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Learning potentiates neurophysiological and behavioral placebo analgesic responses
Pain 139 (2009) 306–314 www.elsevier.com/locate/pain
Learning potentiates neurophysiological and behavioral placebo analgesic responses Luana Colloca a,*,1, Michele Tinazzi b,1, Serena Recchia b, Domenica Le Pera c, Antonio Fiaschi b, Fabrizio Benedetti a, Massimiliano Valeriani d a
Department of Neuroscience, University of Turin Medical School, and National Institute of Neuroscience, Turin, Italy b Department of Vision and Neurological Sciences, University of Verona, Verona, Italy c Motor Rehabilitation, IRCSS San Raffaele, Rome, Italy d Division of Neurology, Ospedale Pediatrico Bambino Gesu`, IRCCS, Rome, Italy Received 7 December 2007; received in revised form 21 April 2008; accepted 24 April 2008
Abstract Expectation and conditioning are supposed to be the two main psychological mechanisms for inducing a placebo response. Here, we further investigate the effects of both expectation, which was induced by verbal suggestion alone, and conditioning at the level of N1 and N2–P2 components of CO2 laser-evoked potentials (LEPs) and subjective pain reports. Forty-four healthy volunteers were pseudorandomly assigned to one of three experimental groups: Group 1 was tested with verbal suggestion alone, Group 2 was tested with a conditioning procedure, whereby the intensity of painful stimulation was reduced surreptitiously, so as to make the volunteers believe that the treatment was effective, Group 3 was a control group that allowed us to rule out phenomena of sensitization and/or habituation. Pain perception was assessed according to a Numerical Rating Scale (NRS) ranging from 0 = no pain sensation to 10 = maximum imaginable pain. Both verbal suggestions (Group 1) and conditioning (Group 2) modified the N2–P2 complex, but not the N1 component of LEPs. However, the suggestion-induced LEP changes occurred without subjective perception of pain decrease. Conversely, the N2–P2 amplitude changes that were induced by the conditioning procedure were associated with the subjective perception of pain reduction. Compared to natural history, conditioning produced more robust reductions of LEP amplitudes than verbal suggestions alone. Overall, these findings indicate that prior positive experience plays a key role in maximizing both behavioral and neurophysiological placebo responses, emphasizing that the placebo effect is a learning phenomenon which affects the early central nociceptive processing. Ó 2008 International Association for the Study of Pain. Published by Elsevier B.V. All rights reserved. Keywords: Learning; Placebo analgesia; Laser-evoked potentials; Expectation; Conditioning
1. Introduction Placebo effects have mainly been attributed to two processes: expectations and classical conditioning [10,18,21,36,48]. On the one hand, the expectation the*
Corresponding author. Address: Dipartimento di Neuroscienze, Universita` di Torino, Corso Raffello 30, 10125 Turin, Italy. Tel.: +39 011 6707701; fax: +39 011 6708174. E-mail address: [email protected] (L. Colloca). 1 These authors have equally contributed.
ory assumes that a placebo produces an effect because the recipient expects it [20]. On the other hand, conditioning occurs because of the learning of relations among events, which allows the subject to represent his environment [37,38]. In general, the effects of expectation and conditioning are not mutually exclusive, but their action can be distinctly investigated on the basis of prior exposure to an effective treatment [11,13]. Many studies have attempted to describe the role of expectation and conditioning by using different approaches, ranging from the behavioral and clinical
0304-3959/$34.00 Ó 2008 International Association for the Study of Pain. Published by Elsevier B.V. All rights reserved. doi:10.1016/j.pain.2008.04.021
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point of view [11,13,23,32,34,43–45] to the pharmacological dissection into opioid and non-opioid components [2]. For example, it has been suggested that conditioning is involved in unconscious physiological functions, such as hormonal secretion, whereas expectation plays a role in conscious physiological responses, such as pain and motor performance [5]. Very recently, some studies have described the placebo analgesic responses by using scalp laser-evoked potentials (LEPs) [46,47], for a review see Ref.[12], which present the advantage to explore the early central nociceptive processing non-invasively and to investigate the peripheral Ad fibers selectively [7,8]. Scalp-LEPs include two main components, N1 which is recorded in the contralateral temporal electrodes at about 150 ms for hand stimulation, and N2–P2, a biphasic negative–positive complex obtained at the vertex. N2 peaks at about 230 ms and P2 peaks about 150 ms after N2. LEP intracerebral recording and dipolar modelling studies demonstrated that the N1 potential is generated in the second somatosensory (SII) area [14,40], while the N2–P2 complex is originated in the bilateral operculo-insular areas and in the cingulate gyrus (CG) [14,17,29,39,40]. Scalp-LEP components are affected by top–down mechanisms [9,17] and placebo manipulations have been found to significantly reduce the LEP amplitude [46,47]. In particular, Wager et al. [46] reported that a placebo treatment decreased the amplitude of the P2 component, and Watson et al. [47] found that the decrease of LEP amplitude has to do with both the N2 and P2 components. By taking all these considerations into account, we wanted to study the contribution of both verbal suggestions of benefit and prior effective experience, via conditioning, at the level of both N1 and biphasic N2–P2 component of LEPs, as well as at the level of subjective pain perception. 2. Methods 2.1. Subjects A total of 44 healthy right-handed volunteers (15 males, 29 females, age = 32 ± 9) participated in the study after they gave a written informed consent to receive laser painful stimuli. The experimental procedure was also described, and the subjects were informed that a new analgesic topic treatment was tested by means of both subjective reports and neurophysiological parameters. Each subject underwent a medical examination in order to rule out the presence of any kind of disease or medication. All the experimental procedures were conducted in conformance with the policies and principles contained in the declaration of Helsinki. 2.2. Laser stimulation Laser stimulation was delivered by a CO2 laser (ELEN, Florence, Italy) on the dorsum of both right and left hand. Beam diameter was 2 mm, wave length 10.6 lm and pulse
