p After achieving extinction of the orienting reflex it is possible specially to investigate the types of changes in the stimulus that may excite the orienting reflex and thus to determine the “configuration” of the nervous trace. A special investigation has shown that the orienting reaction may be excited by any discrepancy between a stimulus and its nerve model formed in the course of repeated applications of the stimulus. The nerve model includes even the intervals between the applied stimuli.

p For the given property to become the stimulus of the orienting reaction the stimulus must be strictly constant during repetition, i.e., in the course of preliminary extinction of the reaction.

p The most interesting are the experiments where the “nerve model of the stimulus" reflects the temporary sequence of signals equal in strength and quality. Experiments with a change in the intervals between the stimuli and a change in the duration of the signal are specific cases. Experiments have shown that the nerve model must be conceived as a dynamic process of elaboration of a “prediction” by the nervous system and comparison of the prediction with the incoming value of the signal. It may 348 be said that by observing the peculiarities of the incoming signal for a long time the nervous system extrapolates its future value. The orienting reaction arises if the prediction elaborated by the nervous system for the given moment of time does not coincide with the value of the incoming signal. In such cases the temporary intervals are differentiated with very high precision.

p Experiments show that the central nervous system exercises continuous control over the external and internal signals which are not in the “focus of attention”.

p A “nerve model of the stimulus" is also formed when the stimulus becomes a signal of the response reaction. As a conditioned reflex is being elaborated, when the conditioned reaction becomes stabilised, the orienting reactions become extinguished as they do under the action of indifferent stimuli. The change in the conditioned stimulus leads to the fact that the stimulus ceases to coincide with the formed model, a phasic orienting reaction arises, and the conditioned reaction becomes the more inhibited, the more the new stimulus differs from the model formed earlier. Thus the signal of discoordination is the agent of the orienting reaction also during the action of the conditioned stimulus.

p At the same time, when the stimuli acquire a signalling value, a sharp difference between the indifferent and signalling stimuli is observed. Orienting reactions to signalling agents are stronger and last longer. Against this background non-signalling stimuli often weaken or disappear altogether (O. S. Vinogradova, 1958,1961).

p The intensification of orienting reactions under the action of signalling stimuli depends on:

p a) the activating action of the reinforcement;

p b) the overall action of the signal and return afferentation during the realisation of the reaction;

p c) an intensification of the local tonic orienting reflex evoked by the instruction and selectively spreading to the analysers which receive definite groups of stimuli.

p Investigation of orienting reflexes against the background of elaboration of conditioned reflexes is noted for the fact that here the tone of the orienting reaction is higher and the phasic orienting reactions become extinguished more slowly. This makes it possible to study in detail the elements of the signal which evoke orienting 349 reactions. The situation in which the subject has to differentiate complex chain stimuli is the most efl’eclive. O. P. Terekhova’s experiments (1957) have shown that the orienting reaction arises the most stably:

p a) at the moment of application of the stimulus;

p b) at the moment when the sign differentiating the stimuli is received.

These investigations were continued by L. Aran (1960) and Eckles (1961) who identified complex sound sequences resembling the signals of Morse’s code. In these experiments it was also shown that the orienting reactions are the most stably preserved in the process of elaborating a differentiation of several sequences in response to application of the signal at the moments which are differentiating signs of these complex signals. If the sound stimuli which make up the complex are equal in strength, pitch and timbre, they differ only as to the place they occupy in the sequence of the sounds which form the complex. With knowledge of the complex signals which the subject has to differentiate and the frequency with which each of them appears in the experiment, as well as by observing the moments at which the orienting reactions appear and when the conditioned motor response arises, it is possible to establish the dependence of the orienting reflex of the conditioned reaction on the statistic properties of the signals.