Dyslexia and a temporal processing deficit: A reply to the commentaries.

A number of points and criticisms were raised in the commentaries on our review paper (Farmer & Klein, 1995), and in this reply we address the most pertinent and major of those points. First, we clarify and expand upon what we mean by a temporal processing deficit. We then address Studdert-Kennedy and Mody's (1995) major claims, which are confined to the auditory modality, that (1) a discriminative deficit underlies what they see as a rate of processing deficit, and (2) discriminative/rate deficits for speech and nonspeech materials are independent. We explain why we believe the first proposal is unlikely to provide an explanation of the temporal processing deficits that we reviewed, and we present a simple framework within which speech and nonspeech perceptual codes are viewed as higher level isolable subsystems that depend on a common, lower level, auditory input system. The speech and nonspeech systems may be influenced similarly by damage to, or impairments of, their common input system, but they can be selectively influenced by insults after the pathways diverge. Then we address some of the issues raised by Rayner, Pollatsek, and Bilsky (1995), relating to visual deficits and oculomotor behavior, and we point to the rapidly growing evidence to diminish skepticism about the occurrence of a transient system deficit in dyslexia. Next, while agreeing that case studies are valuable, we dispute Martin's (1995) endorsement of the case study as the preferred methodology for studying a heterogeneous deficit such as developmental dyslexia. Finally, we affirm our original conclusion that more research aimed at revealing the nature and generality of the visual and auditory temporal processing deficits is warranted, and we reiterate some of our suggestions for the types of study that might help elucidate if and how these deficits might be causally related to the dyslexia with which they are frequently associated.