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Sensation and Perception

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The Ames Room

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Visual illusions often exploit the fact that a person receives input from both eyes to see the world in three dimensions.

For example, if someone is talking to a friend through their open apartment door, they’ll discern this individual as standing a few steps in front of the hallway wall. This perception is due, in part, to the fact that the person’s brain receives and combines two images of the friend—one from the right eye, and another from the left.

However, if the door suddenly slams shut and the renter is forced to look at their friend with one eye through a hole—size and depth become more difficult to judge.

The Ames Room—a structure named after its ophthalmologist inventor Aldebert Ames—manipulates this binocular aspect of vision, and cleverly uses geometry to trick the visual system into identifying objects’ sizes as being different when in fact they are the same.

This video demonstrates how to design an Ames Room, documents the illusion it creates, and discusses how such deception is used in special effects for movies.

In this experiment, participants view a distorted exhibit with a single eye by peering through an aperture—the entire setup is the Ames room. The forced perspective ensures that the brain receives only one image of the scene, and no depth cues from the contralateral eye.

The geometrical trick here is that the display is constructed with sidewalls of irregular length, making the back wall diagonal rather than parallel to the front wall. Such construction does not produce the right angles that are expected in a room.

Instead, a situation is created where two equally sized figurines—placed in the back corners—vary in distance from the aperture. That is, the one on the left is farther away than the one on the right side.

Participants are asked to report what they see, and their responses are recorded as the dependent variable. It’s expected that they will report the character on the right as being much larger, even though they are the same size.

Before participants arrive, assemble a small-scaled version of the Ames Room, using scissors or a knife, tape, a ruler, four, one-foot wide cardboard pieces of varying lengths, and two, equally sized action figures.

First make the aperture: Trace around a penny in the center of one of the foot-long pieces. Cut around this outline and then push the part out to leave a small hole. Stand this segment up, and with tape, attach it to the other one-foot long portion of cardboard to form a 90° angle on what will be its right side.

Now fasten the two-foot piece on the left side, forming another 90° angle. Finally, connect the remaining one-and-a-half-foot section to close the polygon. Note that the backside should not form right angles.

With the display assembled, place two figures of the same height in the back corners—one at each vertex.

To begin the experiment, make sure that the back is covered to avoid revealing the exhibit’s skewed geometry and seat a participant in front. Explain that she is to look into the aperture and report which of the two characters is larger.

At this point, allow the participant to lean in. While she is peering inside, ask her which one is larger and note her response. [Participant says: Right is larger.]

In this demonstration, the participant was tricked into thinking that the characters differed in size. In this case, she reported the right figure as being larger than the left one.

Here, the underlying expectation was based on previous experiences—that walls should attach at right angles. Because the brain could not gather countervailing evidence—like that which could be provided by the other eye—it therefore applied the assumption. This resulted in the room appearing rectangular, with the far wall occupying the position shown.

The implication then was that the two figures were side-by-side. Now, what appeared on the retina—two differently sized projections—was interpreted by the brain as a size difference, rather than one caused by the objects standing at different distances from the viewer.

Now that you are familiar with the Ames Room illusion, let’s look at how its principles are applied in situations involving special effects and virtual-reality viewing.

Many movie scenes have been filmed inside an Ames Room set. Because the frames are captured through the apertures of cameras, directors can depict an actor as being super small and a robot as being much larger than it really is by simply positioning them appropriately around the room.

By helping us understand the cues needed by humans to perceive the visual world in 3D, the Ames Room has also helped researchers design better virtual reality tools that employ size illusions.

Such systems can actually help individuals visualize home remodeling and design projects. By using virtual reality devices, objects can be positioned so that viewers can perceive the actuality of appearance within a given space.

You’ve just watched JoVE’s video on the Ames Room. Now you should have a good understanding of how to design the exhibit and run the demonstration, as well as how to interpret responses and apply the principles behind the illusion.

Thanks for watching!

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