Space-time dependence of fixational saccades
63.43, Wednesday, May 15, 8:30 am - 12:30 pm, Orchid Ballroom
Claudia Cherici1, Michele Rucci1,2; 1Department of Psychology, Boston University, 2Graduate Program in Neuroscience, Boston University
Recent work has shown that microsaccades precisely relocate gaze in high-acuity tasks (Ko et al, 2010). Fixational saccades, however, also occur when attempting to maintain steady fixation on a single point, with no other visual stimulus present. What is the function of these movements? It has long been debated whether fixational saccades correct for fixation errors (i.e., they are elicited by spatial displacement between the current gaze position and the intended fixation point) or their generation depends instead on temporal factors rather than spatial ones. This controversy has never been solved, primarily because of technical limitations on precisely determining where subjects look. Whereas eye-tracking may enable detection of very small changes in gaze position, the absolute localization of the line of sight is typically affected by an area of uncertainty as large as the fovea itself, a problem in part caused by the occurrence of fixational eye movements during preliminary calibration procedures. To overcome this problem, we developed a gaze-contingent calibration procedure, which effectively improved localization of the line of sight by one order of magnitude over standard methods. Using this improved accuracy, we examined the characteristics of fixational saccades in 6 human observers while they maintained fixation on a small dot. Our results show that the generation of fixational saccades depends both on space and time. Fixational saccades occurred relatively frequently even when the center of gaze was very close to the requested location. However, the rate of fixational saccades increased proportionally with the distance of the current location of gaze from the fixation marker. Unexpectedly, saccades were extremely accurate in correcting for fixation errors, especially on the horizontal axis. Thus fixational saccades primarily serve a corrective function, but they occasionally occur when not needed, as they can only be suppressed for a limited period of time.