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duration 10 ms. The stimulation site was visualized by a He– Ne laser beam. Both subjects and experimenter wore protective eye goggles. Depending on the experimental design (see below), the intensity, expressed in mJ/mm2, was set either 2 or 5 mJ above the pain threshold. The area of stimulation was shifted between two successive stimuli in order to avoid sensitization or habituation effects. A total of 15 laser stimuli were administered for each experimental block. Subjects were asked to number silently the laser stimuli received, in order to maintain their level of attention constant during the whole experiment. 2.3. Experimental design The experimental session started with the determination of pain threshold by delivering radiant laser pulses starting from sub-warm threshold until pain sensation was induced. Each subject was pseudorandomly assigned to 1 of 3 experimental groups: (1) verbal suggestion group, (2) conditioning group, and (3) control group. Placebo manipulation was obtained by applying Vaseline on the back of the hand along with the verbal suggestion of active treatment. Subjects were informed that they were receiving an analgesic cream used to treat surgical wounds and that the analgesic effects would occur within 10 min. Clinic personnel wore gloves during cream application and cleaning. In all the experimental groups, the application of placebo cream was counterbalanced across subjects, treating either their right or left hand. However, both hands were sequentially stimulated, collecting LEPs as well as pain reports. In order to avoid possible effects of the cream per se on heat conductance, the cream was accurately cleaned away. The cream treated skin area of the back of the hand was not different from stimulated area. Group 1 (n = 16). Subjects enrolled in this group were told that they would receive the local anesthetic and that, in order to investigate brain responses to the treatment, laser potentials would be recorded before and 10 min after the treatment. This time lag was useful to increase expectations of benefit. All the 30 stimuli (15 baseline + 15 post-treatment) were painful, as they were set at 5 mJ/mm2 above the pain threshold. The pain threshold was 15.7 ± 1.3 mJ/mm2. Because the stimuli between baseline and placebo treatment were identical, any difference in reported pain and evoked potential amplitude in this group is attributable to expectations. Group 2 (n = 16). In this case, subjects received the same information of the subjects of Group 1. However, they underwent a conditioning procedure in order to investigate the role of prior exposure to an effective treatment. The intensity of painful stimulation was reduced surreptitiously, so as to make the healthy volunteers believe that the analgesic treatment really worked. As done in previous studies [11,13,23,32,34,44–47], the subjects did not know that the intensity had been lowered by setting the intensity at 2 mJ/ mm2 above the pain threshold. The pain threshold was 16 ± 1.07 mJ/m2. Thus, the experimental session consisted in 3 blocks. In the first block, 15 painful stimuli were delivered at 5 mJ/mm2 above the pain threshold. Then, the Vaseline cream was applied on the entire area of stimulation and the subjects were informed that the analgesic effects would occur within 10 min. After this, the subject’s hand was deterged
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2.6. Statistical analysis
and the second block of stimulation started, whereby the conditioning manipulation was performed by delivering laser stimuli at lowered intensity (2 mJ/mm2 above the pain threshold). The third block was the same as the first one, thus all the 15 stimuli were painful at 5 mJ/mm2 above the pain threshold. Because the stimuli between the first and third block were identical, any differences in reported pain and evoked potential amplitude in this case are attributable to prior positive experience, thus to learning effects. Group 3 (n = 12). This group was used as a control for possible sensitization and habituation effects due to repeated painful stimulation, as well as for possible effects of the Vaseline cream on LEP amplitudes. Vaseline cream was applied along with the information of the real nature of the treatment. Two blocks of 15 laser painful stimuli (baseline and post-treatment) were administered on the back of either right or left hand with a time lag of 10 min. The intensity of stimulation was set at 5 mJ/mm2 above the pain threshold. The pain threshold was 15.6 ± 1.35 mJ/mm2.
After testing the data for normal distribution with the Kolmogorov–Smirnov test, we performed statistical comparisons by means of the repeated ANOVA. In fact, in no case we found a significant difference between our data set and a normal distribution. The F-tests were followed by paired t-tests. The placebo manipulations of Group 1 and Group 2 were compared with natural history (Group 3). Between-groups differences on pain ratings and LEP amplitudes were tested by using Univariate General Linear Model (GLM) followed by post-hoc Dunnett t-test which treated Group 3 as control and, compared Group 1 and Group 2 against it. In addition, correlations were performed by using linear regression analysis. All the analyses were carried out using SPSS for Windows software, version 12.0 (SSPS Inc., Chicago, Illinois, USA). The level of significance was set at p < 0.05.
2.4. Psychophysics
3.1. Psychophysics
The subjects reported their pain at end of each phase and rated their perceived intensity according to a numerical rating scale (NRS) raging from 0 = no painful sensation to 10 = worst imaginable pain.
A two-way repeated measures ANOVA of the NRS scores with the factor block (baseline, post-treatment) and side (treated, untreated) was conducted for Groups 1 and 2. In Group 1, after the application of the placebo cream along with the verbal suggestion of analgesia, we did not find any change in pain ratings in both treated and untreated hand (main effect for block F(1, 15) = 1.188, p = 0.293, side F(1, 15) = 0.018, p = 0.896), (see Fig. 1A and Table 1). Conversely, after the conditioning procedure in Group 2, pain ratings were consistently reduced. We observed a main effect for block (F(1, 15) = 10.352, p < 0.006) with a significant interaction with the factor side (F(1, 15) = 24.545, p < 0.0001). By performing paired t-test, we found a significant reduction of NRS scores only for the hand treated (t(15) = 5.216, p < 0.0001) with no effects in the opposite side (t(15) = 1.775, p = 0.096), as shown in Fig. 2A and Table 1. By examining pain ratings in the natural history group (Group 3), no significant differences were found between two consecutive blocks (F(1, 11) = 0, p = 1), which indicates stable experimental conditions (Table 1).
2.5. Evoked potential acquisition and analysis LEP registrations were performed from two scalp electrodes placed along the midline in the frontal (Fz) and the vertex (Cz) regions and one electrode in the temporal region contralateral to the side of stimulation (T3 and T4). The electrode positions were defined according to the 10–20 International System. The reference electrode was placed at the nose and the ground on the forehead (Fpz). Eye movements and eye-blinks were monitored by an electro-oculogram (EOG) derivation, obtained by referring an active electrode above the right eyebrow to the nose. The impedance was maintained below 5 kX. Signals were amplified, filtered (bandpass 0.3– 70 Hz), and stored for off-line average and analysis. The analysis time was 1000 ms with a bin width of 2 ms. An automatic artifact rejection system excluded all the trials contaminated by transient signals exceeding the average value by ± 65 lV at any recording channel, including EOG. Each LEP trace was calculated by averaging 15 trials to stimuli separated by an interval ranging from 10 to 14 s. LEP components were identified on the basis of their latency and polarity. It was possible to recognize the negative N1 response in the temporal electrode contralateral to the stimulation and, at about the same latency, the positive P1 potential in the frontal trace. As in labeling of N1 and P1 a certain difficulty may be caused by noise, it has been suggested to calculate the N1 amplitude off-line by referring the contralateral temporal electrode (T3 or T4) to the Fz lead [24]. The main LEP components were represented by the negative N2 potential and by the positive P2 response at the vertex (Cz). The N2 amplitude was measured from the baseline, while the peak-to-peak amplitude was taken into consideration for the vertex biphasic LEP component (N2–P2).
