Temporal brain dynamics of the competition between proximity and shape similarity grouping cues in vision

Perceptual grouping operations are crucial for visual object recognition. From the pioneering proposal of Gestalt psychologists, research has focused mostly on the dynamics of single grouping laws. However, the integration between grouping cues has received relatively less attention. The present event-related potentials (ERPs) study aimed to examine the brain correlates of the competition between multiple grouping cues (namely, shape similarity versus proximity) in visual patterns by means of a selective attention paradigm that allows to measure the contribution of each cue independently to the competition between them. Behavioural results indicated larger interference effects of shape similarity on proximity cues when both cues compete. ERPs data showed two main neural effects. First, the amplitude of a negative component peaking around 250 ms (N200) was modulated by the interaction between proximity and shape similarity cues. Specifically, the single shape similarity relative to competing shape similarity cues elicited enhanced amplitudes. This finding seems to reflect the visual salience and/or the processing fluency of the shape similarity grouping factor. Remarkably, it can be considered an indirect brain signature of the competitive interaction between grouping cues. Second, we found larger P300 amplitudes elicited by single displays compared with competing trials, as well as by proximity relative to shape similarity cues, which presumably reflects higher perceived confidence in decisions during the processes joining perception to action.