Tuesday, December 9, 2008

Conduction aphasia and delayed auditory feedback

Here's an interesting nugget of information: conduction aphasics appear to be less susceptible to the disruptive effect of delayed auditory feedback. Why is this interesting? Because it is more evidence for a link between systems supporting auditory-motor interaction and the deficit in conduction aphasia. Here are the details...

Delayed auditory feedback (DAF) disrupts speech production. You can prove this to yourself either by trying to talk on a microphone in a large stadium (where your echo is delayed) or, if you don't regularly speak in large stadiums, you can simply talk to yourself on two cell phones: call one phone with the other, hold them both to your ears and start talking; there is a slight delay in transmission leading to delayed auditory feedback, and so speaking becomes difficult. DAF is strong evidence that auditory speech information interacts with speech production systems.

While the classic view is that conduction aphasia is a disconnection syndrome resulting from damage to the arcuate fasciculus, this view is no longer tenable. I have been promoting the view that the syndrome results from damage to our favorite brain region, Spt, which we believe is a critical node in a network that supports auditory-motor interaction (e.g., see Hickok et al., 2000). This, we claim, explains why conduction aphasics make phonemic errors in production (because speech planning is guided to some degree by auditory speech systems) and why they have trouble with verbatim repetition under conditions of high phonological load such as with multisyllablic words, unfamiliar phrases, or non-words (because these kinds of stimuli maximally rely on sensory speech guidance). One prediction of this view is that conduction aphasics should exhibit other "symptoms" of a disrupted auditory-motor integration system.

So I was digging through some old papers on conduction aphasia and came across two, both published by Francois Boller in 1978, that suggested that conduction aphasics are less susceptible to DAF than controls and patients with other aphasia types. One was a group study that found that conduction aphasics were the least affected by DAF of the groups studied (Boller, Vrtunski, Kim, & Mack, 1978), and the other was a case study showing no effect of DAF (and even some improvement!) on the repetition of speech in a conduction aphasic (Boller & Marcie, 1978). This decreased DAF effect in conduction aphasia makes sense if the system that supports auditory-motor interaction is disrupted in that syndrome.


F Boller, P Marcie (1978). Possible role of abnormal auditory feedback in conduction aphasia Neuropsychologia, 16 (4), 521-524 DOI: 10.1016/0028-3932(78)90078-7

Boller, F., Vrtunski, B., Kim, Y., & Mack, J.L. (1978). Delayed auditory feedback and aphasia. Cortex, 14, 212-226.

G Hickok, et al. (2000). A functional magnetic resonance imaging study of the role of left posterior superior temporal gyrus in speech production: implications for the explanation of conduction aphasia Neuroscience Letters, 287 (2), 156-160 DOI: 10.1016/S0304-3940(00)01143-5

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