Cortical Pulsed-Inhibition Hinders Identification in Briefly Presented Stimuli at Low Contrast

Cortical and thalamo-cortical oscillations in different frequency bands have been proposed to provide a neuronal basis for discretization of perception [VanRullen & Koch, 2003, TICS, 7, 207-313]. When the amplitude of occipital alpha oscillations is higher than some threshold cortical excitability, a pulsed inhibition might result in discrete perception [Mathewson et al. 2011, Frontiers in Psychology, 2]. Here, we studied the effect of pulsed inhibition on stimuli presented with supra- and near threshold contrast, in an identification task. The task was to report back the orientation of a Landholt ring presented in high and low contrast. Variable fixation period helped avoid task-induced phase locking of alpha. Alpha oscillations were derived by band-pass filtering the raw EEG data between 8 and 15 Hz and behavioral data was fitted with a Poisson Counter model of identification after classifying the trials based on the average phase distribution prior to stimulus onset of correct and wrong responses. A grand average counter-phase stimulus-locked alpha oscillation between correct and wrong responses was present in the low but not in the high contrast condition. When stimuli contrast is high, the preceding oscillatory activity does not play a role since stimulus evoked cortical excitability is well above threshold.