***TASK*** the task is to summarize an input biomedical literature in six sentences 
    ***INPUT*** the input is a biomedical literature 
    ***OUTPUT*** the output is the summary of an input biomedical literature in six sentences 
    ***DOCUMENTATION*** 
    ***EXAMPLES*** 
    Input: The aim of this study was to determine the reliability of the conditioned pain modulation ( CPM ) paradigm assessed by an objective electrophysiological method , the nociceptive withdrawal reflex ( NWR ) , and psychophysical measures , using hypothetical sample sizes for future studies as analytical goals . Thirty-four healthy volunteers participated in two identical experimental sessions , separated by 1 to 3 weeks . In each session , the cold pressor test ( CPT ) was used to induce CPM , and the NWR thresholds , electrical pain detection thresholds and pain intensity ratings after suprathreshold electrical stimulation were assessed before and during CPT . CPM  was consistently detected by all methods , and the electrophysiological measures did not introduce additional variation to the assessment . In particular , 99 % of the trials result ed in higher NWR thresholds during CPT , with an average increase of 3.4 mA ( p<0.001 ) . Similarly , 96 % of the trials result ed in higher electrical pain detection thresholds during CPT , with an average increase of 2.2 mA ( p<0.001 ) . Pain intensity ratings after suprathreshold electrical stimulation were reduced during CPT in 84 % of the trials , displaying an average decrease of 1.5 points in a numeric rating scale ( p<0.001 ) . Under these experimental conditions ,  CPM reliability was acceptable for all assessment methods in terms of sample sizes for potential experiments . The presented results are encouraging with regards to the use of the CPM as an assessment tool in experimental and clinical pain . Trial Registration : Clinical Trials.gov Background Recent studies suggest that participant expectations influence pain ratings during conditioned pain modulation testing . The present study extends this work by examining expectancy effects among individuals with and without chronic back pain after administration of placebo , naloxone , or morphine .  Purpose This study aims to identify the influence of individual differences in expectancy on changes in heat pain ratings obtained before , during , and after a forearm ischemic pain stimulus . Methods Participants with chronic low back pain ( n = 88 ) and healthy controls ( n = 100 )  rated heat pain experience ( i.e. , “ test stimulus ” ) before , during , and after exposure to ischemic pain ( i.e. , “ conditioning stimulus ” ) . Prior to testing , participants indicated whether they anticipated that their heat pain would increase , decrease , or remain unchanged during ischemic pain . Results Analysis of the effects of expectancy ( pain increase , decrease , or no change ) , drug ( placebo , naloxone , or morphine ) , and group ( back pain , healthy ) on changes in heat pain revealed a significant main effect of expectancy ( p = 0.001 ) , but no other significant main effects or interactions . Follow-up analyses revealed that individuals who expected lower pain during ischemia reported significantly larger decreases in heat pain as compared with those who expected either no change ( p = 0.004 ) or increased pain ( p = 0.001 ) . Conclusions The present findings confirm that expectancy is an important contributor to conditioned pain modulation effects , and therefore significant caution is needed when interpreting findings that do not account for this individual difference . Opioid mechanisms do not appear to be involved in these expectancy effects 
    Output:
    
Several studies have shown that participant expectations are significantly related to observed CPM effects .
Furthermore , Cormier et al. reported that CPM expectancy effects were observable not only for pain ratings but also for nociceptive flexion reflex ( NFR ) activity .
In comparison to a control condition , participants who were told to expect analgesia to electrical stimulation during concomitant application of cold pain showed decreased NFR activity , whereas those who were told to expect hyperalgesia showed increases in NFR activity .
Indeed , NFR has shown excellent reliability both within and across testing sessions ; however , spinal reflexes such as NFR are susceptible to descending modulation and therefore have the potential to respond to expectancy in much the same manner as pain ratings .