Event-related potential study on image-symmetry discrimination in the human brain

The human visual system seems to have a highly perceptual sensitivity to symmetry. However, where and when the discrimination of symmetrical properties begins in the context of visual information processing is largely unclear. This study investigates event-related potential (ERP) patterns in humans when perceiving symmetry-varied complex object images. ERP responses were derived from electroencephalography (EEG) data recorded from eight healthy subjects using 128-channel scalp electrodes. Visual stimulation was provided using gray-scaled photographs of a car with six different viewpoints, hence disrupting the vertical symmetry, where one of the stimuli was intentionally made symmetric by mirroring the image about its center vertical axis. The results show that discrimination of image symmetry is revealed by potential deflection in early ERP components recorded at occipito-temporal sites and can be significantly observed around 220 ms after stimulus onset.