Midbrain as a Site for the Motion After-effect

Abstract

FOLLOWING inspection of a rotating spiral, there is an illusion of counter-rotation when the spiral is stationary. This motion after-effect was studied as early as 1849 by Plateau¹, and is believed to be caused by the fatiguing or reduction in the maintained discharge of one set of directionally sensitive units in the visual system^2–5. Locating the part of the human brain where these adapted “movement detectors” reside, however, has been difficult. Two facts suggest that the major component of the motion after-effect is not in the retina: first, the after-effect transfers from one eye to another^6,7; and second, inter-ocular transfer persists even when the previously adapted eye is pressure-blinded^8,9. This evidence has led most workers to conclude that the motion after-effect is a cortical phenomenon. But, considering the wealth of recent electro-physiological data revealing directionally sensitive units in the optic tectum (superior colliculus) of many different mammals^10–14, the midbrain appears to be an equally plausible site.