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...
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.