I gave, is that sometimes conduction aphasics get stuck on the simplest of words. Case in point, in my talk, I showed an example of such an aphasic who was trying to come up with the word cup. He showed the typical conduit d'approche, "its a tup, no it isn't... it's a top... no..." etc. Alfonso justifiably noted that conduction aphasics shouldn't have trouble with such simple words if the damaged sensory-motor circuit wasn't needed as critically in these cases.
So here is a sketch of a possible explanation. I'd love to hear your thoughts. There is a difference between repetition and naming: repetition shows the typical length/frequency effects, whereas naming doesn't. Here's why:
In repetition, a common word like cup can be recognized/understood and then semantic representations can drive the activation of the motor speech pattern. As the word gets more phonologically complicated or less semantically constrained, this route becomes less and less reliable and the sensory-motor system is required. This is the classic explanation of invoked to explain why conduction aphasics sometimes paraphrase in their repetition; a view that has gained some recent support (Baldo, et al. 2008).
In naming, the main hang up in conduction aphasia is in trying to access the phonological word form. Since the lesion in conduction aphasia typically involves the STG, systems involved in representing word forms are likely partially compromised leading to more frequent access failures. Further, in lexical-phonological access simple, high-frequency forms that share a lot of neighbors (cup, pup, cut, cop, cope ...) will actually lead to more difficulty because of the increased competition.
Baldo JV, Klostermann EC, and Dronkers NF. It's either a cook or a baker: patients with conduction aphasia get the gist but lose the trace. Brain Lang 105: 134-140, 2008.
William W. Graves, Thomas J. Grabowski, Sonya Mehta, Prahlad Gupta (2008). The Left Posterior Superior Temporal Gyrus Participates Specifically in Accessing Lexical Phonology Journal of Cognitive Neuroscience, 20 (9), 1698-1710 DOI: 10.1162/jocn.2008.20113
Okada K, Smith KR, Humphries C, and Hickok G. Word Length Modulates Neural Activity in Auditory Cortex During Covert Object Naming. Neuroreport 14: 2323-2326, 2003.
Strub RL, and Gardner H. The repetition defect in conduction aphasia: Mnestic or linguistic? Brain and Language 1: 241-255, 1974.