45. ANIMALS AND PAIN

by Lord Brain

Whether or not animals feel pain is not altogether an easy question to answer. A human being has direct awareness only of the pains which he himself suffers. Our knowledge of the pains even of other human beings is only an inference from their words, and to a lesser extent their behaviour. Animals cannot tell us what they feel. We can, of course, study their bodily reactions to the kind of stimuli which would be painful to human beings and this has often been done. When such stimuli are applied to animals their pupils dilate, their pulse rate and blood pressure rise, they may withdraw the stimulated limb and they may make struggling movements. Nevertheless it has been pointed out that none of these reactions can safely be taken as indication that the animal experiences pain because they can all be evoked when the parts of the body stimulated have been isolated from the higher nervous centres. Furthermore, when disease produces such an isolation in human beings the corresponding stimuli are painless. We must therefore look for other evidence as to the capacity of animals to experience pain.

Because man alone can describe his experiences most of what we know about the physiological basis of pain, and its pathways in the nervous system, is derived from observations upon him. Some of these have been made with volunteers who have exposed themselves to painful stimuli, or even like the late Sir Henry Head, have had their own nerves divided so that they could observe the effects of interruption of "pain pathways ". But most human investigations have been based upon observations of the effects of disease. Pain is essentially a reaction to tissue damage, which is its commonest cause, but since there are pain pathways in the nervous system through which nervous impulses concerned with the appreciation of pain are conducted, pain may arise not only as the result of damage to non-nervous tissues, but also because the nervous pathways themselves are irritated. Moreover, disease may selectively destroy the pathways in the nervous system that conduct pain-impulses: the study of such damage has made it possible to map these within the spinal cord and brain. Once surgeons discovered where these pathways were, they could interrupt them in order to abolish the sensation of pain when this could not be treated in any other way - for example, for the relief of intractable cancer. Finally, studies of human pathology established the anatomical pathways and centres in the nervous system by means of which pain is appreciated. It then became possible for physiologists, using microelectrodes and electrical recording methods, to apply to animals stimuli which would cause pain to human beings and record the electrical responses which these evoked at different levels of the nervous system.

Within the spinal cord there exist two main pathways for sensory impulses. Those concerned with the discriminative aspects, i.e. the recognition of touch and posture, size and shape, run mainly in the columns at the back of the cord on the same side on which they enter it, and those concerned with appreciation of pain, heat or cold cross over and run upwards at the front and at the opposite side to that on which they enter - in the antero-lateral column. This means that interruption of the antero-lateral column on one side abolishes appreciation of pain on the opposite side of the body below the level of the lesion. The fibres concerned with pain, after entering the brain from the spinal cord, form connections with a deeply placed region of the brain-stem known as the central reticular formation, which, among other functions, is concerned with attention, alertness, and the maintenance of the waking state. The pain fibres end in part of the optic thalamus, a complex group of nuclei in the base of the brain.

Basically, all the nervous elements which underlie the experience of pain by human beings are to be found in all mammalian vertebrates at least; this is hardly surprising as pain is a response to a potentially harmful stimulus and is therefore of great biological importance for survival. Is there any reason, then, for supposing that animals, though equipped with all the necessary neurological structures, do not experience pain? Such a view would seem to presuppose a profound qualitative difference in the mental life of animals and men. The difference between the human and subhuman ner vous system lies chiefly in the much greater development of the human fore-brain. This would be significant in the present context only if there were reason to believe that it alone was correlated with the occurrence of conscious experiences. But much of our knowledge of the nervous regulation of consciousness is derived from experiments on animals.

In everyday life we take it for granted that animals see and hear, and there seems no reason to suppose that they do not feel pain. So, while the reactions of the pupils, pulse rate and blood pressure mentioned above can in exceptional circumstances occur without the conscious experience of pain, it seems likely that in the intact animal they are indications that pain is being experienced.

Nevertheless we cannot tell what pain means to an animal. We know from our own experience that pains differ greatly in their quality, severity and emotional accompaniments. We know, too, that experiences which in man would be fraught with intense and prolonged emotion cause only brief and fleeting disturbances in animals. The great development of the human fore-brain is the basis of man's capacity for foresight, apprehension, and memory which so profoundly affects his experiences. It would be rash, therefore, to conclude that the pain which animals feel can be in all respects equated with that which human beings experience.

(from The New Scientist, 22nd August, 1963)