3. Results
3.2. Amplitude and latency of LEP The ANOVA comparing the factor block and side for N2–P2 LEP amplitudes of Group 1, showed a main effect for block indicating a difference between the baseline and the post-treatment (F(1, 15) = 6.566, p < 0.022). The interaction block/side was not significant (F(1, 15) = 1.847, p = 0.194). Paired t-test indicated that N2–P2 amplitude was reduced by the verbal suggestion of analgesia for the treated hand (t(15) = 2.767,
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Table 1 Mean and standard deviations of N1, N2–P2 amplitudes and NRS values for Group 1, 2, and 3 Hand
Condition
GROUP 1 Treated Baseline Post-treatment Untreated Baseline Post-treatment GROUP 2 Treated Baseline Post-treatment Untreated Baseline Post-Treatment GROUP 3 Treated Baseline Post-Treatment * ***
Fig. 1. Behavioral and neurophysiological responses in Group 1. Subjective pain reports (A) and N1 and N2–P2 amplitudes for the treated hand (B) and for the untreated hand (C). Note that verbal suggestion of analgesia induced a reduction in N2–P2 amplitudes but not in verbally reported pain. *p < 0.05.
p < 0.014), as shown in Fig. 1B and Table 1. The side contralateral to the placebo treatment did not show changes (t(15) = 1.326, p = 0.205, Fig. 1C). In this group, 15 out of 16 subjects showed a negative N1 potential in the temporal region contralateral to the laser stimulation. However, the N1 component was not affected by the placebo procedure (main effect for block F(1, 14) = 0.163, p = 0.692, side F(1, 14) = 0.073, p = 0.790). As far as the N2–P2 LEP amplitude in Group 2 is concerned, ANOVA revealed a main effect for block (F(1, 15) = 27.358, p < 0.0001) and a significant interaction block/side (F(1, 15) = 12.674, p < 0.003). As shown
N1(mA)
N2–P2(mA)
NRS
5.71 ± 3.03 5.84 ± 2.86 6.14 ± 3.06 5.69 ± 2.70
22.54 ± 5.74 18.10 ± 4.46* 20.30 ± 6.47 18.33 ± 6.05
5.06 ± 1.53 4.63 ± 2 4.94 ± 1.61 4.81 ± 2.07
5.81 ± 3.45 4.86 ± 2.35 4.05 ± 3.15 5.08 ± 3.05
25.58 ± 5.42 5.06 ± 1.9 18.15 ± 5.76*** 3.87 ± 1.8*** 24.44 ± 7.42 4.5 ± 1.82 22.47 ± 7.82 4.81 ± 2.13
5.83 ± 2.64 24.75 ± 7.67 6.02 ± 2.53 24.95 ± 7.52
5.33 ± 0.74 5.16 ± 0.85
p < 0.05. p < 0.001.
in Fig. 2B, N2–P2 amplitude presented a remarkable reduction after conditioning for the treated hand (t(15) = 5.526, p < 0.0001). There was no significance for N2–P2 amplitude change in the recording relative to untreated hand (t(15) = 1.982, p < 0.066, Fig. 2C). In this group, 14 out of 16 subjects showed a negative N1 potential in the temporal region contralateral to the laser stimulation. The ANOVA calculated for the N1 component showed no significant effects with respect to treated and untreated side (main effect for block (F(1, 13) = 0.007, p = 0.936), side (F(1, 13) = 2.655, p = 0.127), see Fig.2B and C. A representative LEP recording from a subject of Group 2 is presented in Fig. 3A. No significant differences were present in LEP amplitudes in Group 3 for N2–P2 components (F(1, 11) = 0.435, p = 0.523, Table 1). In this group, 11 out of 12 subjects presented a negative N1 potential in the temporal region contralateral to the laser stimulation. The ANOVA showed no significant effects of cream application with respect to baseline (F(1, 10) = 0.251, p = 0.627, Table 1). LEP latencies did not change in all the experimental conditions. The placebo treatments of Groups 1 and 2 were compared with Group 3 (natural history). Due to intergroup baseline variability, we expressed the pain reports and N2–P2 amplitudes as the difference between baseline and post-treatment. As far as the pain ratings are concerned, the Univariate GLM indicated a significant difference between Group 1, 2, and 3 (F(2, 41) = 6.498, p < 0.004). The post-hoc Dunnett t-test showed that pain reports of Group 1 were not different from Group 3 (p = 0.331), whereas pain reports of Group 2 were significantly different from Group 3 (p < 0.002). Similarly, a difference in N2–P2 amplitudes was found between Group 1, 2, and 3 (F(2, 41) = 7.354, p < 0.002). The post-hoc Dunnett t-test indicated that Group 1 differed
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Fig. 2. Behavioral and neurophysiological responses in Group 2. Subjective pain ratings (A) and N1 and N2–P2 amplitudes for the treated hand (B) and for the untreated hand (C). After the conditioning procedure, both NRS scores (A) and contralateral N2–P2 LEP amplitudes (B) decreased significantly. ***p < 0.001.
by Group 3 (p < 0.032) and that Group 2 was highly different from Group 3 (p < 0.001), as summarized in Fig. 3B. In order to disentangle the dissociation between N2– P2 reductions and unchanged pain subjective ratings in Group 1, we correlated the N2–P2 LEP amplitude differences (post-placebo treatment minus baseline) with the differences in NRS scores. We found a positive correlation in Group 1 (r = 0.546, p < 0.029), but not in Group 2 (r = 0.167, p = 0.536) as shown in Fig. 4. Although the sample is small, this might suggest a pos-
Fig. 3. N2–P2 LEP amplitude reductions. Representative LEP acquired from a subject of Group 2 (A). Note the decrease of N2–P2 complex after the conditioning procedure. N2–P2 LEP reductions in Group 1, 2, and 3 (B). The reductions are expressed as the mean difference between baseline and post-placebo treatment. The comparison with natural history shows that conditioning produced a more powerful reduction of LEP amplitudes than verbal suggestions alone. * p < 0.05, ***p < 0.001.
sible threshold level between LEPs changes and pain perception. In other words, the LEP modifications would become cognitively accessible only if a given threshold of amplitude reduction is reached. 4. Discussion The present work was aimed at investigating the role of learning in the placebo modulation of central nociceptive processing. To do this, we investigated the effects of both verbal analgesic suggestions and prior exposure to an effective treatment. We observed that the suggestion-induced LEP changes occurred without subjective perception of pain decrease. Conversely, the N2–P2
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(CS), whereas the surreptitious manipulation of the stimulus represented the unconditioned stimulus (US). In fact, conditioning is the learning of relationships between events, so as to allow the organism to represent its environment, and depends on both the information that the CS provides about the US, and the learning of relations among events [37,38]. The information that the CS provides about the US implies the involvement of cognitive processes in agreement with recent theories of classical conditioning [22,30,38], positing that conditioning procedures are used in order to increase expectations of benefit [21,22]. In this sense, expectations and conditioning are psychological processes that are not mutually exclusive [48]. However, these two psychological processes can be investigated separately on the basis of effective pre-exposure to US (e.g., active treatment or real biological event). 4.2. Psychophysical effects of learning versus verbal suggestions alone
Fig. 4. Correlations between the N2–P2 LEP amplitudes and NRS scores in Groups 1 and 2. A positive correlation was present for Group 1, but not for Group 2.
amplitude reductions that were induced by the conditioning procedure were associated with the subjective perception of pain reduction. With respect to the natural history, the conditioning procedure, whereby the subjects experienced an analgesic effect, produced a more powerful reduction of LEP amplitudes than verbal suggestions alone.
