What leads us to the conclusion that meta-linguistic tasks, such as syllable discrimination, are not valid measures of normal speech sound processing? The data tell the story:
Speech sound processing in comprehension vs. in syllable discrimination double dissociate, even when contextual cues are controlled in comprehension tasks. We reviewed the evidence most thoroughly in our 2004 Cognition paper. There are several reports that examined phoneme identification and/or discrimination that we described in that paper (Basso et al, 1977; Blumstein et al., 1977; Caplan et al., 1995), but the Miceli et al., 1980 (Brain and Language, 11:159-169) paper is worth highlighting again. They studied 60+ aphasics using a CCVC syllable discrimination task and an auditory comprehension task using word-to-picture matching. Critically, the comprehension task employed phonological and semantic foils. The inclusion of phonological foils (e.g., a picture of a pear if the stimulus word is bear) minimizes the possibility of using contextual cues in comprehension. Performance was categorized into normal and pathological based on comparison with age-matched controls. The table, reproduced from our Cognition article, summarizes the findings. Notice that 19 patients had pathological performance on the discrimination task yet were normal on the comprehension task, and 9 showed the reverse pattern. A clear double-dissociation.
Anatomical correlations with syllable discrimination deficits are also revealing. The most severe deficits in syllable discrimination tasks are associated with frontal lobe lesions. For example, Gainotti et al. 1982 (Acta Neurol. Scandinav. 66: 652-665) report error rates as a function of lesion location. Patients with left hemisphere lesions restricted to the frontal or parietal lobes made significantly more errors than patients with lesions restricted to the temporal lobe. The worst performance was found in frontal patients. This is an important observation because (i) it suggests that deficits on syllable discrimination tasks are not particularly related to auditory processes (auditory cortex damage appears neither necessary nor sufficient to produce the deficit), and (ii) since frontal or parietal damage typically spares lexical comprehension, such a finding provides further evidence for the non-relation between auditory comprehension and syllable discrimination tasks.
Conclusion: syllable discrimination is not a valid measure of speech sound processing, at least in the context of aphasia. What we have suggested is that performance of syllable discrimination tasks requires frontal-lobe related cognitive processes, such as working memory, that are not as critical for normal auditory comprehension, and it is these processes that are being disrupted by frontal and/or parietal lesions, rather than the (bilateral!) temporal lobe-based mechanisms involved in speech sound processing that are critical to auditory comprehension.