Article

A word-length effect is often described in pure alexia, with reading time
proportional to the number of letters in a word. Given the frequent association
of right hemianopia with pure alexia, it is uncertain whether and how much of the
word-length effect may be attributable to the hemifield loss. To isolate the
contribution of the visual field defect, we simulated hemianopia in healthy
subjects with a gaze-contingent paradigm during an eye-tracking experiment. We
found a minimal word-length effect of 14ms/letter for full-field viewing, which
increased to 38ms/letter in right hemianopia and to 31ms/letter in left
hemianopia. We found a correlation between mean reading time and the slope of the
word-length effect in hemianopic conditions. The 95% upper prediction limits for
the word-length effect were 51ms/letter in subjects with full visual fields and
161ms/letter with simulated right hemianopia. These limits, which can be
considered diagnostic criteria for an alexic word-length effect, were consistent
with the reading performance of six patients with diagnoses based independently
on perimetric and imaging data: two patients with probable hemianopic dyslexia,
and four with alexia and lesions of the left fusiform gyrus, two with and two
without hemianopia. Two of these patients also showed reduction of the
word-length effect over months, one with and one without a reading rehabilitation
program. Our findings clarify the magnitude of the word-length effect that
originates from hemianopia alone, and show that the criteria for a word-length
effect indicative of alexia differ according to the degree of associated
hemifield loss.
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