Planning Sequential Eye Movements
Studies have shown that future actions in a sequence have an effect on both the mechanics and timing of arm movements. In saccadic eye movements, future saccades in a sequence have been shown to impact the reaction time of the first eye movement, suggesting these movements may be planned as an entire sequence (Inhoff, 1986). The aim of the present study was to determine if the location of a second target had an impact on the metrics of the first saccade since research on this question is limited. In Experiment 1, subjects performed either a saccade to a single target, or a sequence of two saccades to two targets. The first target was located 12, 15, or 18 degrees from fixation, and the second target (when present) was located 12, 15, or 18 degrees from the first target. First and second target location, as well as number of targets presented, was randomized on a trial by trial basis. The presence of a second target was found to have a significant effect on the reaction time, amplitude and horizontal end position of the first saccade. Furthermore, the location of the second target was found to have a significant effect on amplitude, horizontal end position and duration of the first saccade. The first saccade was found to have a significantly shorter amplitude the further the second target was from the first target, with other significant saccade metrics following this same pattern. In Experiment 2, two targets were presented on all trials, with target locations as described in Experiment 1. The second target in Experiment 2 was an “x” or a “+”, which was randomized on a trial by trial basis. Three conditions were performed in Experiment 2 in blocked order randomized by participant. In the “look” condition, participants were instructed to look at the first target, and then the second. In the “ignore” condition, participants were instructed to look at the first target only, and ignore the second target. In the “attend” condition, participants were asked to make a perceptual judgement about the second target (whether it was a “+” or an “x”) before looking at the first target only. Once participants moved their eyes away from the fixation target, the second target changed into its masked form (a star, formed by the overlapping of the two possible second target images). The location of the second target was found to have a significant effect on amplitude, horizontal end position and peak velocity of the first saccade. No significant effect of condition was found for any variable except reaction time. Most importantly, no significant interaction was found between second target location and condition. This suggests that task instructions did not have an impact on the effects of second target location on the metrics of the saccade to the first target. This may be the result of the summation of a motor plan to the first target and the inhibition of a motor plan to the second target (or inhibition of head movements), regardless of whether or not subjects were instructed to make a movement to the second target. Another possibility is that the second target changed the perceived visual location of the first target. Further research is required to differentiate between these possibilities.