The Trapezoidal Window Illusion

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Adelbert Ames designed a number of demonstrations in visual perception showing that our perceptions do not always give us correct information about the physical stimulus. While such demonstrations create visual illusions, their purpose is to show how the human visual system copes with the difficult problems of obtaining information about the three dimensional world from the two dimensional image in the eye. The distorted room, which is often shown in text books and may be seen in some science museums, is one of his more famous efforts. This demonstration, the trapezoidal window illusion, shares some of the features of the distorted room.

Click here to see a video of  the trapizoidal window illusion. Flash version is here.

The window you see here is not rectangular, but trapezoidal. Because of your experience with rectangular windows and the specific shape of the trapezoidal window, you see this window oscillating from right to left and back again, even though it is actually rotating in a complete circle. The larger end of the window creates a larger retinal image, even when that larger end is further away. In the normal world, the end of a rectangular window that created a larger image would be closer to you. That is why you tend see that larger end of this oddly shaped window as being closer to you, even when it is farther away.

When we add another object to the display, you may witness some peculiar things.

Click here to see a video of the window with a tube rotating with it. Flash version is here.

The tube suspended from the top of the window has a normal cylindrical shape. As it rotates, the end that is closer to you always creates a larger image than the end that is farther away. You see the tube going around, but the window still looks like it is going back and forth. When the larger end of the window is actually farther away, people tend to perceive the tube as either cutting through the window, or bending around it

Click here to see a video of the window and the tube rotating the other way. Flash version is here.

Seeing different objects attached to the window may create different experiences.   In the next two demonstrations show a plastic bag placed on top of the windows.

Click here to see a video of the window with a plastic bag rotating with it. Flash version is here.

Click here to see a video of the window and plastic bag rotating the other way. Flash version is here.

In the real world, if two similar objects make different sized images, usually the one making the larger image is closer to the observer. The ability of the visual system to automatically take into account both size and distance usually allows us to see the world accurately. This is an example of one of the perceptual constancies. That is, we see objects in the world as maintaining a constant size and shape, in spite of the changes in the size and shape of our retinal image of the object as we move about the world.