Organization of the Postural Substrate

While the postural substrate does not present as many marked variations in its pattern as does phasic response, it is in a more or less continual state of slow change and flux. Tonic contractions differ in their relative intensity and extensity, depending upon the phasic acts which they sustain. Thus the tonic expression of the set to oscillate the index finger will be localized mainly in those muscles which are utilized in the act. The set to add, on the other hand, is represented by a pattern of tension of wide extent but little intensive differentiation. Pattern a indicates the type of response which precedes and sustains predominantly motor activities. It has as its focus a high degree of tonic contraction centered in particular muscle groups, and it has small spread. Pattern b indicates the response which sustains the more elaborate intercerebral processes. It has a broad base and little focal intensity. Of course, these patterns do not stand for two mutually exclusive types of reaction; rather, they represent extremes of a scale, the postural substrate shaping itself in various degrees of intensity and extensity.

We are now in a better position to evaluate the two theories of set discussed in a foregoing section. Both are explanations of the influence of postural substrate at the cortical level only. Both ignore the essential continuity of these influences with those at lower neural levels, and thus both have mistaken one type of pattern as typical of the whole organization of the postural substrate. The peripheral theory emphasizes pattern a, whose focus is definitely centered in the muscles specifically involved in the phasic act. The central theory emphasizes the canalization of proprioceptive impulses from the more or less undifferentiated pattern b. The reasonable view includes both these features. Since the cortical centers are essentially an overlay development, it is to be expected that behavior integrated at that level will preserve to some degree features characteristic of the cord, particularly when it involves a definite pattern of muscular movements. Preparation or set for such activity will include both central canalization and peripheral intensification of proprioceptive impulses. If the postural pattern is highly concentrated in muscles which are to be utilized in phasic response, the peripheral effect may be so marked that phasic response begins without waiting for the appropriate exteroceptive excitant. Runners often show a tendency to start before they hear the starting gun. The driver gets his car under way before he actually sees the green light. Many of the facilitative effects of set are produced by the optimal localization of tension. We deceive ourselves, however, if we fancy that we can trace the facilitation effect of all sets to specific patterns of muscular tension. Cortical reactions vary considerably in their expression by way of specific movements, selection in these processes frequently involving a central canalization of non-specific proprioceptive excitation. The means by which canalization occurs has not been adequately described; but since the postural reaction system extends longitudinally through all the neural levels, the concept may eventually be reduced to more conventional terms, such as facilitation, inhibition, induction, irradiation, and chronaxie tuning.