The Postural Substrate

The explanation of response-variability under constant external stimulation rests upon the fact that separate aberrations are not strictly phasic in character but are built on tonic patterns. Most of our earlier discussion has centered around the determinants of phasic response--the various exteroceptors, the specific integrations occurring at each neural level, and so on. These are the most obvious features of neuromuscular action, but not the most important! They may be called the postural substrate--a general neuromuscular background which operates to facilitate, inhibit, and otherwise modify phasic responses. The patterning of this substrate must be intimately related to the conditions of set, since it forms the frame of reference which renders consistency to all phasic reactions. Without the postural substrate man's behavior would undoubtedly resemble that of a complicated jumping-jack.

It is significant that postural and phasic influences meet originally and ultimately in the motor sphere, for this is the only possible place where one influence could act selectively upon the other. We have already mentioned the great surplus of afferent over efferent fibers. Most of this surplus is contributed by fibers from the exteroceptors, whose connection with the musculature is diffuse rather than specific. Proprioceptive fibers, on the other hand, are in especially intimate contact with the motoneurones which innervate their muscles of origin. Hence, of the wide variety of transitory exteroceptive impulses playing upon a motoneurone field, only those impulses will cause overt response which "fit" in timing, phase, and direction with the prevalent pattern of proprioceptive influences. This seems to be the basis of selection, variability, and set, as exemplified at the lowest level of coordination.

In order to understand how the relation between the proprioceptors and exteroceptors is preserved and modified at higher levels, one must look at the problem genetically. As the nervous mechanism developed, the postural and phasic influences apparently became organized along systematic lines; and more complexities were thereby introduced. In the interests of efficiency of operation, the muscles used in the same or related types of behavior came under the control of higher motor centers. Paralleling this reorganization and condensation of the efferent flux, additional afferent fibers from the proprioceptors ascended to connect with the new control centers. Phasic influences from the exteroceptors, reaching these control centers, met with the same selective sorting as occurred in the primary motoneurone field. However, the transfer of locus brought with it a greater degree of plasticity in the central organization; that is, the pattern of proprioceptive impulses could be altered by the center so as to permit now one and now another phasic influence to act. The appearance of distance receptors brought about even further changes. Immediate phasic adjustment was unnecessary with this equipment, and a method was developed for continuing and elaborating these exteroceptive excitations in intercerebral fields. Here, again, proprioceptive excitations came into play. Through afferent connections with these fields the postural substrate became the chief contributor to the vigilance of cortical neurones. Mainly as a result of learning, definite patterns of vigilance came to be associated with definite patterns of exteroceptive excitation. The organism was enabled to attain the appropriate canalization of proprioceptive energies in advance of the complete exteroceptive patterns. For example, if some auditory stimuli (such as the instruction to add) always accompanied the presentation of visual symbols, the appearance of these auditory stimuli prior to such presentation could set the organism to add. Here the two aspects of response would attain a completely reciprocal relation, with phasic influence setting the postural substrate and the tonic influences determining the ultimate phasic response.

While we have, perhaps, unravelled some of the tangled threads which underlie the conditions referred to as set, we should lose our orientation if we indulged in further speculation. To summarize, the postural substrate sustains and selects the phasic environmental influences which effect response. These functions are carried out in somewhat different manners at the different levels of neural integration. At the spinal level the selection is determined solely by the pattern of proprioceptive excitation extant in the motoneurone field. At the cortical level the backflow of proprioceptive impulses from the muscles may be canalized into preferential paths by some preparatory exteroceptive excitation (such as an instruction).