CONDITIONED REFLEX, (I) the neural mechanism by which, under certain definite conditions, (a) a new stimulus is substituted for the one originally effective in bringing about a specific reaction or (b) a new response takes the place of the originally adequate reaction to a specific stimulus. Thus, with a horse, the snap of the whip is a substitute stimulus for the smart of the lash. The exclamation of an adult who stubs his toe is a substitute response for the child's howl of pain. (2) Modification of reflex behaviour as a result of alteration of the conditions under which the behaviour in question occurs, without assumption as to the neural mechanisms involved.
The first conditioned reflex experiments were conducted by Pavlov, a Russian physiologist. He discovered that certain signal stimuli, like sight and smell of food, are just as effective as the taste of food for evoking the salivary flow. This raised the ques tion: Can any neural stimulus, such as the ringing of an electric bell, be substituted for the natural stimulus in arousing reflex reaction? In putting this problem to test, Pavlov used dogs as subjects, and worked chiefly with the alimentary and reject re flexes. The dog was fastened in position before a partition which could be opened for the presentation of food. An electric bell was sounded and kept ringing until after the food was presented. After about 3o trials, it was found that the sounding of the bell alone was sufficient to evoke the salivary flow. Thus, a new reflex had been built up. The dog responded to the sounding of the bell in the same way that he did to the taste of food. This new re action was named by Pavlov the conditioned reflex.
Control of Subject and Stimulus.—The subject of a conditioned reflex experiment, whether animal, child or adult, must be in a physical condition which permits the functioning of the natural or unconditioned reflex. That is, if the salivary reflex is being conditioned to a new stimulus, the dog must be hungry. The en vironment must be controlled so that all stimuli more potent biologically than the ones under test are eliminated.
Presentation of Stimulus.—There are three possible ways of combining the neutral stimulus with the adequate stimulus. The bell may be sounded before presentation of food; simultaneously with presentation of food; or after food is given. Only the first two methods result in a conditioned reflex. 427 successive pre sentations of the odour of vanillin S to I o sec. of ter acid was introduced into the mouth failed to develop a conditioned re jection reflex. On the other hand, a single trial with the vanillin presented before the acid sufficed to build up a conditioned response. For the most efficient conditioning the new stimulus must not only precede the natural stimulus, but it must also over lap the other in point of time. It is possible, however, to build up a conditioned reflex, even though an appreciable interval occurs between the cessation of the new stimulus and the be ginning of the natural stimulus. After a considerable number of trials, the response will occur upon presentation of the condi tioned stimulus, but only after an interval of time equal to the period of separation occurring in the original trials. This is called a trace reflex because the proximate cause of the response is not supposed to be the neural stimulus, but rather its trace left on the central nervous system. The longest latent period reported is about 3o minutes.
Conditions for Modifying the Response.—The natural response to an electric shock is withdrawal. The natural response to food may be smacking the lips together with increased salivary flow. By suitable presentation of these two stimuli it has been f ound that the electric shock will combine with the food as stimulus to the extent of inhibiting the defence reflex and establishing salivary flow as the new response to the shock stimulus. If, however, the electric current is applied to skin over bone, it will not become conditioned to the food response. In a situation of this sort the result will depend in each instance on the relative strength of the two unconditioned responses. The one which is physically weaker and biologically less important is lost while the more funda mental reflex wins out. This principle is particularly vital in the reconditioning of children.
Use and Adaptability of the Conditioned Reflex.—In an imal psychology the conditioned reflex has been particularly use ful in investigating sensory discrimination. A conditioned reflex is first built up to a specific sensory stimulus, let us say, auditory. Then a different stimulus of the same category is substituted. If the animal does not respond to the new sound it is inferred that he discriminates between the two. By this method of differential conditioning it has been found that a dog's hearing is more sensi tive than a man's. In experiments with children, Mateer suggests using the speed with which a conditioned reflex can be developed as an index of intelligence. Extension of Pavlov's technique to human subjects was made possible by apparatus designed by Lash ley for catching and measuring salivary secretion. Cason in com bining the eye wink with a substitute stimulus proved that the con ditioned response occurs more quickly than the voluntary eye wink. He also succeeded, after 400 repetitions, in bringing about contraction of the pupil in response to the ringing of the bell, al though the normal response to the bell is a slight dilation. Watson and Jones have proved that emotional responses of children can be reconditioned. Although a child is not naturally afraid of a furry animal, he becomes afraid if a naturally frightening sound accompanies presentation of the animal. In this way many un natural emotional responses are built up. Jones has demonstrated the possibility of removing undesirable fears by what is known as a reconditioning technique. If the animal the child fears is brought into the room when the child is eating, the eating, being a stronger biological response, controls and the withdrawal re sponse is eliminated. (See BEHAVIOURISM.) Evaluation of the Conditioned Reflex.—With regard to the supposed underlying neural mechanism, the question has been raised as to whether or not the conditioned reflex is a true reflex. An experiment by Lang and Olmsted may indicate that it is not. A true reflex is a predetermined necessary response of the organ ism to a given stimulus. If an experimentally created, conditioned reflex wherein saliva flows at the beat of a metronome is a true reflex, it should still function even though the salivary receptor tract is destroyed. Lang and Olmsted found that the removal of afferent connections from the salivary gland abolishes the con ditioned reaction. This demonstrates a dependence upon a second neural mechanism which is not present in a true reflex. Neverthe less the conditioned reflex is an accepted method for changing behaviour. As such it is firmly established in the animal labora tories and widely recognized as a basic method of child training all over the world.