Jamie L. Rhudy

Fear and anxiety: divergent effects on human pain thresholds

Abstract Animal studies suggest that fear inhibits pain whereas anxiety enhances it; however it is unclear whether these effects generalize to humans. The present study examined the effects of experimentally induced fear and anxiety on radiant heat pain thresholds. Sixty male and female human subjects were randomly assigned to 1 of 3 emotion induction conditions: (1) fear, induced by exposure to three brief shocks; (2) anxiety, elicited by the threat of shock; (3) neutral, with no intervention. Pain thresholds were tested before and after emotion induction. Results suggest that findings from animal studies extend to humans: fear resulted in decreased pain reactivity, while anxiety led to increased reactivity. Pain rating data indicated that participants used consistent subjective criteria to indicate pain thresholds. Both subjective and physiological indicators (skin conductance level, heart rate) confirmed that the treatment conditions produced the targeted emotional states. These results support the view that emotional states modulate human pain reactivity.

Pain and Emotion: Effects of Affective Picture Modulation

Objective and Methods Two experiments examined the impact of viewing unpleasant, pleasant, and neutral photographic slides on cold-pain perception in healthy men and women. In each experiment, participants viewed one of three slide shows (experiment 1 = fear, disgust, or neutral; experiment 2 = erotic, nurturant, or neutral) immediately before a cold-pressor task. Skin conductance and heart rate were recorded during the slide shows, whereas visual analog scale ratings of pain intensity and unpleasantness thresholds and pain tolerance were recorded during the cold-pressor task. Results Viewing fear and disgust slides decreased pain intensity and unpleasantness thresholds, but only the fear slides decreased pain tolerance. In contrast, viewing erotic, but not nurturant, slides increased pain intensity and unpleasantness threshold ratings on the visual analog scale in men, whereas neither nurturant nor erotic slides altered pain tolerance. Conclusions These results are consistent with a motivational priming model that predicts that unpleasant affective states should enhance pain and that pleasant affective states should attenuate it.

The role of emotion in pain modulation

Although most agree that emotion can alter pain, little well-controlled research has been conducted to examine this issue. The present review provides psychological and physiological rationales for considering the influence of emotion on pain, followed by an overview of recent work in this area. We conclude that the pain-modulating effects of emotion are best characterized by an interaction between valence and arousal. Positive emotions lead to pain reduction as long as a minimal threshold of arousal is attained. However, negative emotions only lead to pain inhibition when they are highly arousing. Negative emotions coupled with low-to-moderate arousal facilitate pain.

Emotional control of nociceptive reactions (ECON): Do affective valence and arousal play a role?

&NA; Prior research suggests emotional picture‐viewing modulates motoric (nociceptive flexion reflex), autonomic (skin conductance response, heart rate acceleration), and subjective (pain rating) reactions to noxious electrodermal stimulation. The present study sought to determine whether emotional valence and arousal contribute to nociception modulation. To do so, pictures varying in emotional content (erotica, food, neutral, loss, attack) were chosen to manipulate emotional valence (pleasant = erotic and food; unpleasant = loss and attack) and arousal (low = food and loss; moderate = erotica and attack). Pictures were presented in pseudorandom order to elicit emotional processing while noxious electric stimulations were delivered to the sural nerve. Nociceptive flexion reflex (NFR) magnitude, skin conductance response (SCR), heart rate (HR) acceleration, and subjective pain ratings to each stimulation were measured, standardized, averaged by picture content, and analyzed. Results suggested that picture‐viewing explained 52% of the variance in the multivariate combination of the nociceptive reactions and modulated them in parallel. Pleasant pictures inhibited reactions, whereas unpleasant pictures enhanced them. However, only erotica and attack pictures elicited significant modulation relative to neutral pictures, suggesting arousal also contributed. An exploratory multilevel analysis also supported this conclusion. Together, these data suggest emotional control of nociceptive reactions (ECON) is associated with a valence‐by‐arousal interaction. Implications of these findings for how emotional picture‐viewing can be used to study supraspinal modulation are discussed.

