7.5 Dyslexia
  • Developmental dyslexia is characterized by serious reading and spelling difficulties that are persistent and resistant to the usual didactic measures and remedial efforts. At present it is well established that a major cause of these problems lies in the phonological domain (seeSnowling, 2000 for a review). One hypothesis maintains that this phonological deficit results from a more fundamental deficit in the basic perceptual mechanisms that are responsible for auditory temporal information processing. The auditory temporal hypothesisoriginated from studies on children with specific language impairments (SLI) and was later extended to dyslexia. The empirical evidence started with Tallal’s repetition task (Tallal,1980). In this temporal order judgement (TOJ) task, two complex tones with different fundamentals were presented in pairs at various inter-stimulus intervals (ISI) and the listener responded with two button presses to identify the order of the stimuli presented. Tallal found that children with dyslexia, in comparison to normal readers, were impaired in discriminating and sequencing pairs of short-lived stimuli with short ISI, and concluded that the dyslexic deficit was specific to processing stimuli that are brief and occur in rapid succession.
  • The hypothesis that a temporal processing deficit may have a causal relationship with many cases of dyslexia has been gaining popularity in recent years. Findings from researchin diverse areas have appeared to support this hypothesis.
  • Although never stated as explicitly by its proponents, one might assume that the phonological theory of dyslexia would situate the core neurobiological deficit in the auditory-to-motor stream. In contrast, the auditory temporal processing theory would primarily situate it in the auditory-to- meaning pattern recognition stream, somewhere at the level where phonemic representations have to be extracted from the acoustic features in the speech signal. Evidently, according to the auditory temporal processing theory, dysfunctional processing should also be observed along the auditory-to-motor pathway as a secondary consequence of these aberrant phonemic representations.
  • Although theoretically attractive, the auditory temporal processing hypothesis has been hotly debated and has been facing growing criticism in recent years (e.g. Blomert&Mitterer, 2004; Denenberg, 1999; Mody, Studdert-Kennedy, & Brady, 1997; Nittrouer, 1999; Ramus, 2003; Rosen &Manganari, 2001).