Abstract
Background: A characteristic feature of pure alexia is the word-length effect, in which reading speed increases with increasing word length. However, many alexic patients also have right hemianopia, which can itself can cause a hemianopic dyslexia. The degree to which hemianopia causes a word-length effect is therefore an important question. Objective: Our goal was to determine if a word-length effect could be generated under hemianopic conditions in the absence of damage to language processing areas. Method: 13 healthy adults read single words of 3 to 9 letters in length while eye movements were monitored with full-field viewing and then with gaze-contingent displays simulating right or left hemianopia. We also studied 6 patients with reading problems: two had hemianopic dyslexia without fusiform lesions, four had fusiform lesions causing alexia, two of which had associated hemianopia while two had full visual fields. Results: In healthy subjects, there was a small word-length effect with full-field viewing of 14ms/letter, which more than doubled to 37 ms/letter for right and 31ms/letter for left hemianopia. The upper 95% prediction limit was 51 ms/letter for full-field viewing and 160 ms/letter for right hemianopia. These results were corroborated by our patient sample. Our two patients with hemianopic dyslexia fell within the virtual hemianopic range (18.9, 95.1 ms/letter), while the two patients with fusiform lesions causing alexia and hemianopia had word-length effects well beyond this range (1536, 16500 ms/letter). The subjects with alexia without hemianopia had modest word-length effects that were abnormal compared to full-field viewing (53, 182 ms/letter). Conclusions: Hemianopic simulations show a small word-length effect of up to 160 ms/letter. Given that word-length effects of similar magnitude can be seen in alexia without hemianopia, in patients with hemifield loss in the central 5°, word-length effects should be larger before concluding that there is an additional component of alexia.
MA was supported by a grant from the Swiss National Science Foundation.