Before proceeding to attack the problem thus suggested, we must consider certain other features of the response mechanism, particularly as it exists in human beings. In studying the operations which occur in complex nervous systems, we find it necessary to distinguish between a number of levels of central transfer or adjustment. Such a plurality of levels involves the possibility of conduction from the afferent to the efferent sides through more than one junction point. There is a network, so to speak, of long and short circuits. In the human being, the shortest circuit occurs through the centers of the spinal cord, or of the medulla oblongata, and involves the minimum number of neurones. The longest circuit is through the association areas of the cerebral cortex, along which path a very large number of neurones may be concerned in serial order. Intermediate transfer levels occur in such portions of the nervous system as the thalamus, the mid-brain, and the cerebellum. There are also alternative paths of conduction, which involve more than three conductors, even within the spinal cord.
The distinction between the short and the long circuit centers corresponds to that between lower and higher nerve adjustment mechanisms. These adjectives are used for several reasons. Firstly, the adjustments which occur through the longer circuit paths are more complex than those occurring through the shorter circuit ones. Secondly, the former are the result of a later evolutionary development than are the latter. Thirdly, the higher central processes are directly associated with conscious states, whereas the lower ones are only indirectly so associated. Finally, it happens that the long circuit centers are actually located in the upper portion of the nervous system in the erect human being.
If we approach the problem of motivation from an entirely unbiased physiological standpoint, our interest lies in an explanation of the specificity of all kinds of response whatsoever. We shall be just as much concerned as to why irritation of the nose results in sneezing as in an understanding of why traffic officers react with harsh language to the infringement, by automobile drivers, of the rules of the road. From the physiological standpoint, complex voluntary action is no more needful of explanation than is the simplest reflex reaction. Moreover, the general type of explanation which must be given may be the same in both cases, differing only in complexity. We wish to know, first, the exact nature of the mechanism which connects the ingoing and outgoing nerve currents; and, second, how this connection was established. However, if we approach the problem from the psychological angle, we are apt to be interested only in those forms of behavior which are definitely determined by adjustments made in the higher brain centers, since these are the only ones which are represented in consciousnss.
The layman, as we have already noted, limits his interest further within the domain of voluntary behavior to uncommon forms of response. But for us the problem is perfectly general. Nevertheless, the largest number of problems and the greatest difficulty of explanation are found in connection with those adjustments which occur through the intermediation of the cerebral cortex, the highest of all nerve centers, so that we must devote particular attention to the properties of this organ.