Illusions of filled lateral and angular extent. The role of cortical feature detectors.

The common principle of the illusions found by Helmholtz and Botti is the overestimation of a filled extent. They differ in that Helmholtz's illusions of filled angular extent can be accounted for in terms of spatial two-line interactions, whereas Botti's illusions of filled lateral extent cannot. The former observation is consistent with the assumption of interacting orientation-detectors in the visual cortex. The Botti illusion probably depends on global computation in the visual system.