Contextual effects in the parallel lines illusion: some implications for assimilation theory.

32 undergraduates participated in each of two experiments designed to determine how structural manipulations of the parallel lines figure affect the resulting illusion. Collectively and for the range of figures studied, the data show progressive elongations of the contextual lines of the illusion figure first increase, then decrease, the apparent length of the judged line; the misestimation of the length of the judged line diminishes as the separation between it and shorter contextual lines increases; and variations in contour lightness significantly affect the illusion when the contextual lines are shorter than and at any distance from the judged line or when the contextual lines are slightly longer than and at an intermediate distance from the judged line. These data indicate that size-assimilation, as well as size-contrast processes, contributes to the parallel lines illusion and that the assimilation process must be mediated by both sensory and cognitive activity in the visual system.