Abstract
We have recently shown that microsaccades relocate the gaze toward regions of interests in a high visual acuity task (Ko et al., 2010, Nature Neuroscience). This finding raises the question of how precisely humans control these tiny movements. Previous studies investigating this issue could only give a superficial account of the precision and accuracy of microsaccades because of two major technical limitations. First, they could not accurately localize the line of sight in space, because of the inherent uncertainty caused by fixational eye movements during preliminary calibration procedures. Second, they could not compensate for fixational instability as it occurs, and presented saccade targets at fixed positions on the display rather than at fixed retinal locations. To overcome these problems, we developed a gaze-contingent calibration, which effectively reduces uncertainty in localization of the line of sight by one order of magnitude. Second, we compensated for fixational eye movements by placing saccade targets at the desired distance on the retina. Our results show that microsaccades between 7' and 20' produce larger angular errors, but smaller amplitude deviations than larger saccades. Microsaccades also tend to be more precise than larger saccades, yielding a smaller dispersion of their landing position. Although less efficient than larger saccades, they reduce the distance between the center of gaze and the saccade target by more than 50%, particularly for microsaccades larger than 14’. These factors are taken into account by the motor system; the probability of eliciting a microsaccade decreases with targets located at eccentricities for which the microsaccade is likely to increase rather than reduce the visual error. These results have important implications for the role of microsaccades in vision: they demonstrate that the oculomotor system controls microsaccades almost at the same level of larger saccades.
Meeting abstract presented at VSS 2013