Relative Size vs. Familiar Size in the Perception of Represented Depth

Abstract

The monocular cue of distance-familiar size-has recently received considerable attention due largely to concerted attempts to 'explain' the perception of depth-and in general, all perception of the world around us-in terms of learning and past experience.1 This empiristic enterprise, which is by no means new to the history of perception, is certainly one legitimate approach to the problem but the evidence in favor of it is still far from conclusive. Regardless of whether a particular 'cue' is ultimately a product of past experience, we can learn much more about its operation and its limitations, if we attempt to relate its present effects to its stimulus-characteristics, than if we dismiss the measurable characteristics of the stimulus and simply ascribe its power to the life-history of the observer.2 Since familiar size is the only cue of depth that requires, by its very definition, reference to past experience, it seems important to determine its applicability and limitations. This is particularly true since, as has been suggested elsewhere, another monocular distance cue has frequently been confounded with familiar size; namely, that of 'relative size,' a cue which need not invoke past experience.3 Familiar size requires a complex array of intervening variables for its explanation. Probably the simplest version would be that a given retinal image will, by its shape and size, evoke a trace or a memory of a specific object at a specific distance, and this trace then determines the distance at which the object is perceived. One retinal image is all the stimulus that is required for the operation of this cue. Relative size, on the other hand, requires two similar or identical shapes of different size; when that occurs, the larger tends to appear nearer than the smaller. Here a pair of figures is required, and the pair must be similar or identical. This explanation does not refer to past experience; it simply states that images of the same shape but of different size are stimuli for a depth relationship. Very many of the experiments purporting to demonstrate the operation of familiar size in 'immediate' perception (as opposed to judgment, reflection, or whatever) are open to reinterpretation in terms of relative size.4 A playing card casting a large retinal image is seen as being nearer than a playing card casting a small one, a situation which has been repeatedly interpreted as evidence for familiar size and, therefore, for past experience as a determinant of immediate perception of depth.