Developmental dyslexia is a specific reading disability that affects 5-10% of the population. Recent studies have suggested that dyslexics may experience a deficit in the visual magnocellular pathway. The most extensively studied prediction deriving from this hypothesis is impaired contrast sensitivity to transient, low-luminance stimuli at low spatial frequencies. However, the findings are inconsistent across studies and even seemingly contradictory. In the present study, we administered several different paradigms for assessing temporal contrast sensitivity, and found both impaired and normal contrast sensitivity within the same group of dyslexic participants. Under sequential presentation, in a temporal forced choice paradigm, dyslexics showed impaired sensitivity to both drifting and flickering gratings. However, under simultaneous presentation, with a spatial forced choice paradigm, dyslexics' sensitivity did not differ from that of the controls. Within each paradigm, dyslexics' sensitivity was poorer at higher temporal frequencies, consistent with the magnocellular hypothesis. These results suggest that a basic perceptual impairment in dyslexics may be their limited ability to retain-and-compare perceptual traces across brief intervals.
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 Shaul Hochstein for helpful comments on the manuscript. This study was supported by grants from the Israel Foundation Trustees, The Institute for Psychobiology in Israel and the Israeli Science Foundation.
- Contrast sensitivity
- Sequential presentation
- Temporal processing