Psychophysics of Reading IX. The stability of eye position in normal and low vision.


Parish, D.H. & Legge, G.E. (1989). Psychophysics of reading. IX. The stability of eye position in normal and low vision. [Unpublished Manuscript]. (abstract) (PDF)


Slow reading rates are characteristic of people with low vision, particularly those with central-field loss. Fixational inaccuracy may play a role in limiting reading speed. We used a psychophysical method to infer the accuracy of eye placement during reading by people with normal and low vision. Reading speed was measured by timing subjects as they read standardized sentences displayed on a video screen. By measuring reading speed in the presence of controlled amounts of isotropic stimulus jitter (i.e., Gaussian-random horizontal and vertical rigid displacements of the text every video frame), we inferred the tolerance of reading to erratic changes in stimulus position. Using an "equivalent noise" analysis, we estimated the internal jitter, representing stochastic variation of eye placement during reading. Reading rates of subjects with normal vision were little affected by substantial amounts of stimulus jitter, implying a considerable amount of imprecision in the placement of eyes during reading. We estimate a position error equivalent to the area of about 5 characters, independent of angular character size, for readers with normal vision. Subjects with low vision showed a more rapid decline in reading rate as stimulus jitter increased. Surprisingly, the low-vision data indicate greater precision in eye placement during reading. For these subjects we estimate a position error of about 2 characters. When normal subjects were tested with low-contrast text, the precision of eye placement increased significantly. These findings indicate a requirement for greater accuracy in eye placement when reading in reduced visual conditions. We interpret our results in the context of models that attribute slow reading in low vision to prolonged fixations or shortened saccades: We infer from our results that shorter, more precise saccades are characteristic of lowvision reading and account for reduced reading rates.