Why does dream pain occur? A brief review of current theories

The International Association for the Study of Pain (IASP) defines pain as “an unpleasant sensory and emotional experience associated with actual or potential tissue damage, or described in terms of such damage.” The IASP definition of pain includes in its notes, “Pain is always subjective”. The term “subjective” emphasizes that pain is a conscious experience rather than simply a causal result of unconscious nociceptive processing. The intuition underlying the IASP definition of pain is that if a pain is not being consciously felt by its owner then it does not exist.

Up until the late twentieth century, it was widely believed by pain researchers that conscious pain could not be felt by humans in sleep because sleep is an unconscious state. This pre-scientific intuition about pain has since been undermined by numerous scientific studies showing that both stimulus-induced and non-stimulus induced pain reported during rapid eye movement (REM) sleep does not always result in subject wakefulness and that pain can also feature in dreams (e.g., Nielsen et al. 1993; Zadra et al. 1998; Raymond et al. 2002; Knoth & Schredl, 2011). Concerning dream pain, Zadra et al. (1998) found that 48.2% of subjects reported that they have had at least one pain dream in their lives, although only 0.62% of recorded dreams in home diary studies contain clear reference to pain feelings and meanings. In some subjects, dream pain continues to be felt following wakefulness; in other subjects, it is rapidly terminated after awakening. These divergent findings prompt the causal question: Why does dream pain exist and have the nature it does?

To explain dream pain, Schredl (2003) proposed that there is continuity between wakeful and dream pain such that pain regularly felt when awake is causally efficacious concerning its occurrence in dreams. Raymond et al. (2002) investigated Schredl’s “continuity hypothesis” in hospitalized burn patients and healthy control subjects and found that patients reported a significantly higher rate of dreamed pain than controls. The burn patients also reported marginally more intense pain during wakeful medical procedures, a finding which was interpreted by Raymond et al. (2002) to support the continuity hypothesis. The patients reported constant pain during wakefulness, which further supports the continuity hypothesis. However, there are three further competing interpretations of the data presented in Raymond et al. (2002). Since dreamed pain was not always reported as located in injured body regions or in bodily areas patients reported pain in during the night and following awakening, Raymond et al. (2002) speculated that dream pain might not be causally continuous with wakeful pain experiences, but with a personal pain memory trace formed after wakeful pain experiences. In support of this interpretation of the data, Jantsch et al. (2009) showed the existence of a reliable long-term memory trace for experimentally induced pain sensations. This finding would explain the rarity of reported dream pain in controls since pain is rare in their everyday experience. A competing causal explanation of the data in Raymond et al. (2002) is that some dreamers report pain they had never experienced in real life (e.g., dream pain in a fictional fight situation) (Schredl, 2011). In support of this view, Danziger et al. (2009) found that people with congenital insensitivity to pain show patterns of brain activation in shared-circuits for “self” and “other” pain while observing pain in other persons. This finding leads to the proposal that pain observed externally in others or in electronic media might also explain dream pain (Borsook & Beccera, 2009).

Thus, the three explanations on offer to explain why dream pain occurs are: (1) Dream pain is causally continuous with wakeful pain experiences; (2) Dream pain is causally continuous with personal pain memories formed after wakeful pain experiences; and (3) Dream pain is causally continuous with pain observed externally in others or in electronic media during wakefulness. These competing explanations show that the task of explaining why dream pain occurs is still very much an open question in the field, and more research on the topic is needed.

References

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