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Adam J. Reeves, Zhenlan Jin; The Gap Effect revisted; seven wrong explanations and two possibly right ones. Journal of Vision 2006;6(6):485. doi: https://doi.org/10.1167/6.6.485.
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We investigated several explanations for the ‘gap effect’ (saccades to flashed targets are faster if there is a temporal gap from fixation offset to target onset than if there is no gap.) One target appeared on each trial randomly at one of 4 locations (top, bottom, left, right; 10 deg eccentricity). Three trained observers pressed a manual key at target onset (MRTtarg), or pressed a key at fixation offset (MRTfix), or made a single saccade to the target (latency = SRT). The fixation spot was continuous (no gap), or (200 ms gap) was blanked, or dimmed, or brightened, or turned into 4 foveal or 4 extrafoveal spots. Mean SRTs were 232 (no gap), and 183, 192, 208, 202, 199 ms (corresponding gap conditions), generating gap effects of 49, 40, 24, 30, and 33 ms. MRTfix was proportional to MRTtarg (r =0.98). MRTtargs were 274 (no gap), and 196, 223, 216, 198 and 206 ms (gaps). As potential explanations, we (like others) exclude, from data analysis, (1) supression of microsaccades, (2) early triggering of pre-programmed saccades, and (3) express saccades; also, (4) speed-accuracy trade-offs —faster saccades were less accurate within, but not across, conditions, (5) foveal capture of attention, (6) changes in the shapes of the SRT distributions, and (7) salience of the fixation target, in that mean MRTs and mean SRTs were independent across conditions. We conclude the gap effect reflects (8) an overall warning effect and (9) the disengagement of saccade-specific central attention.
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