Hereditarily Determined Specific Responses

Let us now consider what particular aspects of human or animal behavior can be explained in terms of reflex mechanisms such as we have outlined in the previous chapter. The forms of response which can be included under this classification will be primarily hereditary in their determination, since they must depend principally upon anatomical arrangements of neurones which are built into the organism at birth, or which develop as an inevitable consequence of congenital forces. The factors which may be involved in addition to anatomical continuity, such as threshold, chronaxy, and sensitiveness to general organic conditions, must also be regarded as having an hereditary basis. Nevertheless, we may find that the exercise of such reflex mechanisms, or their interactions with one another, produce significant changes in their operation.

Determining Factors in Reflex Response

Let us continue to adhere strictly to our physical point of view regarding response, and endeavor to arrive at a preliminary understanding of the factors which must determine its specificity. The reader is reminded that the concept of response specificity (as defined by Holt) comprises the identification, or selection of a particular kind of reaction (behavior) as a constant function, or accompaniment of a particular object which acts so as to stimulate the nervous system. Reduced to mathematical form, the specificity is represented by a functional equation, such as R & f (O), where O is the object and R is the effector reaction. Although such response functions can seldom be expressed in quantitative form, their logical nature is not affected by this limitation. The statement: "Johnny always cries when he sees a dog, is a functional formulation of this sort which indicates the specificity of Johnny's response to a dog.

Response Conduction Patterns

The processes of convergence and of divergence are of particular interest in relation to the notion of conduction pattern. The idea of such patterns is of the utmost importance to an understanding of the more complex forms of response. It may be well to make this conception more vivid by means of a concrete example. Let us consider a case of visual response, which is naturally the easiest kind to "visualize." Now the notion of pattern can be applied to every stage of the response, beginning with the object and ending with the effect. In each stage it is a matter of the arrangement of parts in space; for the object, it is simply the spatial form of the latter with particular reference to the manner in which various portions of its surface are reflecting, or emitting, light in the direction of the observer's eyes. In the second, or stimulus stage, the pattern is comprised by the configuration of light rays which are passing from the object to the pupils of the eyes. This pattern is primarily an arrangement of directions of movement rather than of points on a surface. In passing through the eye the pattern is again changed as a consequence of the shifting of the directions of the rays, which is brought about by the refractive action of the ocular media. As a result of this an image or optical picture of the object is formed on the sensitive retina of the eye. This retinal image pattern is followed by the formation of a corresponding or registering pattern of visual receptor processes in the retinal rods and cones.

The response now passes into the fibres of the optic nerve, of which there are about a million. All of these are simultaneously excited in any act of vision, and the kind of behavior which results must obviously depend to a large degree upon the pattern of distribution of the excitations over the individual conducting components in these complex bundles of nerve fibres. At the moment when the nervous disturbance passes from the receptor cells to the conducting nerve fibres, the pattern has a close geometrical resemblance to the retinal image; but within the conducting layers of the retina itself there is almost immediately a radical condensation and change in geometrical form. This is referable to the fact that, except in the center of the retina, a considerable number of receptors connect in party line fashion with a single nerve conductor; and also to the convergence of the nerve fibres upon the point in the eye-ball where they emerge as a compact cable. Along the further course of the visual impulses from the eyes to the brain, there are other regions of transfer and of redistribution of the fibres, which cause still more radical changes in the exact geometrical arrangement of the individual nerve fibre excitations. When the impulses reach the cerebral cortex the pattern bears hardly any geometrical resemblance to the structure of the original object. Nevertheless, the exact configuration of the excitation in the cortex is determined in a fairly reliable manner by that of the object.

Starting at the cerebral cortex and proceeding in an efferent or outgoing direction, there will ordinarily be a complex outflow of nerve impulses along the so-called pyramidal system of neurones; and the exact pattern of this outflow may be controlled by that of the visual processes. However, the motor pattern will never have any resemblance whatsoever to the sensory one, the connection between the two being entirely arbitrary, so far as similarity of process is concerned. The pattern of the impulses in the pyramidal neurones will determine that in a larger number of fibres which innervate the skeletal muscles and in turn will govern the behavior pattern, or the posture and movements of the organism. All along the course of the conduction from the object to the "effect," there is a continuous and at many points radical modification of the pattern, but, nevertheless, the pattern in each stage determines that in the next following stage, working of course in conjunction with the inherent structures which characterize the stages in question.