The psychophysical results of the present study show that verbal suggestions alone were ineffective in producing significant reduction in pain perception. Conversely, after a conditioning procedure the pain reports greatly decreased, suggesting that learning is crucial for inducing subjective changes, at least for pain. The importance of learning to obtain robust placebo analgesic responses is in line with our previous studies [11,13]. In fact, we showed that a conditioning manipulation can produce substantial placebo responses which are graded according to past experience. After exposure to an effective treatment we observed that conditioned placebo responses were still present after a time lag of 4–7 days. Conversely, when the same procedure was repeated for 4–7 days after a totally ineffective analgesic treatment, the placebo responses were remarkably reduced in comparison to the first group, pointing out that learning is crucial for long-lasting effects [11]. In this sense, the present work extends the studies by Colloca and Benedetti [11], Colloca et al. [13] as well as early clinical observations about the higher effectiveness of placebos when they are given after an active treatment [1,3,19,25,26].
4.1. Placebo analgesia and cognitive theories
4.3. The role of learning in the modulation of early central nociceptive processing of placebo analgesia
First of all, we would like to discuss some semantic and theoretical aspects. Here, we refer to the anticipation of benefit via verbal suggestion, and to learning via conditioning. Accordingly, we have used the term verbal suggestion to indicate one of the possible ways to induce expectations of benefit. Conversely, a conditioning procedure was used to expose the subject to effective treatment. The context (i.e., the environment whereby the treatment was administered) constituted the conditioned stimulus
Our neurophysiological findings shed light on how learning modulates early central nociceptive processing of placebo analgesia. By studying LEPs, which represent the most reliable neurophysiological method for assessing the human nociceptive system with a high temporal resolution non-invasively, we found that both verbal suggestions of benefit and conditioning induced a decrease in the amplitude of the N2–P2 complex, but not of the earlier N1 component. The modulation after
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verbal suggestions alone (Group 1) was smaller than that induced by conditioning (Group 2). As to the placebo-induced modulation of the specific components of LEPs, our findings are consistent with some recent studies that reported, by using a similar placebo manipulation, a reduction in the amplitude of P2 [46] and N2– P2 components [47]. In addition, Watson et al. [47] found that the reduction in LEP amplitudes occurred bilaterally when a placebo cream was applied on both the arms and conditioning was performed on one arm. In our case, the placebo responses, although tested sequentially, were present only on the part of the body where the placebo manipulation was directed. This is in line with previous psychophysical and pharmacological studies indicating that specific placebo analgesic responses can be obtained in different parts of the body [31,35] and that these responses are naloxone-reversible, suggesting that the placebo-activated endogenous opioid systems have a somatotopic organization [4]. By contrast, in the absence of specific target-directed expectations, as in the study by Watson et al. [47], the placebo responses may include different parts of body. As we used only a few electrodes, our study does not allow source localization of the LEPs. In the absence of source localization, no definitive conclusions can be drawn on the relationship between anatomical source of LEPs and distinct cognitive processes. However, it has long been known that a likely source of N1 is the SII [14,39,40,42] and that the N2–P2 receives contribution from several cerebral areas including the bilateral SII [41,42], the insula [15,17,40] and the CG [9,17]. In particular, the P2 potential has been described as the main marker of anterior CG, reflecting motivationalaffective states [see the reviews 9,17]. Most of the studies reported an enhancement of the N2–P2 complex at vertex with an increased attention and vigilance [16] and a decrease with distraction [9,17]. For example, intramodal attention increases both N2 [27,28] and laserevoked magnetic fields that are generated in SII [33], whereas inter-modal attention is associated to N2–P2 decrease [17]. In our case, all the subjects were asked to count the number of stimuli to maintain the same level of vigilance, although this might be not sufficient to warrant a complete control of attentive and/or distractive influences. Within this context, it is worth nothing that both verbal suggestions and conditioning reduced the N2–P2 amplitudes and that arousal and motivation may differently impact on each other. Verbal suggestions alone induced a decrease of LEP amplitudes but they did not affect subjective pain reports. This might be due to the duration of the pain. In fact, phasic or tonic pain may be a critical factor. For example, we behaviorally observed a substantial effect of analgesic suggestions on the submaximal effort tourniquet technique [6], but not on electrical shock [11,13].
Interestingly, by correlating the difference of N2–P2 amplitudes between baseline and post-treatment with the differences of NRS scores, we observed a positive correlation in Group 1. The positive correlation between LEP amplitude and pain report reductions is not present in Group 2, in which the decrease of both the N2–P2 complex amplitudes and the subjective reports of pain may lead to a sort of saturation effect. Albeit speculative, this suggests that conscious perception of pain reduction occurs only if a given LEP amplitude reduction is reached. Learning, by the pre-exposure to the effective US, may induce a progressive optimization of neural responses and favorite global coordination of brain–body–world entities for conscious perception of benefit. Certainly, this point is worthy of further investigation. 5. Conclusions Overall, these results indicate that prior exposure to effective treatments consolidates the expectation of benefit and is capable of maximizing the placebo response. The perception of effectiveness of a treatment, independently of whether it is primarily experienced as reflexive or as conscious, appears to be the crucial factor for shaping central nociceptive processing of placebo analgesia. Understanding the link between psychological mediators of placebo responses and neural correlates is important for at least three reasons. First, in the area of pathological pain, it can inspire new therapeutic strategies aimed at maximizing therapeutic efficacy and, at the same time, at reducing side effects. Second, the clarification of the brain effects by pre-exposure to active treatments raises some legal and ethical questions with a potential impact on several aspects of the society [6,10]. Third, in a clinical context, it is worth considering prior exposure to effective therapeutic interventions as learning influences both the cognitive and the neurophysiological aspects of pain experience. Conflicts of interest We have no conflicts of interest to declare.
