Fig. 1.
Experimental paradigm. A: one stimulation electrode was placed on the vertex (Cz; black) and one on PO7–PO3 (red). Subjects were instructed to fixate on the central black dot on the screen throughout the experiment. The visual stimulus was a sinusoidal grating that could drift outward (solid arrows; Ao and To) or inward (dashed arrows; Ti). B: structure of individual trials. In outward (O) trials, the outward moving adapter (Ao) was followed by an outward-moving test stimulus (To). In inward (I) trials, the same outward-moving adapter was followed by an inward moving test stimulus (Ti). Red lines show a prediction of the neural activity in human motion area (hMT+) in each of these trials; adaptation to outward motion should reduce the response to the outward test stimulus (To) but not to the inward test stimulus (Ti). C: structure of an experimental run. Each run started with a 30-s presentation of Ao followed by alternating blocks of 3 O and 3 I trials. Red lines show the prediction of neural activity at the block level. In a voxel that adapts, neural activity should be lower in 3 successive O trials than in 3 successive I trials. Blue line shows the shape of the predicted hemodynamic signal (based on a canonical hemodynamic response function; see materials and methods). Note that the vertical axes of B and C are not to scale. D: structure of a session. Each subject participated in 2 runs without transcranial alternating current stimulation (tACSOFF) followed by 2 runs with stimulation (tACSON).