Length illusions and attentional deployment

Abstract

Length illusions are caused by the systematic manipulation of size ratios. The principle of relational determination (i.e., framing ratio) holds for different classical illusional displays and is described as a failure of selective attention that facilitates size constancy. Implementing a spatial distance (D) between the focal stimulus (of length F) and the contour marks (of length B) leads to illusions that contradict the description in terms of a simple framing ratio. The stimulus configuration rather than the framing ratio determines the illusions (two different ratios are independently effective, D/F and B/F). The relational determination of length illusions contradicts explanations in terms of either efferent readiness or attentional deployment. In a developmental approach, it was shown that a failure of selective attention to the length of the focal stimulus due to relational determination holds only for adults (aged 21–32 years). The B/F ratio tended to effectiveness in determining the illusions for children aged 7–9 years, but not for children aged 5–6 years. The absolute size of D, regardless of F, determined the illusional trends of both groups of children. The failure of selective attention, which facilitates size constancy, in judging length illusions seems to develop first for contour size and later for spatial distance.