Visual symmetry in objects and gaps

It is known that perceptual organization modulates the\udsalience of visual symmetry. Reflectional symmetry is\udmore quickly detected when it is a property of a single\udobject than when it is formed by a gap between two\udobjects. Translational symmetry shows the reverse\udeffect, being more quickly detected when it is a gap\udbetween objects. We investigated the neural correlates\udof this interaction. Electroencephalographic data was\udrecorded from 40 participants who were presented with\udreflected and translated contours in one- or two-object\uddisplays. Half of the participants discriminated regularity,\udhalf distinguished number of objects. An event-related\udpotential known as the Sustained Posterior Negativity\ud(SPN) distinguished between reflection and translation.\udA similar ERP distinguished between one and two object\udpresentations, but these waves summed with the SPN,\udrather than altering it. All stimuli produced\uddesynchronization of 8?13 Hz alpha oscillations over the\udbilateral parietal cortex. In the Discriminate Regularity\udgroup, this effect was right lateralized. The SPN and\udalpha desynchronization index different stages of visual\udsymmetry discrimination. However, neither component\uddisplayed the Regularity · Objecthood interaction that\udis observed in speeded discrimination tasks, suggesting\udthat integration of visual regularity with objectness is\udnot inevitable. Instead, both attributes may be\udprocessed in parallel and independently