Saccadic Interception of a Moving Visual Target after a Spatiotemporal Perturbation

Abstract

Animals can make saccadic eye movements to intercept a moving object at the right place and time. Such interceptive saccades indicate that, despite variable sensorimotor delays, the brain is able to estimate the current spatiotemporal (hic et nunc) coordinates of a target at saccade end. The present work further tests the robustness of this estimate in the monkey when a change in eye position and a delay are experimentally added before the onset of the saccade and in the absence of visual feedback. These perturbations are induced by brief microstimulation in the deep superior colliculus (dSC). When the microstimulation moves the eyes in the direction opposite to the target motion, a correction saccade brings gaze back on the target path or very near. When it moves the eye in the same direction, the performance is more variable and depends on the stimulated sites. Saccades fall ahead of the target with an error that increases when the stimulation is applied more caudally in the dSC. The numerous cases of compensation indicate that the brain is able to maintain an accurate and robust estimate of the location of the moving target. The inaccuracies observed when stimulating the dSC that encodes the visual field traversed by the target indicate that dSC microstimulation can interfere with signals encoding the target motion path. The results are discussed within the framework of the dual-drive and the remapping hypotheses.

Updating of an Occluded Moving Target for Interceptive Saccades

“…In some trials, eye position was perturbed at occlusion onset through microstimulation in deep SC. Fleuriet and Goffart (2012) showed that, even when the saccades started from a different position than saccades without microstimulation, they always landed near the (occluded) target. Moreover, the subsequent interceptive saccade often adequately compensated for the different amounts of invisible target motion occurring during additional delays induced by the microstimulation.…”

“…A recent study published in The Journal of Neuroscience (Fleuriet and Goffart, 2012) addressed the distinction between these schemes on the basis of experiments involving microstimulation of the SC. Monkeys made interceptive saccades to, and subsequently pursued, moving targets; these targets were occluded during the interceptive saccades.…”

“…In discussing these findings, Fleuriet and Goffart (2012) identified several problems with the dual-drive scheme. Because of these concerns, they suggested that the ability to make accurate interceptive saccades is better explained by the alternative remapping scheme.…”

“…Accordingly, the oculomotor system must account for target displacement between final SC read-out and saccade end, which is a more predictable interval. The only difference between this scheme and the remapping scheme advocated by Fleuriet and Goffart (2012) is the stage at which target motion compensation occurs. In both schemes, interceptive saccades only land at the correct (occluded) target position if SC activity is updated for the microstimulation-induced change in eye position, just like for stationary targets (Sparks and Mays, 1983).…”

“…The saccades observed by Fleuriet and Goffart (2012) were influenced by the target trajectory in several ways. Inaccuracies of interceptive saccades were observed for targets initially moving in the direction of the induced saccade; these also depended on the vector encoded at the stimulation site (Fleuriet and Goffart, 2012, their Fig.…”