An fMRI study

Human Brain Activation Under Controlled Thermal Stimulation and Habituation to Noxious Heat:An fMRI StudyLino R.Becerra,2,6Hans C.Breiter,2,7Milan Stojanovic,1,3Scott Fishman,1,3Annabel Edwards,1,3Alison ite,1,3R.Gilberto Gonzalez,2,6and David Borsook1,3,4,5*Brain activity was studied with functional magnetic resonance imaging(fMRI)following thermal stimulation.Two groups(n؍6/ group)of human male volunteers were given up to four noxious (46°C)and four non-noxious(41°C)stimuli.In the46°C experi-ment,positive signal changes were found in the frontal gyri, anterior and posterior cingulate gyrus,thalamus,motor cortex, somatosensory cortex(SI and SII),supplementary motor area, insula,and cerebellum.Low-level negative signal changes appeared in the amygdala and hypothalamus.All regions acti-vated by46°C were also activated by41°C.However,except for SI and thalamus,significantly more activation was observed for the46°C stimulus.A significant attenuation of the signal change was observed by the third stimulus for the46°C,but not for41°C experiment.Similarfindings were replicated in the second group.These fMRIfindings specify differences between somato-sensory and pain sensation and suggest a number of rich avenues for future research.Magn Reson Med41:1044–1057, 1999.௠1999Wiley-Liss,Inc.Key words:fMRI;pain;heat;Peltier thermodeThe use of fMRI offers significant advantages over positron emission tomography(PET)for the study of pain,includ-ing ability to image the time course of a physiological response,lack of exposure to radiation,improved anatomi-cal localization,and the potential to image pain in indi-vidual patients.The capability of fMRI to capture func-tional images over time can facilitate insights into the habituation of the nervous system to specific stimuli(1).To date,only a few fMRI pain studies have been reported(2,3). Studies are now needed to determine the sensitivity and specificity of fMRI to physiological responses after expo-sure to painful stimuli,such as noxious thermal heat. Functional MR images of such responses may allow us to examine novel central nervous system(CNS)regions of activation,not observed in PET studies,and may help demonstrate the utility of fMRI studies of pain in evaluat-ing other conditions,such as hyperalgesia or central sensi-tization.In this study,we describe the use of fMRI to image human CNS regions activated by a controlled thermal stimuli.We tested the hypotheses that a)the same activa-tion maps will be observed in two separate groups follow-ing either a41ЊC or46ЊC stimulus;and b)the two tempera-tures will produce similar patterns of activation that differ in the magnitude of signal change.The first hypothesis allows us to test the reliability of our findings and the second to assess similarities and/or differences between noxious and non-noxious stimuli.A priori regions for the first hypothesis were based on results for noxious heat obtained in the literature for PET studies(see Activation Analysis in the Materials and Methods section). MATERIALS AND METHODSHuman SubjectsThe study was approved by the Subcommittee on Human Studies for Research at the Massachusetts General Hospi-tal,Harvard Medical School,and written informed consent was obtained before each study.Healthy male right-handed volunteers(24Ϯ3.2years;meanϮSD,nϭ8)were re-cruited to measure pain intensity levels outside the fMRI environment(off-line/psychophysics group).Since prior exposure to the experimental paradigm might confound the results,none of these subjects were used in the fMRI experiments.For the fMRI studies,we examined the effects of heat on CNS activation in two separate groups of healthy right-handed men(33.7Ϯ7.7years,group1;29.0Ϯ3.8 years,group2;nϭ6/group).Two groups were selected to demonstrate that reliable and consistent results could be acquired.In both groups,the subjects had no medical, neurological,or psychiatric illness,were pain-free,and were not on any medications.Thermal Stimulus ParadigmNociceptive fibers in monkeys show little or no response to stimulus temperatures less than45ЊC;correlations in hu-mans in the same study show that such temperatures produce sensations primarily of warmth but not pain(4).In previous PET experiments,a41ЊC temperature was used to produce