Introduction
Visual symmetry contributes to perceptual organisation and object formation (Bertamini et al., 2018; Makin et al., 2023). Psychophysical research demonstrates that the detection of symmetry is fast and noise tolerant. Symmetry can be discriminated from random within 25 ms (Locher & Wagemans, 1993) and when it is presented in the visual periphery (Barlow & Reeves, 1979; Rampone et al., 2016). The neural basis of symmetry perception has been researched in the last two decades. Converging evidence from fMRI and TMS has found that symmetry is coded in extrastriate visual areas, with the strongest response the shape-sensitive Lateral Occipital Complex. V1 and V2 have smaller receptive fields and do not respond to symmetry (Audurier et al., 2022; Bona et al., 2015; Keefe et al., 2018; Kohler et al., 2016; Sasaki et al., 2005; Tyler et al., 2005; Van Meel et al., 2019).
The extrastriate symmetry response can also be measured with Electroencephalography (EEG). Both symmetrical and random patterns produce Event Related Potential (ERPs) at posterior electrodes. After the P1 and N1 components of the visual evoked potential, amplitude is lower for symmetrical patterns (Höfel & Jacobsen, 2007; Jacobsen & Höfel, 2003; Norcia et al., 2002). This symmetry-random difference wave is called the Sustained Posterior Negativity (SPN, Makin et al., 2012). SPN amplitude scales with the salience of different visual regularities (Figure 1, Makin et al., 2016, 2020, Palumbo et al., 2015) The SPN is generated whatever the participant’s task, but amplitude is enhanced when symmetry is task relevant (Figure 2).