Multi-component correlate for lateral collinear interactions in the human visual cortex

Perceptual facilitation, a decrease in detection threshold for low-contrast Gabor patches (GPs) occurs when the GP is flanked by collinearly oriented high-contrast patches. There is earlier evidence suggesting a spatial architecture of excitatory and inhibitory interactions. Here we used Visual Evoked Potentials (VEPs) to study the temporal structure of this process. We measured VEPs elicited by a foveal near-threshold target GP presented in isolation (T), T in the presence of two flanking collinear high-contrast GPs (lateral masking, LM), or the flankers alone (F). Stimuli were presented for 50 ms every 1000 ms. The choice of the set parameters elicited behavioral facilitation of T detection. Significant modulation of peak amplitudes in LM compared with linearly summed waveforms elicited by T and F was found for five alternating polarity components, ranging from 65 to 290 ms after stimulus onset. In the frequency domain, suppression at lower frequencies (up to 0.8 log units) was followed by facilitation at higher frequencies (4-6 Hz, up to 0.8 log units). Although no differences in the latencies were found, lateral interactions were reflected by non-linear waveform modulation of multiple components and frequencies, including components as early as 65-75 ms. Spectrum analysis suggests that both suppression and facilitation may be found for the same configuration of stimuli, simultaneously, distributed at different temporal frequencies and/or sources. The physiological correlates of lateral interactions may thus originate at multiple sources, only some of which are explicitly facilitatory. The final perceptual outcome of this complex spatio-temporal representation is determined by combining sensory and cognitive factors.