A study of the conditioned patellar reflex.

Abstract

Experiments performed on 50 subjects show that the knee jerk can be con-ditioned to certain simple auditory and tactile stimuli. As a rule at least 200 presentations of the conditioned and unconditioned stimuli were necessary to bring this conditioning about. The subjects varied in the rapidity with which they became conditioned, as well as in the extent and stability of the resulting conditioned response. When the subjects were requested to make some "voluntary" response, such as clenching the fists, as soon as the conditioned stimulus occurred, con-ditioned knee jerks were obtained about twice as fre-quently as they were when no such "facilitating" stimulus was used. It was also found that decreasing the interval between the conditioned and unconditioned stimuli below .11 sec. made it very difficult to condition the subjects. In its temporal aspects, the conditioned knee jerk was very similar to the "voluntary" contraction of the quadriceps muscle, and quite different from the simple knee jerk. It had a latency of .2 -.5 sec, about the same as that of the voluntary contraction, and ten times that of the simple knee jerk. In the slow rise of the early portion of the curve of contraction, it again resembled the former, rather than the latter. In most of the work, the leg was not permitted to swing freely, and records were made directly from the thickening of the muscle. Under optimal conditions, when the conditioned stimulus was presented alone, contractions of the quadriceps occurred 75% of the time.