Size constancy is the tendency for any given object to appear the same size whether its size in the retinal image is large or small. For example, if someone walks towards you, their retinal image increases progressively, but their size seems to remain the same.
Why do we show size constancy? Many factors are involved. However, an object’s apparent distance is especially important when judging its size. For example, an object may be judged to be large even though its retinal image is very small if it is a long way away. Thus, there are close connections between size and distance judgements. Many other factors influence the extent to which size constancy is observed. Below we briefly consider some of these factors before considering the relationship between size constancy and depth perception.
Findings
Haber and Levin (2001) argued that size perception of objects typically depends on memory of their familiar size rather than solely on perceptual information concerning their distance away. They found participants estimated the sizes of common objects with great accuracy using only memory. Then they presented observers with various objects at close (0–50 metres) or distant viewing range (50–100 metres) and asked them to make size judgements.
The objects belonged to three categories: (1) those almost invariant in size or height (e.g., tennis racquet, bicycle); (2) those varying in size (e.g., television set, Christmas tree); and (3) unfamiliar stimuli (e.g., ovals, triangles).
What findings would we expect? If familiar size is of major importance, size judgements should be better for objects of invariant size than those of variable size, with size judgements worst for unfamiliar objects. What if distance perception is all-important? Distances are estimated more accurately for nearby objects than more distant ones, so size judgements should be better for all categories of objects at close viewing range.
Haber and Levin’s (2001) findings indicated the importance of familiar size to accuracy of size judgements (see Figure 2.22). However, we cannot explain the fairly high accuracy of size judgements with unfamiliar objects in terms of familiar size. Note that the viewing distance had practically no effect on size judgements.
Witt et al. (2008) found good golfers perceived the hole to be larger than not-so-good golfers when putting. They also found golfers facing a short putt perceived the hole’s size to be larger than those facing a long putt. They concluded objects look larger when we can act effectively with respect to them, which could explain why the hole always looks remarkably small to the first author when playing golf! Note, however, that A.D. Cooper et al. (2012) obtained evidence suggesting these effects involve memory rather than perception.
Van der Hoort et al. (2011) found evidence for the body size effect, in which the size of a body mistakenly perceived to be one’s own influences the perceived sizes of objects in the environment. Participants equipped with head-mounted displays connected to CCTV cameras saw the environment from the perspective of a doll (see Figure 2.23). The doll was small or large.
KEY TERMS
Size constancy
Objects are perceived to have a given size regardless of the size of the retinal image.
Body size effect
An illusion in which misperception of one’s own bodily size causes the perceived size of objects to be misjudged.
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Size constancy
Van der Hoort et al. (2011) found other objects were perceived as larger and further away when the doll was small than when it was large. These effects were greater when participants misperceived the body as their own – this was achieved by touching the participant’s and the doll’s body at the same time. Thus, size and distance perception both depend in part on our lifelong experience of seeing everything from the perspective of our own body.
Figure 2.22
Accuracy of size judgements as a function of object type (unfamiliar; familiar variable size; familiar invariant size) and viewing distance (0–50 metres vs. 50–100 metres).
Based on data in Haber and Levin (2001).
We turn now to research considering the relationship between size estimation and perceived distance. If size judgements depend on perceived distance, size constancy should not be found when the perceived distance of an object differs considerably from its actual distance. The Ames room provides a good example (Ames, 1952; see Figure 2.24). It has a peculiar shape: the floor slopes and the rear wall is not at right angles to the adjoining walls. In spite of this, the Ames room creates the same retinal image as a normal rectangular room when viewed through a peephole. The fact that one end of the rear wall is much farther from the viewer is disguised by making it much higher.
What happens when observers look into the Ames room? The cues suggesting the rear wall is at right angles to the viewer are so strong observers mistakenly assume two adults standing in the corners by the rear wall are at the same distance from them. This leads them to estimate the size of the nearer adult as much greater than that of the adult who is farther away. (See the Ames room on YouTube: Ramachandran – Ames room illusion explained).
The illusion effect with the Ames room is so great that an individual walking backwards and forwards in front of the rear wall seems to grow and shrink as he/she moves! Thus, perceived distance seems to drive perceived size. However, observers are more likely to realise what is going on if the individual is someone they know very well. On one occasion, a researcher’s wife arrived at the laboratory to find him inside the Ames room. She immediately said, “Gee, honey, that room’s distorted!” (Ian Gordon, personal communication).
Similar (but more dramatic) findings were reported by Glennerster et al. (2006). Participants walked through a virtual reality room as it expanded or contracted considerably. Even though they had considerable information from motion parallax and motion to indicate the room’s size was changing, no participants noticed the changes! There were large errors in participants’ judgements of the sizes of objects at longer distances. The powerful expectation that the size of the room would not alter caused the perceived distance of the objects to be very inaccurate.
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Ramachandran explains the Ames room
Figure 2.23
This shows what participants in the doll experiment could see. From the viewpoint of a small doll, objects such as a hand look much larger than when seen from the viewpoint of a large doll. This exemplifies the body size effect.
From Van der Hoort et al. (2011). Public Library of Science. With kind permission from the author.
Figure 2.24
(a) A representation of the Ames room. (b) An actual Ames room showing the effect achieved with two adults.
Photo © Peter Endig/dpa/Corbis.
Nguyen et al. (2011) also showed the close relationship between size and distance perception. They used a virtual environment in which two poles were presented in a tunnel. There were initial training trials in which the size of the poles remained constant and observers judged their distance away. After that, the poles changed in size. When the poles became smaller, observers overestimated the distance away, whereas observers underestimated the distance away when the poles became larger. Thus, the familiar size of the poles established during initial training strongly influenced perceived distance.
Evaluation
Size perception and size constancy depend mainly on perceived distance. Some of the strongest evidence for this comes from studies in which misperceptions of distance (e.g., in the Ames room or in virtual environments) produce systematic distortions in perceived size. In addition, perceived distance can be strongly influenced by perceived or familiar size. Numerous other factors influence size perception and we have mentioned only a few.
What is lacking so far are comprehensive theories of size judgements. Little is known about the relative importance of the factors influencing size judgements or the circumstances in which any given factor is more or less influential. In addition, we do not know how the various factors combine to produce size judgements.
PERCEPTION WITHOUT AWARENESS
Can we perceive aspects of the visual world without any conscious awareness we are doing so? In other words, is there such a thing as unconscious perception or subliminal perception (perception occurring even though the stimulus is below the threshold of conscious awareness?). Common sense suggests the answer is “No”. However, there is strong evidence that the correct answer is “Yes”. However, we need to be careful in the terms we use. A thermostat responds appropriately to temperature changes and so could be said to exhibit unconscious perception!
Some of the most important evidence suggesting visual perception does not require conscious awareness comes from research on blindsight patients with damage to early visual cortex (V1). Blindsight “refers to the rare ability of V1-damaged patients to perform visual tasks … even though these patients claim not to consciously see the relevant stimuli” (Ko & Lau, 2012, p. 1401).
KEY TERMS
Subliminal perception
Perceptual processing occurring below the level of conscious awareness that can nevertheless influence behaviour.