Colliding saccades may reveal the secret of their marching orders

Saccades are fast eye movements by which objects of interest are sought and captured. Their brevity means that they can often be executed within the time taken by visual impulses to reach the brain. Microstimulation, used in conjunction with single-unit recording, has become a powerful tool for analysing the role of central visuomotor structures in producing accurate saccades. This review presents the rationale of a test in which saccades are artificially elicited (evoked) while the eyes are moving. The presumed role in saccade production played by stimulated local neurons is inferred from the way trajectories of evoked saccades are deviated. The results suggest that the forebrain structures tested designate a goal expressed in retinocentric coordinates. To take into account an eye displacement that occurs during visual processing, the goal location must be recomputed; in this article, current views on how this might be accomplished are discussed in relation to observations made by unit recording.