Cerebral Hemispheric Dominance In The Auditory System Of Automatic Processing Of Chinese Tones And Consonants

Language is the product of human evolution and an important tool for human communication. The mechanisms through which the human brain perceives and discriminates complex and rapidly changing components of human speech are not well understood and remain as an important question for brain research. Although speech is a compound sound and there exists a variety of speech in the world, it is built with syllables composed of the phonemes. Variations in fundamental frequency (f0) form pitch patterns of speech, or lexical tones which have different functions in the different languages. The speech pitch can be used to identify the speaker's voice, mood and doubt. But the pitch pattern, or a lexical tone, of a tonal language such as Mandarin Chinese has a unique linguistic function: they encode word meaning as vowels and consonants do.In tonal languages, lexical tones acoustically correlate with variations of the fundamental frequency (f0) of a syllable. The same syllable can represents different word meanings when it is accented in different lexical tones. For example, syllable /mal/ means "mother", /ma2/ means "hemp". How the brain processes lexical tones is an important issue in neurolinguistic research. In particular, it remains as an interesting question which hemisphere dominates the processing of a lexical tone in the brain. Currently, there are two competing hypotheses about hemisphere dominance for processing lexical tones in tonal languages. The functional hypothesis predicts that a lexical tone will be preferentially processed in the left hemisphere based on its linguistic functions. Alternatively, the acoustic hypothesis predicts that a lexical tone will be preferentially processed in the right hemisphere based on its acoustic structures. The two hypotheses are competing and neither can account for a full range of experimental data.When non-speech sounds are used as the stimulus, the lateralization patterns as revealed with fMRI or PET are well predicted by the acoustic hypothesis. Arguing against this hypothesis are data from the fMRI or PET experiments which use lexical tones as the stimulus to demonstrate that additional areas of the left hemisphere will be activated in native tonal language speakers versus English speakers, suggesting the dependence of hemisphere lateralization on linguistic functions and language experience, rather than on acoustic features. Because fMRI or PET measures hemodynamic responses with a temporal resolution ranging from seconds to tens of seconds, the observation from these neuroimaging studies, which require subjects to execute a discrimination task, likely represents the temporally aggregated brain activities including those at an attentive stage of auditory processing. However, the hemisphere lateralization for an auditory response to speech sound can occur as early as 144 ms after the onset of stimulus. Thus, there is a possibility that the auditory response to lexical tones at such an early stage was actually lateralized to the right hemisphere based on their acoustic features but confounded with the response components at a later stage of processing and not explicitly revealed in the previous fMRI or PET studies. The present study attempted to identify which hypothesis prevails in early auditory processing with whole-head electric recordings of the mismatch negativity (MMN) from native Mandarin Chinese speakers under a passive auditory odd-ball paradigm. The recorded MMN response occurred around 200 ms after the onset of stimulus and reflected the brain's automatic processing at a pre-attentive stage. This study demonstrates that early auditory processing of a lexical tone at a pre-attentive stage is actually lateralized to the right hemisphere. A meaningful auditory word with a consonant-vowel structure is frequently presented to native Mandarin Chinese speakers and either its lexical tone or initial consonant using an odd-ball paradigm infrequently varied to create a contrast and result in a change in word meaning. The lexical tone contrast evoked a stronger pre-attentive response, as revealed by whole-head electric recordings of the mismatch negativity, in the right hemisphere than in the left whereas the consonant contrast produced an opposite pattern. Given the distinct acoustic features between a lexical tone and a consonant, this opposite lateralization pattern suggests the dependence of hemisphere dominance mainly on acoustic cues before speech input is mapped into a semantic representation in the processing stream.