Abstract
Large intentional saccades to a peripheral target that is crowded (surrounded by flankers) have been shown to reduce the harmful effects of crowding. Here we investigate if the same holds true for smaller eye movements (with amplitudes matched to fixational eye movements) to a crowded target 20 arc minutes from fixation. 5 normally sighted observers performed an orientation judgement task (4-AFC), with the target consisting of a rotated T of size 6.25 arcminutes that was either unflanked or surrounded by 4 plus signs at one of 3 nominal spacings (multiples of letter height: 1.25x, 1.75x, & 2.5x). Eye movements were monitored using a tracking scanning laser ophthalmoscope (TSLO) with a tracking fidelity of <0.5 arc minute to image the retina of the better eye of each observer. The imaging raster was optically overlaid with an LCD display (3840x2160, ~5 pixels per arcminute) used to present stimuli. The target duration was determined individually for each subject (33-66 ms) to provide 75% performance for the widest spacing used. In 80% of trials, subjects made voluntary microsaccades to the target when cued, and in the remaining 20% of the trials subjects continued to maintain fixation. Targets were presented at different time points from cue onset, to get an equal distribution of stimulus presentations relative to microsaccade onset (50-200ms). We found that visual crowding affected recognition even when close to the fovea for these briefly presented stimuli (average critical spacing (CS): 13.59 ± 2.45 arcminutes, or Bouma ratio (CS/eccentricity): 0.67±0.12). For most subjects there was an overall benefit in behavioral performance causing a shift of the psychometric function in the microsaccade condition (CS:13.04 ± 2.55 arc minutes) compared to the fixation condition (CS:14.13 ± 2.51 arc minutes). However, this apparent reduction in crowding could be driven by a general enhancement in performance.