Inertia and Perseveration of Neuromuscular Sets

While neuromuscular sets are most easily identified with the prestimulus period, it should be recognized that they accompany and frequently outlast the briefer responses which they help initiate. Once aroused, these sets predispose the organism to a continuance of the same type of activity. It may be difficult to "get set" to write upon a particular topic or to study a particular lesson; but once the organism is warmed up to this type of organization, it is difficult to shift to a radically different one. The student may have observed this inertia in connection with his class work when he is called upon to think psychology one hour, chemistry the next, and English the next. The difficulty is especially apparent if quizzes are taken in each of the three subjects on the same day. Another good example of inertia is found in the complications which are introduced when a person who is postured or set to react to one stimulus situation is suddenly confronted with and must react to a different situation. The skillful halfback utilizes this principle to escape the tackler.

He runs as if to pass directly into the latter's clutches; but when the tackler's set has been established he suddenly alters his course and catches his opponent unprepared to do anything but lunge harmlessly in what is now the wrong direction. Most detective stories owe their appeal to the fact that they prepare the reader for one outcome and then towards the end suddenly shift his set. Likewise, a sudden and necessary change in our "way of taking" a stimulus is basic to a humorous situation, such as the sudden slip of a pompous individual upon a banana peel.

It is doubtful whether the concept of postural fixation will account for all varieties of perseveration in human behavior, but something very much like it must occur in the continuance of interrupted tasks. Experiments have shown that simple tasks, such as counting odd numbers or writing the alphabet backwards, when interrupted short of completion, will be resumed at the first possible instant. If they are not resumed, they are subsequently recalled more easily than are completed tasks. The postural basis of such perseverative consequences was suggested in a study which compared changes in the tonus of the quadriceps muscles during related periods of interrupted and continued performance. Subjects were given a series of twenty different tasks, some of which were interrupted half a minute after they were begun. The interruptions included such things as cutting off the illumination and appropriating the subject's pencil. No matter how serious the interruption, opportunity was presented for the resumption of the task at a later period. Although the subjects were not instructed to resume work, they almost invariably did so. Many showed a tendency to become "frozen" in a definite posture until the disturbance was removed. For example, one subject held his pencil at the same point in the cancellation test until the light was again switched on; and another kept his hands momentarily poised in mid-air after the experimenter had removed the ring puzzle on which he was working. More obvious than these specific postural fixations were the general increments in muscular tension during and after the interruptions. Such increments probably served to reinforce the original task-set and to inhibit responses to irrelevant stimuli.

Task-sets are normally ended when reaction to a group of stimuli is completed. Interruption introduces a complication in this normal process of equilibration, and it becomes necessary for the task-set to obtain compensatory reinforcement in order for it to compete successfully for the directive control of behavior. If the irrelevant stimuli introduced by the obstruction are not too strong or long-continued, reinstatement occurs almost immediately. But if such reinstatement proves impossible, the reinforcement derived from the muscles may help to fix the neural remnants of the displaced activity, which will persevere and crop out under more favorable conditions.

The relative resistance of various sets to competing tendencies has never been specifically explored, but a number of observations point to a marked difference in the efficacy of various task-sets. Experience and training apparently operate to render more stable and persistent those neural activities which sustain the performance of professional duties. Interesting and entertaining work resists distraction more easily than monotonous and routine performance. Fluctuations in attention are less noticeable in some types of activity than in others.