IMPLICIT MUSCULAR ACTIVITIES DURING THINKING
There are two general theories concerning the physiological basis of thinking. According to the older of these theories, which we may term the "central" theory, processes of thought, reasoning, imagination, or ideation depend upon or are aspects of complex brain activities. The implied assumption is that motor processes are not a necessary condition of thinking, and that thoughts and ideas may run their courses, even in the absence of all motor activities. On the other hand, the somewhat newer "motor" or ("peripheral") theory assumes that processes of thinking, reasoning, or imagining are as much dependent upon motor responses as upon brain action. Thinking, so viewed, is not a correlate or aspect of cortical brain activity alone but is intrinsically a reaction which functions in terms of complete sensorimotor patterns, with the motor part of each circuit just as essential to the process as the central segment is.
A few adherents of the motor theory of thinking have insisted that thought processes are to be identified with activities of the speech mechanisms. According to this view, when a person is thinking, he is "talking to himself" and his vocal organs are active, even though the movements are microscopic and are detectable only with the aid of sensitive instruments. Thinking and imagining are merely "subvocal" talking. Other exponents of the motor theory have proposed broader interpretations, suggesting that thinking may be based upon implicit muscular contractions involving many other muscle groups as well.
Early Experimental Investigations. Prior to the studies which will be described in the following pages, several attempts had been made to test experimentally the role of motor processes in thinking, but the results obtained by different investigators had been somewhat contradictory. Some of the reported results favored the motor theory, but quite as many findings were of a negative sort. In all probability,