Ethics of Research involving animals
Representations of pain and suffering and their neurological context
4.15 In most mammals, the ascending pain pathways not only relay nervous impulses in the brain stem, but also in the thalamus before ascending to the somatosensory or ‘touch’ neocortex, which enables the localisation of pain. In humans, this localisation can be exceptionally accurate for primary pain, which can result, for example, from a knife cut or burn, but inaccurate for chronic deep-organ pain because there is no mapped representation of these areas in the human brain.14
4.16 Pain pathways also extend to other areas of the cortex, known as the association cortex, the great expansion of which is unique to humans and certain other primates, such as the great apes. These areas are virtually non-existent in the brains of rodents, where more than 70 percent of the cortical structures are responsible for processing olfactory information (in humans, less than one percent of cortical structures have this function). It is significant that the embeddedness of pain processing in the association cortex in humans contributes to the emotional dimension of pain, which is a characteristic of suffering. It is therefore possible to interpret suffering as a higher-order phenomenon in that it relates to the experience of chronic pain in a predominantly negative way. Furthermore, this finding suggests that animals such as mice, which lack similarly developed brain structures, may be very unlikely to experience suffering resulting from pain in a similar way, although they do suffer pain itself. Therefore, evidence about differences in the way in which pain is embedded in the brains of different animals supports the view that care is required when ascribing states such as suffering to mice.
4.17 The embeddedness of pain processing in the association cortex also appears to contribute to the phenomenon that suffering can be extremely variable between, and within, individuals. Some humans, and possibly also some closely related animals, have the ability to feel pain and suffering when there is no pain stimulus, to be untroubled by pain when there is what others would objectively describe as pain and even to enjoy pain being inflicted in sexual contexts. In adults, the fear of the dentist can intensify innocuous sensations, but the belief that it is a price worth paying in order to avoid far greater suffering can also render the experience of the treatment less significant. The latter capacity is not usually found in children, which may suggest that beings with less developed rational capacities are not necessarily suffering less, but more, since they are not in a position to conceptualise the pain as a means to an end.