Donatella Spinelli, Gabriella Antonucci, Pierluigi Zoccolotti

L'illusione del rod-and-frame

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The rod-and-frame illusion has been studied in psychology since the 1950s. Initial investigations developed the topic of individual differences in performance on the rod-and-frame, producing the concept of field dependence; the psychological correlates of this dimension have also been studied in areas other than perception. Interest then shifted to the problem of the mechanisms underlying the illusion. In the 1970s and 1980s two mechanisms were singled out: a visuo-vestibular one, integrating retinal and gravitational information; and a purely visual one, based on the local interaction of oriented contours, analogous to the one postulated for other orientation illusions. At first studies on the mechanisms were guided by a dichotomous model based on the size of the figure: large inducing figures would produce the activation of visuo-vestibular mechanisms and small ones would produce the activation of only local visual mechanisms. However, this model was surpassed by the demonstration that visuo-vestibular mechanisms can also enter into play with small inducing figures and that effects of a purely visual type are also present with large figures in several specific conditions. Finally, subsequent research specified the action of a third, visual type of mechanism, able to act long distance and linked to the analysis of the shape and extraction of the bilateral axes of symmetry of the figure. In this review a three-component model is illustrated for the rod-and-frame illusion, and various experiments are described which have allowed better defining of the conditions of action of the three mechanisms implied in the illusion. It is shown that they contribute more or less relevantly depending on the specific presentation conditions of the stimuli. One important element is the size of the inducing figure, but other elements (such as the distance between inducing figure and test figure, the erect/supine position of the observer and the presence of additional references) are also relevant. After the mechanisms underlying the rod-and-frame illusion are singled out, the topic of individual differences is reproposed to clarify the mechanisms responsible for this variability. It is shown that visuo-vestibular mechanisms contribute greatly to differences between individuals, mechanisms of local interaction to a lesser degree and mechanisms of global analysis of the figure do not seem to have any effect on the production of individual differences.

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