The Principle of Reafference: Interactions Between the Central Nervous System and the Peripheral Organs

A major question for the physiology of the central nervous system (CNS) has always been this: what lawful relationships hold between impulses which are generated by external stimulation and travel inward into the CNS and those which-either directly or indirectly-reemerge from it; that is, the question of the relations between afference and efference? The CNS has been characterized as a sort of automat, which reflexly delivers a given ticket when a particular coin is inserted in it. For simple protective reflexes-like sneezing and withdrawal from painful stimuli-this idea is easy to accept; for more complicated reactions such as balancing and orienting responses, the same conceptual scheme has been advocated. Even rhythmic 1ocomotor patterns can be understood in these terms, if one assumes that each single movement reflexly evokes its counterpart, that each component sets off its successor in time (reflex chain theory). The higher forms of behavior, which are modifiable through experience, are subsumed under the same rubric by way of "conditioned" reflexes. This "classical reflex theory7' largely still holds sway, although there are Illany facts which are inconsistent with it. W e know that the breathing centre maintains its activity without external rhythmic stimulation, that the central locoillotor rhythms of illany invertebrates remain intact in the absence of afference (von Holst, 1932, 1933, 1938), that in fish (von Holst, 1935; Lissman, 1946) and amphibians (Weiss, 1941; Gray, 1946, 1950) only a very small part of the afferent innervation is sufficient to ensure that all parts of the body can continue to locomote properly. The reflex chain theory cannot handle such facts. The analysis of relative coordination in anthropods, fish, mammals and humans has taught us to recognize central forces of organization-of coordination and control-whose interplay leads to the establishment of laws which are formally quite similar to those laws of perception which were discovered by Gestalt psychology.