The pattern of the response on the entrant or sensory side of the cerebral cortex is correlated very closely with the psychical pattern, or configuration which is found in the accompanying consciousness. This psychical pattern is identical, as a rule, with the object, as the latter is presented in consciousness. It bears, however, only a remote resemblance to the physical object which initiates the response.

Now the transformations of response patterns which occur between the object and the brain are of great technical interest in general physiology, but the transformation which is of primary interest in the theory of motivation is that occurring between the entrant and the emergent brain processes. This is the relationship which determines the specificity of the response. From the standpoint of the brain, it is quite immaterial how the entrant activities are determined, and from the standpoint of the specification of behavior the only thing which counts is the form of the impulses which leave the centers. In order to understand behavior, we must know how the input and output phases of the central activity are associated, and what the specific causes of such association are in particular cases. For example, suppose that we react to an apple by taking a bite out of it. This involves a linkage of the cortical representation of the apple with a complex series of motor innervations which are prerequisite to the grasping of the apple, the lifting of the same to the mouth and the application of the teeth thereto. General physical principles do not enable us to infer the behavior from the object, so that we shall be forced to appeal to the peculiar structure and properties of the cerebral machine.

The General Mechanism of Response

Suppose, now, that we attempt a general physical analysis of the mechanism or apparatus which modern physiology offers in at least partial explanation of human and higher animal behavior. This apparatus, which we may designate as the response mechanism, is essentially that of the nervous system, although it of course involves in its processes all portions of the body and particularly the muscles. Response is ordinarily said to be initiated by objects or stimuli in the environment of the organism. However, in many cases, particularly where we are dealing with human behavior, it is very difficult to identify the objects in question, unless perhaps we look for a clue in the accompanying introspective consciousness. The activities of living beings would not be so characteristically spontaneous if it were always evident that their causes lay in the environment.

Nevertheless, the most helpful approach to a study of these more difficult cases is by way of an understanding of the more typical kind of response, which begins with an object. This object must act upon a sense-organ by means of a force or energy, which is called the stimulus, if it is to call forth a reaction from the organism. Thus, in the case of visual response, the object emits or reflects light which enters the eye and stimulates the retina. The latter is an example of a receptor mechanism, in which the stimulus initiates an excitation of the nervous system. The receptor process, in turn, is followed by a transference of the excitation to a so-called afferent nerve fibre, along which the disturbance is propagated to a nerve center, such as those of the spinal cord or brain. The processes which transpire at the nerve center determine along what path the excitation will be conducted in an outgoing direction to reach the muscular system. In other words, the central process is of critical importance in determining the character of the reaction or behavior in any given instance. For this reason, it is sometimes called the adjustor process. Its mechanism always involves one or more junction points between individual nerve fibres, such points being known as synapses.

After leaving the central, synaptic, region the disturbance passes along an afferent or outgoing nerve, and is transferred, through a mechanism known as the end-plate, usually into a muscle. Muscles are the most typical examples of the class of effectors, which also include glands and electrical organs (in certain fishes). The arrival of the excitation at the effector is marked by the characteristic reaction, which controls the relation of the organism to its environment. This may be regarded as the last stage of the response activity, and be designated as the effect. When we study response in the superficial manner which characterizes behaviorism, or the everyday observations of the behavior of men or animals, we usually recognize only the first and the last links in this chain of events, and very frequently we miss the first link.

The mechanism of response, as above outlined, belongs to a class of processes known as propagation or conduction. A disturbance is set up a certain point in space at a certain instant, and this gives rise to a series of further disturbances at successively different points in space and instants in time. Other examples of processes of this sort are to be found in the propagation of radiant energy, or of sound, in space; or the conduction of fluids through pipes, or of electricity through wires. They are controlled by a number of fairly simple general principles. Firstly, the path of the conduction depends upon the structure of the conducting medium. Secondly, the nature of the process at any point in this path is a function of the nature of the conducting medium at the same point, as well as of the character of the activity at the immediately preceding point. The propagation of the disturbance is energized, or pushed by an agency which corresponds to pressure or voltage, but the process is impeded by another factor exemplified by viscosity or resistance. The conduction may be regarded as an outcome of the struggle, or balance between these two determining factors. In the case of electrical conduction this relationship is expressed mathematically in the formula known as Ohm'slaw, according to which the current strength is proportional to the voltage and inversely proportional to the resistance. At a later point in our discussion we shall consider in considerable detail the analogy between the laws of electrical and of nervous conduction.