Disabled readers suffer from visual and auditory impairments but not from a specific magnocellular deficit

Abstract

The magnocellular theory is a prominent, albeit controversial view asserting that many reading disabled (RD) individuals suffer from a specific impairment within the visual magnocellular pathway. In order to assess the validity of this theory we tested its two basic predictions. The first is that a subpopulation of RD subjects will show impaired performance across a broad range of psychophysical tasks relying on magnocellular functions. The second is that this subpopulation will not be consistently impaired across tasks that do not rely on magnocellular functions. We defined a behavioural criterion for magnocellular function, which incorporates performance in flicker detection, detection of drifting gratings (at low spatial frequencies), speed discrimination and detection of coherent dot motion. We found that some RD subjects (six out of 30) had impaired magnocellular function. Nevertheless, these RD subjects were also consistently impaired on a broad range of other perceptual tasks. The performance of the other subgroup of RD subjects on magnocellular tasks did not differ from that of controls. However, they did show impaired performance in both visual and auditory non-magnocellular tasks requiring fine frequency discriminations. The stimuli used in these tasks were neither modulated in time nor briefly presented. We conclude that some RD subjects have generally impaired perceptual skills. Many RD subjects have more specific perceptual deficits; however, the "magnocellular" level of description did not capture the nature of the perceptual difficulties in any of the RD individuals assessed by us.

Patterns of deficit in auditory temporal processing among dyslexic adults

“…Rate and accuracy of phonological decoding were assessed with single nonwords written with diacritics (shallow orthography). Reading rate was also assessed for an academic level passage written without diacritics (deep orthography, for a description of Hebrew orthography see [4]). Other tests included: single word reading, spelling, speed of naming letters and numbers (RAN [16]; 30 dyslexic, 22 control) and phonological awareness (Spoonerism task).…”

“…Although many studies found poorer performance in dyslexics than in normal readers [4,7,10], others did not [11]. Two inter-study differences may underlie this variance.…”

“…language delay [2]) and the extent of perceptual difficulties [3]. Perceptual deficits in dyslexia have been found on both visual [4] and auditory [5][6][7] tasks. The relationship between perceptual deficits and subtypes of dyslexia is still an open question.…”

“…There is substantial evidence for dyslexics' impaired performance with brief stimuli (see [6,9] for reviews); however deficits with longer stimuli were also reported [5][6][7]9]. For example, in frequency discrimination tasks, using a variety of procedures, significantly higher thresholds were found in disabled compared with agematched normal readers [4,5,7,10]. Ahissar et al [5] measured two-tone frequency discrimination with both temporal order judgments of brief tones and intervals, and same/different judgments of longer tones and intervals.…”

The Neurophysiology of Dyslexia: A Selective Review with Implications for Neurofeedback Remediation and Results of Treatment in Twelve Consecutive Patients

“…A hypothesis suggesting that dyslexics have a specific impairment within the visual magnocellular pathway (Stein & Walsh, 1997) has been largely discredited (Amitay, Ben-Yehudah, Banai, & Ahissar, 2002). Over-activation of Broca's area in dyslexics may represent increased effort in performing language related tasks (Shaywitz et al, 1998).…”