Reafference Theoryprinciple

The term reafference, coined by the German physiologists E. von Holst and H. Mittelstaedt in 1950, refers to a distinction between "active" sensory input that is the result of some movement of the animal, and "passive" input that occurs independently of the organism. The principle of reafference has come to be a cover term for those sensory events that are produced by voluntary movements of a sense organ (e.g., events resulting from the movement of an image across the retina that accompany voluntary movements of the eye). This is contrasted with the concept of exaffer-ence that refers to those sensory events produced by changes in the stimulus itself (e.g., events resulting from movement of an image across the retina that accompany real displacements of the physical object). The seminal studies by von Holst and Mittelstaedt examined the optokinetic reflex in the fly - this is a reflex movement of the fly in response to movement of the visual world and compensates for external movement. As a result of the optokinetic reflex, the fly's eye is able to look at the same part of the environment successively, an event that is similar to nystagmus in the human eye where rapid movement of the eyes in one direction is followed by a slow drift in the opposite direction. During operation of this reflex system, an efference/ef-fector copy is left, theoretically, at some place in the nervous system, and the effector that is activated by the efferent message has some influence on new stimulation that enters the receptor. The afferent/sensory message is compared to the efference/effector copy and, according to the theory, if they match, the copy is nullified. If the afference matches the efference copy, it is called reafference. However, afferent stimulation that does not match an efference copy is called exafference. Re-afference theory is example of the application of control and feedback systems to behavior analysis. In control systems, the mismatch between the afference and the motor copy results in an error signal that, in the normal fly, is corrected by negative feedback. When the fly's head is rotated, the error signal becomes positive feedback. An example of re-afference/feedback in human behavior may be seen in the classical perception studies by the American psychologist George Malcolm Stratton (1865-1957) who reported that any movement made while wearing inverting lenses causes the participants' world to swing and whirl about them (cf., Kohler, 1962); the left-right reversal of the customary relation of image displacement to body movement causes the field to appear to move in the direction of the person's movement, only faster. As with the fly, there is a positive rather than a negative feedback loop involving the efference copy. Humans have the capability to cope and adapt with such inverted-image-lens conditions (as in Stratton's studies) so that the world eventually appears stationary during the participant's head movements. The fly, however, does not have such an adaptive capability and, as a consequence, circles to exhaustion. See also CONTROL/SYSTEMS THEORY.

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