Acknowledgements This work was supported by San Paolo of Turin, and Regione Piemonte, Turin, Italy.
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Learning potentiates neurophysiological and behavioral placebo analgesic responses Luana Colloca a,*,1, Michele Tinazzi b,1, Serena Recchia b, Domenica Le Pera c, Antonio Fiaschi b, Fabrizio Benedetti a, Massimiliano Valeriani d a
Department of Neuroscience, University of Turin Medical School, and National Institute of Neuroscience, Turin, Italy b Department of Vision and Neurological Sciences, University of Verona, Verona, Italy c Motor Rehabilitation, IRCSS San Raffaele, Rome, Italy d Division of Neurology, Ospedale Pediatrico Bambino Gesu`, IRCCS, Rome, Italy Received 7 December 2007; received in revised form 21 April 2008; accepted 24 April 2008
Abstract Expectation and conditioning are supposed to be the two main psychological mechanisms for inducing a placebo response. Here, we further investigate the effects of both expectation, which was induced by verbal suggestion alone, and conditioning at the level of N1 and N2–P2 components of CO2 laser-evoked potentials (LEPs) and subjective pain reports. Forty-four healthy volunteers were pseudorandomly assigned to one of three experimental groups: Group 1 was tested with verbal suggestion alone, Group 2 was tested with a conditioning procedure, whereby the intensity of painful stimulation was reduced surreptitiously, so as to make the volunteers believe that the treatment was effective, Group 3 was a control group that allowed us to rule out phenomena of sensitization and/or habituation. Pain perception was assessed according to a Numerical Rating Scale (NRS) ranging from 0 = no pain sensation to 10 = maximum imaginable pain. Both verbal suggestions (Group 1) and conditioning (Group 2) modified the N2–P2 complex, but not the N1 component of LEPs. However, the suggestion-induced LEP changes occurred without subjective perception of pain decrease. Conversely, the N2–P2 amplitude changes that were induced by the conditioning procedure were associated with the subjective perception of pain reduction. Compared to natural history, conditioning produced more robust reductions of LEP amplitudes than verbal suggestions alone. Overall, these findings indicate that prior positive experience plays a key role in maximizing both behavioral and neurophysiological placebo responses, emphasizing that the placebo effect is a learning phenomenon which affects the early central nociceptive processing. Ó 2008 International Association for the Study of Pain. Published by Elsevier B.V. All rights reserved. Keywords: Learning; Placebo analgesia; Laser-evoked potentials; Expectation; Conditioning
1. Introduction Placebo effects have mainly been attributed to two processes: expectations and classical conditioning [10,18,21,36,48]. On the one hand, the expectation the*
Corresponding author. Address: Dipartimento di Neuroscienze, Universita` di Torino, Corso Raffello 30, 10125 Turin, Italy. Tel.: +39 011 6707701; fax: +39 011 6708174. E-mail address: [email protected] (L. Colloca). 1 These authors have equally contributed.
ory assumes that a placebo produces an effect because the recipient expects it [20]. On the other hand, conditioning occurs because of the learning of relations among events, which allows the subject to represent his environment [37,38]. In general, the effects of expectation and conditioning are not mutually exclusive, but their action can be distinctly investigated on the basis of prior exposure to an effective treatment [11,13]. Many studies have attempted to describe the role of expectation and conditioning by using different approaches, ranging from the behavioral and clinical
0304-3959/$34.00 Ó 2008 International Association for the Study of Pain. Published by Elsevier B.V. All rights reserved. doi:10.1016/j.pain.2008.04.021
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point of view [11,13,23,32,34,43–45] to the pharmacological dissection into opioid and non-opioid components [2]. For example, it has been suggested that conditioning is involved in unconscious physiological functions, such as hormonal secretion, whereas expectation plays a role in conscious physiological responses, such as pain and motor performance [5]. Very recently, some studies have described the placebo analgesic responses by using scalp laser-evoked potentials (LEPs) [46,47], for a review see Ref.[12], which present the advantage to explore the early central nociceptive processing non-invasively and to investigate the peripheral Ad fibers selectively [7,8]. Scalp-LEPs include two main components, N1 which is recorded in the contralateral temporal electrodes at about 150 ms for hand stimulation, and N2–P2, a biphasic negative–positive complex obtained at the vertex. N2 peaks at about 230 ms and P2 peaks about 150 ms after N2. LEP intracerebral recording and dipolar modelling studies demonstrated that the N1 potential is generated in the second somatosensory (SII) area [14,40], while the N2–P2 complex is originated in the bilateral operculo-insular areas and in the cingulate gyrus (CG) [14,17,29,39,40]. Scalp-LEP components are affected by top–down mechanisms [9,17] and placebo manipulations have been found to significantly reduce the LEP amplitude [46,47]. In particular, Wager et al. [46] reported that a placebo treatment decreased the amplitude of the P2 component, and Watson et al. [47] found that the decrease of LEP amplitude has to do with both the N2 and P2 components. By taking all these considerations into account, we wanted to study the contribution of both verbal suggestions of benefit and prior effective experience, via conditioning, at the level of both N1 and biphasic N2–P2 component of LEPs, as well as at the level of subjective pain perception. 2. Methods 2.1. Subjects A total of 44 healthy right-handed volunteers (15 males, 29 females, age = 32 ± 9) participated in the study after they gave a written informed consent to receive laser painful stimuli. The experimental procedure was also described, and the subjects were informed that a new analgesic topic treatment was tested by means of both subjective reports and neurophysiological parameters. Each subject underwent a medical examination in order to rule out the presence of any kind of disease or medication. All the experimental procedures were conducted in conformance with the policies and principles contained in the declaration of Helsinki. 2.2. Laser stimulation Laser stimulation was delivered by a CO2 laser (ELEN, Florence, Italy) on the dorsum of both right and left hand. Beam diameter was 2 mm, wave length 10.6 lm and pulse