a‘‘warm’’stimulus(5–7).Similarly,a46ЊC tem-1MGH Pain Center,Massachusetts General Hospital and Harvard MedicalSchool,Boston,Massachusetts.2MGH-Nuclear Magnetic Resonance Center,Massachusetts General Hospitaland Harvard Medical School,Boston,Massachusetts.3Department of Anesthesia and Critical Care,Massachusetts General Hospitaland Harvard Medical School,Boston,Massachusetts.4Neural Plasticity Research Group,Massachusetts General Hospital andHarvard Medical School,Boston,Massachusetts.5Department of Neurology,Massachusetts General Hospital and HarvardMedical School,Boston,Massachusetts.6Department of Neuroradiology,Massachusetts General Hospital and HarvardMedical School,Boston,Massachusetts.7Department of Psychiatry,Massachusetts General Hospital and HarvardMedical School,Boston,Massachusetts.Grant sponsor:Public Health Service;Grant numbers:DA012581,NS34626,and RR13213;Grant sponsor:Ruth Freeman Fund for Pain;Grant sponsor:Melvin Fisher Fund for Pain Research.*Correspondence to:David Borsook,MGH Pain Center,ACC-324,Massachu-setts General Hospital,Fruit Street,Boston,MA02114.E-mail:Borsook@Received19March1998;revised4December1998;accepted6January1999.Magnetic Resonance in Medicine41:1044–1057(1999)1044௠1999Wiley-Liss,Inc.perature has been shown to produce a ‘‘noxious’’heat stimulus (6,8).Therefore,two temperatures (41ЊC and 46ЊC)were chosen for these experiments.Figure 1shows the stimulus paradigm.The thermode was strapped to the dorsum of the left hand.Subjects were informed 30sec in advance that stimulation would start.The experimental paradigm employed alternating stimuli of a baseline temperature of 35ЊC for 36sec,and then a thermal heat stimulus of 41ЊC for 29sec.Five minutes later,the experiment was then repeated with the higher tempera-ture (46ЊC instead of 41ЊC).In our experimental design we considered ordering effects by the two temperatures.A noxious stimulus such as 46ЊC might produce hyperalgesia that would clearly have an effect on activation by a subsequent 41ЊC stimulus,but not vice versa,since 41ЊC is an innocuous temperature (9,10).Furthermore,in prelimi-nary experiments the attenuation observed with a 46ЊC stimulus was not affected if it followed 41ЊC stimuli.Hence,we elected to give the 41ЊC stimulus ahead of the 46ЊC stimulus.Each experiment had four stimuli for the psychophysics group.In the fMRI experiment,four stimuli were used for group 1and three stimuli for group 2.We reduced the number of stimuli in group 2from four to three because initial analysis demonstrated attenuation of the signal after two stimuli (see results of group 1),thus limiting the duration of pain experience of the subjects.The heat stimulus was delivered using a ramp and hold method,with a rate of temperature rise of 4ЊC/sec.Data analyses focused on brain activation during the time of maintained (i.e.,plateau)temperature.Previous studies have used higher temperatures (49–50ЊC),shorter (5–12.5sec)stimulus duration,or smaller thermode surface area (6,11,12).In our studies,we used a fixed temperature of 41ЊC or 46ЊC,with a stimulus duration of 29sec and a larger thermode surface area (9cm 2).The relationship between the temperature and surface area of the thermode contribut-ing to spatial (13)and temporal summation (14,15)of noxious stimuli allowed us to design our protocol to produce subject responses to pain similar to those obtained in PET studies.Psychophysical Assessment of the Different Thermal Stimuli Measurement of pain intensity within the magnet would require interference with the protocol and might add spurious activation due to the cognitive operation of interceptive processing.Therefore,the stimulus paradigm used in the magnet was independently tested in the psychophysics group.Off-Line Measurements (Psychophysics Group)Subjects rated their pain intensity after each stimulus on an unmarked scale that measured 10cm [visual analog score (VAS)].One end of the analog scale read ‘‘no pain,’’and the other read ‘‘maximal pain intensity.’’The ratings scored on the VAS scale were measured and recorded in centimeters.After the four stimuli,subjects were asked to circle verbal descriptors derived from the McGill Question-naire for heat pain(16).FIG. 1.Summary of the experimental paradigms used in the off-line and on-line