Why do the two identical twins in this room appear to be drastically different in size?
What's Going On?
The distorted room seen above is named after the American ophthalmologist Adelbert Ames, Jr., who first constructed such a room in 1946. He based his design on a concept originally conceived by Hermann Helmholtz in the late 19th century.
There are two illusions associated with the Ames Room. First the room appears cubic when viewed monocularly from a special viewing point (the true shape of the room is trapezoidal). Secondly, within an Ames Room people or objects can appear to grow or shrink when moving from one corner to the other.
The Perception of a Cubic Room
When you look (through a peephole -- to remove any cues from stereopsis) into an Ames Room, the room looks normal and cubic, but its true shape is cleverly distorted. The floor, ceiling, some walls, and the far windows are actually trapezoidal surfaces. Although the floor appears level, it is actually at an incline (the far left corner is much lower than the near right corner). The walls appear perpendicular to the floor, although they are actually slanted outwards.
This diagram shows how the Ames Room forms an identical image of a normal cubic room on your retina. If a straight line (representing a ray of light) is drawn from one corner of an imaginary cubic room to your eye, the corner can meet this ray at any point along its length and still appear cubic.
Since the two visible corners of the room subtend the same visual angle to the eye through the peephole, the two corners appear to be the same size and distance away. The left corner, however, is actually twice as far away as the right corner. When the view sees the room from another angle the true shape of the room is revealed.
The retinal image produced by the distorted room is identical with (and therefore indistinguishable from) that of a normal cubic room. In fact, there are an infinite number of possibilities that will give rise to this same retinal image. How does your visual system discard this infinity of possible Ames Rooms and settle on one single interpretation?
This is one of the central problems of perception.
The generally accepted explanations states that given this ambiguity of perspective, your visual system relies partly on past experience with normal cubic rooms to judge the shape of the room. This explanation is an oft repeated argument in favor of top down processing, i.e., that your visual system resolves ambiguity based upon knowledge of the external world.
Al Seckel and Alice Klarke, however, make the point that the Ames Room consists of equal visual angles and equal lengths, rather than unequal angles and unequal lengths. There are no strong visual cues to the contrary. In addition, they argue that the room's positioning of edges and corners relative to the horizon is crucial to seeing it as cubic. It has nothing to do with your past experience with cubic environments.
Seckel and Klarke's explanation is consistent with studies that have shown that people in cultures who have not been brought up in "carpentered" environments (rectangular, cubic, houses, angles, etc.) have no difficulty in perceiving the cubicity of the room.
"Oddness of the Room"
When you look into a full-sized Ames Room, such as the one in the Exploratorium (a fabulous science museum in San Francisco), you get the strange feeling that there is something is odd about the room, but it is difficult to articulate what it is. English visual psychologist Richard Gregory, who has written extensively about the Ames Room, felt that this "oddness" was perhaps due to "irregularities" in its construction.
Seckel and Klarke, however, maintain that this "oddness" come from trying to accommodate to the two far corners. Your visual system assumes that the two corners are equally far away, but they do have two different focal lengths. Accommodation, however, is not a strong enough cue to break the stronger constraints that give rise to a cubic perception. Seckel and Klarke fitted the peephole with a pinhole, which removed any cues from accommodation (normally thought to work over only small distances). Observing through a peephole, any signs of oddness disappeared.
The Element of Surprise and the Generic View Point
There is an element of surprise when you move away from the peephole and the true shape of the room is revealed. This suggests that in addition to one's perception of the cubic nature of the room, a mental expectation of the room's shape is also formed separate from one's perception of it. This expectation assumes that you are looking at the room from a non-accidental point of view -- The Generic Viewpoint. One is surprised when one moves to a different point of view and one's perception does not coincide with one's expectation.
Seckel and Klarke maintain that there is a split between one's perception of something and one's expectation of it. This lies at the heart of many illusions, which means that many illusions are conceptual in nature.
The Size Illusion
When you look through a peephole into an Ames Room, a person seen standing in the left corner will always appear substantially smaller than when seen standing in the right corner. The person looks too small because the image is smaller than what would be expected for the apparent distance of that part of the room. The illusion of normality is so convincing that as the person is seen walking about the room, he or she appears to be growing and shrinking rather than approaching and receding.
The generally accepted explanation for the Ames Room size illusion is that the apparently cubic perspective overrides your perception of size constancy. In other words, the size illusion is somehow caused by the strange shape of the room (A rather peculiar explanation, since one does not normally have such effects when walking into oddly designed rooms).
Seckel and Klarke have shown, however, that the room contributes little (if only charm) to the illusion. By using scale models, they found that only an apparently horizontal path against a perspective background is necessary to produce the size illusion. The figure's relative height (or elevation to the apparent horizon is what's important). The room (walls and ceiling) can be omitted.
While the additional apparent perspective lines are among several factors enhancing the illusion, their main affect is to hide correct perspective cues, not to create the illusion. In addition, the perspective cue of relative head height is not significant, compared to ground level.
In the Ames Room, the person appears to travel along an apparently horizontal and level surface. There is no observed rise in relation to the horizon as the figure recedes.
This is important, because a figure receding on a level surface will rise in the visual field, given the fixed relative position of the viewer and the figure.
On a level surface as someone recedes into the distance the level of their feet rise and the level of their head lowers in relation to a true horizontal.
This illustration shows how the figure in the background appears perfectly normal when compared to the foreground figure (even though its visual angle is much smaller).
Notice what happens when the background figure is moved to the same elevation as the foreground figure. You get the Ames Room size illusion without the room!
This photograph by English visual psychologist Richard Gregory shows two people, placed at differing distances, so that their images differ in size just as in the Ames Room. Most people looking at this photograph stated that the nearer person looks a little nearer, but also a lot larger. The two subjects maintained a constant horizontal foot level independent of thier distance from the viewer (this was accomplished by having the camera placed at ground level). The effect still occured, but it is ambiguous due to the lack of a perspective background.
In a normal situation, the smaller figure should be on a higher level in the visual plane than the larger and closer figure. A perspective background would also be present.
In this illustration by Stanford psychologist Roger Shepard, two figures of equal size are placed against a perspective background. The top figure is perceived to be larger than the figure on the bottom. If your visual system is using the relation to the horizon as a cue for judging size/distance relationships and then constructing a three-dimensional mental representation, then a truly equal sized figure would be smaller (or have a smaller visual angle) if it is closer to the horizon.
Since the top figure is the same size, your visual system assumes that it is larger than the figure below.
Gregory, R. L. (1987) "Analogue Transactions with Adelbert Ames," Perception, 16, 277-282
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