A crosslinguistic positron emission tomography (PET) study investigated the influence of linguistic experience on the perception of segmental (consonants, vowels) and suprasegmental (tones) information. Chinese and English subjects (10 per group) were presented binaurally with lists consisting of five Chinese monosyllabic morphemes or low-pass filtered versions of the same stimuli, and asked to make same-different judgements of the first and last items. A speeded-response, selective attention paradigm required subjects to make discrimination judgments of the target items while ignoring intervening distractor tones. PET scans were acquired for five tasks presented twice: one passive listening (pitch), and four active (consonant, vowel, tone, pitch). Results from subtractions of group-averaged images between tasks indicate that Chinese subjects show increased activity in the left premotor cortex, pars opercularis, and pars triangularis across the four tasks. English subjects, on the other hand, show increased activity in the left inferior frontal regions only in the vowel task and in the right inferior frontal regions in the pitch task. Our findings support a task-dependent hypothesis of cerebral lateralization whereby left hemisphere mechanisms selectively mediate processing of linguistic information irrespective of acoustic cues or type of phonological unit (i.e., segmental or suprasegmental), and right hemisphere mechanisms are engaged for prosody-specific cues. Our findings also illustrate the interaction between memory and language systems in phonological processing. A functional separation between anterior and posterior areas of the left frontal lobe corresponding to executive processes and phonological loop, respectively, of verbal working memory suggests that executive processes may be mediated in discrete regions of the left frontal lobe. Consistent activation in the left inferior frontal regions on tone and pitch tasks for Chinese listeners provides neurological evidence for prelexical phonological processing of lexical tones. Our findings further support a revision of the classical model of language localization. Left frontal regions appear to mediate perception as well as production of both segmental and suprasegmental aspects of the speech signal. Finally, functional lateralization of anterior insular may depend on linguistic relevance of auditory input.
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