Abstract
When perceiving symmetry, an ERP component known as the Sustained Posterior Negativity (SPN) is produced. From around 250ms, amplitudes in posterior electrodes are more negative for symmetrical than random patterns (Makin et al., 2012). This component is thought to be spread over both hemispheres. A role for the corpus callosum has also been highlighted when symmetrical patterns are presented at fixation (Herbert & Humphrey, 1996). We tested this callosal hypothesis by measuring SPN response to peripheral stimuli. Twenty-four participants were presented with a reflection and a random dot pattern. Patterns were both either light or dark red in different trials, and participants made a colour discrimination. Patterns were presented to a single hemisphere by simultaneously positioning one to the left of the fixation cross and the other to the right. We contrasted trials where symmetry was on the left and random on the right, and trials where symmetry was on the right and random on the left. There are two ways to describe the results: Amplitude was lower in the hemisphere contralateral to the symmetrical pattern, than in the opposite hemisphere, contralateral to the random pattern. Put another way – we found an SPN within each hemisphere, where the SPN is defined as lower amplitude for reflection than random. This pattern of results shows that each hemisphere has its own symmetry processing mechanism, which is sensitive to symmetry in the contralateral visual field. This is in sharp contrast to the callosal hypothesis, which suggests a special role for cross-hemispheric connections in symmetry perception. References Herbert, A. M. H. & Humphrey, G. K. (1996). Bilateral symmetry detection: Testing a 'callosal' hypothesis. Perception, 25, 463-480. Makin, A. D. J., Wilton, M. M., Pecchinenda, A., & Bertamini, M. (2012). Symmetry perception and affective responses: A combined EEG/EMG study. Neuropsychologia, 50, 3250–3261.
Meeting abstract presented at VSS 2015