Abstract
Humans use rapid eye movements (saccades) to inspect stimuli with the foveola, the region of the retina where receptors are most densely packed. It is well established that visual sensitivity is generally attenuated during these movements, a phenomenon known as saccadic suppression. This effect is commonly studied with large, often peripheral, stimuli presented during instructed saccades. However, little is known about how saccades modulate the foveola and how the resulting dynamics unfold during natural visual exploration. Here we measured the foveal dynamics of saccadic suppression in a naturalistic high-acuity task, a task designed after primates' social grooming, which-like most explorations of fine patterns-primarily elicits minute saccades (microsaccades). Leveraging on recent advances in gaze-contingent display control, we were able to systematically map the perisaccadic time course of sensitivity across the foveola. We show that contrast sensitivity is not uniform across this region and that both the extent and dynamics of saccadic suppression vary within the foveola. Suppression is stronger and faster in the most central portion, where sensitivity is generally higher and selectively rebounds at the onset of a new fixation. These results shed light on the modulations experienced by foveal vision during the saccade-fixation cycle and explain some of the benefits of microsaccades.
Highlights
Humans use rapid eye movements to inspect stimuli with the foveola, the region of the retina where receptors are most densely packed
We show that contrast sensitivity is not uniform across this region and that both the extent and dynamics of saccadic suppression vary within the foveola
Unbeknownst to the subject, the probes were activated on the basis of the position and movement of the eye to measure visibility within selected regions of the fovea and at various time lags around saccades. This was possible due to three state-of-the-art components: 1) high-resolution eye tracking achieved via the Dual Purkinje image method [30]; 2) accurate gaze localization obtained by means of an iterative gazecontingent calibration, a procedure that improves accuracy by approximately one order of magnitude over standard methods [6]; and 3) real-time control of retinal stimulation, obtained via a custom system for flexible gaze-contingent display control, Eye movement Real-time Integrated System (EyeRIS) [31]
Summary
Humans use rapid eye movements (saccades) to inspect stimuli with the foveola, the region of the retina where receptors are most densely packed. Microsaccades, gaze shifts so small that the attended stimulus remains within the foveola, are the most frequent saccades when examining a distant face [7], threading a needle [8], or reading fine print [9], tasks in which they shift the line of sight with surprising precision Because of their minute amplitudes, microsaccades pose specific challenges to the mechanisms traditionally held responsible for saccadic suppression [10,11,12,13,14,15,16,17,18,19]. These movements yield broadly overlapping pre- and postsaccadic images within the fovea, which would appear to provide little masking in visual stimulation [20] They result in reduced retinal smear [21], as they rotate the eye at much lower speeds than larger saccades, delivering luminance modulations that are well within the range of human temporal sensitivity.
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