Eye movements and the span of the effective stimulus in visual search

Bertera, James H.; Rayner, Keith · 2000 · OpenAlex-citations

DOI: 10.3758/bf03212109

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Summary

This study investigates the "span of the effective stimulus" in visual search, defined as the region from which useful information is acquired during a single eye fixation. While previous research often ignored eye movements or relied on saccade size to estimate visual span, Bertera and Rayner argue that eye-contingent display change paradigms provide a more accurate measure. The authors aimed to determine how much information is processed per fixation when searching an unstructured alphanumeric array and to assess the relative importance of foveal versus parafoveal vision. The experiment employed two eye-contingent techniques: a moving window, which preserved information within a specific radius of fixation while masking the periphery, and a foveal mask, which obscured information within a radius of fixation while preserving the periphery. Five subjects searched for a target letter ("y") among distractors in arrays of three different sizes (large, medium, small), which varied in density but contained the same number of items. Window sizes ranged from 1.0° to 5.7°, and mask sizes ranged from 0.3° to 3.0°. Eye movements were tracked using a Dual Purkinje Eyetracker, with display changes occurring within 6–9 milliseconds of saccade completion. Results from the window conditions showed that search performance improved as window size increased, reaching an asymptote at a 5.0° window for all array sizes. This indicates that subjects utilized information from a 5.0° region around fixation. In contrast, foveal masks were significantly more detrimental to performance. Even small masks (0.3°) reduced accuracy and increased search times, with larger masks causing severe degradation. For instance, accuracy dropped from 100% with no mask to 39% with a 3.0° mask. The study also found that while array density influenced search metrics like fixation duration and saccade length, the critical window size for asymptotic performance remained constant across densities. The findings demonstrate that the span of the effective stimulus in visual search is approximately 5.0°, corresponding to the processing of three to six characters per fixation depending on array density. The severe impact of foveal masking highlights the critical role of high-acuity foveal vision in identifying targets, whereas parafoveal information supports the search process up to the 5.0° limit. The authors conclude that both visual acuity and task difficulty influence the effective span, and they caution against using saccade length as a proxy for visual span, as it does not accurately reflect the amount of information processed during fixation.

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Topics

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• visual search
• useful field of view