Research Article Fixational Eye Movements Are Not an Index of Covert Attention

ABSTRACT—The debate about the nature of fixational eye movements has revived recently with the claim that mi-crosaccades reflect the direction of attentional shifts. A number of studies have shown an association between the direction of attentional cues and the direction of micro-saccades. We sought to determine whether microsaccades in attentional tasks are causally related to behavior. Is reaction time (RT) faster when microsaccades point toward the target than when they point in the opposite direction? We used a dual-Purkinje-image eyetracker to measure gaze position while 3 observers (2 of the authors, 1 naive observer) performed an attentional cuing task under three different response conditions: saccadic local-ization, manual localization, and manual detection. Critical trials were those on which microsaccades moved away from the cue. On these trials, RTs were slower when microsaccades were oriented toward the target than when they were oriented away from the target. We obtained similar results for direction of drift. Cues, not fixational eye movements, predicted behavior. People’s eyes are never completely still. Even when one is fix-ating on a small point in space, one’s eyes are in constant motion. There are three types of fixational eye movements: tremor, drift, and microsaccades (for a recent review, see Martinez-Conde, Macknik, & Hubel, 2004). Fixational eye movements may arise, in part, from noise in the oculomotor system, but they are also known to maintain direction of gaze and prevent image fading due to neural adaptation (Cornsweet, 1956; Martinez-Conde, Macknik, Troncoso, & Dyar, 2006). Recently, several re-searchers have proposed that fixational eye movements (spe-cifically, microsaccades) are related to shifts in attentio