Eye movements allow us to optimize the perception of objects of interest. Perception is particularly accurate when the projection of the objects on the retina falls within the fovea, an area of the retina that contains an especially high density of photoreceptors.
When an object is moving, slow eye movements (smooth pursuit) are used in order to maintain the retinal projection of the object onto the fovea.
Smooth pursuit eye movements are guided both by retinal inputs (difference in velocity between the eye and the target) and by an internal representation of target motion when available (Orban de Xivry et al. 2008). To study how internal representation of target motion drives eye movements, scientists transiently extinguishes the target for a few hundreds of millisceconds (a period often referred to as occlusion). Usually, when the target is extinguished eye velocity quickly drops to zero as it is impossible to produce smooth eye movements voluntarily without any moving target. However, when the reappearance of the stimulus is expected, eye velocity can be maintained at a lower gain (~60%). For instance, after a few repetitions, subjects are quickly able to predict target motion during occlusion and to continue pursuing the now invisible target (Bennett et al. 2010). In addition, when target motion does not repeat but changes from trial-to-trial, our brain is able to extrapolate the pre-occlusion target motion and to use this extrapolation in order to guide eye movements during the transient disappearance of target motion (Bennett et al. 2007).
Guiding eye movements with external cues
The internal representation of target motion is also used to anticipate target motion onset (Barnes and Asselman 1991). For instance, if you are looking at an object while knowing when and in which direction it will move, your eyes will start moving one or two hundreds of milliseconds before the object actually does.
Even in unpredictable situation, the brain still tries to predict what the future target motion is. For instance, uncertainty about target direction is not sufficient to abolish anticipatory target motion. The presentation of a target with known timing of motion onset but unknown motion direction (left or right) does not suppress anticipatory eye movements. Rather, the brain will predict future target motion on the basis of the past experience (Kowler et al. 1984). For instance, if the two previous trials contained a rightward target motion, anticipatory eye movements will be directed rightward.
Anticipation and prediction improves perception during object tracking
Prediction and anticipation result in a smaller difference in velocity between the eye and the target, hence maximizing perception. Without them, the large difference between eye and target velocities would result in a large difference in position over time. This difference in position would force the oculomotor system to trigger a catch-up saccade in order to realign the fovea on the target. Those saccadic eye movements are fast and short but disrupt target perception as vision is suppressed during them. Therefore, anticipation and prediction help minimizing the frequency of saccades during smooth pursuit eye movements, hence optimizing target perception.
Kowler, E. (1989). Cognitive expectations, not habits, control anticipatory smooth oculomotor pursuit. Vision Research, 9, 1049-1057.
Barnes, G. R., & Asselman, P. T. (1991). THE MECHANISM OF PREDICTION IN HUMAN SMOOTH PURSUIT EYE MOVEMENTS. Journal of Physiology, 439-461.
Bennett, S. J., Orban de Xivry, J., Barnes, G. R., & Lefèvre, P. (2007). Target acceleration can be extracted and represented within the predictive drive to ocular pursuit. Journal of Neurophysiology, 98(3), 1405. doi: 10.1152/jn.00132.2007.
Bennett, S. J., Orban de Xivry, J., Lefèvre, P., & Barnes, G. R. (2010). Oculomotor prediction of accelerative target motion during occlusion: long-term and short-term effects. Experimental brain research. doi: 10.1007/s00221-010-2313-4.
Orban de Xivry, J., Missal, M., & Lefèvre, P. (2008). A dynamic representation of target motion drives predictive smooth pursuit during target. Journal of Vision, 8, 1-13. doi: 10.1167/8.15.6.
Kowler, E., MARTINS, A., & PAVEL, M. (1984). The effect of expectations on slow oculomotor control—IV. Anticipatory smooth eye movements depend on prior target motions. Vision Research, 24(3), 197-210. doi: 10.1016/0042-6989(84)90122-6.