Defining the nociceptive flexion reflex (NFR) threshold in human participants: A comparison of different scoring criteria

Abstract Despite the widespread use of the nociceptive flexion reflex (NFR) paradigm in clinical and experimental pain research, there is currently no consensus on how best to define NFR threshold. Accordingly, the present studies were designed to assess the accuracy and reliability of different NFR threshold scoring criteria. Study 1 compared 13 scoring criteria in their accuracy for identifying the presence of the NFR, then generated empirically derived cut‐points for the best criteria, and examined the test–retest reliability of NFR thresholds derived from these cut‐points. Study 2 evaluated the replicability of these findings in an independent sample. Results from the two studies suggested that standardized peak (NFR Interval Peak z score) and mean (NFR Interval z score) biceps femoris electromyogram (EMG) activity were accurate and reliable criteria for defining NFR threshold. Acknowledging that cut‐points may need to be adjusted for different research designs, graphs depicting sensitivity and specificity across a range of cut‐points have been provided to facilitate researchers decision‐making. It is hoped that the results of these studies will promote a standard NFR threshold assessment methodology, and further encourage the application of the NFR paradigm in the investigation of mechanisms and characteristics of both painful and non‐painful diseases.

Negative affect: effects on an evaluative measure of human pain

Prior work indicates that exposure to fear‐inducing shock inhibits finger‐withdrawal to radiant heat in humans (hypoalgesia), whereas anxiety induced by threat of shock enhances reactivity (hyperalgesia; Pain 84 (2000) 65–75). Although finger‐withdrawal latencies are thought to reflect changes in pain sensitivity, additional measures of pain are needed to determine whether pain perception is altered. The present study examined the impact of negative affect on visual analog scale (VAS) ratings of fixed duration thermal stimuli. One hundred twenty‐seven male and female human subjects were randomly assigned to one of three emotion‐induction conditions: (1) negative affect induced by exposure to three brief shocks; (2) negative affect elicited by the threat of shock without presentation; and (3) neutral affect, with no intervention. VAS ratings were tested before and after emotion‐induction. Results suggest that both negative affect manipulations reduced pain. Manipulation checks indicated that the emotion‐induction treatments induced similar levels of fear but with different arousal levels. Potential mechanisms for affect induced changes in pain are discussed.

Psychological Risk Factors in Headache

Headache is a chronic disease that occurs with varying frequency and results in varying levels of disability. To date, the majority of research and clinical focus has been on the role of biological factors in headache and headache‐related disability. However, reliance on a purely biomedical model of headache does not account for all aspects of headache and associated disability. Using a biopsychosocial framework, the current manuscript expands the view of what factors influence headache by considering the role psychological (i.e., cognitive and affective) factors have in the development, course, and consequences of headache. The manuscript initially reviews evidence showing that neural circuits responsible for cognitive–affective phenomena are highly interconnected with the circuitry responsible for headache pain. The manuscript then reviews the influence cognitions (locus of control and self‐efficacy) and negative affect (depression, anxiety, and anger) have on the development of headache attacks, perception of headache pain, adherence to prescribed treatment, headache treatment outcome, and headache‐related disability. The manuscript concludes with a discussion of the clinical implications of considering psychological factors when treating headache.

Gender Differences in Pain: Do Emotions Play a Role?

