Objective: To investigate whether the subthalamic nucleus (STN) deep brain stimulation (DBS) improves the memory guided saccade (MGS) amplitude in the same degree in small and large eccentricities in Parkinson’s disease (PD) patients. We hypothesize that the deviation ratios of MGS amplitude to target eccentricity improve equally among several eccentricity conditions of saccade target.

Background: Although not only limb movements but also eye movements are involved in PD patients, they rarely notice the latter and also its improvements by the STN DBS. We reported the impaired saccade latencies and amplitudes were improved by the STN DBS (Yugeta, et al. 2010). Last year, we reported the improvements of the deviation ratios of visually guided saccade (VGS) amplitude to target eccentricity by the STN DBS did not differ among eccentricity conditions.  In this study, we planned to clarify whether the STN DBS improves the deviation ratios of MGS amplitude to target eccentricity equally among different target eccentricity conditions.

Methods: We recorded DC electro-oculograms in 32 PD patients with and without bilateral STN stimulation during horizontal MGS task using target eccentricities of 5, 10, 20, and 30 degree. We analyzed the first saccade amplitude by paired t-test (DBS on and off), as well as the deviation angle and ratio of saccade amplitude to the target in each target eccentricity. We also analyzed the improvements of MGS amplitudes and of deviation ratios of those to target eccentricity conditions by one-way analysis of variance.

Results: Saccade amplitudes and deviation angles and ratios of saccade amplitudes from targets were improved by the STN DBS in any target eccentricities (p<0.01). Improvements of MGS amplitudes significantly increased with target eccentricities (p<0.01). The deviation ratios of MGS amplitude to target eccentricity increased with target eccentricities (p<0.05), in contrast to VGS (p>0.05).

Conclusions: The deviation ratios of MGS amplitude to target eccentricity were significantly different among the target eccentricities. The differential STN DBS effects on the deviation ratio of saccade amplitude to the target eccentricity indicate that the STN DBS improves not only saccadic eye movements but also estimation of target positions, retention of visuo-spatial working memories, and/or initiation of voluntary eye movements.

References: Yugeta et al. (2010) Neurology;74:743-748

Terao et al. (2013) Clin Neurophysiol;124:1491-1506

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MDS Abstracts - https://www.mdsabstracts.org/abstract/improvement-of-memory-guided-saccade-amplitude-by-the-stn-dbs-and-target-eccentricity/