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 nonmagnocellular 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.
Bibliographical noteFunding Information:
We wish to thank Avital Deutch for permission to use the tests she designed for reading single words and non-words, Ilana Ben-Dror and Sharon Peleg for the spoonerism test they designed, and Z. Shalem and D. Lachman for their spelling tests. We also thank John Stein, Udi Zohary, Israel Nelken and David Moore for helpful comments on early versions of the manuscript. This study was supported by the Institute for Psychobiology in Israel, the Israeli Science Foundation and the Israel–USA Binational Foundation.
- Coherent motion
- Contrast sensitivity
- Reading disability
- Temporal processing