Decades of psychophysical tests have demonstrated that the perception of low luminance contrast defies Webers’ law: contrast discrimination works best at low (but non-zero) pedestal intensities. The “dipper effect” was long assumed to result from an early stage of visual processing sigmoidal neural transducer function that is expansive at low contrasts. For a study, researchers investigated – if the transducer that drove the global form perception had comparable nonlinearity, as well as whether sensitivity was skewed towards a specific axis of pattern alignment such as translational, radial, or concentric. Investigators in the study examined the observers’ sensitivity to global form by altering the dipole orientation coherence of Glass patterns by using a mix of psychophysics and steady-state VEPs.
The psychophysical results demonstrated a Glass pattern “dipper effect” that was the highest for concentric patterns. In contrast, the study’s EEG results showed nonlinear transduction but not as expected by psychophysical thresholds. Therefore, the findings implied that, like low-level contrast, mid-level form discrimination was influenced by some modest facilitation (compared to detection) at low global form coherence. It was more likely to be mediated by neurons with nonlinear transducer functions.