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Fixation conditions, the foveola and saccadic latency
1Department of Physiology, University of Toronto, Ontario, Canada.
Abstract:
Saccades are often elicited in the laboratory by the abrupt step-displacement of a single lit point which is initially the foveolar fixation point and then the eccentric refixation target. This was our Control condition. Four experiments modified the fixation arrangements to examine the effect of altered foveolar stimulation on saccadic latency and accuracy to targets within the central +/- 6 deg of the visual field. (1) No foveolar fixation point: The subject fixated the empty space midway between a pair of fixation guides, which later collapsed into a single refixation target. Latencies for small saccades were similar to the Control values. (2) No foveolar fixation point and no real refixation target: A pair of fixation guides underwent a yoked displacement, and it was easy to fixate and track the invisible midpoint. The smallest saccades were hypermetric, and the typical pattern of latency variation with retinal eccentricity was exaggerated in scale. (3) Spatial effects of a persistent non-target: The precise position of a non-target was important, latency increases being in the ipsilateral hemifield when the non-target was intrafoveolar and unilateral, bilateral when intrafoveolar and on the midline, and local when the non-target was extrafoveolar. (4) Temporal effects of a foveolar fixation point: Blanking an otherwise persistent fixation point for as little as 1 msec at the time of target presentation reduced the expected latency increase. We conclude that the position and timing of foveolar illumination can be critical for saccades of all sizes.
Insights
Altering foveolar stimulation significantly impacts eye movements. The position and timing of foveal illumination critically influence saccadic latency and accuracy, affecting even small saccades.
Area of Science:
- Ophthalmology
- Neuroscience
- Visual Perception
Background:
- Saccades are rapid eye movements crucial for visual exploration.
- Laboratory studies often use step-displaced targets to elicit saccades.
- The role of foveolar stimulation in saccade control is not fully understood.
Purpose of the Study:
- To investigate how altered foveolar stimulation affects saccadic latency and accuracy.
- To examine the influence of fixation point presence, absence, and timing on saccade characteristics.
- To determine the spatial and temporal effects of foveolar illumination on saccades within the central visual field.
Main Methods:
- Four experiments manipulated fixation arrangements.
- Conditions included no foveolar fixation point, no refixation target, and varied non-target presentations.
- Saccadic latency and accuracy were measured for targets within +/- 6 degrees of the visual field.
Main Results:
- Small saccade latencies were similar to controls when a midpoint was fixated without a foveolar point.
- Absence of a refixation target exaggerated latency variations with retinal eccentricity, causing hypermetric saccades.
- Non-target position influenced latency increases based on hemifield and proximity to the fovea.
- Briefly blanking a fixation point (1 msec) reduced expected latency increases.
Conclusions:
- Foveolar stimulation plays a critical role in saccadic control.
- Both the spatial positioning and temporal presentation of foveolar stimuli significantly impact saccade latency and accuracy.
- These findings have implications for understanding visual attention and eye movement disorders.