Constancy of size

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As size constancy refers to the observation that objects of vision perceived despite different distance in an approximately constant magnitude. One of the most important achievements of visual perception is to be able to estimate the real size of the objects seen. This requires a specific performance of the brain , which generates a largely constant perceived variable from the retinal image of variable size. Objects are imaged on the retina in a size that changes with distance, their apparent size or also angular size .

A number of known neurophysiological mechanisms generate a correction of the distance from the variable retinal image in the perception. Different criteria are used for this, e.g. B. stereoscopic vision , but above all comparison objects of known size as well as the perspective frame and other optical depth signals. In the optical cortex of the cat's brain, it has even been shown that the “receptive fields” of individual neurons change in size with attention to an object ( David H. Hubel and Torsten N. Wiesel ).

The constancy of size can be demonstrated by images for so-called optical illusions . Examples of deceptive perceptions are the moon illusion , the Ames space and a series of figures of actually constant size in a line of flight representation. Here, the mechanism of constancy of size leads to the opposite: objects of the same size are perceived as being of different sizes by a higher-ranking reference system.

From a geometrical point of view the following relationship arises:

(1) g = e * tan (w) ;

where g is the real object size, e is the distance and w is the angle at which the object appears. The object size can be calculated with this formula; the same, constant value always results for an object regardless of the distance. A realistic perception must correspond to these conditions. The size of the retinal image o 'of the observed object is largely proportional to the tan (w) from (1) and corresponds directly to the number of retinal cells covered by the image. The distance ef would have to be derived in a laborious manner from the muscular work involved in the fixation of the object or by measuring the differences in the image position on the retinas of both eyes caused by equatorial factors and their calculation. The use of the subjunctive is intended to make it clear that the perception of size could arise in this way - but there are strong indications that distances are not measured at all. Nevertheless, the perceived object size g obeys the following relationships:

(2) g = ef * o '* K

K is a proportionality factor . (2) is an equation with a numerical result, while perception shows us the objects relatively, in the correct proportions. One therefore also writes:

(3) gr ~ ef * o ' ,

gr is the relative size here.

(3) corresponds to the so-called Emmert's law . It accurately describes the relative perception of size and was derived from experiments with afterimages . It was found that afterimages, which do not change their size on the retina while they are visible, were nevertheless perceived as different sizes, the larger the further away the background was at which one was looking. The afterimages maintain their size and position on the retina regardless of the movements of the eyes. This not only means that they always follow the eye movements, but also that they can be seen at the same distance as the currently fixed object. If you look at a room wall 6 m away, the afterimage is also apparently there; when you look out of the window it is e.g. B. with the tree in 100 m or with a mountain range in 4 km. Its relative size appears exactly like that of a real object of the same retinal image size at the same distance. From these considerations it follows:

An object is perceived as having a constant size if its apparent size is inversely related to the respective distance. In contrast, an object of constant apparent size is perceived as having a relative size that increases proportionally to the distance . There is one essential requirement: the presence of other objects. If this is missing, the relative perception of size does not apply; we only see the object in its apparent size, which is variable with distance. If there are no or misleading removal instructions, the perceived sizes may be incorrect. For example, the (coincidentally) identical apparent sizes of the sun and moon are considered to be the same relative sizes: here the illusion arises from a perception rule that deduces the equidistance between two objects from the unrecognizable differences in distance between them .

Individual evidence

  1. ^ E. Bruce Goldstein: Perceptual Psychology. The basic course (Chapter 10) . 9th edition. Springer, Berlin 2015, ISBN 978-3-642-55073-7 .