Spatial resolution gradient of the visual field and flexibility of visual sampling

Shirama, Aya; Ishiguchi, Akira · 2009 · OpenAlex-citations

DOI: 10.4992/jjpsy.80.223

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Summary

This study investigates the mechanisms underlying the eccentricity effect in visual search, specifically examining how the spatial resolution gradient (SRG) of the visual field influences visual sampling. The SRG refers to the decline in spatial resolution from the fovea to the periphery, a structural constraint often viewed as a limitation requiring eye movements for compensation. The authors aim to disentangle two competing explanations for the eccentricity effect: the "spatial resolution degradation hypothesis," which attributes performance declines to insufficient resolution in the periphery, and the "attentional bias hypothesis," which posits that attention is inherently prioritized toward central vision. To test these hypotheses, the researchers conducted three experiments using visual search tasks with varying target discriminability. They employed three scaling methods to manipulate stimulus size relative to retinal eccentricity: Unscaling (constant small size), M-scaling (size magnified according to cortical magnification factors to compensate for resolution loss), and D-scaling (constant large size, using the maximum size from M-scaling, to ensure high visibility while preserving the SRG). Experiment 1 used a high-discriminability task (searching for notched squares), while Experiments 2 and 3 used a low-discriminability task (searching for complete squares among notched distractors). Stimuli were presented briefly (180 ms) to prevent saccades, allowing the isolation of covert attentional processes. The results revealed distinct patterns depending on task difficulty and scaling method. In the high-discriminability task (Experiment 1), both M-scaling and D-scaling eliminated the eccentricity effect, indicating that when spatial resolution is adequate, peripheral search is as efficient as central search. However, in the low-discriminability task (Experiment 2), D-scaling failed to eliminate the eccentricity effect; performance remained significantly better in central vision despite high peripheral visibility. This residual effect was not due to crowding, as confirmed by Experiment 3, which adjusted stimulus spacing. Instead, the persistence of the eccentricity effect in difficult tasks suggests an attentional bias toward central vision. The authors conclude that the visual system employs a flexible sampling strategy that balances simultaneous wide-field monitoring with attentional prioritization of central vision. While the SRG allows for efficient parallel processing of salient targets across the visual field, the system adaptively shifts to a sequential, attention-driven strategy prioritizing high-resolution central vision when target discriminability is low. This challenges the view of the SRG as merely a constraint, suggesting instead that it supports a dynamic interplay between global sampling and focused attention.

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• visual search