P 47. Does intermittent theta burst stimulation on visual cortex modulate visual acuity?

Abstract

Introduction Repetitive transcranial magnetic stimulation (rTMS) in form of theta burst stimulation (TBS) is a potent tool to modulate motor cortex function. Several studies demonstrate effects of TBS on non-motor cortical areas. However, it is not yet clear whether TBS can change network properties in any cortical area. Objectives Our aim was to determine the effect of intermittent theta burst stimulation (iTBS) applied to the visual cortex on visual acuity. Expecting a facilitation, i.e., an increase in acuity, we tested several stimulation intensities. Methods Eleven healthy subjects participated in the study. First, a coil position at left occipital pole was determined to evoke a bright phosphene in the right lower quadrant of the visual field. The position was registered by neuronavigation. In four separate sessions iTBS (600 pulses) was applied with the following stimulation intensities: 60%, 80%, 100% and 120% of individual phosphene threshold, respectively. In order to control for putative carry-over effects, we started the four sessions with either 60% or 120% intensity. The visual task consisted of a simultaneous interleaved acuity measurement in the four quadrants of the visual field. Four rings were presented simultaneously in the four quadrants (eccentricity 10.6°). Three rings were closed, and one ring had a gap (Landolt C optotype) oriented either up, right, down, or left. The rings were displayed for 100 ms followed by a mask consisting of random noise pixels. Subjects had only to report on the direction of the gap neglecting the quadrant of presentation (four alternatives forced choice). Acuity was determined for each quadrant separately by varying the diameter of the Landolt C using an adaptive method. In each presentation the diameter of the three closed rings was identical with the optotype. The variation occurred in steps of one pixel (0.0375°). In the sessions, prior to iTBS, subjects had to complete 4 acuity trials of the visual acuity task. Immediately after stimulation they passed another acuity trial. Results Only 8 of 11 subjects tolerated iTBS at 120% phosphene threshold. Intensities up to 100% were tolerated by each of the subjects. Visual acuity in the right lower quadrant decreased with 60% iTBS. After 100% iTBS we observed an increase of visual acuity in this quadrant. No significant changes were found either with 80% or with 120% intensity in this part of the visual field. With respect to the other three quadrants no significant changes were observed at any intensity with the exception of one condition: 120% iTBS decreased acuity in the left upper quadrant. Conclusions Our preliminary data suggest a facilitatory effect of iTBS applied with100% of individual phosphene threshold on visual acuity in the corresponding part of the visual field. Although 60% iTBS decreased visual acuity in the same part of the visual field, a clear intensity dependency was not found since with 120% iTBS no modulation of visual acuity was observed in the critical quadrant. In principle, iTBS seems to work in visual cortex. Download : Download high-res image (80KB) Download : Download full-size image ITBS-induced change of visual acuity in the four quadrants of visual field, represented in the four quadrants of the graph. Stimulation intensity was scaled on individual phosphene threshold. Positive acuity values depict an improvement of acuity.