Mémoire de la douleur

Abstract

The nociceptive input sets off a short sensation, leading to a behavioural response and an emotional reaction, which are the only facts stored in the memory, whereas the precise nociception cannot be re-evoked by a visual reminder, for example. However, this classical view is debated in light of many data which prove the possible reappearance of a former pain after an injury of the nervous system. There are many kinds of associations between memory and pain, which leads to many confusing terminologies. This relation could not be tackled without the knowledge of the differents boxes of memory in cognitive neuropsychology: the dichotomy between explicit and implicit memories (or conditional learning) may be useful. If a pain may facilate the encoding of the context, is it conceivable that isolated context may induce the pain? The implicit memory has been proved in animals after a Pavlov conditioning. It has been extensively proved in humans, particularly in infants. The explicit recall of an acute pain may be measured by using verbal or visual scale a long time after the acute phase: the memory is related to the context with affective patterns rather than the precise somatic pain. It has to be understood if the patient describes the pain intensity by re-evoking the nociceptive input or by recalling only the verbal or visual scale filled in the hospital. However, the recognition of a past acute pain is possible, which proves that a trace of the painful stimulus is stored in the brain, even though it is not accessible spontaneously. The pain influences the encoding of the associated context in two directions: hyper-encoding and the “flash-bulb memory” associated to some strongly emotional events where the subject may recall spontaneously the entire context as if it was etched in one’s mind. Such a fact has been reported in acute pain with stressful event. But a perturbation of other memories may be induced by an acute or chronic pain: the disruption may concern some components of the episodic verbal memory or the pastauto biographical memory. The most caricatural situation is the transient global amnesia induced by an acute pain. More frequently, a chronic pain patient who was treated with many opioids or psychotropes has lower results on the psychometric tests in the domains of memory and attention. Rarely a past pain reappears without somatic injury as a reminiscence: that kind of recall is implicit since the subject is unaware of the phenomenon until he or she recognized it later by remembering his past. This possibility of remembering pain or suffering may explain some psychogenic phenomena in adults exposed to chronic pain and stress before the acquisition of language when the pain is necessarily encoded in an implicit way.