Cognitive Neuroscience and Neuropsychology

Auditory language comprehension is a multistage process relying on many different cognitive functions as an acoustic signal is perceived, parsed into subphonemic features, and the features synthesized into a semantically coherent utterance. In approximately 96% of right-handers and 70% of lefthanders, the left hemisphere is preferentially involved in the later (phonological and semantic) stages of this process,1 and it is possible that speech processing proceeds asymmetrically from its earliest stages. Indeed, it has been hypothesized that the left hemispheric specialization for speech may be based, in part, on prepotent mechanisms within this hemisphere for the processing of auditory patterns of a fine temporal grain, such as are found in speech.2–4 Some phonetic features (such as voicing and place of articulation) are instantiated in spectral changes of an acoustic signal that happen with a time course of tens of milliseconds. For example, the perception of place of articulation contrasts (like /ba/ vs /da/) requires accurate tracking of formation trajectories at constant-vowel transitions which are typically no more than 40 ms long. Evidence for a left hemispheric specialization for the processing of acoustic transients comes from several sources. First, patients with left hemisphere lesions and aphasia appear to be impaired at discriminating phonemes on the basis of voicing and place of articulation, but such patients do not appear to show impairment of vowel discrimination, which is made on the basis of spectral information present in the auditory signal over a longer period.5–9 Second, dichotic listening studies in normal populations demonstrate a greater right ear (left hemisphere) advantage for speech sounds that require perception of short duration spectral changes for their accurate identification than for speech sounds that do not.2,3,10,11 For example, Shankweiler and StuddertKennedy2 found no differences in accuracy of identification of steady-state vowels between stimuli presented to the left ear and those presented to the right. In contrast, when perception of stop consonant–vowel syllables was examined (/pa, ta, ka, ba, da, and ga/), they found that subjects were more accurate at reporting stimuli presented to the right ear, to a highly significant degree. Finally, two recent positron emission tomography (PET) studies12,13 revealed increased activation in left hemisphere areas in response to rapidly changing auditory cues. In a study in which normal subjects listened passively to speech-like syllables incorporating formation transitions, Belin and his colleagues12 found bilaterally symmetrical activity in auditory areas when the formation transitions were long (200 ms), but a leftward asymmetry in activation in response to short (40 ms) formant transitions. Fiez et al.12 observed that increases in cerebral blood flow (CBF) in the left frontal operculum were larger when their subjects performed an auditory detection task upon stimuli that incorporated stop consonants than for steadyCognitive Neuroscience and Neuropsychology