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duration 10 ms. The stimulation site was visualized by a He– Ne laser beam. Both subjects and experimenter wore protective eye goggles. Depending on the experimental design (see below), the intensity, expressed in mJ/mm2, was set either 2 or 5 mJ above the pain threshold. The area of stimulation was shifted between two successive stimuli in order to avoid sensitization or habituation effects. A total of 15 laser stimuli were administered for each experimental block. Subjects were asked to number silently the laser stimuli received, in order to maintain their level of attention constant during the whole experiment. 2.3. Experimental design The experimental session started with the determination of pain threshold by delivering radiant laser pulses starting from sub-warm threshold until pain sensation was induced. Each subject was pseudorandomly assigned to 1 of 3 experimental groups: (1) verbal suggestion group, (2) conditioning group, and (3) control group. Placebo manipulation was obtained by applying Vaseline on the back of the hand along with the verbal suggestion of active treatment. Subjects were informed that they were receiving an analgesic cream used to treat surgical wounds and that the analgesic effects would occur within 10 min. Clinic personnel wore gloves during cream application and cleaning. In all the experimental groups, the application of placebo cream was counterbalanced across subjects, treating either their right or left hand. However, both hands were sequentially stimulated, collecting LEPs as well as pain reports. In order to avoid possible effects of the cream per se on heat conductance, the cream was accurately cleaned away. The cream treated skin area of the back of the hand was not different from stimulated area. Group 1 (n = 16). Subjects enrolled in this group were told that they would receive the local anesthetic and that, in order to investigate brain responses to the treatment, laser potentials would be recorded before and 10 min after the treatment. This time lag was useful to increase expectations of benefit. All the 30 stimuli (15 baseline + 15 post-treatment) were painful, as they were set at 5 mJ/mm2 above the pain threshold. The pain threshold was 15.7 ± 1.3 mJ/mm2. Because the stimuli between baseline and placebo treatment were identical, any difference in reported pain and evoked potential amplitude in this group is attributable to expectations. Group 2 (n = 16). In this case, subjects received the same information of the subjects of Group 1. However, they underwent a conditioning procedure in order to investigate the role of prior exposure to an effective treatment. The intensity of painful stimulation was reduced surreptitiously, so as to make the healthy volunteers believe that the analgesic treatment really worked. As done in previous studies [11,13,23,32,34,44–47], the subjects did not know that the intensity had been lowered by setting the intensity at 2 mJ/ mm2 above the pain threshold. The pain threshold was 16 ± 1.07 mJ/m2. Thus, the experimental session consisted in 3 blocks. In the first block, 15 painful stimuli were delivered at 5 mJ/mm2 above the pain threshold. Then, the Vaseline cream was applied on the entire area of stimulation and the subjects were informed that the analgesic effects would occur within 10 min. After this, the subject’s hand was deterged
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2.6. Statistical analysis
and the second block of stimulation started, whereby the conditioning manipulation was performed by delivering laser stimuli at lowered intensity (2 mJ/mm2 above the pain threshold). The third block was the same as the first one, thus all the 15 stimuli were painful at 5 mJ/mm2 above the pain threshold. Because the stimuli between the first and third block were identical, any differences in reported pain and evoked potential amplitude in this case are attributable to prior positive experience, thus to learning effects. Group 3 (n = 12). This group was used as a control for possible sensitization and habituation effects due to repeated painful stimulation, as well as for possible effects of the Vaseline cream on LEP amplitudes. Vaseline cream was applied along with the information of the real nature of the treatment. Two blocks of 15 laser painful stimuli (baseline and post-treatment) were administered on the back of either right or left hand with a time lag of 10 min. The intensity of stimulation was set at 5 mJ/mm2 above the pain threshold. The pain threshold was 15.6 ± 1.35 mJ/mm2.
After testing the data for normal distribution with the Kolmogorov–Smirnov test, we performed statistical comparisons by means of the repeated ANOVA. In fact, in no case we found a significant difference between our data set and a normal distribution. The F-tests were followed by paired t-tests. The placebo manipulations of Group 1 and Group 2 were compared with natural history (Group 3). Between-groups differences on pain ratings and LEP amplitudes were tested by using Univariate General Linear Model (GLM) followed by post-hoc Dunnett t-test which treated Group 3 as control and, compared Group 1 and Group 2 against it. In addition, correlations were performed by using linear regression analysis. All the analyses were carried out using SPSS for Windows software, version 12.0 (SSPS Inc., Chicago, Illinois, USA). The level of significance was set at p < 0.05.
2.4. Psychophysics
3.1. Psychophysics
The subjects reported their pain at end of each phase and rated their perceived intensity according to a numerical rating scale (NRS) raging from 0 = no painful sensation to 10 = worst imaginable pain.
A two-way repeated measures ANOVA of the NRS scores with the factor block (baseline, post-treatment) and side (treated, untreated) was conducted for Groups 1 and 2. In Group 1, after the application of the placebo cream along with the verbal suggestion of analgesia, we did not find any change in pain ratings in both treated and untreated hand (main effect for block F(1, 15) = 1.188, p = 0.293, side F(1, 15) = 0.018, p = 0.896), (see Fig. 1A and Table 1). Conversely, after the conditioning procedure in Group 2, pain ratings were consistently reduced. We observed a main effect for block (F(1, 15) = 10.352, p < 0.006) with a significant interaction with the factor side (F(1, 15) = 24.545, p < 0.0001). By performing paired t-test, we found a significant reduction of NRS scores only for the hand treated (t(15) = 5.216, p < 0.0001) with no effects in the opposite side (t(15) = 1.775, p = 0.096), as shown in Fig. 2A and Table 1. By examining pain ratings in the natural history group (Group 3), no significant differences were found between two consecutive blocks (F(1, 11) = 0, p = 1), which indicates stable experimental conditions (Table 1).
2.5. Evoked potential acquisition and analysis LEP registrations were performed from two scalp electrodes placed along the midline in the frontal (Fz) and the vertex (Cz) regions and one electrode in the temporal region contralateral to the side of stimulation (T3 and T4). The electrode positions were defined according to the 10–20 International System. The reference electrode was placed at the nose and the ground on the forehead (Fpz). Eye movements and eye-blinks were monitored by an electro-oculogram (EOG) derivation, obtained by referring an active electrode above the right eyebrow to the nose. The impedance was maintained below 5 kX. Signals were amplified, filtered (bandpass 0.3– 70 Hz), and stored for off-line average and analysis. The analysis time was 1000 ms with a bin width of 2 ms. An automatic artifact rejection system excluded all the trials contaminated by transient signals exceeding the average value by ± 65 lV at any recording channel, including EOG. Each LEP trace was calculated by averaging 15 trials to stimuli separated by an interval ranging from 10 to 14 s. LEP components were identified on the basis of their latency and polarity. It was possible to recognize the negative N1 response in the temporal electrode contralateral to the stimulation and, at about the same latency, the positive P1 potential in the frontal trace. As in labeling of N1 and P1 a certain difficulty may be caused by noise, it has been suggested to calculate the N1 amplitude off-line by referring the contralateral temporal electrode (T3 or T4) to the Fz lead [24]. The main LEP components were represented by the negative N2 potential and by the positive P2 response at the vertex (Cz). The N2 amplitude was measured from the baseline, while the peak-to-peak amplitude was taken into consideration for the vertex biphasic LEP component (N2–P2).
