Abstract: The distribution of spatial attention in visual field has been one of the most interesting topics in visual studies. However to date it remains controversial due to different paradigms used and many confounding factors involved. One such factor, the retinal eccentricity, can potentially spatial attention but has been largely overlook in previous reports. In the current study we aimed to investigate whether and how retinal eccentricity affects gradient distribution of visual spatial attention using Scharlau’s paradigm.
In Experiment 1, the participants were presented with two target stimuli with different stimulus onset asynchrony (SOA), and were asked to make untimed simultaneous judgment (SJ). Two target stimuli were separated at 6.4° or 3.2° distribution of retina. In addition, two conditions were set for the fixation point and attentional point, including an overlapping condition where two points overlapped and a separate condition where the attentional point was covertly pulled away from the fixation point by a peripheral exogenous cue. To avoid occurrence of response bias, the cue was backward-masked by the following target. The points of subjective simultaneous (PSS) across each conditions and distances were compared. The results showed that only PSS in the separate condition with long distances apartment between two points was significantly different with 0 ms.
In order to prevent apparent movement which might confound the results of Experiment 1, the two target stimuli were assigned different topological structures in the Experiment 2, which confirmed that there was no significant difference of the subjective brightness between two stimuli. A follow-up study Experiment 3 replicated the procedure of Experiment 1 with the following modifications: the two target stimuli were topologically different; the contrast degree was reduced; and four dots were converged to the center cross before each trial to ensure the subjects focused their attentions on the center. The result showed that a distance effect was found only under the overlapping condition but not the separate condition, indicating that the gradient distribution of visual spatial attention was affected by retinal eccentricity.
To sum up, the present study demonstrates that retinal eccentricity affects the gradient distribution of visual attention. Current findings also provide further evidences to support the “two attentional gradients theory” which argues for an inhibition gradient centered at the fixation point and an excitation gradient centered at the cued location in the visual field.

Key words: retinal eccentricity, metacontrast masking, excitation gradient, simultaneous judgment, inhibition gradient