They look virtually the same: extraretinal representation of symmetry in virtual reality

The brain must identify objects from different viewpoints that change the retinal image. This study will determine the conditions under which the brain spends computational resources to construct view-invariant, extraretinal representations in a 3D virtual environment. We will focus on extraretinal representation of visual symmetry. Visual symmetry activates the extrastriate visual cortex and generates an Event Related Potential (ERP) called Sustained Posterior Negativity (SPN). Amplitude is lower for symmetrical compared to asymmetrical stimuli at posterior electrodes. Given a symmetric pattern on a plane, regularity in the retinal image is degraded by perspective. Previous studies have found that the SPN is selectively reduced for perspective symmetry. However, this perspective cost might be reduced when sufficient visual cues are available to support view invariance. Our previous work was concerned with adding more cues on a 2D screen. The current study explored the use of a 3D virtual environment. We adopted a sequential approach to data collection, with a minimum sample of 48. This was sufficient to confirm our hypothesis. Participants completed 2 tasks in a counterbalanced order. In one task they discriminated stimulus regularity (symmetry or asymmetry). In the other they discriminate stimulus luminance (light or dark). We computed perspective cost as the difference between frontoparallel SPN and perspective SPN. We found no SPN perspective cost in the Regularity task. Perspective cost was significantly < 0.35 microvolts – our a priori definition of a small SPN modulation. The results from the Luminance task were less clear. SPN perspective cost was not significantly more than 0 microvolts, but not significantly less than 0.35 microvolts. We conclude that the extrastriate cortex can construct extraretinal representations of symmetry when sufficient visual depth cues are available. This certainly happens during regularity discrimination and may happen automatically during luminance discrimination.