BACKGROUND Research suggests that the influence of gender on the processing and experience of pain is a result of several mechanisms. One mediating variable is emotion, which may modulate pain through an interaction of valence (pleasant-unpleasant) and arousal (calm-excited). OBJECTIVE This review examines whether gender differences in the experience and processing of emotion contribute to differences in the modulation and perception of pain. METHODS An English-language search of MEDLINE and PsycINFO was conducted from 1887 to May 2005. Additional literature was obtained from reference lists of articles retained in the initial search. RESULTS Emotion appears to influence pain through a valence-by-arousal interaction. Specifically, negatively valenced emotions with low to moderate arousal (eg, anxiety) enhance pain, whereas negatively valenced emotions with high arousal (eg, fear) reduce pain. In contrast, positively valenced emotions always reduce pain, as long as minimal arousal is achieved. Some evidence suggests that women are more sensitive than men to threat-related stimuli and thus experience more negative affect than men. This would generally lead to enhanced pain perception in women. It is also possible that women are more likely than men to experience negative affect with high arousal (intense fear) and thus pain inhibition. However, the relatively lower base rate of intense negative emotions is not likely to contribute much to gender differences in pain. Evidence also suggests that men may be more sensitive to positive events, particularly sexual/erotic stimuli, which may lead to more positive emotion-induced pain reduction in men, relative to women. CONCLUSIONS This review suggests that gender differences in the experience of pain may arise from differences in the experience and processing of emotion that, in turn, differentially alter pain processing. Specifically, the system associated with negative affect may be more attuned to threatening stimuli in women, and the system associated with positive affect may be more attuned to pleasurable stimuli in men. However, there is a paucity of research directly addressing this issue; much of the research on this topic has failed to test a comprehensive model of emotion, failed to use adequate manipulation checks, or failed to use within-subject experimental designs that control for intra- and interindividual differences. Therefore, it is concluded that additional research is warranted.

Emotional modulation of spinal nociception and pain: The impact of predictable noxious stimulation

&NA; Recent evidence suggests that emotional picture‐viewing is a reliable method of engaging descending modulation of spinal nociception. The present study attempted to replicate these findings and determine the effect of noxious stimulus predictability. Participants viewed pictures from the International Affective Picture System (IAPS), during which pain and nociceptive flexion reflexes (NFR) were elicited by electric shocks delivered to the sural nerve. For half of the participants (n = 25) shocks were preceded by a cue (predictable), whereas the other half received no cue (unpredictable). Results suggested emotion was successfully induced by pictures, but the effect of picture‐viewing on the NFR was moderated by the predictability of the shocks. When shock was unpredictable, spinal nociception (NFR) and pain ratings were modulated in parallel. Specifically, pain and NFR magnitudes were lower during pleasant emotions and higher during unpleasant emotions. However, when shocks were predictable, only pain was modulated in this way. NFRs from predictable shocks were not altered by pictures. Further, exploratory analyses found that pain ratings, but not NFRs, were lower during predictable shocks. These data suggest emotional picture‐viewing is a reliable method of engaging descending modulation of spinal nociception. However, descending modulation could not be detected in NFRs resulting from predictable noxious stimuli. Although preliminary, this study implies that separate mechanisms are responsible for emotional modulation of nociception at spinal vs. supraspinal levels, and that predictable noxious events may disengage modulation at the spinal level. The current paradigm could serve as a useful tool for studying descending modulation.

Using normalized EMG to define the nociceptive flexion reflex (NFR) threshold: further evaluation of standardized NFR scoring criteria.

ABSTRACT The nociceptive flexion reflex (NFR) has been used as a psychophysiological tool to study spinal nociceptive processes in numerous clinical and experimental studies. Despite widespread use of the NFR, few attempts have been made to empirically test and compare different scoring criteria to detect the presence/absence of the reflex. The present studies were conducted to address this issue. Study 1 (N = 56 healthy participants) examined the reliability of 15 different scoring criteria that were examined in a previous report. Study 2 (N = 73 healthy participants) extended this work by examining normalized scoring criteria based on biceps femoris activity unrelated to noxious stimulation (reference contraction, maximal contraction). In both studies, receiver operating characteristics (ROCs) analyses were used to evaluate and compare different scoring methods. The results indicate that a number of different criteria were acceptable for defining an NFR threshold based on the area under the ROC curve and its statistical significance; however, NFR Interval z score [(NFR Interval Mean − baseline mean)/baseline SD] emerged as the scoring criterion with the greatest accuracy and with cut‐points that are reliable across samples. These findings support the application of a common NFR scoring criterion to enhance direct comparison of results across different research laboratories and study samples.