Propixx lite projector

Stimuli were presented using a Propixx lite projector (VPixx Technologies Inc) which allows refresh rates of up to 1440 Hz. To achieve this high-frequency mode, the projector separates the screen (initial resolution: 1920 × 1080 pixels) into quadrants and treats them as separate frames, resulting in a display resolution of 960 × 540 pixels. The RGB color codes for each quadrant, red, green and blue, are converted to a gray scale, separately for each frame and color, and presented consecutively within one refresh interval. The twelve frames are presented at a refresh rate of 120 Hz, resulting in 12 × 120 Hz = 1440 Hz. This approach allows to drive the luminance of each pixel with high temporal precision, allowing for smooth sinusoidal modulations, reducing unwanted harmonics (see Fig. 1C,D). In this study, we applied rapid rhythmic stimulation at frequencies ranging from 52 to 90 Hz in 2-Hz increments.

We presented a counterphase flickering checkerboard visual stimuli using Matlab and Psychtoolbox [57 (link)]. The stimuli were presented on a VPixx Technologies PROPixx Lite Projector (VPixx Technologies, Quebec, Canada) with a refresh rate of 120 Hz. Subjects viewed the stimuli with a mirror that was mounted on the 64-channel head coil. Each run lasted 254 s, with fixed 16 s ON and 16 s OFF periods, beginning with an OFF period. To maintain attention, participants were asked to fixate on a dot at the center of the checkerboard and press a button on a response box when they detected a color change. The flicker frequency of the checkerboard varied (4, 8, 12, or 40 Hz) across runs.