Thinking of anything else does not always reduce pain: It depends on timing

In this issue, you will find a paper by Silvestrini and Rainville (2013), entitled ‘After-effects of Cognitive Control on Pain’. In their study, the authors investigated the effect of a cognitive task on subsequent pain perception and nociceptive flexion reflex (NFR). It was found that both pain rating and NFR score increased after a highly demanding cognitive task, as compared to a less demanding cognitive task. The subject is interesting and original. Indeed, it is widely accepted that employment of cognitive resources during a noxious stimulation may reduce the subjective perception of pain through a mechanism of distraction from the stimulation itself (Chen, 2009; Ossipov et al., 2010). In other words, if one suffering from pain is induced, automatically or by a voluntary process, to diverge his/her attention from his/her own painful feeling, he/she will downgrade the subjective pain perception. It has also been demonstrated that distraction from a painful stimulus can reduce an objective measure of pain, such as the laser-evoked potential amplitude (Lorenz and Garcia-Larrea, 2003). However, what happens when the cognitive task precedes the noxious stimulation? Is the level of cognitive task difficulty important in the interaction between the initial cognitive resource allocation and the subsequent pain perception? The study by Silvestrini and Rainville (2013) answers these questions since the authors show that performing a numerical Stroop task in a condition with a high cognitive demand increases the subsequent pain perception, as compared with the same task performed in a condition with a low cognitive demand. Previous papers demonstrated a reduced pain tolerance following acts of self-control (Muraven et al., 2006; Vohs et al., 2008; Schmeichel and Vohs, 2009). However, in all these studies, reduced pain tolerance was suggested by a faster withdrawal of subject’s hand from the cold water following the self-control act. Therefore, there was not a direct pain measurement, nor an objective evidence of the level of the nociceptive sensitization. Silvestrini and Rainville (2013), beyond showing an increased subjective pain perception after a highly demanding cognitive task, demonstrated that the sensitization to the painful stimuli begins from the nociceptive input processing at the spinal cord level (increased NFR score). This last finding means that the increased pain perception is a top-down process, beginning with the allocation of cognitive resources (numerical Stroop task) and leading to a reduced efficiency of the descending pain control mechanisms. Some points remain to be elucidated and will have to be investigated in future studies. First, as was also underlined by Silvestrini and Rainville (2013), it is not clear whether the effective depletion of cognitive resources per se or merely the subjective perception of such depletion is responsible for the increased subsequent pain perception. Indeed, if submitted to a highly demanding cognitive task, one could feel less able to cope with a subsequent painful stimulus, independent of the effective consumption of cognitive resources. On the ground of the treatment of an expected pain, this is not a secondary point. While in the first hypothesis (quantitative effect of cognitive resource depletion), the subject could feel less pain only by reducing his/her own resource engagement in the prior cognitive task, in the other case (belief about one’s own willpower), a psychological treatment encouraging the subject to downgrade his/her subjective perception of the preceding cognitive effort may make him/her less sensitive to following painful stimuli. This is nothing else than what sportsmen define as ‘mental energy recovery’ between two consecutive competitions. Second, in the study by Silvestrini and Raiville’s (2013), the subjects were asked to rate pain due to a train of electrical stimuli. Each train was very short, lasting 30 ms. Such a phasic stimulus is very different from the far longer-lasting clinical pain referred by patients. This prevents any immediate application of Silvestrini and Rainville’s (2013) results to the clinical practice. However, it will be very interesting to investigate whether also a more ecological expected pain, as compared to that due to electrical stimuli, is influenced by a preceding depletion of cognitive resources.