The tilt illusion (TI) is one of the basic orientation-based visual illusions. A TI occurs when viewing a test line against an inducing grating of uniformly oriented lines with an angle difference of &phis; between the orientations of test line and background grating. Two aspects of the TI have been described: the direct TI, where the perceived &phis; difference in orientations seems to be increased, that is, the test line appears to be repelled by the grating; and the indirect TI, where the test line appears to be attracted to the orientation of the grating, i.e. the perceived &phis; is smaller than actuality. So far only the orientation aspects of the TI have been explained through mathematical models of cortical network. The present work reviews some of the biological structures in question and investigates the tilt illusion further; showing how its spatial frequency aspects can be derived from a recently developed model for the structure of the visual cortex.
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