Acupunctural analgesia? Evaluation by signal detection theory

Selective and replicable neuroimaging-based indicators of pain discriminability

Neural indicators of pain discriminability have far-reaching theoretical and clinical implications but have been largely overlooked previously. Here, to directly identify the neural basis of pain discriminability, we apply signal detection theory to three EEG (Datasets 1-3, total N = 366) and two fMRI (Datasets 4-5, total N = 399) datasets where participants receive transient stimuli of four sensory modalities (pain, touch, audition, and vision) and two intensities (high and low) and report perceptual ratings. Datasets 1 and 4 are used for exploration and others for validation. We find that most pain-evoked EEG and fMRI brain responses robustly encode pain discriminability, which is well replicated in validation datasets. The neural indicators are also pain selective since they cannot track tactile, auditory, or visual discriminability, even though perceptual ratings and sensory discriminability are well matched between modalities. Overall, we provide compelling evidence that pain-evoked brain responses can serve as replicable and selective neural indicators of pain discriminability.

During vigilance to painful stimuli: slower response rate is related to high trait anxiety, whereas faster response rate is related to high state anxiety

A pathological increase in vigilance, or hypervigilance, may be related to pain intensity in some clinical pain syndromes and may result from attention bias to salient stimuli mediated by anxiety. During a continuous performance task where subjects discriminated painful target stimuli from painful nontargets, we measured detected targets (hits), nondetected targets (misses), nondetected nontargets (correct rejections), and detected nontargets (false alarms). Using signal detection theory, we calculated response bias, the tendency to endorse a stimulus as a target, and discriminability, the ability to discriminate a target from nontarget. Owing to the relatively slow rate of stimulus presentation, our primary hypothesis was that sustained performance would result in a more conservative response bias reflecting a lower response rate over time on task. We found a more conservative response bias with time on task and no change in discriminability. We predicted that greater state and trait anxiety would lead to a more liberal response bias. A multivariable model provided partial support for our prediction; high trait anxiety related to a more conservative response bias (lower response rate), whereas high state anxiety related to a more liberal bias. This inverse relationship of state and trait anxiety is consistent with reports of effects of state and trait anxiety on reaction times to threatening stimuli. In sum, we report that sustained attention to painful stimuli was associated with a decrease in the tendency of the subject to respond to any stimulus over time on task, whereas the ability to discriminate target from nontarget remains unchanged.NEW & NOTEWORTHY During a series of painful stimuli requiring subjects to respond to targets, we separated response willingness from ability to discriminate targets from nontargets. Response willingness declined during the task, with no change in subjects' ability to discriminate, consistent with previous vigilance studies. High trait anxious subjects were less willing to respond and showed slower reaction times to hits than low anxious subjects. This study reveals an important role of trait anxiety in pain vigilance.

Behavioural and neural evidence for self-reinforcing expectancy effects on pain

Beliefs and expectations often persist despite evidence to the contrary. Here we examine two potential mechanisms underlying such 'self-reinforcing' expectancy effects in the pain domain: modulation of perception and biased learning. In two experiments, cues previously associated with symbolic representations of high or low temperatures preceded painful heat. We examined trial-to-trial dynamics in participants' expected pain, reported pain and brain activity. Subjective and neural pain responses assimilated towards cue-based expectations, and pain responses in turn predicted subsequent expectations, creating a positive dynamic feedback loop. Furthermore, we found evidence for a confirmation bias in learning: higher- and lower-than-expected pain triggered greater expectation updating for high- and low-pain cues, respectively. Individual differences in this bias were reflected in the updating of pain-anticipatory brain activity. Computational modelling provided converging evidence that expectations influence both perception and learning. Together, perceptual assimilation and biased learning promote self-reinforcing expectations, helping to explain why beliefs can be resistant to change.

