Neural processing stages during object-substitution masking and their relationship to perceptual awareness.

The extent of visual perceptual processing that occurs in the absence of awareness is as yet unclear. Here we examined event-related-potential (ERP) indices of visual and cognitive processes as awareness was manipulated through object-substitution masking (OSM), an awareness-disrupting effect that has been hypothesized to result from the disruption of reentrant signaling to low-level visual cortical areas. In OSM, a visual stimulus array is briefly presented that includes a parafoveal visual target denoted by a cue, typically consisting of several surrounding dots. When the offset of the target-surrounding cue dots is delayed relative to the rest of the array, a striking reduction in the perception of the target image surrounded by the dots is observed. Using faces and houses as the target stimuli, we found that successful OSM reduced or eliminated all the measured electrophysiological indices of visual processing stages after 130ms post-stimulus. More specifically, when targets were missed within the masked condition (i.e., on trials with effective OSM that disrupted awareness), we observed fully intact early feed-forward processing up through the visual extrastriate P1 ERP component peaking at 100ms, followed by reduced low-level activity over the occipital pole 130-170ms post-stimulus, reduced ERP indices of lateralized shifts of attention toward the parafoveal target, reduced object-generic visual processing, abolished object-category-specific (face-specific) processing, and reduced late visual short-term-memory processing activity. The results provide a comprehensive electrophysiological account of the neurocognitive underpinnings of effective OSM of visual-object images, including evidence for central roles of early reentrant signal disruption and insufficient visual attentional deployment.