Backward masking suppresses collinear facilitation in the visual cortex

Perceptual facilitation in detecting low-contrast Gabor patches (GPs) is induced by collinearly oriented high-contrast flankers. Our recent Visual Evoked Potentials (VEPs) study provided new physiological evidence for these collinear interactions, reflected by nonlinear modulation of multiple waveform components and frequencies [Sterkin, A., Yehezkel, O., Bonneh, Y. S., Norcia, A., & Polat, U. (2008). Multi-component correlate for lateral collinear interactions in the human visual cortex. Vision Research, 48(15), 1641-1647]. Here we used VEPs to study the temporal structure of this process. Low-contrast, foveal target GP (T) was simultaneously flanked by two collinear high-contrast GPs with a spatial separation that induces facilitation of T (lateral masking, LM). Another mask, identical to LM, was presented at different time-intervals (ISIs) after LM (backward masking, BM-on-LM). The responses were compared to separate waveforms evoked by T-alone and mask-alone at different ISIs. BM canceled the physiological markers of facilitation at an ISI of 50 ms, in agreement with earlier psychophysical findings, whereas no BM effect on T-alone was observed. This ISI coincides with the active time-window of lateral interactions, confirming our working model. The waveform amplitude of the negative N1 peak of LM was modulated toward the linear prediction of no interactions and the spectrum was shifted toward suppression, with no evidence of facilitation. Moreover, the P1 peak amplitude of BM was decreased at the same ISI, indicating that there is a mutual interference in cortical representation of both events. Waveform subtraction between BM-on-LM and LM suggests a mechanism of extended persistence of the target representation underlying facilitation in LM. We suggest an explanation for the role of improved detection of collinear stimuli in grouping of contours.