Overcoming pain thresholds with multilevel models-an example using quantitative sensory testing (QST) data

The assessment of somatosensory function is a cornerstone of research and clinical practice in neurology. Recent initiatives have developed novel protocols for quantitative sensory testing (QST). Application of these methods led to intriguing findings, such as the presence lower pain-thresholds in healthy children compared to healthy adolescents. In this article, we (re-) introduce the basic concepts of signal detection theory (SDT) as a method to investigate such differences in somatosensory function in detail. SDT describes participants’ responses according to two parameters, sensitivity and response-bias. Sensitivity refers to individuals’ ability to discriminate between painful and non-painful stimulations. Response-bias refers to individuals’ criterion for giving a “painful” response. We describe how multilevel models can be used to estimate these parameters and to overcome central critiques of these methods. To provide an example we apply these methods to data from the mechanical pain sensitivity test of the QST protocol. The results show that adolescents are more sensitive to mechanical pain and contradict the idea that younger children simply use more lenient criteria to report pain. Overall, we hope that the wider use of multilevel modeling to describe somatosensory functioning may advance neurology research and practice.

Effects of acupuncture on sensory perception: a systematic review and meta-analysis

BACKGROUND

The effect of acupuncture on sensory perception has never been systematically reviewed; although, studies on acupuncture mechanisms are frequently based on the idea that changes in sensory thresholds reflect its effect on the nervous system.

METHODS

Pubmed, EMBASE and Scopus were screened for studies investigating the effect of acupuncture on thermal or mechanical detection or pain thresholds in humans published in English or German. A meta-analysis of high quality studies was performed.

RESULTS

Out of 3007 identified articles 85 were included. Sixty five studies showed that acupuncture affects at least one sensory threshold. Most studies assessed the pressure pain threshold of which 80% reported an increase after acupuncture. Significant short- and long-term effects on the pressure pain threshold in pain patients were revealed by two meta-analyses including four and two high quality studies, respectively. In over 60% of studies, acupuncture reduced sensitivity to noxious thermal stimuli, but measuring methods might influence results. Few but consistent data indicate that acupuncture reduces pin-prick like pain but not mechanical detection. Results on thermal detection are heterogeneous. Sensory threshold changes were equally frequent reported after manual acupuncture as after electroacupuncture. Among 48 sham-controlled studies, 25 showed stronger effects on sensory thresholds through verum than through sham acupuncture, but in 9 studies significant threshold changes were also observed after sham acupuncture. Overall, there is a lack of high quality acupuncture studies applying comprehensive assessments of sensory perception.

CONCLUSIONS

Our findings indicate that acupuncture affects sensory perception. Results are most compelling for the pressure pain threshold, especially in pain conditions associated with tenderness. Sham acupuncture can also cause such effects. Future studies should incorporate comprehensive, standardized assessments of sensory profiles in order to fully characterize its effect on sensory perception and to explore the predictive value of sensory profiles for the effectiveness of acupuncture.

The existence of a linear system consisting of sympathetic endings in rat skin

In a previous pilot study we suggested the novel notion that the catecholaminergic sympathetic nerve endings are non-homogeneously distributed in the rat skin. In the present study we have utilized several independent methods to determine the in vivo distribution of catecholamine-containing fibers in rat skin. Using whole body macro-autoradiography with an iodine125-labeled tyrosine, we localized the distribution of iodine-125-catecholamine in rat skin. The images on the film showed various pairs of symmetrical linear arrays running from the head through the back and to the hind limbs of the animal that we arbitrarily termed sympathetic substance lines (SSLs). The distribution of catecholamine in rat skin was also visualized by light microscopy autoradiography with tritiated tyrosine. The majority of silver grains in the sections were located among hair follicles along a band or zone. Furthermore, a modified sucrose-phosphate-glyoxilic acid (SPG) method was adapted to observe sympathetic fibers in the skin sections. Dense clusters of fluorescent nerve fibers in correspondence of arrector pili muscles (AP muscles) were located along lengthwise lines of the body, in a pattern coinciding with the linear arrays identified by macro-autoradiography. We concluded that concentrated clusters of noradrenergic nerve fibers innervate AP muscles and form a longitudinal linear system in the whole skin. These results are discussed in terms of physiological functions associated with hair follicles, sensory signal pathways and Meridians in Chinese traditional medicine.

