Size Constancy in Visual Perception

Size constancy is the tendency of objects to keep the same apparent size even as they approach us or move farther away. Why do we not think a car is shrinking when it drives away from us, even though the retinal image is shrinking? We know it is going further away, and we know this on a basic level that does not require us to pause for thought. We make this calculation automatically. That is why Helmholtz used the term "unconscious inference." The visual system unconsciously infers the size of an object from cues about its distance.

What is size constancy? How can you make your own hand look suddenly smaller?

You can experience a momentary collapse of size constancy with a simple demonstration. Hold one of your hands at arm's length, palm toward your face. Hold the other hand closer to you. Close one eye (to eliminate the depth cue of binocular disparity). Now pretend a small child has just thrust a hand up next to yours, saying, "Look! My hand is just like yours, only smaller!" Suddenly your hands will not look the same size. The more distant one will look smaller. Why? Because you have reinterpreted the scene. By using imagination you have given your visual system a reason to interpret the smaller retinal image as a small hand rather than a distant hand.

The occasional failure of size constancy is a source of wonder to children. My youngest son, when he was six, exclaimed, "Hey! Buildings aren't even as big as your finger!" He was holding his finger up in front of his eye, and it dwarfed the size of distant buildings.

What is Emmert's Law and how does it explain size constancy? no item

Psychologists explain size constancy with Emmert's Law: known distance determines apparent size. Normally you know how far away things are, thanks to depth cues. Therefore you can compensate for the size of the image entering your eye, producing size constancy.

How do moviemakers make you think a small model is actually a huge object?

Moviemakers use a lack of distance cues to make you think a small object is large. In effect, they reverse Emmert's Law. If you don't know how far away a model is, you cannot determine its actual size. A moviemaker surrounds a model of the Titanic or a space ship with a featureless background (ocean or sky) so you have no definite information about the distance of the object. Then you can assume the small model is something huge. In the various Star Trek series, for example, starships and space stations are usually models a foot or two across. They are surrounded by a sky full of stars (actually a curtain on the television set) that contains no depth cues. Therefore the small models are readily interpreted as large objects.

Now it is time to leave visual perception and review the other senses. We will return to the topic of how humans interpret the visual world in Chapter 7 (Cognition), in the section on visual scene analysis.


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