Your eye sharpens edges before your brain even sees them. Each cell suppresses its neighbours, so at the border between light and dark the bright side looks brighter and the dark side darker — ghostly stripes (Mach bands) that aren’t in the light at all, only in the seeing. Slide the inhibition and watch the edge pop.
Each receptor’s output is its own input minus a fraction k of its neighbours’ (a center-surround, on-center off-surround field). On a flat region the neighbours cancel and nothing changes; but at a step edge the cell just on the DARK side loses more (it borders bright) and dips below the plateau, while the cell on the BRIGHT side borders dark and overshoots — the Mach bands. A fail-loud self-check throws unless, at a light/dark step, the output shows a local dark undershoot on the dark side and a bright overshoot on the light side — contrast the retina manufactures.
A 1-D difference-of-neighbours model; real retinae use graded difference-of-Gaussians receptive fields with light adaptation (Hartline’s Limulus work, 1967 Nobel). The edge over/undershoot is exact for this kernel; Mach bands are a real perceptual illusion, shown here as the signal that causes them.