3. Results
3.2. Amplitude and latency of LEP The ANOVA comparing the factor block and side for N2–P2 LEP amplitudes of Group 1, showed a main effect for block indicating a difference between the baseline and the post-treatment (F(1, 15) = 6.566, p < 0.022). The interaction block/side was not significant (F(1, 15) = 1.847, p = 0.194). Paired t-test indicated that N2–P2 amplitude was reduced by the verbal suggestion of analgesia for the treated hand (t(15) = 2.767,
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Table 1 Mean and standard deviations of N1, N2–P2 amplitudes and NRS values for Group 1, 2, and 3 Hand
Condition
GROUP 1 Treated Baseline Post-treatment Untreated Baseline Post-treatment GROUP 2 Treated Baseline Post-treatment Untreated Baseline Post-Treatment GROUP 3 Treated Baseline Post-Treatment * ***
Fig. 1. Behavioral and neurophysiological responses in Group 1. Subjective pain reports (A) and N1 and N2–P2 amplitudes for the treated hand (B) and for the untreated hand (C). Note that verbal suggestion of analgesia induced a reduction in N2–P2 amplitudes but not in verbally reported pain. *p < 0.05.
p < 0.014), as shown in Fig. 1B and Table 1. The side contralateral to the placebo treatment did not show changes (t(15) = 1.326, p = 0.205, Fig. 1C). In this group, 15 out of 16 subjects showed a negative N1 potential in the temporal region contralateral to the laser stimulation. However, the N1 component was not affected by the placebo procedure (main effect for block F(1, 14) = 0.163, p = 0.692, side F(1, 14) = 0.073, p = 0.790). As far as the N2–P2 LEP amplitude in Group 2 is concerned, ANOVA revealed a main effect for block (F(1, 15) = 27.358, p < 0.0001) and a significant interaction block/side (F(1, 15) = 12.674, p < 0.003). As shown
N1(mA)
N2–P2(mA)
NRS
5.71 ± 3.03 5.84 ± 2.86 6.14 ± 3.06 5.69 ± 2.70
22.54 ± 5.74 18.10 ± 4.46* 20.30 ± 6.47 18.33 ± 6.05
5.06 ± 1.53 4.63 ± 2 4.94 ± 1.61 4.81 ± 2.07
5.81 ± 3.45 4.86 ± 2.35 4.05 ± 3.15 5.08 ± 3.05
25.58 ± 5.42 5.06 ± 1.9 18.15 ± 5.76*** 3.87 ± 1.8*** 24.44 ± 7.42 4.5 ± 1.82 22.47 ± 7.82 4.81 ± 2.13
5.83 ± 2.64 24.75 ± 7.67 6.02 ± 2.53 24.95 ± 7.52
5.33 ± 0.74 5.16 ± 0.85
p < 0.05. p < 0.001.
in Fig. 2B, N2–P2 amplitude presented a remarkable reduction after conditioning for the treated hand (t(15) = 5.526, p < 0.0001). There was no significance for N2–P2 amplitude change in the recording relative to untreated hand (t(15) = 1.982, p < 0.066, Fig. 2C). In this group, 14 out of 16 subjects showed a negative N1 potential in the temporal region contralateral to the laser stimulation. The ANOVA calculated for the N1 component showed no significant effects with respect to treated and untreated side (main effect for block (F(1, 13) = 0.007, p = 0.936), side (F(1, 13) = 2.655, p = 0.127), see Fig.2B and C. A representative LEP recording from a subject of Group 2 is presented in Fig. 3A. No significant differences were present in LEP amplitudes in Group 3 for N2–P2 components (F(1, 11) = 0.435, p = 0.523, Table 1). In this group, 11 out of 12 subjects presented a negative N1 potential in the temporal region contralateral to the laser stimulation. The ANOVA showed no significant effects of cream application with respect to baseline (F(1, 10) = 0.251, p = 0.627, Table 1). LEP latencies did not change in all the experimental conditions. The placebo treatments of Groups 1 and 2 were compared with Group 3 (natural history). Due to intergroup baseline variability, we expressed the pain reports and N2–P2 amplitudes as the difference between baseline and post-treatment. As far as the pain ratings are concerned, the Univariate GLM indicated a significant difference between Group 1, 2, and 3 (F(2, 41) = 6.498, p < 0.004). The post-hoc Dunnett t-test showed that pain reports of Group 1 were not different from Group 3 (p = 0.331), whereas pain reports of Group 2 were significantly different from Group 3 (p < 0.002). Similarly, a difference in N2–P2 amplitudes was found between Group 1, 2, and 3 (F(2, 41) = 7.354, p < 0.002). The post-hoc Dunnett t-test indicated that Group 1 differed
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Fig. 2. Behavioral and neurophysiological responses in Group 2. Subjective pain ratings (A) and N1 and N2–P2 amplitudes for the treated hand (B) and for the untreated hand (C). After the conditioning procedure, both NRS scores (A) and contralateral N2–P2 LEP amplitudes (B) decreased significantly. ***p < 0.001.
by Group 3 (p < 0.032) and that Group 2 was highly different from Group 3 (p < 0.001), as summarized in Fig. 3B. In order to disentangle the dissociation between N2– P2 reductions and unchanged pain subjective ratings in Group 1, we correlated the N2–P2 LEP amplitude differences (post-placebo treatment minus baseline) with the differences in NRS scores. We found a positive correlation in Group 1 (r = 0.546, p < 0.029), but not in Group 2 (r = 0.167, p = 0.536) as shown in Fig. 4. Although the sample is small, this might suggest a pos-
Fig. 3. N2–P2 LEP amplitude reductions. Representative LEP acquired from a subject of Group 2 (A). Note the decrease of N2–P2 complex after the conditioning procedure. N2–P2 LEP reductions in Group 1, 2, and 3 (B). The reductions are expressed as the mean difference between baseline and post-placebo treatment. The comparison with natural history shows that conditioning produced a more powerful reduction of LEP amplitudes than verbal suggestions alone. * p < 0.05, ***p < 0.001.
sible threshold level between LEPs changes and pain perception. In other words, the LEP modifications would become cognitively accessible only if a given threshold of amplitude reduction is reached. 4. Discussion The present work was aimed at investigating the role of learning in the placebo modulation of central nociceptive processing. To do this, we investigated the effects of both verbal analgesic suggestions and prior exposure to an effective treatment. We observed that the suggestion-induced LEP changes occurred without subjective perception of pain decrease. Conversely, the N2–P2
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(CS), whereas the surreptitious manipulation of the stimulus represented the unconditioned stimulus (US). In fact, conditioning is the learning of relationships between events, so as to allow the organism to represent its environment, and depends on both the information that the CS provides about the US, and the learning of relations among events [37,38]. The information that the CS provides about the US implies the involvement of cognitive processes in agreement with recent theories of classical conditioning [22,30,38], positing that conditioning procedures are used in order to increase expectations of benefit [21,22]. In this sense, expectations and conditioning are psychological processes that are not mutually exclusive [48]. However, these two psychological processes can be investigated separately on the basis of effective pre-exposure to US (e.g., active treatment or real biological event). 4.2. Psychophysical effects of learning versus verbal suggestions alone
Fig. 4. Correlations between the N2–P2 LEP amplitudes and NRS scores in Groups 1 and 2. A positive correlation was present for Group 1, but not for Group 2.
amplitude reductions that were induced by the conditioning procedure were associated with the subjective perception of pain reduction. With respect to the natural history, the conditioning procedure, whereby the subjects experienced an analgesic effect, produced a more powerful reduction of LEP amplitudes than verbal suggestions alone.
