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p Introducing the concept of the orienting reflex, as a special unconditioned reflex, Pavlov emphasised both the peculiarity of the factors which evoke this reflex and the specific character of the adaptational importance of the separate reactions (components), which make up this reflex act, to the organism.

p As a special stimulus of the orienting reflex, he pointed out the “novelty” of the stimulus and, as ils biological significance—the set of receptor to facilitate the reception of the stimulus. Pavlov wrote: “Upon appearance of new agents in the animal’s environment (by this I also mean the strength, new intensity of old agents) the organism sets up in their direction appropriate receptor surfaces for the best possible impression of the external stimulations.”  [334•* 

p The fact that the orienting reflex is associated with a corresponding set of receptor at first gave Pavlov reason to regard this reflex as a set reflex. “Before us is again a fatal reaction of the organism—a simple reflex which we call an orienting, set reflex.”  [334•** 

p Subsequently, however, he relinquished such identification of orienting and set reflexes. As a matter of fact, there are set reflexes (muscular reactions of change in posture), which are not excited by the “novelty” of the stimulus. Afterwards Pavlov used only the term “orienting reflex”, emphasising again, as an essential characteristic, the “novelty” as a special agent, and the set of a receptor surface as an expression of the biological significance of this reflex. It may be said that the orienting 335 reflex is such a set reflex whose special agent is the “novelty” of the stimulus.

p Inasmuch as the orienting reflex was usually studied by observation of its external manifestations in the reactions of the skeletal muscles, the motor components (movements of the ears, eyes, head, body) began to be regarded as the only manifestation of the orienting reflex, and the participation of the skeletal muscles—as its essential characteristic. This point of view was given expression by A. A. Biryukov: “...We see the essence of the concept of the orienting reflex in emphasising its muscle-setting nature.”  [335•* 

p From this point of view the respiratory, vascular and other components are regarded as collateral, secondary manifestations. Suffice it to analyse the animal’s “sniffing” as a manifestation of the orienting reaction, directly connected with the change in the respiratory acts, to see the groundlessness of this assertion. Analogous examples may be cited with regard to “orienting” salivation associated with an increased reactivity of the taste analyser.

p Since the reactivity of an analyser depends not only on setting up a peripheral receptor surface, but also on the functional state of the higher parts of the brain, the muscle-setting is not the only way to increase the reactivity of the analyser, and all the reactions associated with the intensification of the functional state of the analysers may be ascribed to the system of the orienting reflex, providing they arise in connection with the “novelty” of the stimulus.

p Those who regard the orienting reaction as a reaction to the “novelty” of the stimulus usually imply two different phenomena:

p a) the reflex arising at the moment of application of the stimulus later disappears against the background of the continuing action of the stimulus; upon removal of the stimulus the reflex reappears;

p b) arising upon the first appearance of the temporarily acting stimulus the reflex then weakens and completely disappears during repeated applications of the stimulus.

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p These two characteristics of “novelty” are not equivalent. The first may occur in decorticated animals, the second is, in higher animals, clearly associated with the cortical mechanism.

p The ability of the reflex to disappear in the course of the continuing action of a stimulus is not a sufficient indication of the orienting reaction. The point is that, as the stimulus continues to act, such unconditioned reflexes which are clearly not orienting may also disappear. For example, the heat regulating vascular reflex may disappear in the course of a long-acting weak or locally applied thermal (for instance, cold) stimulus.

p Thus it is not the disappearance of the reaction upon prolonged application of the stimulus that is an essential characteristic of the orienting reaction. This also occurs in other unconditioned reflexes. Removal of the stimulus in this case evokes an opposite reaction.

p In evaluating the orienting reflex it is necessary to consider the sign of the reaction upon the change in the stimulus.

p Only the reaction whose sign does not depend on the direction of the change in the stimulus may be regarded as a true orienting reaction to the “novelty”.

p In other words, the critical characteristic is the nonspecific character of the reaction with regard to the changes in the intensity of the stimulus.

p The simplest explanation of the mechanism of such non-specificity of the orienting reaction with regard to the power characteristic of the stimulus may be the idea of the role of the “on” and “off” elements as agents of the orienting reflex.

p The aforesaid criteria for differentiating the unconditioned reactions are applicable to a characterisation of the origin of the orienting reaction against the background of its extinction as the result of repeated application of a stimulus. It was precisely the extinction that many authors assumed to be the main characteristic of the orienting reflex. Thus, according to L. A. Orbeli, the specific feature of the orienting reflex is that it arises as an unconditioned reflex, proceeds as a conditioned reflex and is extinguished upon repeated application of the stimulus.  [336•* 

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p However, extinction cannot be considered a characteristic sign of the orienting reflex without essential reservations. The thing is that the phenomenon of extinction can also be observed in other reflexes upon repeated application of a stimulus. Such, for example, is the extinction of the adaptational pupillary reaction during repeated application of weak light stimuli (Y. N. Sokolov, 1958). An anologous phenomenon is observed in cases of adaptational vascular reflexes (O. S. Vinogradova, 1961).

p A characteristic feature of the orienting reflex is not only the effect of extinction, but also that the reaction is independent of the direction of the change in the agent applied.

p The criterion of the “independence of the sign of the reaction from the direction of the change in the stimulus" requires a more precise definition. The point is that after extinction of the adaptational reflexes to weak specific stimuli, for example, after the pupil ceases to contract in response to the action of weak light, both weakening and intensification of the light evoke the specific reaction of contraction of the pupil.

p The same sign of the reaction does not make it possible to differentiate the extinction of the orienting reaction from the extinction of specific reflexes. This is precisely what A. R. Shakhnovich fails to take into account in regarding the contraction of the pupil as an orienting light reflex during a change in the stimulus.  [337•* 

p A specific, for example, heat-regulating reflex is also restored with the same sign that was possessed by the reaction which was being extinguished. The change in the stimulus after extinction of the reactions to heat restores the reaction of dilatation of the vessels, whereas the change in the stimulus after extinction of the reactions to cold restores the effect of contraction.

p The parallel registration of the heat-regulating and electrodermal reactions has shown that the “novelty” stimulates both the extinguished orienting and the extinguished heat-regulating reflexes.

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p Thus it may be concluded thai il is not the “novelty” of the stimulus which is itself a characteristic of the orienting reaction, if it is a question of the effect of extinction, but the peculiarity of the unconditioned orienting reaction which may be characterised as non-specific with regard to all changes in the stimulus. It follows from this that, speaking of the “novelty” factor during extinction, it is necessary to remember the possibility of disinhibition of the adaptational reflexes upon a change in the stimulus used during extinction. The effect of appearance of the orienting reaction proper and the restoration of the specific adaptational reflex may coexist, entering into complex interaction if the effector mechanisms of these reflexes have common executive organs.  [338•* 

p Since during extinction a stimulus may be new only with regard to the traces left in the nervous system by the stimulus used earlier, it is necessary to suppose that not the afferent signals themselves are the source of excitation of the orienting reaction, but the impulses of discoordination which arise at the moment when the new stimulus does not coincide with the formed trace—“nervous model of the stimulus”.

Thus in characterising the orienting reflex it is necessary to emphasise not only the property of extinction, not only the fact that the agent of the reaction is the “novelty” of the stimulus which is determined by the degree of the failure of the signal being received to coincide with the “nervous model of the stimulus" formed in the nervous system but also the non-specific character of the reaction in all changes.