Thursday, October 16, 2008

More on speech recognition and the left hemisphere: Important comment from Dorte Hessler

Dorte Hessler has posted an important comment in on my entry Speech recognition and the left hemisphere. Indeed, the comment is thoughtful, thorough, and important enough that I have decided to repost it here as it's own entry. This is exactly the kind of informal (but informed) discussion that I hoped the blog would support. I'll post a response in a new entry shortly.


dörte hessler said...

Hi again,

First thanks to Greg for your response, which made me think quite a while. Especially your comment of atypical cortical organization. So I went through the articles on phonemic processing deficits I read before, because I seemed to remember that there was a substantial amount of patients with unilateral damage.
But to clarify some things first: Of course my earlier comment was on the acute stroke study – sorry, I should have mentioned it more clearly. Furthermore I definitely did not want to claim that the right hemisphere is does not play any role in phonological processing, I think there is a vast amount of evidence for that it is in fact involved (some of it cited in the comments above). However I did want to claim (and still want to do so), that a damage to solely the left hemisphere can lead to word sound deafness (as e.g. defined by Franklin, 1989): thus problems to identify or discriminate speech sounds in the absence of hearing deficits. I quote Sue Franklin here because she looked at this phenomenon in the light of aphasia and not as a pure syndrome, which, indeed, is very rare. But looking at aphasic cases, quite a lot of aphasic patients suffering from left hemisphere damage have shown problems in discriminating or identifying speech sounds. I won’t quote the single case studies here, but limit myself to larger group studies. I will particularly mention 4 of them which did not investigate only patients with a proven disorder in auditory discrimination, but those who investigated a broader aphasic group:

- Basso, Casati & Vignolo (1977): Of 50 aphasic patients (with unilateral left hemisphere damage) only 13 (26%) were unimpaired in a phoneme identification task (concerning VoiceOnsetTime), the remainder of 37 patients showed impaired performance.

The three other studies are concerned with minimal pair discrimination

- Varney & Benton (1979): Of 39 aphasic patients (with unilateral left hemisphere damage) 10 (~25,6%) showed defective performance on the minimal pair discrimination task and the other 29 showed normal performance

- Miceli, Gainotti, Caltagirone & Masullo (1980): Of 66 aphasic patients (with unilateral left hemisphere damage) 34 (~51,5%) showed pathological performance on a phoneme discrimination task. The other 32 scored normal.

- Varney (1984): Of 80 aphasic patients (with unilateral left hemisphere damage) 14 (17,5%) showed defective performance on the same task as used in Varney & Benton, the remainder was unimpaired.

To sum up 235 aphasic patients (all with unilateral left hemisphere damage) took part in these studies. 95 of them (~40%) were impaired on tasks investigating phonemic processing (discrimination and identification tasks).

For me this seems to underline the notion that a damage in the left hemisphere is definitely sufficient to cause a substantial problem in the recognition/processing of speech sounds!
Also these results differ of course quite from those of the acute stroke study of Rogalsky and colleagues (2008), which I claimed is due to the material used in that study.

Franklin, S. (1989). Dissociations in auditory word comprehension: evidence from nine fluent aphasic patients. Aphasiology 3(3), 189-207.

Basso, A., Casati, G. & Vignolo, L. A. (1977). Phonemic identification defects in aphasia. Cortex, 13, 84-95.

Varney, N.R. & Benton, A.L. (1979). Phonemic discrimination and aural comprehension among aphasic patients. Journal of Clinical Neuropsychology 1(2), 65-73.

Miceli, G., Gainotti, G., Caltagirone, C. & Masullo, C. (1980). Some aspects of phonological impairment in aphasia. Brain and Language 11, 159-169.

Varney, N.R. (1984). Phonemic imperception in aphasia. Brain and Language 21, 85-94.

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 and Language 107(2), 167-169.

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