The effect of visibility on eye-movement parameters in reading.

Abstract

The effect of on eye-movement parameters was investigated. As a measure of visi­ bility, the notion of was introduced. A first experiment, in which a simple letter­ recognition task was used, directly measured the changes in visual span produced by changing viewing distance and character spacing. The results of this experiment were used as a reference for a second experiment in which the same changes were made, but in which subjects read short texts while their eye movements were monitored. Saccade sizes were affected not primarily by visual but by other factors, possibly related to word boundary detection or linguistic processing. Fixation durations appeared to be strongly affected by the proximity of the letters to the subject's acuity threshold. The purpose of the present study was to investigate the effect of changes on eye-movement be­ havior in reading. Visibility was studied in relation to eye movements by Hackmann and Tinker (1947), Paterson and Tinker (1947), and Tinker and Paterson (1955), but these authors' purpose was to find the optimal conditions for reading, and not the reason why these conditions were optimal. A measure of visibility can be obtained by de­ termining the probability of recognizing letters in the fovea. But this measure may not be relevant to un­ derstanding reading behavior, because it neglects the effect of parafoveal and peripheral vision. Visual span, that is, the size of the region around the eye's fixation point in which letters can be recognized with a given accuracy, would appear to be more relevant in determining saccade size and fixation durations in reading. In adjusting to poor conditions, in