Symmetry detection and orientation perception: Electrocortical responses to stimuli with real and implicit axes of orientation

The present study was designed to investigate the electrocortical correlates of visual anisotropy with particular reference to the anisotropies reported for symmetrical patterns. On the basis of earlier studies of visual evoked potentials (VEPs) to lines of differing orientation, it was predicted that VEP parameters would differ according to the orientation of the axis of symmetry of a stimulus pattern. It was further predicted that a direct comparison of explicit orientation (by line presentation) and implicit orientation (via patterns symmetrical about axes differing in orientation) would yield data consistent with the notion that symmetry detection is a special case of orientation detection. The present findings indicated that for components of VEP waveforms up to 500 ms from stimulus onset, amplitude was generally greater for patterns with a vertical axis of symmetry than for patterns with a horizontal axis of symmetry, while patterns with positive and negative oblique axes evoked VEPs with the lowest amplitude components. In addition, it was found that the early VEP components exhibited latency differences for different axes of symmetry, with shortest latencies occurring for figures with vertical symmetry. The data are discussed within the context of theories that suggest symmetry detection is an acquired process dependent on the development of cell populations responsible for particular stimulus orientations.