P151. The effects of occipital tDCS on phosphene thresholds in dependence of visual demand

Abstract

Effects of transcranial direct current stimulation (tDCS) were reported in motor and cognitive domains. From the motor system we know that anodal tDCS causes facilitation whereas cathodal stimulation has a slight inhibitory effect. Same effects have been reported in visual cortex using phosphene thresholds (Antal et al., 2003), whereas contrast detection thresholds were only changed by cathodal tDCS (Antal et al., 2001). The aim of this study is to investigate the effect of either anodal or cathodal tDCS on the phosphene threshold in the absence and presence of a controlled activity in visual cortex. Phosphene thresholds were measured with transcranial magnetic stimulation (TMS) pulses using the method of constant stimuli. We applied 7 TMS pulses per intensity using 7 different ones in steps of 3% stimulator output. After each pulse participants had to respond by key press whether they saw a phosphene or not. Thresholds were calculated by fitting a sigmoid psychometric function to the data. After establishing a stable phosphene threshold within 4 measurements, TDCS was applied for 15 min (1 mA, 15 s ramp in/out) with electrodes placed over occipital cortex at the site of optimum phosphene stimulation and over Cz. Following tDCS two phosphene thresholds were measured beginning at minute 2 and at minute 22 after the end of tDCS. In a within-subjects design after the first threshold measurement a visual acuity task using a Landolt-C optotype lasting 10 min was applied in one session, whereas in a second session subjects just waited until minute 22 without visual task. Polarity of tDCS is varied between subjects. So far we have measured 6 participants with anodal stimulation at the occipital site. Preliminary analysis demonstrates a decrease of phosphene threshold with anodal tDCS, confirming previous observations. It is planned to measure up to 32 participants (16 anodal, 16 cathodal). The experimental design is similar to another study presented on the conference, where theta burst stimulation (TMS) instead of tDCS was applied as modulatory intervention. Thus, a direct comparison of effects in visual cortex with two different modulatory brain stimulation techniques is provided.