Studying eye movements produced whilst making visual decisions

In everyday situations we move our eyes around the world 3-5 times every second to obtain visual information. Frequently, the information we require is at different locations in depth within a complex 3-D scene. How do the eyes move in these situations? Does the task being performed influence gaze-shifts? Here, we examine the binocular and monocular eye movements produced by subjects whilst performing different visual tasks using real world objects. Subjects were required to make decisions about small cuboids presented within a purpose-built viewing-box. The cuboids were painted different shades of grey, and varied in size: some were perfect cubes, whilst others had square faces and rectangular sides. On a single trial, subjects performed a 2AFC regarding which of two cuboids was biggest or which had the lightest shade of grey. Viewing was either binocular or monocular. The cubes were laterally separated by 11deg, and were located at either the same distance from the observer or at different distances. Whilst subjects performed the task their eyes were tracked using an infra-red video-based binocular eye-tracking system sampling eye position at 120Hz. Subjects were naïve, and were given no instructions regarding eye movement. Eye position data were classified into regions of fixations, gaze shifts and local adjustments of gaze. The task (shape vs. grey-level) was not found to influence the number or duration of gaze shifts, fixations, or gaze adjustments. During binocular viewing, vergence changes primarily occurred during saccadic eye movements. Subjects made more fixations, and for longer periods during monocular viewing, although other aspects of the eye movements were comparable with binocular viewing. Whilst task dependent differences in gaze-shift dynamics have previously been reported (Epelboim et al., Vis Res, 1997, 37, 2597), we fail to find differences in eye movements produced under different tasks or different viewing conditions.