Response characteristics of the pigeon's pretectal neurons to
illusory contours and motion. ABSTRACT Misinterpretations of
visual information received by the retina are called visual
illusions, which are known to occur in higher brain areas.
However, whether they would be also processed in lower brain
structures remains unknown, and how to explain the neuronal
mechanisms underlying the motion after-effect is intensely
debated. We show by extracellular recording that all motion-
sensitive neurons in the pigeon's pretectum respond similarly to
real and illusory contours, and their preferred directions are
identical for both contours in unidirectional cells, whereas
these directions are changed by 90 deg for real versus illusory
contours in bidirectional cells. On the other hand, some
pretectal neurons produce inhibitory (excitatory) after-
responses to cessation of prolonged motion in the preferred
(null) directions, whose time course is similar to that of the
motion after-effect reported by humans. Because excit