Pain, Rejection, and the Optimal Calibration Hypothesis

A paper by Chester et al. was published in the July 2012 issue of Frontiers in Evolutionary Neuroscience under the title:

"The optimal calibration hypothesis: how life history modulates the brain's social pain network."

This work presents a novel way of looking at adaptations in the sensitivity of the "social pain network" throughout the human lifespan.

The authors start out by reviewing the evidence of overlap between the brain networks involved in pain perception, and the neural substrates of the affective response to social threats, such as rejection. Reviewed are the two neocortical structures which appear to be involved in both. The anterior cingulate cortex (ACC) is thought to support the affective features of pain, but activation in this region is also observed in response to perceived social rejection. Similarly, the anterior insula appears to be involved in affective responses to both physical pain and social rejection.

Additional background for the hypothesis of this paper comes from Attachment theory, which provides three broad categories of early life attachment styles arrived at, purportedly, through people's drive to avoid social pain. The styles are: "anxious", "avoidant", and "secure", each referring to a distinct response pattern to social threats. Chester et al. suggest that anxious attachment may be thought of hyperactivation and, presumably, hyper-reactivity of the social pain system, whereas avoidant attachment is associated with hypoactivation and hyporeactivity of the system. Indeed, a recent study by DeWall et al. (2012) showed that anxiously-attached individuals showed increased activity in the dorsal ACC and the anterior insula in response to social rejection. Likewise, avoidantly-attached individuals showed decreased activity in these areas. How do these differences come about? Enter the Optimal Calibration Hypothesis.

In short, the reasoning goes as follows:

There are fitness benefits to having a pliable social pain network in childhood, as it will be able to adapt to its environment, whereby chronic social pain in childhood will be alleviated in adulthood by desensitization of the social pain network, whereas "unpredictable" rejection in childhood will equip the social pain network with heightened sensitivity for better detection of such acute rejection events. "The social pain network's plasticity likely evolved due to the fitness benefits that are commensurate with a flexible response to rejection" (p.6)

Predictions:

• Early life experiences define the responsiveness of the social pain network (SPN) 
• Greater responsiveness of the social pain network in individuals with a childhood history of volatile social rejection 
• Reduced responsiveness of the social pain network in individuals with a childhood history of chronic rejection 
• Increased activation of the social pain network at any given age in response to rejection from individuals which fall into the most relative relationship categories for that age. 

Points of Contention:

This otherwise attractive framework is unfortunately garnished with the dubious claim that "avoidant and anxious attachment styles result from early social ecologies characterized by inadequate care-giving" (p.3). Saying that childhood experiences shape the SPN is not the same as saying that attachment styles result from childhood experiences (at least some credit must go to genetics here).

"Given that the evolution of overlapping neural substrates for physical and social pain would be predicated on the existence of social threats, it is expected that social threats preceded the aforementioned neural overlap in time." (p.2)

This point sounds reasonable, and I can see its intuitive appeal, but it cannot be taken for granted. Social threats (e.g. ostracism) can only exist in a species which has already embraced and depends upon life in communities. For such living conditions to have come about, organisms must already be in one way or another drawn towards social living and/or motivated to avoid "anti-social" action (actually, both, since not obtaining the benefits of communal life when others are, becomes an evolutionary risk). In essence, if the risks of anti-social behavior were present, individuals less prone towards such behaviors were at an advantage. Evolution, being a tinkerer, makes use of already existing mechanisms for threat detection and selects for individuals who rely on their existing networks for physical threat detection and response in socially threatening situations.

So far so good. However, the implicit premise of this argument is that networks for pain pre-date social threats and, therefore, social threat-detecting networks. This premise needs to be defended. It is perfectly plausible that humans, as well as many other species were always social. I.e. physical and social pain were never separate and social threats were threatening in the same way as physical threats were. Thus, the existing pain network was from its inception employed for both types of threats. These authors' position necessitates that at some point in our evolutionary history our ancestors were "loners" and thus were only concerned with physical stressors (which may or may not be the case, of course). But one shouldn't easily fall into the trap of assuming that pain was ever only "physical" (at least not the type of pain which is supported by structures such as the anterior cingulate and the anterior insula).

Perhaps the biggest point of contention is the reasoning which the authors give as justification for their hypothesis, namely that

"individuals with a flexible social pain system are able to maintain social pain as an informative signal, recruit parental investment, retain and acquire mates, and avoid the health issues that are comorbid with social rejection" (p.6)

The claim that the retention of social pain as an informative signal drove the evolution of "optimal calibration" of the pain network has some flaws. The authors suggest that under conditions of chronic rejection, chronic social pain would be uninformative, counterproductive and, indeed, unhealthy (as explained in their short overview of the effects of chronic stress on the immune system). This may be the case, of course, but chronic physical pain is no more informative, productive, or healthy, yet it occurs fairly often. Moreover, I fail to see how desensitization to social rejection in individuals who have suffered chronic rejection in their childhood would result in the maintenance of social pain as an informative signal. If the network becomes desensitized, signals will be reduced throughout.

References

Chester, D. S., Pond Jr, R. S., Richman, S. B., & DeWall, C. N. (2012). The optimal calibration hypothesis: how life history modulates the brain's social pain network. Frontiers in evolutionary neuroscience, 4.

Eisenberger, N. I., Lieberman, M. D., & Williams, K. D. (2003). Does rejection hurt? An fMRI study of social exclusion. Science, 302(5643), 290-292.

Panksepp J. (2011). The neurobiology of social loss in animals: some keys to the puzzle of psychic pain in humans, in Social Pain: Neuropsychological and Health Implications of Loss and Rejection, eds MacDonald G., Jensen-Campbell L. A., editors. (Washington, DC: American Psychological Association; ), 11–51.