Modulation of Microsaccades by Spatial Frequency Content of Objects

Abstract

Microsaccades are miniature involuntary eye movements which occur even during fixation. The rate of occurrence of microsaccades is typified by a decrease immediately after stimulus onset and a subsequent peak; many results in the EEG gamma band may be attributable to this peak, which occurs approximately 200ms to 300ms after stimulus onset in object recognition tasks. This rate may be modulated by a number of stimulus and task factors. While recording eye movements, we presented images of living and non-living objects and phase scrambled textural patches produced from those objects, as either broadband unfiltered (BB), high-pass filtered (HP), or low-pass filtered (LP) images. Participants classified objects as living or non-living. We observed a bimodal distribution of eye movements during fixation. Only the rate of occurrence of the larger eye movements (∼1 deg or more) was modulated by our manipulations. These larger movements were present only for objects, and showed a smaller peak for LP images. In contrast, the rate of the smallest population of eye movements (less than 0.4 deg) was unaffected by spatial frequency or object presence. The results imply an early and automatic input of high-level information into eye movement control during fixation, probably driven by information on edges and lines present in BB and HP images.