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).