EEG and behavioral responses during the prolonged (40 min) Necker cube classification task

We recorded EEG signals and responses of 18 healthy subjects during the visual stimuli classification task. During this task, the ambiguous visual stimuli, Necker cubes, with a different ambiguity degree (a) were presented in random order. We instructed the subjects to press the left or the right key to report their interpretation of each Necker cube orientation (see supplementary Fig1. A).Ambiguity (a) depends on the contrast of the middle edges and affects the Necker cube orientation. In our experiment, we used the Necker cube images with eight different ambiguity degrees, (a) = {0.15, 0.25, 0.4, 0.45, 0.55, 0.6, 0.75, 0.85}. Half of them, (a) = {0.15, 0.25, 0.4, 0.45}, are left-oriented, and another half, (a) = {0.55, 0.6, 0.75, 0.85}, are right-oriented. While for (a) ≈ 1 the Necker cubes have a clear orientation (left and right), for (a) ≈ 0.5 the identification of the cube orientation is a more difficult task since the image becomes ambiguous. The entire experimental session was divided into 4 equal time intervals T1-T4 (see supplementary Fig1. B).The proposed MATLAB structure contains 2 cells corresponding to the EEG data time-locked to the Necker cube demonstration and the EEG data time-locked to the subject's response. Each cell includes 4 cells, corresponding to the 4 intervals (T1-T4). In turn, these cells contain 18 cells corresponding to 18 subjects. Each one consists of a field <em>trial</em> containing EEG trials, recorded for 31 EEG channels. The field <em>label</em> includes the names of the EEG channels. Each EEG trial starts 2 seconds before the Necker cube demonstration (for data during the demonstration) or 2 seconds before the button is pressed (for data during the subjects click on the button) and lasts 4 seconds. EEG signals are recorded at a sampling rate of 250 Hz. The field <em>timeOfPresentation</em> contains the time moments when the stimulus was presented in the experiment. The field <em>ReactionTime</em> includes the response time to this stimulus, i.e. the time from presenting the stimulus to clicking the button. The field <em>Ambiguity</em> contains the value of the parameter (a) for the stimulus. The field <em>Button</em> includes information on which button was pressed by the subject (0 – left, 1 - right).EEG signals were filtered by a band-pass FIR filter with cut-off points of 1 Hz (HP) and 100 Hz (LP), and by a 50 Hz notch filter. Eye blinking and heartbeat artifacts were removed using independent component analysis (ICA).