another image of the same face or object--called the test image--was flashed on the screen. But this time, the image was either head-on or turned very slightly (three or six degrees) to one side or the other. Whatever the orientation of the test image, subjects were required to choose whether it was turned to the right or the left.

When subjects were presented with an adapting image turned 30 degrees to one side, then tested with an image of the same thing in head-on view, they tended to say the test image was tilted in the opposite direction of the adapting image. That is, if they first saw the face of a man turned 30 degrees to the left, then saw his face head-on, they said the face was turned to the right. This "adaptation effect" occurred 80 percent of the time; normally, responses for both directions would be equally likely. Even if the test image was turned three degrees in the same direction as the adapting image, the subjects guessed wrong half the time, saying the test image was turned in the other direction.

The reason for the errors is that when a person stares at an image, neurons that respond to the viewing angle of the image get "tired" and become less responsive when a very similar image is presented again, He said. The brain interprets this lack of response as the object not being turned in the direction the neurons are attuned to. This suggests that there are separate populations of neurons, each responding to a particular narrow range of orientations. The neurons are likely located in the lateral occipital cortex, an area of the cerebral cortex very far back on either side of the head.

The researchers also performed experiments that suggested that for faces, at least, subjects were not deciding the orientation of test images based on "local" features such as noses. When subjects saw unorganized fragments of faces--as if parts of the face were simply erased--as adapting images, no adaptation effect occurred.

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2-Mar-2005

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