When do we find a third neural response to visual symmetry?

The human visual system is tuned to symmetry, and the neural response to visual symmetry has been well studied. One line of research measures an Event Related Potential (ERP) component called the Sustained Posterior Negativity (SPN). Amplitude is more negative at posterior electrodes when participants see symmetrical patterns compared to asymmetrical patterns. Source localization confirms that the SPN is generated by two dipoles in the left and right extrastriate cortex, in line with fMRI results. However, exploratory analysis by Tyson-Carr, Bertamini, Rampone, and Makin (2021) found a third symmetry response located approximately in the posterior cingulate peaking at around 600 msec. The third symmetry response was only generated in conditions where symmetry was 1) task relevant and 2) salient. We tested whether these findings are reliable by running source localization analysis on all suitable datasets from the complete Liverpool SPN catalogue (an online repository of all 40 SPN projects with 2215 participants https://osf.io/2sncj/). We predicted that less variance would be explained by a two-dipole model in experiments where participants classified regularity (hypothesis 1), and, when the third dipole is present, amplitude would correlate with that of the sensor-level SPN (hypothesis 2). Hypothesis 1 was not supported, while hypothesis 2 was. We conclude that the bilateral extrastriate symmetry response is sometimes followed by a third activation near the posterior cingulate. However, this third symmetry response is not as predictable as we had assumed. One possibility is that it may sometimes be hidden from average waveforms by temporal inconsistency between trials. This may happen more in experiments with longer presentation durations.