The effect of pain on human time perception

Abstract

This thesis explored the effect of pain on human temporal perception. This aim was achieved firstly by systematically testing the way in which pain experience affects duration estimates, and memory for duration and secondly by examining whether it was possible to reduce perceived duration of pain in clinical and no clinical population. Chapter 5 examined the effect of different pain intensities on perceived duration when pain was the to-be-timed stimulus (i.e., task-relevant) and when pain was in the background (i.e., task-irrelevant). Participants were required to verbally estimate the duration of no pain, low pain and high pain electro-cutaneous stimulations and the duration of a neutral visual stimulus whilst being exposed to no pain, low pain and high pain thermal stimulation. Increases in the intensity of the electro-cutaneous stimulation were associated with longer verbal estimates, reflecting a multiplicative effect. However, low pain thermal stimulation did not affect the perceived duration of the visual stimulus and high pain thermal stimulation led to shorter verbal estimates. The lengthening effect of pain therefore appeared to be limited to circumstances when pain was task-relevant. Chapter 6 examined whether changes in physiological arousal mediated the effect of task-relevant and task-irrelevant pain on time perception. Participants’ physiological activity (skin conductance level and high frequency heart rate variability) was measured while they were asked to verbally estimate the duration of an electro-cutaneous stimulation at different intensities and a neutral stimulus whilst perceiving a thermal stimulation at different intensities. The lengthening effect of task-relevant pain on time perception, although did not replicate the multiplicative effect, was mediated by sympathetic arousal, supporting previous suggestions that temporal distortions due to pain are caused by changes in the arousal level. However, task-irrelevant pain did not affect verbal estimates of participants, despite it increased their physiological arousal, and there was no relationship between physiological arousal and verbal estimates. This suggests that changes in arousal do not affect time perception when arousal arises from sources other than the to-be-timed stimulus. Chapter 7 examined whether pain enhanced or disrupted the memorization of duration by using a temporal generalisation task. Participants were required to encode the duration of a tone whilst experiencing neutral or painful thermal stimulation and to recall the duration immediately after learning or after a delay. Delay affected neutral and pain related durations in a comparable way, suggesting that pain does not have any unique effect on the memorization of duration: pain does not enhance nor disrupt the memorization of duration information. Chapter 8 tested whether a mindfulness intervention could reduce the lengthening effect of pain in heathy people and in chronic pain patients. Participants were asked to estimate the duration of visual, vibrotactile and electro-cutaneous stimuli before and after practicing mindfulness meditation for a week. Healthy participants gave similar verbal estimates before and after the intervention, suggesting that mindfulness was not able to modulate the perceived duration in any stimulus modality. In chronic pain patients mindfulness practice led to longer verbal estimates in any stimulus modality including pain, suggesting that mindfulness was not an appropriate intervention to reduce the lengthening effect of pain, however, caution should be taken when interpreting this latter finding due to the small sample. Together the finding of this thesis show that task relevant pain distorts time, in part due to its capacity to increase sympathetic nervous system activity. Pain, however, appears to have no influence on memory for duration. Furthermore, interventions which reduce the intensity of pain do not appear to be effective in reducing the perceived duration of pain. Further research is therefore required to understand how the lengthening effect of pain can be mitigated in clinical and non-clinical settings.