Experimental setting and behavioural task
Experimental setup and details of recordings have been explained in detail previously (Murty et al., 2020, 2021) and are briefly summarised here. EEG was recorded from 64-channel active electrodes (actiCap) using BrainAmp DC EEG acquisition system (Brain Products GMbH) and were placed according to the international 10-10 system. Raw signals were filtered online between 0.016Hz (first-order filter) and 1kHz (fifth order Butterworth filter), sampled at 2.5kHz, and digitized at 16-bit resolution (0.1µV/bit). It was subsequently decimated to 250 Hz. Average impedance of the final set of electrodes was (mean±SEM) 7.82 ± 0.02 kΩ. EEG signals recorded were in reference to electrode FCz during acquisition (unipolar reference scheme).
All subjects sat in a dark room facing a gamma-corrected LCD monitor (dimensions: 20.92 x 11.77 inches; resolution: 1289 x 720 pixels; refresh rate: 100 Hz; BenQ XL2411; maximum luminance of 120 cd/m2) with their head supported by a chin rest. It was placed ~58 cm from the subjects and subtended 52º x 30º of visual angle for full screen gratings.
In the experiment, subjects performed a visual fixation task, wherein 2-3 full screen grating stimuli were presented for 800ms with an inter-stimulus interval of 700ms after a brief fixation of 1000ms in each trial using a customized software written in Objective C with OpenGL libraries running on MAC OS. The stimuli were full contrast sinusoidal luminance achromatic gratings with one of the three spatial frequencies (1, 2, and 4 cycles per degree (cpd)) and four orientations (0º, 45º, 90º, and 135º). Details of this experimental paradigm, including images of these stimuli, can be found in (Murty et al. , 2021; Murty & Ray, 2022).