| In natural manipulation tasks, gaze and hand movements are organized around object contact events with gaze typically shifting from one object of interest to the next just after contact but before visual and haptic information about contact could be used to trigger the saccade. We examined the control of these predictive gaze shifts by manipulating both the presence and timing of visual and haptic feedback in a simple manipulation task in which subjects contacted a series of virtual objects that either moved when contacted or, in some conditions, were fixed. As expected, when both visual and haptic feedback was provided, gaze shifts between successive targets were largely predictive, occurring approximately 50 ms after contact on average. Removing or delaying haptic feedback led to an increase of saccadic latency of about 50 ms relative to baseline. Removing vision of the hand (represented by a cursor) increased saccadic latency by about 100 ms when hitting moving objects and, thus, many of these saccades may have been triggered by sensory feedback. However, removing vision of the hand did not affect saccadic latency when hitting fixed objects. When visual feedback of the hand and object motion was delayed, saccades were delayed by approximately the same amount. Consequently, when aligned to visual perceived contact, there was no change in saccadic latency relative to baseline. Two alternative hypotheses that can at least partially explain this pattern of results are considered. One possibility is that changes in saccadic latency across conditions arise because the motor system anchors predictive gaze shifts on different events depending on the available sensory feedback. Alternatively, increases in saccadic latency may arise due to poorer prediction of forthcoming contact event when sources of sensory feedback are removed or delayed. |
