Role of the Postural Substrate in Neural Economy

The function assumed by the postural reaction system makes it of vital importance in the economy of neuromuscular action. Had the organism developed only a phasic system of response, there could be little consistency or continuity to behavior. Without the postural substrate the organism would, on the one hand, invariably be exposed to the mercies of the most intense external stimulus and, on the other hand, be occupied constantly in making movements of high metabolic cost.

Tonic contraction is much more economical of energy than is phasic contraction; hence it is the logical form of activity to be utilized in preserving organic balance and equilibrium. If we conceive of the postural substrate as changing its fundamental pattern without difficulty for each new stimulus attended and each new act executed, we have missed one of its fundamental features. It has been suggested that the prolonged refractory period between successive phasic responses may be a function of the time required by the neuromuscular mechanism to return to a state of equilibrium. In terms of our analysis this would mean that the phasic act, which has disturbed the pattern of the more static tensions, cannot be repeated until that pattern is reinstated. Considered in such light, refractory phase may turn out to be an important key to the understanding of fluctuations in attention. It has been repeatedly shown that this phenomenon is not due primarily to the inertia of the reacting end-organs; hence it must lie in the relation of the phasic act to the underlying postural pattern.

These patterns exhibit considerable inertia to change; they are slow to reach their maximum effectiveness and slow to subside. Phasic excitants come and go, but the stream of proprioceptive impulses is measured and continuous. It does not take much imagination to see that the postural substrate is effective in regulating and determining the duration of phasic response; but use of the term set to cover all these effects tends to obscure the subtleties of the problem. Ultimately we may be able to discard the concept entirely and speak in more conventional physiological terms. Until that time, however, the grouping of all such conditions in a single chapter will serve to remind the physiological psychologist of their common matrix and to point to the necessity for additional work.