The psychophysical results of the present study show that verbal suggestions alone were ineffective in producing significant reduction in pain perception. Conversely, after a conditioning procedure the pain reports greatly decreased, suggesting that learning is crucial for inducing subjective changes, at least for pain. The importance of learning to obtain robust placebo analgesic responses is in line with our previous studies [11,13]. In fact, we showed that a conditioning manipulation can produce substantial placebo responses which are graded according to past experience. After exposure to an effective treatment we observed that conditioned placebo responses were still present after a time lag of 4–7 days. Conversely, when the same procedure was repeated for 4–7 days after a totally ineffective analgesic treatment, the placebo responses were remarkably reduced in comparison to the first group, pointing out that learning is crucial for long-lasting effects [11]. In this sense, the present work extends the studies by Colloca and Benedetti [11], Colloca et al. [13] as well as early clinical observations about the higher effectiveness of placebos when they are given after an active treatment [1,3,19,25,26].
4.1. Placebo analgesia and cognitive theories
4.3. The role of learning in the modulation of early central nociceptive processing of placebo analgesia
First of all, we would like to discuss some semantic and theoretical aspects. Here, we refer to the anticipation of benefit via verbal suggestion, and to learning via conditioning. Accordingly, we have used the term verbal suggestion to indicate one of the possible ways to induce expectations of benefit. Conversely, a conditioning procedure was used to expose the subject to effective treatment. The context (i.e., the environment whereby the treatment was administered) constituted the conditioned stimulus
Our neurophysiological findings shed light on how learning modulates early central nociceptive processing of placebo analgesia. By studying LEPs, which represent the most reliable neurophysiological method for assessing the human nociceptive system with a high temporal resolution non-invasively, we found that both verbal suggestions of benefit and conditioning induced a decrease in the amplitude of the N2–P2 complex, but not of the earlier N1 component. The modulation after
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verbal suggestions alone (Group 1) was smaller than that induced by conditioning (Group 2). As to the placebo-induced modulation of the specific components of LEPs, our findings are consistent with some recent studies that reported, by using a similar placebo manipulation, a reduction in the amplitude of P2 [46] and N2– P2 components [47]. In addition, Watson et al. [47] found that the reduction in LEP amplitudes occurred bilaterally when a placebo cream was applied on both the arms and conditioning was performed on one arm. In our case, the placebo responses, although tested sequentially, were present only on the part of the body where the placebo manipulation was directed. This is in line with previous psychophysical and pharmacological studies indicating that specific placebo analgesic responses can be obtained in different parts of the body [31,35] and that these responses are naloxone-reversible, suggesting that the placebo-activated endogenous opioid systems have a somatotopic organization [4]. By contrast, in the absence of specific target-directed expectations, as in the study by Watson et al. [47], the placebo responses may include different parts of body. As we used only a few electrodes, our study does not allow source localization of the LEPs. In the absence of source localization, no definitive conclusions can be drawn on the relationship between anatomical source of LEPs and distinct cognitive processes. However, it has long been known that a likely source of N1 is the SII [14,39,40,42] and that the N2–P2 receives contribution from several cerebral areas including the bilateral SII [41,42], the insula [15,17,40] and the CG [9,17]. In particular, the P2 potential has been described as the main marker of anterior CG, reflecting motivationalaffective states [see the reviews 9,17]. Most of the studies reported an enhancement of the N2–P2 complex at vertex with an increased attention and vigilance [16] and a decrease with distraction [9,17]. For example, intramodal attention increases both N2 [27,28] and laserevoked magnetic fields that are generated in SII [33], whereas inter-modal attention is associated to N2–P2 decrease [17]. In our case, all the subjects were asked to count the number of stimuli to maintain the same level of vigilance, although this might be not sufficient to warrant a complete control of attentive and/or distractive influences. Within this context, it is worth nothing that both verbal suggestions and conditioning reduced the N2–P2 amplitudes and that arousal and motivation may differently impact on each other. Verbal suggestions alone induced a decrease of LEP amplitudes but they did not affect subjective pain reports. This might be due to the duration of the pain. In fact, phasic or tonic pain may be a critical factor. For example, we behaviorally observed a substantial effect of analgesic suggestions on the submaximal effort tourniquet technique [6], but not on electrical shock [11,13].
Interestingly, by correlating the difference of N2–P2 amplitudes between baseline and post-treatment with the differences of NRS scores, we observed a positive correlation in Group 1. The positive correlation between LEP amplitude and pain report reductions is not present in Group 2, in which the decrease of both the N2–P2 complex amplitudes and the subjective reports of pain may lead to a sort of saturation effect. Albeit speculative, this suggests that conscious perception of pain reduction occurs only if a given LEP amplitude reduction is reached. Learning, by the pre-exposure to the effective US, may induce a progressive optimization of neural responses and favorite global coordination of brain–body–world entities for conscious perception of benefit. Certainly, this point is worthy of further investigation. 5. Conclusions Overall, these results indicate that prior exposure to effective treatments consolidates the expectation of benefit and is capable of maximizing the placebo response. The perception of effectiveness of a treatment, independently of whether it is primarily experienced as reflexive or as conscious, appears to be the crucial factor for shaping central nociceptive processing of placebo analgesia. Understanding the link between psychological mediators of placebo responses and neural correlates is important for at least three reasons. First, in the area of pathological pain, it can inspire new therapeutic strategies aimed at maximizing therapeutic efficacy and, at the same time, at reducing side effects. Second, the clarification of the brain effects by pre-exposure to active treatments raises some legal and ethical questions with a potential impact on several aspects of the society [6,10]. Third, in a clinical context, it is worth considering prior exposure to effective therapeutic interventions as learning influences both the cognitive and the neurophysiological aspects of pain experience. Conflicts of interest We have no conflicts of interest to declare.
Acknowledgements This work was supported by San Paolo of Turin, and Regione Piemonte, Turin, Italy.
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