Contributed Session II: Evidence for a dipper effect in the perception of global form: findings from psychophysics and EEG.

Abstract

Decades of psychophysical experiments have shown that the perception of low luminance contrast violates Webers' law: contrast discrimination is best at low (but non-zero) pedestal intensities. This "dipper effect" is thought to be the product of a sigmoidal neuronal transducer function in the early stages of visual processing which is expansive at low contrasts. Here, we ask 1) whether the transducer that governs the perception of global form is subject to a similar nonlinearity and 2) if sensitivity is biased towards a certain axis of pattern alignment (translational, radial, concentric). Using a combination of psychophysics and steady-state VEPs, we examined observers' sensitivity to global form by manipulating the dipole orientation coherence of Glass patterns. Our psychophysical data indicate a Glass pattern "dipper effect" that is strongest for concentric patterns, while our EEG results show some evidence of nonlinear transduction but do not have the form predicted by psychophysical thresholds. Our findings indicate that, like low-level contrast, mid-level form discrimination is subject to mild facilitation (relative to detection) at low global form coherences, and suggest that this could be driven by neurons with nonlinear transducer functions.