Prism Adaptation

When people first look through prisms that displace the visual world left or right, they experience errors in their behavior. For example, they reach to the side of their coffee cup and may bump into a door frame when walking. With continued exposure to the prisms, however, these errors disappear. The person has adapted to the prisms. When the prisms are removed, the person may initially make similar errors, but now in the opposite direction. These aftereffects demonstrate that a change has occurred in the person's perceptual-motor system (i.e., the organization of senses and limbs), a change that persists until the person relearns the normal manner of interacting with the world. Once an amusing curiosity good for classroom demonstrations, prism adaptation is proving to be a valuable tool for investigating adaptive perceptual-motor performance and the rehabilitation of braindamaged patients. This entry describes some of the mechanisms that have been proposed to account for prism adaptation and a practical application of prism adaptation.

Research has shown that prism adaptation involves four kinds of adaptive processes. (1) The person may consciously guide the visible hand toward the coffee cup, in which case, aftereffects will not occur. (2) The person may unconsciously adjust head posture so that the head is turned in the direction of the displacement, in which case the coffee cup appears and is straight ahead, performance error is reduced, and head posture aftereffects will occur. (3) The person may learn to correct for the prisms in the same way that everyday reaching errors are corrected: Aftereffects of such perceptual-motor learning can occur, especially if the person does not recognize that the prisms have been removed. (4) The person may correct the underlying spatial mappings that are disrupted by the prisms. This last kind of adaptive change is the unique feature of prism adaptation.

Current research suggests that our perceptual-motor system is composed of multiple sensorimotor systems, each of which can operate independently of the others, but which can be coordinated to perform a more complex task. For example, the eye-hand perceptual-motor system includes the visual eye-in-head sensorimotor system, which is exercised autonomously when we sit quietly reading, and the proprioceptive hand-to-head sensorimotor system, which operates autonomously when we struggle to find the alarm clock in a dark room. Coordinating the eyes and hand to pick up the visible morning coffee cup poses a problem because visual and proprioceptive systems do not have the same spatial maps.

When we stand up, our eyes and limbs change position relative to the coffee cup, and we have to change how we look at and reach for the coffee cup on the table. Similarly, our eyes and limb have different origin positions: The eyes are located in the head and the limb is attached at the shoulder. The seen coffee cup is located relative to our head, whereas the cup in hand is located relative to our shoulder. This difference in cup location relative to the head and shoulder must be taken into account when we reach for the seen cup. The position of the seen cup must be adjusted for the distance between the head and shoulder if the hand is to successfully pick up the cup: That is, position in visual space must be transformed into position in proprioceptive space or vice versa if we want to see if there is coffee in the cup in hand.

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