A signal detection theory analysis of the placebo effect

Some instances of the placebo effect may be understood as a particular type of error made by the patient--a false positive error. False positive errors are common (indeed, frequently encouraged) in medical decision making, both by diagnosticians and by patients, and are the inevitable consequence of concluding that an ambiguous signal (e.g., attenuation of pain, relief of depression) did, or did not, occur. Signal detection theory (SDT) was developed to model errors in the detection of ambiguous signals. The authors use SDT to understand the false positive errors that might be made by patients administered a placebo and termed a placebo effect.

Cerebral cortex participation in the physiological mechanisms of acupuncture stimulation: a study by auditory endogenous potentials (P300)

Although acupuncture has traditionally used the acupoints formula to treat diseases, the physiological mechanisms involved and the effectiveness of therapy remain unclear. This study investigated the physiological mechanism(s) and response to acupuncture stimulation using the acupoints formula. Scalp-recorded potentials P300 were evoked by auditory stimulation of non-target and target in 13 normal adult volunteers. Latencies and amplitudes were measured. Three assessments were performed in each subject over a period of at least one week. Each assessment was divided into a control period with no acupuncture stimulation, followed by an acupuncture period and then a post-acupuncture period. Acupuncture needles were inserted into the body as follows: 1) non-acupoint: acupuncture needles were inserted 2 cm lateral to both Zusanli acupoints; 2) acupoint: acupuncture needles were inserted into both Zusanli acupoints; 3) acupoints formula: acupuncture needles were inserted into both Zusanli and Shousanli acupoints. Our results showed that both acupoint and acupoints formula assessments resulted in a significant decrease of P300 amplitudes during the acupuncture and post-acupuncture periods. However, there was significant difference in P300 amplitudes in the non-acupoint assessment during these periods. P300 changes in latencies and amplitudes were not significantly different between the acupoint assessment and the acupoints formula assessment. We concluded that acupuncture stimulation of both Zusanli acupoints resulted in a decrease of P300 amplitudes, suggesting the involvement of the cerebral cortex in sensory interaction when simultaneous sensations of the two types are received. No similar changes were observed in the non-acupoint assessment, which have been suggested to be related to so-called acupoint specificity. Results obtained using the acupoints formula were not significantly different from those using acupoints alone. These findings suggested that neuropsychological effects from stimulation of Zusanli acupoints and Shousanli acupoints are different.

Sensory decision theory and visual analogue scale indices predict status of chronic pain patients six months later

Thirty-nine outpatients suffering from chronic pain were studied in a multidisciplinary program. Pain intensity on a visual analogue pain scale (VAPS), sensory decision indices of thermal discriminability, P(A), and pain report criterion, B, age and sex obtained before treatment, were used to predict the patients' status, determined by a follow-up questionnaire 6 mo later. The results showed that patients who were high on the VAPS at intake had shorter pain relief and decreased physical activities on follow-up. Patients with better thermal discriminability had greater pain relief, while those with low pain report criterion, that is, less stoical, demonstrated improved physical activity, and more social and hobby activities. Patients who were less stoical to thermal stimuli (lower pain criterion) took fewer centrally active drugs after treatment. Younger patients showed greater improvement at follow-up. The data indicate that the VAPS, thermal discriminability, and pain report criterion all predict the duration of pain relief after treatment. Nevertheless, each of these variables had its individual character. The VAPS was most efficient in predicting physical activities, thermal discriminability related best to pain relief, and pain report criterion to social and hobby activities as well as drug intake.

