Second-order illusions: Mach bands, Chevreul, and Craik--O'Brien--Cornsweet

Area of Science:

• Visual perception
• Computational neuroscience
• Image processing

Background:

• Mach bands are perceptual phenomena typically observed at luminance gradients.
• Previous research focused on first-order luminance processing for these illusions.
• The role of second-order stimulus properties in visual illusions was less understood.

Purpose of the Study:

• To investigate whether Mach bands occur in stimuli modulated by contrast rather than luminance.
• To determine the rectification mechanism (fullwave vs. halfwave) underlying second-order visual illusions.
• To compare the magnitude of contrast-based Mach bands with luminance-based ones.

Main Methods:

• Utilized dynamic, fullwave-rectified random textures (pixels, 'Mexican hats') with ramp contrast modulation and constant expected luminance.
• Presented stimuli dynamically (new texture every 67 msec) to enhance illusion magnitude.
• Compared perceived contrast bands in fullwave stimuli with brightness bands in conventional luminance Mach-band stimuli.

Main Results:

• Perceptual Mach bands were observed in fullwave contrast-modulated stimuli, appearing as changes in apparent texture contrast without brightness changes.
• The magnitude of perceived contrast bands in fullwave stimuli was comparable to brightness bands in luminance Mach-band stimuli.
• Similar magnitudes were found for Chevreul and Craik-O'Brien-Cornsweet illusions in both luminance and fullwave contrast conditions.
• These illusions were not perceived in halfwave-modulated textures.

Conclusions:

• Second-order visual illusions, including Mach bands, arise from fullwave rectification processes.
• The spatial interactions involved in second-order texture illusions are analogous to those in first-order luminance processing.
• These findings suggest a unified processing mechanism for both luminance and contrast-based visual illusions.