Adaptation of micro-saccades reveals active control during fixation
21.13, Saturday, May 11, 8:15 - 9:45 am, Royal Ballroom 1-3
Katharina Havermann1,2, Claudia Cherici3, Michele Rucci3, Markus Lappe1,2; 1Institute for Psychology, University of Muenster, Germany, 2Otto Creutzfeldt Center for Cognitive and Behavioral Neuroscience, University of Muenster, Germany, 3Departments of Psychology and Program in Neuroscience, Boston University, Boston, USA
When asked to maintain their gaze steady on a given location, humans perform small involuntary eye movements, which include drifts and fixational saccades. It has long been speculated that fixational saccades may contribute to the maintenance of fixation, but direct evidence that these movements are under oculomotor control has remained contradictory. Here we show that these fixational saccades are indeed precisely monitored and controlled. We used saccadic adaptation, an experimental procedure in which the stimulus is shifted during saccades, to expose observers to an altered sensory-motor contingency. We first examined whether target-directed saccades with amplitudes within the microsaccade range (30 arcmin) could be adapted. Subjects were instructed to look at a small dot (3'), which shifted by a fixed amount every time the subject made a saccade toward the target. Repeated exposure to this procedure led to changes (both shortening and lengthening, depending on the intra-saccadic shift direction) in microsaccade amplitude. We then examined whether fixational saccades, the microsaccades that occur during fixation of a stationary marker, are also adaptable. Since the occurrence of fixational saccades is unpredictable, we estimated the direction and amplitude of a fixational saccade during its occurrence and shifted the fixation dot proportionally. Fixational saccades also changed during prolonged exposure to this procedure: the average amplitude of fixational saccades decreased following backward shifts of the fixation marker and increased following forward shifts. These findings show that the success of fixational saccades is linked to the post-saccadic position of the fixation point, so that the motor program of fixational saccades is modified if the fixation point is not at the expected retinal location after the saccade. Thus fixational saccades serve to position the eye with respect to the fixation point and are actively controlled at a minute level of detail.