The phenomena of geometrical illusions of extent suggest that the metr
ic of a perceived held is different from the metric of a physical stim
ulus. The present study investigated the Muller-Lyer and Oppel-Kundt i
llusions as functions of spatial parameters of the figures, and constr
ucted a neurophysiological model. The main idea of the modelling is ba
sed on the uncertainty principle, according to which distortions of si
ze relations of certain parts of the stimulus, socalled geometrical il
lusions, are determined by processes of spatial filtering in the visua
l system. Qualitative and quantitative agreement was obtained between
psychophysical measurement of the strength value of the illusions and
the predictions of our model.