Constancy Scaling Research Articles

Haptic illusions and inappropriate constancy scaling.

THE widespread use of the term “optical” illusion has implied both that spatial illusions are restricted to vision and that the explanation of illusions is to be sought in terms of the operating characteristics of the visual system. Equivalent visual and haptic illusions are, however, obtained with many of the well known illusion figures1. During haptic inspection a blindfolded subject is required to trace his finger over the raised contours of a figure. Some variables known to affect the magnitude of a visual illusion similarly influence the haptic illusion found with the same figure. Varying the angle of the arrowheads of the Muller–Lyer figure modifies visual and haptic judgments in the same way2. The magnitude of the illusion diminishes over practice trials at the same rate for the two modalities, and crossmodal transfer of the practice decrement is found3. These data suggest that it is unlikely that quite different processes coincidentally produce equivalent spatial illusions in the two modalities. Haptic illusions are found with congenitally blind subjects1,4; thus it cannot be argued that haptic judgments are controlled by a framework developed through visual learning.

THE SIZE‐CONSTANCY THEORY OF VISUAL ILLUSIONS

This article criticizes the view that visual illusions, conceived as flat projections of typical views of objects lying in three‐dimensional space, result from inappropriate constancy scaling. It points out that visual illusions are multiply determined and that the theory in some cases leads to contradictions. It suggests that perspective interpretations of visual illusions have overemphasized the influence of geometrical environments. Finally it criticizes attempts to distinguish between two types of scaling responsible for constancy.

Susceptibility to the Müller-Lyer Illusion and its Relationship to Differences in Size Constancy

The hypothesis that susceptibility to the Müller-Lyer illusion is the result of normal constancy scaling, misapplied, was submitted to direct test. No significant correlations between illusion error and size constancy estimates were obtained. Also invalidated were hypotheses that under-constancy is correlated with non-susceptibility to the illusion, and that over-constancy is correlated with greater illusion error. The results suggest that an approach to the explanation of illusion effects by means of analysing individual differences in size constancy, in intelligence and preferred “perceptual style,” might be fruitful. Some tentative suggestions are made concerning the role of perceptual inference, abstraction and analysing.

Constancy and the Geometric Illusions

GREGORY has proposed1 and defended2 an account of the geometric illusions based on the notion of ‘misplaced constancy scaling’. Following Tausch, he suggests that all illusion figures have features indicating depth by perspective which bring into play size constancy scaling, leading to expansion of some parts of the figure relative to others. For this theory he makes the claim that “so far no valid objections seem to have been raised”2. The theory nevertheless needs challenging.

Constancy and the geometric illusions.

PERHAPS it is difficult to demonstrate primary constancy scaling independently of the distortion illusions, but its effects are not limited to plane figures. For example, the apparent size of the Moon is not a simple function of its apparent distance—when low on the horizon it looks both large and near, which contravenes the classical Emmett's Law, and demands some kind of size scaling which is not locked to apparent distance. The worry about lack of perception of depth in luminous plane figures having exaggerated perspective but which give illusory distortions was in fact expressed by myself, in the (unpublished) paper cited by Humphrey and Morgan. If illusions do occur in figures not seen in depth though the countermanding depth-cues are removed, it would have to be supposed that primary constancy can work like a ‘super releaser’ (as found in ethology), working beyond the normal range at which it is useful. This is quite possible, but if true it would make precise predictions difficult. On present evidence, however, it does not occur: Measuring the apparent depth in luminous Muller–Lyer figures of various fin angles, with the technique briefly described1, I found that the apparent depth of the illusion figures correlates extremely closely with the measured illusions for each angle tested, over the full range of possible angles of the fins. The distortion and the apparent depth of the figures are both reduced for extreme angles, when normal perspective breaks down. To argue that the relation discovered between apparent depth and the illusions is fortuitous is to fly in the face of a correlation coefficient better than 0.9. There are other and quite different predictions which follow from the theory, and it is compatible with the curious finding that peoples who live in environments largely free of right angular corners and parallel lines, such as the Zulus who live in a ‘circular culture’ of round huts, do not suffer these illusions to anything like the normal extent2,3.

ILLUSORY PERCEPTION AS A CONSTANCY PHENOMENON.

GREGORY1 offers a general account of visual illusions, relating the apparent distortions in them to the perceptual process of size constancy. He observes that Tausch is the only previous writer to have considered constancy in relation to illusions. However, Gibson2 has incorporated illusory perceptions within the context of size constancy, and suggests that perception of size is a by-product of a constant scale, which Gregory calls ‘constancy scaling’, at different distances. Arguing that illusory figures are “flat projections of typical views of objects lying in three-dimensional space”, Gregory notes that “the parts of the figure corresponding to distant objects are expanded and the parts corresponding to nearer objects reduced”. He states the same principle more simply when he writes: “Those parts of the figures which would normally be further away in 3-D space appear too large in the illusion figures”3. These principles are applied to a series of illusions, including those of Müller-Lyer, Ponzo and Hering. Gregory therefore postulates a common process modifying retinal images in constancy scaling and in the perception of illusory figures.

DISTORTION OF VISUAL SPACE AS INAPPROPRIATE CONSTANCY SCALING.

DISTORTION OF VISUAL SPACE AS INAPPROPRIATE CONSTANCY SCALING.