experiments.A:Off-line/psychophysics group.Psychophysical ratings [visual analog scale (VAS)]were scored immediately after each of the four stimuli (35–41°C or 35–46°C).At the end of the paradigm,subjects were asked to rate the stimulus using verbal descriptors (VD).B:On-line/fMRI group.Anatomical scans were obtained prior to the functional scans (shaded blocks).For groups 1and 2a verbal score (VS)was obtained at the end of the functional scan.For group 2,only three stimuli were delivered (see text).fMRI of Thermal Pain 1045On-Line Measurements(fMRI Groups)In the fMRI experiments,subjects rated their experience of pain intensity on a verbal scale from0(no pain)to10 (maximal pain intensity)immediately after completion of each functional scan.In50%of the subjects,randomly selected in each group,heart rate and respiratory rate/ ETCO2were continuously monitored(Omnitrack,FL)dur-ing the41ЊC and46ЊC scans to assess probable correlation with CNS activation.Peltier Thermal Probe Use in the fMRI Environment Controlled thermal stimuli were applied by using a commer-cially available Peltier thermode with a3ϫ3cm thermo-conducting surface(MEDOC,Haifa,Israel).The thermode is designed to provide noxious heat,noxious cold,and non-noxious stimulation over a range of0Њ–50ЊC.We specially adapted the thermode to be used in the fMRI environment by using non-magnetic parts and a radiofre-quency(RF)filter designed for this purpose.ImagingSubjects were scanned at the Massachusetts General Hospi-tal(MGH)-NMR Center with a quadrature head-coil in a1.5 T MR scanner(General Electric,Milwaukee,WI),modified for echoplanar imaging(Advanced NMR,Wilmington, MA).Prior to the fMRI scans,an axial localizer scan was performed.A conventional three-dimensional(3D)sagit-tal,T1-weighted,SPOILED/GRASS gradient echo GE sequence(through-plane resolution2.8mm;in-plane1.2 mm;60slices)was acquired for the anatomical atlas(17) transformation and for correct placement of experimental slices.Twenty contiguous slices(7mm thick)were pre-scribed perpendicular to the anterior/posterior commisure (AC-PC)line,extending from the anterior frontal pole through part of the cerebellum.The fourth scan was a high-resolution,T1-weighted echoplanar inversion recov-ery sequence(TI1100msec;in plane resolution1.57mm) for high-resolution structural images to be used in prelimi-nary statistical maps,but not with Talairach-transformed or averaged ones.For the functional studies,we utilized an asymmetric spin-echo(ASE),T2*-weighted sequence (TR/TE2500/70msec;flip angle90Њ;in-plane resolution 3.125ϫ3.125mm),performed with the same20-slice prescription.One hundred images were acquired per slice in all experiments,leading to a scan time of4min and10 sec.Data AnalysisData were analyzed as previously described in detail elsewhere(1,17,18).Briefly,data were motion corrected, Talairach transformed,normalized,and averaged.Any data displaying movement between acquisitions of struc-tural and functional scans of more than7mm in magnitude were discarded from the study;furthermore,all data were analyzed after motion correction for residual motion,and no studies were found to display residual motion.Voxel-by-Voxel Statistical MappingNon-parametric[Kolmogorov-Smirnov(KS)]statistical maps were constructed from the averaged data sets(1)and smoothed with a Hanning filter.Statistical maps weretransformed into-log(P)maps,and color-coded by magni-tude of the-log P value.Volumes of a priori regions werebased on data from the Center for Morphometric Analysisat MGH(19).Time courses were analyzed to determine thatactivation did not precede,but followed the stimulus,andto calculate the mean signal change between conditions. Anatomic LocalizationStatistical maps were superimposed over high-resolution,conventional T1-weighted images,which were also trans-formed into the Talairach domain.Regions activated werechecked against functional image data using objectivecriteria(20)to ensure that they did not overlap areas ofsusceptibility artifact(i.e.,near sinuses or ventricles).Apriori regions of interest were systematically inspected.Following such inspection,the entire extent of brainsampled was systematically inspected post hoc for unex-pected areas of activation.The specificity of anatomicalareas was determined by using Talairach coordinates. Percent Signal ChangeWe calculated percent signal change for the voxel with theminimum P value in a region of interest(ROI).