The Formation of Symmetrical Gestalts Is Task-Independent, but Can Be Enhanced by Active Regularity Discrimination

Abstract

The brain can organize elements into perceptually meaningful gestalts. Visual symmetry is a useful tool to study gestalt formation, and we know that there are symmetry-sensitive regions in the extrastriate cortex. However, it is unclear whether symmetrical gestalt formation happens automatically, whatever the participant's current task is. Does the visual brain always organize and interpret the retinal image when possible, or only when necessary? To test this, we recorded an ERP called the sustained posterior negativity (SPN). SPN amplitude increases with the proportion of symmetry in symmetry + noise displays. We compared the SPN across five tasks with different cognitive and perceptual demands. Contrary to our predictions, the SPN was the same across four of the five tasks but selectively enhanced during active regularity discrimination. Furthermore, during regularity discrimination, the SPN was present on hit trials and false alarm trials but absent on miss and correct rejection trials. We conclude that gestalt formation is automatic and task-independent, although it occasionally fails on miss trials. However, it can be enhanced by attention to visual regularity.

The extrastriate symmetry response can be elicited by flowers and landscapes as well as abstract shapes

“…We re-ran the regression analysis again and confirmed that flower and landscape SPNs were not statistical outliers. For consistency with Makin et al (2016) , we calculated all SPNs from PO7/8 in the 300–400 ms window (where amplitude is typically maximal), and we only use published SPNs from experiments where participants were explicitly discriminating regularity ( Makin et al., 2015 ; Makin, Rampone, Morris, & Bertamini, 2019 ; Makin et al., 2013 , 2012 , 2016 ; Palumbo et al., 2015 ). Therefore, we used only SPNs from the Symmetry task in the current study, not the Color or Color control tasks.…”

Source dipole analysis reveals a new brain response to visual symmetry

“…This confirmed that the PCC response was a unique component, with its own distinct time course. Finally (and importantly), Analysis 6 replicated all results from the Regularity task of Makin et al 24 by reanalyzing data from a similar experiment by Palumbo et al 32 .…”

“…This investigation involved six analyses, which built on each other to reveal new properties of the brain response to symmetry. In Analysis 1, we re-examined the sensor-level ERP data from Makin et al 24 . In addition to the classic bilateral SPN, we identified a new vertex positivity in the 80 and 100% PSYMM conditions of the Regularity task.…”

“…The scaling of the SPN response with PSYMM is represented by increasing bilateral negativity (blue) across posterior electrodes. This effect was observed across all tasks, but it was enhanced in the Regularity task (for a more detailed sensor-level analysis, see Makin et al 24 ).…”

“…Makin et al 24 , we computed the difference between random trials (0% symmetry) and symmetry trials (20%, 40%, 60%, 80%, 100%). Figure 1 illustrates the SPN at posterior electrodes (top row) and global field power (GFP) for the topographic difference map for each task and level of PSYMM (bottom row).…”