Computational protocol: Gestalt Perceptual Organization of Visual Stimuli Captures Attention Automatically: Electrophysiological Evidence

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[…] EEG was recorded by using a 32-channel Brain Vision amplifier and an ElectroCap cap with Ag/AgCl electrodes placed at 28 scalp sites (Fp1, Fp2, F3, F4, T7, C3, Cz, C4, T8, CP5, CP1, CP2, CP6, P7, Pz, P8, PO7, PO3, PO4, PO8, O1, Oz, O2, I1, Iz, I2, according to the modified 10–20 system; ) and at the left and right mastoid. Electrooculogram (EOG) was recorded using three electrodes placed at the outer canthi of each eye and below the right eye, respectively. The EEG and EOG were low-pass filtered during acquisition (half-amplitude cutoff: 250 Hz) and digitized at 1 kHz.Analyses were conducted with software packages EEGLAB () and ERPLAB (). Signals were re-referenced offline to the average of the mastoid electrodes and hi-pass filtered (half-amplitude cutoff: 0.1 Hz) for reducing low-frequency drifts. Epochs were created with a duration of 800 ms (starting 200 ms before stimulus onset and ending 600 ms after stimulus onset) and baseline correction was applied on the pre-stimulus interval (from -200 to 0 ms) and used for all analyses. Artifact detection was conducted by identifying epochs where either the EOG signal (re-referenced bipolarly) exceeded ±75 μV within a 200 ms moving window or any EEG channel exceeded ±200 μV within a 500 ms moving window.All epochs with detected artifact were visually inspected prior to artifact rejection, which led to the exclusion of a proportion of epochs ranging between 2.2 and 8.8% across participants. Averaged ERP waveforms were computed for each subject and experimental condition, and then group averages were computed for each experimental condition (GG, Gn, nG, nn). For display purposes only, EEG waveforms were additionally filtered with a fourth-order low-pass filter (half-amplitude cutoff: 25 Hz, roll-off: 24 db/octave). EEG data for analysis purposes were not low-pass filtered. [...] Behavioral measures of response time (RT) and detection rates were extracted from the non-rejected trials using ERPLAB. Responses faster than 150 ms (anticipations) and slower than 600 ms (late responses) were considered outliers (on average, 0.5% of total trials) and excluded from the analysis. Importantly, trials excluded from the behavioral analysis were also excluded from the ERP analysis and vice-versa.In order to obtain a measure of the response evoked by visual stimuli at the net of any influence of the preceding tone, ERPs relative to catch trials were subtracted from the group-averaged waveforms of each condition. The main dependent variable of the ERP analysis was the mean amplitude within specific time ranges and at electrode sites defined on the basis of the existing literature. The within-trial analysis of Gestalt configurations was conducted in the N2pc time range (220–300 ms post-stimulus onset, electrodes: PO7/8). The across-trial analysis of Gestalt configurations was conducted within four time windows: (i) the P1-range (90–150 ms, electrodes O1/2, PO7/8, P7/8); (II) the N1-range (150–210 ms, electrodes O1/2, PO7/8, P7/8); (iii) the N2-range (220–300 ms, electrodes O1/2, PO7/8, P7/8); (iv) the P3 range (250–350 ms, electrodes CP1/2, C3/4). The a priori power analysis was conducted using G-Power. Behavioral and EEG data were analyzed with paired t-tests and with fixed-effect repeated-measure analysis of variance (ANOVA) as implemented in the software Statistica 6.0 (StatSoft). In ANOVA, degrees of freedom were corrected for non-sphericity when appropriate. […]

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