Detection thresholds for visually presented targets can be influenced by the nature of information in adjacent regions of the visual field. For example, detection thresholds for low-contrast Gabor patches decrease when flanked by patches that are oriented collinearly rather than orthogonally with the target. Such results are consistent with the known microanatomy of primary visual cortex, where long-range horizontal connections link cortical columns with common orientation preferences. To investigate the neural bases of collinearity effects, we recorded event-related brain potentials (ERPs) together with psychophysical measures for targets flanked by collinear vs. orthogonal gratings. Human volunteers performed a contrast discrimination task on a target grating presented at a perifoveal location. For targets flanked by collinear stimuli, we observed an increased positive polarity voltage deflection in the occipital scalp-recorded ERPs between 80 to 140 ms after stimulus onset. Such a midline occipital scalp voltage distribution of this ERP collinearity effect is consistent with a generator in primary visual cortex. Two later negative voltage ERP deflections (latencies of 245-295 and 300-350 ms) were focused at lateral occipital scalp sites, a pattern consistent with activity in extrastriate visual cortex. These ERP effects were correlated with improved contrast discrimination for central targets presented with collinear flanks. These results demonstrate that the integration of local flanking elements with a central stimulus can occur as early as 80 ms in human visual cortex, but this includes processes occurring at longer latencies and appears to involve both striate and extrastriate visual areas.