Altered pain and visual sensitivity in humans: the effects of acute and chronic stress

In the runner study, as measured by tourniquet ischemic pain, exercise stress produced hypoalgesia 20 minutes post-run, followed by hyperalgesia and euphoria at 30 minutes. The hypoalgesia and euphoria were reversed by naloxone. Exercise stress also produced a decrease in P(A), suggesting hypoalgesia to the thermal cutaneous stimulation. However, this analgesia was not naloxone reversible. Nor did exercise stress produce analgesia to cold-pressor pain. In the acupuncture study, noxious electrical stimulation of classical acupuncture sites failed to produce analgesia either during or after stimulation. However, expectation did produce a change in the pain report criterion, but only in the acupunctured arm. Noxious electrical stimulation (TENS) of the median nerve produced no analgesia outside of the related segmental area, that is, acute electrical pain did not produce generalized hypoalgesia. Thus, the effects of the stress produced by noxious electrical stimulation differ from that produced by exercise. In contrast to the results of the acute pain studies, chronic clinical pain, which combines mental stress and pain stress, produced strong hypoalgesia and anesthesia. Again, in contrast to the acute experimental pain studies, the emotional stress of mental illness produces hypoalgesia, but not anesthesia. Finally, the somatosensory system is not the only the sensory system affected by stress. Cold-pressor pain decreases visual sensitivity both during and for a few minutes following stimulation, and does not interfere with short-term (supra-digit span) memory.

Psychological aspects of components of pain

The present study investigated the inhibitory effects of acupuncture on the separate sensory and emotional components of pain and their relation to each other. Electro-acupuncture as practiced in China was applied to the same nerve used for initiating pain, and the responses of 9 adult volunteers (5 males, 4 females) were assessed. The results indicate that (a) pain involves at least two related but different components (the sensory component and the emotional component) and (b) electro-acupuncture has a stronger inhibitory effect on the emotional component of pain. The findings provide a new outlook for the improvement of the clinical effects of acupuncture and for further study of the pain mechanism.

Anxiety, pain experience, and pain report: a signal-detection study

The effect of experimentally induced anxiety on pain perception was examined using a signal detection discrimination experiment. The experimental condition consisted in unsignalled application of very painful stimuli which substantially raised state anxiety. The discrimination task included the total range of painful stimuli. The results indicated a range-specific effect of anxiety on pain, particularly on increased sensitivity in the upper range of intensities. The importance of testing the effect of anxiety over the whole range of intensities is stressed. That prior work considered only one intensity level may be the main reason why previous signal detection studies about pain and anxiety showed contradictory results.

Acupuncture--cultural perspectives. 1. The Western view

While acupuncture has been popularized in the lay press for the past decade, its therapeutic efficacy has often been clouded by issues of Oriental mystique and difficulties in understanding a foreign medical system. Numerous studies have explored the mechanism by which acupuncture works. The effects of acupuncture constitute a generalized systemic reaction with far-reaching consequences. Acupuncture has numerous clinical applications in obstetrics and dentistry, as well as use in the treatment of asthma, musculoskeletal disorders, various addictions, angina pectoris, peptic ulcer disease, functional bowel disorders, and acute bacillary dysentery.

Painful sensation induced by a thermal cutaneous stimulus

This review emphasizes how little we know about pain induced by a thermal stimulus. The study of the intensity of pain evoked by heat is relatively exhaustive: the influence of various local, stimulus-dependent or general factors upon threshold values has been well studied, as has the relation between pain and stimulus intensities. On the contrary, few studies have used very cold stimuli, since highly efficient stimulators allowing accurate control of the stimulus parameters have been obtainable only recently. Only the influence of stimulation area and stimulation rate on cold pain thresholds have been studied. Moreover, old results obtained on pain quality cannot be used since the conditions of stimulation were not specified or not controlled accurately. It is only known that stimulus duration and stimulation area are determinant for thermal pain quality. There is still much work to be done in this field. All the more so as this type of study is absolutely necessary for the understanding of pain mechanisms--it describes what must be explained by the function of the nervous system. We have seen that at the periphery the intensity of heat pain is coded by the response of polymodal nociceptors, mechanothermal nociceptors, thermal nociceptors and possibly by the paradoxical discharge of cold receptors. If the stimulus is lower than 45 degrees C the activity of certain heat receptors comes into play. Although we lack information which would allow confirmation of this as a fact it seems likely that the activity of polymodal nociceptors, cold mechanothermal nociceptors and possibly certain cold receptors sensitive to very low temperatures code cold pain. These nociceptive impulses carried by A delta and C fibers reach the dorsal horn of the spinal cord through the dorsal roots. They are notably at the origin of the activation of the neurons in Rexed's layers I, V and VIII which are to a large extent at the origin of the spinothalamic and spinoreticulothalamic tracks [21,115,168] moving in the anterolateral quadrant of the spinal cord. At supraspinal level, the thermal information reappears in the reticular formation; there it appears to be solely relative to the pain threshold and not to the intensity of a supraliminary stimulus [55]. In the posterior group of nuclei [134] and the ventroposterolateral nucleus of the thalamus [103], on the contrary, the activity of the neurons reflects the intensity of the stimulation. It has been proved that the neurons of the ventroposterolateral nucleus project onto the SI cortex [103].(ABSTRACT TRUNCATED AT 400 WORDS)