(ROIs werenot selected on the basis of anatomical parcellation,but onthe basis of level of activation.)We also calculated thepercentage signal change for the whole ROI.Activation AnalysisA priori regions were defined for analysis of the firstexperiment(group1),based on results for noxious heatobtained in the literature for PET studies(6,8,11,21).Brainregions were included in the a priori group if they werereported as significantly activated in50%or more of thePET studies.These a priori regions included the anteriorcingulate gyrus bilaterally;the contralateral thalamus,insula,supplementary motor area,somatosensory regions Iand II;and the ipsilateral posterior cingulate gyrus.In thecase of the cerebellum,we only assessed activation in apost hoc manner since we did not scan the whole structure.To minimize errors due to multiple comparisons,activa-tions in group1were listed for a priori regions that met thethreshold of P aprioriϽ5ϫ10Ϫ4.(P apriori was obtained by dividing0.05by the number of voxels sampled in the apriori regions.)Post hoc analysis was performed on CNSregions that were seen to be activated outside these re-gions.Regions with P values of less than P post hocϽ5ϫ10Ϫ5 were considered to be activated.(P post hoc was obtained by dividing0.05by the total number of pixels of brain sampled.)For group2and the41ЊC experiment in group1, P apriori was determined by including all significantly acti-vated voxels found in the46ЊC experiment in group1;this calculation also produced a corrected threshold of PϽ5ϫ10Ϫ4.Post hoc analysis on group2and on the41ЊC experiment in group1was performed to determine unex-pected areas of activation.KS statistical maps were con-structed and analyzed for each of the four stimuli alone,for all four together,and for a combination of the first two and the last two stimuli.KS statistical maps were constructed and analyzed for each of the four stimuli alone,for all four1046Becerra et al.together,and for a combination of the first two and the last two stimuli.Comparison of ActivationAcross Groups (Group 1vs.Group 2)To compare activation across groups,data from both groups were normalized with regard to each other’s baseline and concatenated for each experiment.KS maps were gener-ated by analyzing the time courses with a combined paradigm including both experiments (i.e.,the test deter-mines those areas that were commonly activated in both groups).To investigate the differences,KS maps were calculated assuming the response to the first experiment as the ‘‘on’’condition while the response to the second experiment was the ‘‘off’’condition;the opposite compari-son was also computed.Between Experiments (41ЊC vs.46ЊC)To test whether the response to the 46ЊC stimuli was different from the 41ЊC stimuli,we first inspected tables of localized activations and statistical maps.Then to quantify any differences,we used the ROIs defined by the 46ЊC experiment to generate corresponding average time courses from the 41ЊC experiment (1).We performed a Student’s t -test between the average time courses using a threshold of statistical significance of 0.05divided by 13,the number of activated areas in the 46ЊC experiment (P Ͻ0.004).Negative Signal Change (Negative Activation)Negative activation has been reported in PET studies on pain (12),as well as other fMRI studies (22).Although the meaning of negative activation from a functional point of view (23)remains a topic of current investigation,we examined all data for negative activation and reported it as such.We generated statistical (KS)maps by assuming reverse activation (i.e.,the signal intensity decreases dur-ing the stimulus).RESULTSPsychophysical Assessment of Repeated 41°C and 46°C Stimuli on Pain IntensityOff-Line Measurements (Psychophysics Group)Figure 2depicts the psychophysics group VAS scores for each individual stimulus and the verbal scores of the fMRI subjects in the magnet.Average ratings reported by the psychophysics group are as follows.For the 41ЊC stimuli,VAS ratings were 1.80Ϯ1.41and 7.24Ϯ2.0for the 46ЊC stimuli.As shown in Fig.2,there is a trend to report lower VAS scores for the 41ЊC