Receptors reduce response to sustained stimuli — Ca²⁺ feedback in photoreceptors, Na⁺ channel inactivation in mechanoreceptors. Change matters more than absolute level.
Adjacent neurons suppress each other, sharpening edges. Explains Mach bands — illusory brightness at luminance borders — and improves spatial resolution.
Perceived intensity ∝ log(stimulus). Just-noticeable difference is constant fraction of background. Fechner: sensation S = k·log(I/I₀).