Contextual Effects of High Dynamic Range (HDR) Luminance on Orientation Discrimination

Abstract

Contextual cues are known to affect orientation discrimination and brightness perception, but how these two modalities interact in real-world perception is poorly understood. Studies of brightness perception based on laboratory stimuli are typically at ~100:1 luminance contrast ratio ('standard dynamic range', SDR), but real-world scenes have contrast ratios up to 1,000,000:1, and recent reports suggest that, at contrast ratios over 1000:1 ('high dynamic range', HDR), luminance normalization expands the perceived shadings of gray at the mode of the luminance distribution (Allred et al 2012). We hypothesized that the contextual mechanisms of luminance normalization and feature processing interact at the earliest stage of visual cortical processing, in primary visual cortex. We are measuring EEG, eye tracking, and visual recognition behavior under a two-alternative forced choice (2AFC) task and under rapid serial visual presentation (RSVP, 0.5-2 Hz). In both tasks, a contrast mixture of two orthogonal Gabors appears as a central target, and the subject is instructed to indicate via keypress the orientation with the stronger contrast. The target is surrounded by a 5 x 5 grid of Gabor flankers, each on a separate luminance patch from an HDR luminance distribution. In a baseline condition with uniform flanker orientation and isoluminant patches, we reliably reproduced the classic 'pop-out' effect (Li 1999). We predict that, at HDR luminance, the contextual orientation effect is stronger for patches that are more similar to the target luminance. We also predict that this luminance dependence is stronger for HDR than for SDR luminance distributions. We predict that the ERPs will be consistent with the integration of orientation and luminance cues in visual cortex and that the RSVP task will reveal the spatial dependency of this contextual effect. Meeting abstract presented at VSS 2018