The Ames Room Illusion: How Cultural Norms Influence How The Brain Sees

While it is true that certain optical illusions result from imperfections in the biology of our visual system (see Biological Cause of Optical Illusions), some types of illusions are implicit and unavoidable consequences of the nature of visual information - it is impossible to even construct a machine capable of perfect resistance to optical illusions. To understand why, we have to first appreciate that visual information is infinitely ambiguous, and that as a result, we require an interpreter of the information - the brain - to assist in telling us what we are seeing. The problem is, the tricks that the brain uses to help resolve ambiguities can sometimes backfire.

The issue of infinite ambiguity arises because a 3-dimensional world gets projected as a 2-dimensional image on each eye. The simplest example of this consequence is that a circle 6 feet in diameter located 10 feet away projects the same exact image on our eye as a circle 3 feet in diameter located 5 feet away - both circles look precisely the same size to us. Any other adjustment to the dimensions and distance of the circle that keeps its "proximal size" constant will likewise make it appear to be the same size, and it is easy to see that these adjustments can be infinite. Thus, our brain must be creative and use other information, such as depth cues, to make what amounts to an educated guess about the real characteristics of the visual information. When it guesses wrong, an optical illusion occurs.

In addition to depth cues, the brain's arsenal for deciphering visual information includes probabilities from prior experience. What is meant by this will become clear in the following example. The Ames Room is a famous optical illusion that takes advantage of both visual ambiguity and the brain's propensity to rely on prior experience in the face of it. The illusion consists of an irregularly-shaped room that from a certain vantage point projects the retinal image of a rectangular room. In reality, one back corner of the room recedes further away from the observer than the other back corner, but the observer cannot make out this discrepancy because from his position he sees the room as a simple rectangle.

The craziness ensues when the observer views two people enter and move about the room. Referring to the above diagram, let's say person A stands at the back left corner of the room, while person B stands at the back right corner. In reality, person A is further away from the observer than person B, and so person A looks smaller to the observer than person B. To us, this much makes sense. But to the observer, it makes absolutely no sense because the room appears perfectly rectangular to him and thus it appears that person A and person B are the same distance from him! In other words, from the observer's perspective, person A has inexplicably shrunk. Below is an example of what a stunned observer would see:

Even though we know that the image we are viewing, which can make a child appear to dwarf an adult, seems absurd, apparently the brain's familiarity with rectangular rooms is the overriding assumption that is made in the interpretation of the image. In other words, the force that compels our brain to choose the visual interpretation that makes the room seem rectangular is stronger than the force to choose a visual interpretation that corrects the size of the people - perhaps because we are used to seeing people of different sizes, but not used to non-rectangular rooms.

Astonishingly, this last idea is affirmed by the fact that people who live in a culture of circular rooms do not fall victim to the Ames Room illusion. The Zulus, an ethnic group of South Africa, lack an environment that contains features like perfect parallel lines or rectangular corners. Thus when they are presented with the Ames Room, their brains do not draw on a history of experience that says almost all rooms are rectangular. They correctly identify the true shape of the room, observing the people in it at the correct sizes and distances.

This case study demonstrates that our perception of reality is not purely objective, but is influenced by our prior experiences in a deep way that is out of our control. Even if you revisit an Ames Room a second time knowing that you will encounter the illusion, this awareness still cannot prevent your brain from falling victim to it - your accumulation of prior experiences is too strong of an influence. And if our perception of our visual present is at least partially constructed from our visual past, and if everyone has a unique visual past, it seems a valid conclusion that two people looking at the same thing may not see "eye to eye", just as an American and a Zulu looking at the same Ames Room see two completely different things.