Sample sizes were based on power calculations to detect large effects (effect size d > 0.5) in a within-subjects comparison, which have previously been reported in a similar study design. Twenty-five right-handed subjects (8 male) between 20 and 49 years of age (mean: 25.32, SD = 5.79) participated in study 2 (sample 2). Twenty-six right-handed subjects (13 male) between 18 and 43 years of age (mean: 25.5, SD = 5.81) participated in the study (sample 1). A high chance locus of control, the belief that powerful others control one's life, high catastrophizing, and high fear of pain were assumed to reduce the positive effects of perceived control on suffering. We expected reductions in SCR and EMG, but not HR. We hypothesized that control over pain would positively affect all 3 dimensions, with the strongest reduction related to suffering. The current study examined the influence of controllability on pain intensity, unpleasantness, and pain-related suffering in 2 experiments that differed in the instructions that were used to announce controllability. 36, 36, 47 We hypothesized that an internal locus of control would modulate higher effects of uncontrollability on pain than an external or chance locus of control. We therefore examined locus of control of reinforcement, which is the degree to which people believe that they have control over the outcome of positive or negative events in their lives as opposed to external forces beyond their control. The authors hypothesized that the individuals' locus of control might explain these interindividual variations. 59 reported that exerted control over painful stimulation led to reduced pain perception in half of the subjects, whereas the other half showed increased pain intensity ratings. The effect of the experimental manipulation of controllability is modulated by individual differences in the perception of control. 8 This would indicate multilevel effects and would permit bias-free assessments of pain and suffering also in groups that may not easily give verbal reports such as children or incapacitated persons. 56 In addition to observing the effects of control on verbal reports of pain and suffering, a secondary goal of the study was to see if there are also effects on physiological indicators of pain and suffering such as skin conductance responses (SCRs), corrugator electromyogram (EMG), and heart rate (HR). 6, 17 Ongoing but unsuccessful efforts to influence the pain make patients especially vulnerable to suffering. 10 The lack of controllability might be especially important for the experience of pain-related suffering in both experimental pain studies and in patients. 8, 14– 16, 54 We have demonstrated that pain-related suffering is an additional component of pain that can be assessed independently of pain intensity and unpleasantness. Suffering correlates only moderately with fear and anxiety where avoidance of a feared object prevails, whereas suffering focusses on the self. However, the meaning assigned to distressing events may vary between persons, based on their personality and personal history. Distress evoked by pain can induce suffering. 3 Suffering is an overwhelming experience, which is perceived when the intactness of the person is threatened. This is also true for patients with chronic pain. Carnevale 13 suggested that suffering is the most important factor that drives patients to seek medical attention. Anxiety, 59 helplessness, 60 or pain unpleasantness 7 were not assessed consistently across studies, although these variables may modulate the effects of control on pain perception. 21, 28, 49 Although actual control was more effective than perceived control alone in reducing pain, 40 this dissociation cannot fully explain these ambiguous findings because exerted control did not reduce pain intensity 59 and perceived control was found to decrease pain intensity 7 in other studies. 31, 45, 46 Studies in healthy volunteers showed that controllable situations reduce pain intensity 7, 40, 60 and unpleasantness 7 however, controllability did not always change pain perception. Pain perception is modulated by cognitive and emotional variables such as predictability, 12, 39 controllability, 7, 60 attentional focus, 1, 48 or fear of pain.
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