Donald Shankweiler

Learning to read requires an awareness that spoken words can be decomposed into the phonologic constituents that the alphabetic characters represent. Such phonologic awareness is characteristically lacking in dyslexic readers who, therefore, have difficulty mapping the alphabetic characters onto the spoken word. To find the location and extent of the functional disruption in neural systems that underlies this impairment, we used functional magnetic resonance imaging to compare brain activation patterns in dyslexic and nonimpaired subjects as they performed tasks that made progressively greater demands on phonologic analysis. Brain activation patterns differed significantly between the groups with dyslexic readers showing relative underactivation in posterior regions (Wernicke's area, the angular gyrus, and striate cortex) and relative overactivation in an anterior region (inferior frontal gyrus). These results support a conclusion that the impairment in dyslexia is phonologic in nature and that these brain activation patterns may provide a neural signature for this impairment.

Earlier experiments with dichotically presented nonsense syllables had suggested that perception of the sounds of speech depends upon unilateral processors located in the cerebral hemisphere dominant for language. Our aim in this study was to pull the speech signal apart to test its components in order to determine, if possible, which aspects of the perceptual process depend upon the specific language processing machinery of the dominant hemisphere. The stimuli were spoken consonant-vowel-consonant syllables presented in dichotic pairs which contrasted in only one phone (initial stop consonant, final stop consonant, or vowel). Significant right-ear advantages were found for initial and final stop consonants, nonsignificant right-ear advantages for six medial vowels, and significant right-ear advantages for the articulatory features of voicing and place of production in stop consonants. Analysis of correct responses and errors showed that consonant features are processed independently, in agreement with earlier research employing other methods. Evidence is put forward for the view that specialization of the dominant hemisphere in speech perception is due to its possession of a linguistic device, not to specialized capacities for auditory analysis. We have concluded that, while the general auditory system common to both hemispheres is equipped to extract the auditory parameters of a speech signal, the dominant hemisphere may be specialized for the extraction of linguistic features from those parameters.