Reduced orthographic learning in dyslexic adult readers: Evidence from patterns of letter search

Abstract

Visual letter search performance was investigated in a group of dyslexic adult readers using a task that required detection of a cued letter target embedded within a random five-letter string. Compared to a group of skilled readers, dyslexic readers were significantly slower at correctly identifying targets located in the first and second string position, illustrating significantly reduced leftward facilitation than is typically observed. Furthermore, compared to skilled readers, dyslexic readers showed reduced sensitivity to positional letter frequency. They failed to exhibit significantly faster response times to correctly detect target letters appearing in the most, compared to least, frequent letter position within five-letter words, and response times correlated with positional letter frequency only for the initial, and not the final, letter position. These results are compatible with the SERIOL (sequential encoding regulated by inputs to oscillations within letter units) model of orthographic processing proposed by Whitney and Cornelissen (2005). Furthermore, they suggest that dyslexic readers are less efficient than skilled readers at learning to extract statistical regularities from orthographic input.

Keywords:

• Dyslexia
• Reading
• Orthography
• Letter search

Acknowledgments

We are grateful for the constructive and helpful comments of Carol Whitney and those of another anonymous reviewer

Notes

¹ For dyslexic readers the degree of asymmetry in identifying letter targets presented to the left and right of the array did not correlate significantly (r = .006) with reading performance. A total of 30 irregular words and 30 nonwords (Castles & Coltheart, 1993) were given to each dyslexic reader to quantify the extent to which they relied on lexical (irregular-word reading) and sublexical (nonword reading) processes. Of the 7 dyslexic readers (24.1%) that showed a leftward processing advantage, 2 were better at irregular word than nonword reading, 2 showed the opposite pattern, and 3 showed no difference.

² The most frequent letter that occurs in the final position of five-letter English words is the vowel “E”. The lack of an effect of positional letter frequency at the final letter (see Figure 4) may have been influenced by the dyslexic readers failing to encode higher frequency letters (e.g., vowels) faster than lower frequency letters.