The neuropathology of developmental dysphasia: Behavioral, morphological, and physiological evidence for a pervasive temporal processing disorder

Over the past twenty years, Tallal and colleagues have directed their research toward defining the neuropathological mechanisms responsible for developmental dysphasia. We have hypothesized that higher level auditory processing dysfunction, which has previously been associated with developmental dysphasia, may result from more basic temporal processing deficits which interfere with the resolution of rapidly presented, brief duration stimuli. This temporal processing deficit interferes with adequate perception of specific verbal stimuli which require resolution of brief duration formant transitions, resulting in disordered language development. The temporal processing deficit occurs across multiple sensory modalities, and also affects rapid and sequential motor production skills. Despite relatively normal clinical neuroradiological examinations, in vivo morphological analysis, utilizing magnetic resonance imaging techniques for quantitative volumetric measurements of specific brain structures, has identified abnormalities in superior parietal, prefrontal, and temporal cortices, as well as diencephalic and caudate nuclei. Abnormalities in structures which are involved in multimodal processing and sensory motor integration is consistent with the behavioral profile of developmental dysphasia. Two alternative hypotheses regarding the neurophysiological basis of the multimodal temporal processing disorder include: dysfunction in specific cellular systems which subserve rapid, transient processing; and abnormal gating of sensory relay by intralaminar and reticular thalamic nuclei.