Symmetry perception for patterns defined by colour and luminance

Perception of visual symmetry is fast, efficient and relies on both early, low-level and late, mid-and high-level neural mechanisms. To test for potential influences of early, low-level mechanisms on symmetry perception, we used isoluminant, achromatic and combined (colour + luminance) patterns in a psychophysical and an event-related potential (ERP) experiment. In the psychophysical experiment, pattern contrast was fixed at individual symmetry discrimination threshold. Participants then judged whether a pattern was symmetric or random. Stimuli at isoluminance were associated with a large bias towards symmetry, achromatic stimuli introduced the opposite bias, while stimuli containing a balance of both colour and luminance were perceived without bias. These findings are in line with distinct contrast sensitivity functions for colour and luminance, with colour providing low frequency information useful for symmetry detection and luminance providing high frequency information useful for detection of detail. The subsequent ERP experiment was run at high contrasts to assess processing of symmetry in suprathreshold conditions. Sustained Posterior Negativity, a symmetry-sensitive ERP component, was observed in all conditions and showed the expected dependence on symmetry. However, interactions between symmetry and contrast type were not observed. In conclusion, while our findings at threshold support models that propose an important contribution of low-level mechanisms to symmetry perception, at suprathreshold, these low-level contributions do not persist. Therefore, under everyday viewing conditions, symmetry perception engages a relatively broad cortical network that is not constrained by low-level inputs.