Bidirectional Modulation of Primary Visual Cortex Excitability: A Combined tDCS and rTMS Study

Abstract

In the motor cortex (M1), transcranial direct current stimulation (tDCS) can effectively prime excitability changes that are evoked by a subsequent train of repetitive transcranial magnetic stimulation (rTMS). The authors examined whether tDCS can also prime the cortical response to rTMS in the human visual cortex. In nine healthy subjects, the authors applied tDCS (10 minutes; +/-1 mA) to the occipital cortex. After tDCS, they applied a 20-second train of 5 Hz rTMS at 90% of phosphene threshold (PT) intensity. A similar rTMS protocol had previously demonstrated a strong priming effect of tDCS on rTMS-induced excitability changes in M1. PTs were determined with single-pulse TMS before and immediately after tDCS and twice after rTMS. Anodal tDCS led to a transient decrease in PT, and subsequent 5 Hz rTMS induced an earlier return of the PT back to baseline. Cathodal tDCS produced a short-lasting increase in PT, but 5 Hz rTMS did not influence the tDCS-induced increase in PT. In a control experiment on four subjects, a 20-second train of occipital 5 Hz rTMS left the PT unchanged, whereas a 60-second train produced a similar decrease in PT as anodal tDCS alone. Compared with previous work on the M1, tDCS and rTMS of the visual cortex only produce short-lasting changes in cortical excitability. Moreover, the priming effects of tDCS on subsequent rTMS conditioning are relatively modest. These discrepancies point to substantial differences in the modifiability of human motor and visual cortex.