REFLEX

in biology, a type of action consisting of comparatively simple segments of behaviour that usually occur as direct and immediate responses to particular stimuli uniquely correlated with them. Reflex actions have a widespread occurrence among complex animals. Many reflexes of placental mammals appear to be innate. They are transmitted by heredity and are the common property of the species, and often of the genus. They include not only such simple acts as mastication, swallowing, the blink reflex, the knee jerk, and the scratch reflex, but also stepping, standing, the cat's righting reflex, basic sexual acts, etc. Built up into complex patterns of many coordinated muscular actions, reflexes form the basis of much instinctive behaviour in animals. Humans also exhibit a variety of innate reflexes, which are variously concerned with adjusting the musculature for optimum performance of the body's distance receptors (the eye and the ear), with orienting parts of the body in spatial relation to the head, and with managing the complicated acts involved in ingesting food. Among the innate reflexes concerning just the eyes, for example, are: (1) paired shifting of the eyeballs, often combined with turning of the head, to perceive some interesting object in the field of vision; (2) contraction of the intraocular muscles to adjust the focus of the retina for the viewing of near or far objects; (3) constriction of the pupil of the eye to reduce excessive illumination of the retina; and (4) blinking due to intense light or touching of the cornea. In its simplest and most elementary form, a reflex is now viewed as a function of an idealized mechanism called a reflex arc. The primary components of the reflex arc have been identified as the sensory-nerve cell (or receptor) that receives the stimulation, in turn connecting to another nerve cell that activates the muscle cell (or effector), which thus performs the reflex action. In most cases, however, the basic physiological mechanism is more complicated than this simple arc theory would suggest. Additional nerve cells capable of communicating with other parts of the body (beyond the receptor and effector) are invariably present in reflex circuits. As a result of the integrative action of the nervous system in higher animals, the behaviour of such organisms is more than the simple sum of their reflexes; it is a unitary whole that exhibits coordination between many individual reflexes and is characterized not by inherited, stereotyped responses but by flexibility and adaptability to circumstances. Many automatic, unconditioned reflexes can thus be modified by or adapted to new stimuli, making possible the conditioning of reflex responses. The experiments of the Russian physiologist I.P. Pavlov, for example, showed that if an animal salivates at the sight of food while another stimulus, such as the sound of a bell, occurs simultaneously, the sound alone can induce salivation after several trials. The animal's behaviour is no longer limited by fixed, inherited reflex arcs but can be modified by experience and exposure to an unlimited number of stimuli. The reflex concept has exerted a great influence on psychological thinking, and initially led to premature attempts to develop a psychology based on reflexes. Pavlov's groundbreaking work gave rise to an enormous spate of research in the early 20th century on the physiology of behaviour, and for a long time the conditioned reflex provided the best technique for enabling at least a part of the learning process to be investigated quantitatively and to be subjected to an exact analysis. The principles proposed by such behaviourists as E.R. Guthrie, C.L. Hull, and B.F. Skinner to explain psychological actions as conditioned or learned responses to external and internal stimuli were based in part on earlier reflex notions and upon the fundamental model of the conditioned reflex as demonstrated by Pavlov. It is now generally recognized, however, that the reflex relationship between stimulus and response is not nearly as simple as was previously thought. It has become evident that the use of the conditioned reflex as a model for learning in classical-conditioning experiments artificially isolates, to an extreme degree, part of the total learning process in higher animals, and is by itself inadequate in attempting to analyze the complex physiological and mental interactions that ultimately determine the behaviour of humans and other mammals.