No deficiency in left-to-right processing of words in dyslexia but evidence for enhanced visual crowding

Abstract

Whitney and Cornelissen hypothesized that dyslexia may be the result of problems with the left-to-right processing of words, particularly in the part of the word between the word beginning and the reader's fixation position. To test this hypothesis, we tachistoscopically presented consonant trigrams in the left and the right visual field (LVF, RVF) to 20 undergraduate students with dyslexia and 20 matched controls. The trigrams were presented at different locations (from –2.5° to + 2.5°) in both visual half fields. Participants were asked to identify the letters, and accuracy rates were compared. In line with the predictions of the SERIOL (sequential encoding regulated by inputs to oscillations within letter units) model of visual word recognition, a typical U-shaped pattern was found at all retinal locations. Accuracy also decreased the further away the stimulus was from the fixation location, with a steeper decrease in the LVF than in the RVF. Contrary to the hypothesis, the students with dyslexia showed the same pattern of results as did the control participants, also in the LVF, apart from a slightly lower accuracy rate, particularly for the central letter. The latter is in line with the possibility of enhanced crowding in dyslexia. In addition, in the dyslexia group but not in the control group the degree of crowding correlated significantly with the students’ word reading scores. These findings suggest that lateral inhibition between letters is associated with word reading performance in students with dyslexia.

Keywords:

• Dyslexia
• Visual word recognition
• Letter position encoding
• SERIOL model
• Crowding
• Lateral inhibition

Acknowledgments

We would like to thank the anonymous reviewers for their helpful comments on the manuscript. In particular, we would like to thank the editor, Kate Nation, for her valuable contribution to the improvement of this research paper.

Notes

¹ We restrict ourselves to the predictions made for small eccentricities, within –5°/5° of fixation, as these pertain to the present study. The letter perceptibility weights are slightly different for larger eccentricities.

² Five students from the group with dyslexia performed significantly worse on the spelling test than on the reading tests. To make sure that our findings were not distorted by this subgroup, we repeated the analyses with the scores of the remaining 15 dyslexic participants. The results were the same as those reported here.

³ In the next sections it will become clear that our procedure did not completely succeed in getting equivalent levels of performance in the group with dyslexia and the controls. If full equivalence is required, it may be better to adjust the stimulus duration as a function of the accuracy level throughout the entire experiment. Another way to better match the performance levels may be to use a screen with a higher refresh rate than the presently used 70 Hz, so that finer adjustments can be made. Our adjustments were inspired by the consideration that large differences in overall performance would make the interpretation of interaction effects difficult, and we succeeded in the objective of avoidance them.

⁴ To make sure that the correlation with OMT could be interpreted as the outcome of crowding, we additionally looked at the correlations between OMT and performance on each of the letter positions. This analysis conformed that the correlation OMT performance and L2 accuracy in the dyslexic group was significantly larger than the correlation between OMT performance and L1 accuracy (p = .006, Hotelling–Williams test; see Steiger, 1980) or the correlation between OMT and L3 accuracy (p = .059).