P68. Theta burst stimulation effects on visual cortex: Dependency of coil geometry

Abstract

Transcranial magnetic theta burst stimulation (TBS) is an efficient method to modulate excitability of the motor cortex. There have been some demonstrations of TBS effects in non-motor cortical regions, but it is still not clear whether the scheme from motor cortex, i.e. inhibitory effects by continuous TBS (cTBS) and excitatory effects by intermittent TBS (iTBS) can be transferred to other cortical regions. Applying TBS with 100% of individual phosphene threshold (PT) intensity on visual cortex with a figure-of-eight coil we observed a modulation of PT neither with cTBS nor with iTBS (Bruckner & Kammer 2015, this conference). As our findings were in contrast to Franca et al. (2006), who described an increase in PT following cTBS (80% of individual PT) we aimed to directly replicate their results. To that end we compare cTBS effects elicited with a focal figure-of-eight-coil with those elicited by a simple, less focal round coil, since Franca et al. (2006) were using a round coil. In a between-group-design continuous TBS (cTBS, 600 pulses) with 80% of individual PT intensity was applied using either a figure-of-eight-coil or a round coil. PT was measured before and 2 min after cTBS application. So far (October 2014), 14 subjects (7 men, mean age 24.6 ± 3.7 years) participated in the figure-of-eight-coil-group and 4 subjects in the round coil-group. Mean baseline PT value of the figure-of-eight-coil group was 39.3 ± 7.3% of maximum stimulator output. After cTBS PT values slightly decreased (trend to significance in paired two-tailed t -test, t = 2.07, p = 0.059). First visual inspection of the round coil group data indicate that cTBS applied with 80% of individual PT might increase PT, comparable to Franca et al. (2006). We will complete the data set up to 16 subjects in each group. CTBS effects in the visual system seem to depend on the focality of the applied electromagnetic field varied by the different coil geometries used.