Different Effects of Double-Pulse TMS of the Posterior Parietal Cortex on Reflexive and Voluntary Saccades

Abstract

Gap and overlap tasks are widely used to promote automatic versus controlled saccades. This study examines the hypothesis that the right posterior parietal cortex (PPC) is differently involved in the two tasks. Twelve healthy students participated in the experiment. We used double-pulse transcranial magnetic stimulation (dTMS) on the right PPC, the first pulse delivered at the target onset and the second 65 or 80 ms later. Each subject performed several blocks of gap or overlap task with or without dTMS. Eye movements were recorded with an Eyelink device. The results show an increase of latency of saccades after dTMS of the right PPC for both tasks but for different time windows (0–80 ms for the gap task, 0–65 ms for the overlap task). Moreover, for rightward saccades the coefficient of variation of latency increased in the gap task but decreased in the overlap task. Finally, in the gap task and for leftward saccades only, dTMS at 0–80 ms decreased the amplitude and the speed of saccades. Although the study is preliminary and needs further investigation in detail, the results support the hypothesis that the right PPC is involved differently in the initiation of the saccades for the two tasks: in the gap task the PPC controls saccade triggering while in the overlap task it could be a relay to the Frontal Eye Fields which is known to control voluntary saccades, e.g., memory-guided and perhaps the controlled saccades in the overlap task The results have theoretical and clinical significance as gap-overlap tasks are easy to perform even in advanced age and in patients with neurodegenerative diseases.

Objective Evaluation of Vergence Disorders and a Research-Based Novel Method for Vergence Rehabilitation

“…Variability of eye movement parameters is approximately 20% in healthy adults. 53 Variability increases in the elderly, as well as in patients with neurodegenerative disorders. 44 , 54 , 55 The present study suggested that variability is a biomarker of the vergence disorder and the Vd-s rehabilitation protocol acts primarily on this parameter.…”

Up/down anisotropies of vertical saccades in healthy children depending on the mode and the depth of execution

“…Below, we present the effects of age on each of the experimental paradigms for which a significant effect is present. As we mentioned earlier, Kapoula et al (2011) propose that the cortical and subcortical circuits and mechanisms involved in programming saccade are different according to the initiation mode, automatic or more controlled. In the case of automatic saccades (gap paradigm), these circuits will involve mainly the parietal lobe, whereas in the case of controlled saccades (overlap paradigm), the frontal lobe seems activated (Pierrot-Deseilligny et al, 1995Gaymard 2012).…”

“…a longer circuit of oculomotor command. Kapoula et al (2011) argued that in the overlap test saccade preparation is done via the FEF similarly to voluntary saccades discussed by Pierrot-Deseilligny. Other researchers (Isa and Kobayashi 2004) proposed models that give an important role to the superior colliculus for the initiation of the saccade in the gap paradigm.…”

“…The parietal-frontal oculomotor circuits involved in saccade preparation may be different for the two types of eye movements; namely, for the automatic initiation might be done by the posterior parietal cortex while controlled initiation the posterior parietal cortex would relay information to the frontal eye fields which would be in charge of the initiation of the saccades. Such conceptual framework has been presented by Kapoula et al (2011); these authors provided some evidence for such model using double-pulse TMS on the posterior parietal cortex in the gap versus overlap saccades in healthy adults and showing differential effects in variability in the two tasks. To our knowledge, developmental studies of vertical saccades depending on the mode of initiation are inexistent.…”

“…Saccades can be elicited under automatic versus controlled mode of initiation with the use of gap and overlap paradigm respectively (e.g. Kapoula et al, 2011). The parietal-frontal oculomotor circuits involved in saccade preparation may be different for the two types of eye movements; namely, for the automatic initiation might be done by the posterior parietal cortex while controlled initiation the posterior parietal cortex would relay information to the frontal eye fields which would be in charge of the initiation of the saccades.…”