P69. Does theta burst stimulation on visual cortex only modulate phosphene threshold after high visual demand?

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Transcranial magnetic theta burst stimulation (TBS) modulates excitability of motor cortex in dependence of prior activity of the motor system. Little is known about TBS effects in other cortical areas. We investigated the effect of TBS with 100% phosphene threshold (PT) intensity on the excitability of the visual cortex, using PT as dependent variable. Additionally, as hypothesized in previous work (Bruckner and Kammer, 2014), we examined whether enhanced activity in the visual cortex might influence TBS modulation. In a within-design, 32 healthy subjects (12 men) so far underwent 2 TBS sessions with either enhanced demand on visual cortex following TBS or no demand. Visual cortex demand consisted in measuring visual acuity using a Landolt C optotype. At the beginning of a session, PT was measured 4 times, 2 times to get familiar with the procedure and 2 times for mean baseline value. Then, TBS was applied with 100% of the individual baseline PT. 14 subjects (4 men, mean age 24.2 ± 3.6 years) received continuous TBS (cTBS) and in another 18 subjects (8 men, mean age 23.1 ± 2.4 years) intermittent TBS (iTBS) was applied (between group design). Two minutes after the end of TBS, PT was again estimated. Then, for the next 10 min, subjects in one session kept their eyes open looking at a white wall doing nothing else, or performed the visual acuity task (about 5–6 runs of the task) in the other session. After these 10 min, PT was measured again. The acuity task was performed to enhance activity in the visual cortex only, acuity thresholds were not analysed further. Mean baseline PT value was 38.5 ± 8.8 of maximum stimulator output. Baseline PT did not differ between sessions (F1,30 = 0.51, p > 0.4). Pre and post TBS PTs were subjected to an rmAnova (separately for iTBS and cTBS) with the within factor SESSION (with or without visual acuity task) and TIME (pre, post1, post2). For the cTBS group, no main effect was found but the interaction of SESSION × TIME was significant (F2,26 = 5.3, p = 0.012). Post hoc analysis revealed that cTBS had no direct effect on PT but an increased PT was found after the visual acuity task. For the iTBS group, there was no main effect for SESSION nor for TIME, and no interaction. We aim to complete measurements up to 20 subjects per TBS group. Our preliminary data suggest that neither cTBS nor iTBS applied with 100% of individual PT had an effect on PT 2 min after the end of stimulation. PT increased after cTBS (but not after iTBS) followed by enhanced activity in the visual cortex. Our findings are in contrast to the observation by Franca et al., (2006) showing an increase of PT after cTBS at 80% of PT. It is conceivable that cTBS directly modulates PT only at subthreshold intensity levels. Applied with PT intensity, cTBS might only have an influence on visual cortex excitability if visual demand is high after stimulation, whereas iTBS seems not able to modulate visual cortex excitability at all.