The perceptions of size and distance are intimately linked. For example, a change in the apparent distance of an object almost invariably leads to a corresponding change in its apparent size. These changes need not be optical, but often reflect an internal change of state, for they may occur with invariant retinal images such as after-images. Two kinds of neuronally-induced size-distance transformations may be distinguished: “voluntary” transformations correlated with changes in convergence of the eyes (i.e. micropsia), and “involuntary” transformations based upon changes in the relative disparities of objects in the visual field. In the voluntary case, the convergence of the eyes leads to a reduction in the apparent size of a retinal image, as if the oculomotor command “to converge” generates a correlated efferent signal that modifies the incoming afferent information about object size (von Holst and Mittelstaedt, 1950). This centrifugal effect occurs early in the visual pathway, before binocular interaction and probably at the level of the lateral geniculate (Richards, 1970). As will become obvious in the forthcoming discussion, the act of converging affects both size and distance perception, and complicates the prediction of size transformations.
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