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).