Distortions of vision and pain: two functional facets of D-lysergic acid diethylamide

D-lysergic acid diethylamide (LSD) produces distortions of visual perception and analgesia. Evidence is advanced from a functional standpoint that the observed visual effects result from an attenuation of light-evoked input to the dorsal lateral geniculate nucleus (LGN) from the purely centripetal pathways of the retina. More slowly responding visual afferents or those with more complex receptive fields seem to be affected most. LSD analgesia, accompanied by severe psychotic symptoms, appears to result from drug actions on a centrifugally controlled pain system involving neurons of the midbrain raphe.

Transcutaneous electrical nerve stimulation and the reaction to experimental pain in human subjects

The effect of peripheral transcutaneous electrical nerve stimulation (TENS) on the reaction to experimental pain in human volunteers has been assessed. Placebo stimulation and electrical stimulation at moderate intensities failed to modify the response to the pain produced by conducted thermal stimuli. TENS at very high intensities did however elevate both the thermal pain threshold and the tolerance temperature. TENS at moderate intensities failed completely to alter the response to graded mechanical stimuli. The subjective pain assessment and the maximum pain tolerance produced by ischaemic pain after a submaximal effort tourniquet test were significantly modified by peripheral electrical stimulation at non-noxious intensities. The response to experimental pain can therefore be altered in man by peripheral electrical stimulation in a manner partly dependent on the sensory modality used for producing the experimental pain and on the intensity of the electrical stimulation.

[Abolition by naloxone of the inhibitory effect of peripheral electrical stimulation on the blink reflex]

The effects of a low frequency (2 c/sec) peripheral stimulation (electro-acupuncture, EA) on the nociceptive (R2) response of the blink reflex elicited by supra-orbital nerve stimulation (0.1 msec, 1 shock/8 sec) were studied in 10 healthy subjects. EA stimulation produced a very significant inhibition of the reflex in 8 subjects. Double-blind injection of naloxone (0.8 mg) reversed this inhibition while no signiificant change was observed with placebo. These results suggest that EA stimulation induces the release of endogenous opiates.

Signal detection theory pain measures: empirical validation studies and adaptation-level effects

Proponents of the use of signal detection theory (SDT) in the assessment of pain modulation have generally looked for changes in d' to indicate a reduction of sensory function, and a change in criterion to indicate a modification of the subject's response bias or attitudinal predisposition. In the first experiment, both assumptions failed to receive empirical verification. Discrimination d' was eqivalent before and after two strong levels of electrical current was reduced. The criterion parameter appeared to shift in a more conservative direction after the stimulus diminution. These results are used to question the validity of both detection and discrimination indices in the measurement of pain. An alternative means for describing the experimental results revealed a striking adaptation-level effect with implications for the assessment of both experimentally induced and endogenous pain. The outcome of a second experiment reinforced the adaptation-level theory interpretation of the results and provided additional evidence concerning the difficultuies in evaluating SDT parameters in studies of potential analgesics.

