Unconscious processing of shape-pair relationship

Lin & Murray (2014) recently showed that processing of the sameness or difference of two stimuli could occur without conscious awareness. Participants learned a go/no-go response to pairs of visible shapes; slower reaction times were observed in stimulus pairs associated with response inhibition even when they were rendered invisible by metacontrast masking. We expanded on this paradigm to address common concerns that strongly-masked yet visible stimuli could drive reaction time differences between conditions. By considering “no-go” error rates for each visibility rating on a per-participant basis, we revealed and corrected for inconsistent use of the Perceptual Awareness Scale and identified participants who showed no or weak metacontrast masking. Furthermore, we added a control experiment to compare processing of masked sameness or difference (Shape-Relation) to the simpler task of responding to the shape of a single stimulus in the periphery (Shape-Identity). In our Shape-Relation experiment, participants provided a “go” response to same-shape stimulus pairs or to unseen pairs, withholding their response only when a different-shape pair was consciously perceived. Even with our more stringent criteria for determining masking effectiveness, the results closely matched those of Lin & Murray, with unseen different-shapes eliciting slower responses than unseen same-shapes. In the Shape-Identity control experiment, the same participants followed a similar procedure but responded based on the shape of a single stimulus, presented left or right of fixation to match locations used in the Shape-Relation experiment. Here, unlike previous demonstrations of unconscious processing of a stimulus presented foveally, we found no unconscious slowing for unseen “no-go” shapes despite the task being cognitively less demanding. We hypothesize that the uncertainty of stimulus location prevented participants from sufficiently processing the shape identity, while in the Shape-Relation experiment, attention was consistently diffused across the two stimulus locations, allowing unconscious effects to occur.