Reduced Crowding Accounts for Enlargement of the Visual Span After Training

摘要

The visual span for reading is the number of text letters that can be recognized accurately without eye movements, and may be a sensory bottleneck limiting reading speed. The visual span is smaller in peripheral vision than in central vision, contributing to the slower reading speeds of people with central-field loss. The visual-span size can be enlarged through training, accompanied by an increase in reading speed (Chung, Legge & Cheung, Vision Research, 2004). Here, we report on a visual-span training experiment and decomposition analysis to determine the underlying factors responsible for the training-related enlargement of the visual span. Proposed factors include visual resolution, crowding, and mislocations (errors in the spatial order of letters). In pre- and post-tests, visual-span profiles were measured with trigrams (three horizontally-adjacent letters) displayed at eleven positions along a horizontal line at 10?° in the upper or lower visual field. Letter-recognition accuracy was calculated in two ways: exact accuracy (with correct identity and spatial order) and mislocational accuracy (with correct identity regardless of spatial-order errors). An isolated-letter profile was measured with single letters to assess visual resolution. The magnitude of crowding was calculated as the difference between the isolated and mislocational profiles, and the magnitude of mislocations was the difference between the mislocational and exact profiles. Training consisted of four daily sessions of repetitive trigram visual-span measurements in the lower visual field. Consistent with previous research, we found visual-span enlargement following training, with transfer from the lower to the upper visual field. There was a substantial reduction in the magnitude of crowding and a smaller reduction for mislocations, both statistically significant. The visual-resolution profile remained unchanged. These results demonstrate that a reduction of crowding is the major factor underlying the enlargement of the peripheral visual span following training, with a minor contribution from the reduction in mislocations.