Response to cold pressor pain and to acupuncture analgesia in Oriental and Occidental subjects

On a no treatment trial, a group of 24 oriental subjects rated cold pressor pain as significantly more painful and distressing than did a group of 24 occidental subjects. For half of the Orientals and half of the Occidentals, a second trial was conducted after acupuncture analgesia had been induced. The remaining 12 Orientals and 12 Occidentals served as no treatment controls on trial 2. Regardless of racial group, there was no difference between the pain of those experimental subjects who received acupuncture and the pain of those controls who did not. As they had on trial 1, Orientals reported significantly more pain and distress in response to ice water on trial 2. It is concluded that: (1) if acupuncture does work better for the Chinese than for other racial groups, the likely cause is a more refined patient selection procedure rather than an inherent difference in response to acupuncture; (2) evidence does not support the stereotyped view of Orientals as stoical in the face of physical pain.

Failure of acupuncture to produce localized analgesia

The pain sensitivity of 49 healthy volunteers has been tested both before and during electro-acupuncture at two or three widely separated places on the body surface. Test areas fell into two categories. The "target" areas were within the part of the body that experienced acupuncturists predicted would be most affected by acupuncture of specified traditional points. "Non-target" control areas fell outside this zone of predicted maximal effect. The subjects and the observers who administered the tests knew that acupuncture anesthesia was being studied, but they did not know which areas were the "targets". Each subject was tested using at least two sensory testing methods. Altogether 4 different tests of pain sensitivity (pinprick discrimination, cold pain ratings, heat pain thresholds and pinch pain thresholds) were used. Small, but statistically highly significant, decreases in pain sensitivity occurred with all 4 tests during acupuncture. However, pain sensitivity fell by the same amount at "target" and "non-target" areas. The variability of the data was sufficiently low that moderate differences between areas would have been detected. The mechanism of "acupuncture anesthesia" is discussed in the light of these negative findings.

The possible role of enkephalin in the mechanism of acupuncture analgesia in man

On the basis of recent research on the presence and function of an endogenous peptide with opiate-like activity present in the brain, and the developing understanding of the physiological mechanism of acupuncture analgesia, it is postulated that the analgesic effects of acupuncture are mediated through the action of this substance on the neural mechanism(s) of pain inhibition.

Effects of intrasegmental electrical acupuncture on dental pain: evaluation by threshold estimation and sensory decision theory

The effect of 80 min of low frequency (2 Hz) electric acupunctural stimulation at facial sites on the perception of induced dental pain was evaluated using both pain threshold and sensory decision theory (SDT) procedures. The demonstration of a 187% increase in threshold over a 20 min period of acupunctural stimulation replicated earlier work by Swedish investigators. SDT analyses indicated that the threshold increase reflected a relatively pure sensory change with no significant modification of response bias. However, subjects were able to perceive some of the stimuli presented below threshold level following acupuncture, thus indicating that the threshold concept has been an inadequate description of the phenomenon. This study demonstrated that intrasegmental analgesic stimulation is more efficacious than the extrasegmental meridian point stimulation used in our earlier studies. Possible mechanisms for the observed effect were discussed.

Modulation of shock-elicited pain by acupuncture and suggestion

An experimental investigation of acupuncture's analgesic potency, separated from suggestion effects, is described, in which judgments of shock-elicited pain of the forearm were recorded along two separate scales: intensity and aversiveness. Data from experimental subjects, who received electrical acupunctural stimulation through surface electrodes located over acupuncture points and were given explicit counter-suggestions of sensitization, were compared to those obtained from control subjects, who received such stimulation over incorrect loci and were given explicit suggestions of analgesia. No alterations in the perception of pain intensity, either by acupunctural stimulation or suggestion, could be detected at any time during the experiment, but stimulation of the correct points produced a significant analgesic reduction in aversiveness which was specific to the arm so stimulated, overcame the counter-suggestion of sensitization, and persisted after stimulation was discontinued. No significant reductions were recorded from control subjects. These findings are interpreted in light of other experimental acupuncture research, and of the demonstrated mode of action of analgesics of proven efficacy.