Characterization of transient events during intermittent photic stimulation

Analyzing single-trial neuroelectrophysiological data complements average processing pipelines aiming to better understand generation mechanisms of brain oscillations and transient events. Given a lack of characterization of alpha-band transient events during photic stimulation, we aim to describe and characterize these transient events. EEG/MEG were simultaneously recorded during an intermittent photic stimulation paradigm employing each subject's individual alpha peak frequency. Transient events were detected in the sensor space in the time-frequency domain and were characterized by the number of occurrences, the frequency, and the width in time and frequency. The source-level localizations of single-trial and trial-averaged reconstructions were compared using a minimum norm estimation. Volunteers SNR values of the averaged signals showed a high correlation with the number of transient events with a median (IQR) correlation value of 0.87 (0.06). The median number of transient events per subject was variable, with 76 (78) events identified in EEG, 80 (103) in magnetometers, and 89 (116) in gradiometer signals, with 25% overlap across modalities. The frequency and temporal widths were approximately 2 Hz and 250 ms with the highest values for the gradiometers. Topographical analysis in sensor-space showed enhanced activity over posterior head regions, MNE distributions in source-space showed locations in or close to the occipital lobe for all detected transient events. The median distance between the average transient event reconstructions and the trial-averaged reconstructions was 1 cm, indicating transient events to be considered as building blocks for the averaged signal. We provide a first characterization of transient events in single-trial recordings during photic stimulation.