Objective: To investigate whether the effect of levodopa on saccade is the same to visual targets of small and large eccentricities in Parkinson’s disease (PD) patients. We hypothesized the effects of levodopa on amplitudes and onset latencies of saccades  are equal among the eccentricity conditions.

Background: We reported the effect of levodopa on saccade amplitude and onset latency in de novo PD patients [1]. Levodopa did not alter the accuracy of saccade amplitude. However, levodopa prolonged onset latencies of saccades. PD patients rarely noticed the impairment of saccades clinically, and no subjects noticed alteration of saccades by levodopa. We also reported that the subthalamic stimulation improved saccade amplitude, and the improvements of that were larger at lager target eccentricities [2]. It is unknown whether levodopa affects amplitudes and onset latencies of saccades equally in both saccades towards visual targets of small and large eccentricities.

Method: We recorded DC electro-oculography in 14 de novo PD patients before and after taking levodopa/carbidopa (100mg/10mg) during horizontal prosaccade task presented at target eccentricities of 5, 10, 20, and 30 degree. We analyzed the first amplitudes, as well as onset latencies of saccades, at each target eccentricity conditions using two-way analysis of variance (levodopa conditions and target eccentricities).

Results: Saccade amplitudes were not significantly different between before and after levodopa at any target eccentricities. Onset latencies of saccade were significantly longer after levodopa than before ledvodopa at any target eccentricities. Prolongation of saccade latency by levodopa were significantly longer at larger target eccentricities.

Conclusion: Saccade amplitudes of prosaccades were not altered, but onset latencies were prolonged, by levodopa at any target eccentricities. Montaron et al. reported levodopa enhanced beta rhythms in ventral tegmental area and immobility in normal cats during hunting [3]. It is hypothesized that prolongation of saccade latency in our study may be due to enhanced immobility by levodopa, as indicated in Montaron’s study. Furthermore, prolongation of saccade onset latency was significantly longer at larger target eccentricities. Levodopa may have effects more on attention rather than motor functions of eye movement, thus the change of attention may induce different level of awareness in between central and marginal area.

References: [1]Yugeta A, Terao Y, Fukuda H, Ugawa Y. Effects of levodopa on saccade performance in Parkinson’s disease, Mov Disord, 2008; 23: S296 [2]Yugeta A, Terao Y, Ugawa Y, Improvement of saccade amplitude by the STN DBS and visual target eccentricity, Mov Disord, 2018; 31: S432 [3]Montaron MF, Bouyer JJ, Rougeul A, Buser P, Ventral mesencephalic tegmentum (VMT) controls electrocortical beta rhythms and associated attentive behaviour in the cat. Behav Brain Res. 1982;6(2):129-45.

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MDS Abstracts - https://www.mdsabstracts.org/abstract/effects-of-levodopa-on-prosaccade-performance-and-visual-target-eccentricity/