The Brain Response to Reflectional Symmetry Is Not Uniquely Preattentive

EEG can be used to measure the brain response to visual regularity. Reflectional symmetry generates an event-related potential (ERP) named the sustained posterior negativity (SPN). The SPN for reflectional symmetry is generated automatically, whatever the participant's task. This may be because reflectional symmetry has a fundamental role in perceptual organization and guiding adaptive behaviour. In contrast, other types of regularity, such as glass patterns, do not have this ecological significance. We thus predicted that the glass pattern SPN would be more susceptible to experimental variations of task than the reflection SPN. To test this prediction, we ran three experiments on three different groups of 52 participants. All participants saw the same random, reflection and glass dot dipole stimuli. The stimuli were either black or white. In the Regularity Task, participants discriminated whether the patterns were random or regular (where regular means reflection or glass). In the Luminance task, participants discriminated whether the patterns were black or white. In the Cross task, participants discriminated whether the vertical or horizontal arm of the central fixation cross was longer. As predicted, both the reflection and glass SPN were largest in the Regularity task, reduced in the Luminance task and reduced again in the Cross task. Contrary to predictions, glass pattern SPNs were less affected by task than reflection SPNs. This unexpected result suggests that glass patterns may even be processed more automatically than reflection, although this would require replication before it is treated as secure knowledge.