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Hee-kyoung Ko, Martina Poletti, Michele Rucci; Fixational eye movements in a high-acuity visual task. Journal of Vision 2009;9(8):430. doi: 10.1167/9.8.430.
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During the periods in between saccades, fixational eye movements continually jitter the location of gaze. Recent experiments that eliminated retinal image motion have shown that fixational instability facilitates discrimination/detection of high spatial frequency patterns (Rucci et al, 2007). This finding suggests an involvement of fixational eye movements in the processing of fine spatial detail. Previous studies, however, reported a reduction in the rate of microsaccades in tasks requiring high visual acuity. Microsaccades are one of the components of fixational instability. To explore the relationship between these two sets of results, we examined fixational eye movements during execution of a high-acuity visuomotor task, which consisted of threading a needle in a simulated virtual environment. Subjects used a joypad to control the relative positions of a needle and a thread on a CRT. We show that the rate of microsaccades highly depends on the threshold used to define them. In agreement with previous reports, microsaccades with amplitude smaller than 10' were relatively rare. However, fixational saccades with amplitudes up to 30' occurred frequently. Their average amplitude decreased and their rate increased as the thread approached the needle, a consequence of the fact that these small saccades moved fixation back and forth between the eye of the needle and the tip of the thread. Furthermore, evaluation of changes in the overall rate of microsaccades, i.e. whether the task led to an increment or decrement in microsaccadic rate, depended on the baseline used as a reference. Microsaccades were on average more frequent during sustained fixation, a condition that in some observers elicited a high rate of microsaccades, and less frequent during free-viewing, a condition in which microsaccades were always rare. These results suggest that fixational saccades are part of the strategy by which the visual system acquires fine spatial detail.
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