Attention effects on steady-state visual evoked potentials in response to 3-80 Hz flicker

Rhythmic visual stimuli elicit rhythmic brain responses at the frequency of the stimulus, and can be used to "frequency-tag" processing of several stimuli presented at the same time. The amplitude of steady state visual evoked potentials (SSVEPs) is modulated by attention, and SSVEP amplitude to attended stimuli is generally enhanced. Although SSVEP responses are documented for flicker frequencies up to 100 Hz [Herrmann, 2001], effects of attention on SSVEP amplitude only have been reported for lower frequencies (up to ~30 Hz), with no systematic comparison across a wide, finely sampled range of tagging frequencies. Does attention modulate SSVEP amplitude at higher flicker frequencies (e.g. gamma band, 30-80 Hz), and is attentional modulation constant across frequencies? We addressed these questions by recording EEG from twenty participants while they performed a covert spatial attention task, in which a flickering white-light LED grid had to be attended or ignored (i.e. attending a non-flickering white-light LED grid shown on the other side). Participants were instructed to detect targets (brief blue-light LED flashes) appearing in the cued hemifield (validity 80%) and ignore distractors presented in the other hemifield. Detection rate was significantly higher for validly cued trials, and targets that were presented in the left hemifield. Attentional modulation (operationalized as SSVEP amplitude difference between attend-flicker and ignore-flicker trials, normalized by the average amplitude) of the gamma-band flicker was also significantly higher for the left hemifield. Moreover, attentional modulation was positive for tagging frequencies in theta (3-7 Hz) and gamma bands, but negative for the alpha band (8-13 Hz). These effects were consistent between electrode-level and source-level analyses using linear spatial filters. In conclusion, the effects of spatial attention on SSVEP amplitude are not constant across tagging frequencies, and may depend on the interaction between flicker frequency and endogenous brain rhythms implicated in attentional processes. Meeting abstract presented at VSS 2017