Multiple Saccades Are More Automatic Than Single Saccades

Abstract

In this paper, we demonstrate that when a peripheral object is foveated by a sequence of multiple saccades, the initial saccade in the sequence is initiated markedly faster than a single accurate saccade to the same object. We suggest that multiple saccades represent a more automatic form of oculomotor planning that may be the result of a reduced influence from the cerebral cortex. To test this, we compared single and multiple saccade characteristics across development. We find that in contrast to the reduction in the latency of single saccades that is observed across development, the latency of initial saccades in multiple saccade sequences is remarkably stable across all age groups. Moreover, the longer the latency of this initial saccade, the more accurate it is, suggesting that there is a relation between the degree of procrastination and the accuracy of the response. Finally, the frequency with which multiple saccades occurred within each age group was positively correlated with the tendency to generate erroneous saccades during a fixation control task. Taken together, the present data suggest that multiple saccades are generated in a more automatic manner than single saccades.

The Human Frontal Oculomotor Cortical Areas Contribute Asymmetrically to Motor Planning in a Gap Saccade Task

“…In conclusion, we have directly demonstrated using TMS that the multiple saccades we have previously characterized in young children [9] can be induced in adults by perturbing the balance in activity between frontal cortical oculomotor sites; most likely resulting in inappropriate inhibitory control of the brainstem saccade generation circuitry. Furthermore, we have shown that these inhibitory processes are direction- and time-dependent being maximally perturbed by the TMS during the ∼50 ms period surrounding the appearance of the saccade target ipsiversive to the site of stimulation.…”

“…We have shown previously that multiple saccades occur much more frequently in young adolescents than in older children or adults [9]. However, the latencies of multiple saccades remain remarkably invariant across development.…”

“…However, when TMS was delivered coincident with peripheral target appearance, there was a tendency to produce multiple saccades to the ipsiversive, but not contraversive, side relative to the site of stimulation. The multiple saccades were characterized by shorter latencies and a relatively small amplitude initial saccade followed by a second saccade to foveate the target [9]. The reduction in SRT for multiple saccades is shown for each of the experimental TMS sites in Figure 3 collapsed across saccade direction.…”

“…An alternative explanation, however, is that this type of output reflects a more automatic form of saccade execution that occurs before planning is fully complete. In a recent developmental study, we demonstrated that although the frequency of this type of saccade decreases substantially from 4 years of age to adulthood, the rapidity with which it is generated remains remarkably invariant across age groups [9]. This suggests that multiple saccades represent a form of oculomotor planning that occurs without coordinated input from the cerebral cortex; reflecting instead a more automatic form of output by the midbrain and brainstem saccade generation circuitry – structures that are well developed within the first decade of life.…”

“…The user could subsequently adjust the automatically determined onset if it was judged to be inaccurate. Multiple saccades were defined as two or more changes in eye position in which the initial saccade covered <90% of the distance to the target [9]. Single saccades were defined as one discrete change in eye position covering ≥90% of the distance to the target.…”