stimulus over the four stimuli (2.34for the first two stimuli vs.1.37for the final two stimuli).This trend,however,is not statistically significant (P Ͻ0.2,ANOVA).The ratings for the 46ЊC experiment do not show a decreasing trend in VAS scores and remain similar for each of the four stimuli.Based on the McGill Questionnaire,the most common descriptors used by the subjects were ‘‘warm,mild’’and ‘‘hot,mild’’for the 41ЊC stimulus;for the 46ЊC stimulus,descriptors included ‘‘burning,intense’’and ‘‘scalding,intense.’’On-Line Measurements (fMRI Groups)Subjects undergoing the fMRI experiments reported their pain intensity on a scale of 0–10immediately after imag-ing.The mean VAS reported for the 46ЊC stimulus was 7.3Ϯ2.9for group 1and 6.6Ϯ1.5for group 2.For the 41ЊC stimulus,verbal ratings were 3.7Ϯ2.3for group 1and 3.25Ϯ0.83for group 2.The 35ЊC baseline was rated as 0by all subjects.The ratings reported for the 41ЊC stimulus were signifi-cantly different from those reported for 46ЊC for both the off-line and on-line studies (P Ͻ0.001off-line;P Ͻ0.004on-line;Student’s t -test).A comparison of the off-line with the on-line ratings revealed no statistical difference (P Ͻ0.25for the 41ЊC and P Ͻ0.97for the 46ЊC stimuli;Student’s t -test).In the off-line group,no significantadapta-FIG.2.Psychophysical ratings of pain intensity.The histograms show the mean ϮSD (n ϭ8)visual analog scale (VAS)scores measured in centimeters for ratings (see text)of each of the four stimuli in the 41°C and 46°C paradigms (Fig.1).Although there is no significant differ-ence in ratings of the four stimuli for each temperature,there is a decreasing trend for the 41°C paradigm.The horizontal lines indicate the mean ratings (n ϭ6)from group 1(fMRI group),and the height of the boxes represents 2stan-dard deviations.There is no significant differ-ence for pain intensity ratings between the fMRI and the off-line scores (see text).fMRI of Thermal Pain 1047tion to either the41ЊC or46ЊC stimuli was observed.Based on the off-line results,it appears that scanned subjects did not experience a significant adaptation to the stimuli during the course of the functional scans(see Rapid Attenuation of fMRI Signal below).Figure3displays an example of on-line physiological monitoring(respiratory and cardiac)as well as the result of motion correction for the data from one subject undergoing four46ЊC stimuli.For this and the other subjects,corrected displacement was estimated at less than1.5mm.After motion correction,no subject displayed any residual move-ment.In six subjects measured,no significant correlation(Stu-dent’s t-test,PϾ0.05),was observed in heart rate,respira-tory rate,or ETCO2with the thermal stimuli during the fMRI experiment.No significant changes in these param-eters on analysis for possible phase shift were observed. These results indicate that pain-induced changes in blood flow/cardiac rate did not produce CNS activation.Rapid Attenuation of fMRI Signal With NoxiousThermal Heat StimulusStatistical maps were constructed and analyzed for each of the four stimuli alone,for all four together,and for a combination of the first two and the last two stimuli.When all four stimuli together were analyzed,a decrement of statistical significance was observed relative to the combi-nation of the first two stimuli.For instance,the anterior cingulate gyrus produced a significance value of PϽ8.4ϫ10Ϫ5for the first two stimuli,and a significance value of PϽ2.1ϫ10Ϫ4for all four stimuli together.No significant activation was seen for the last two stimuli in any anatomi-cal region(i.e.,stimulus3or4alone,or stimuli3and4 combined;P values were above threshold,i.e., PՆ5ϫ10Ϫ4),although subthreshold activation from the last two stimuli alone was observed for all activated regions defined from the first two stimuli.Thus,using the example of the anterior cingulate region,mild to signifi-cant signal changes were seen for all four stimuli presenta-tions:PϽ8.5ϫ10Ϫ4,4.1ϫ10Ϫ4,6.3ϫ10Ϫ2,and1.2ϫ10Ϫ2.Given these observations,data for the41ЊC and46ЊC experiments are presented for the first two stimuli together. Further inspection of the data reveals evidence of differen-tial activation(Table1).Some regions(the frontal gyri, cingulate gyrus,insula,middle temporal gyrus,and precen-tral gyrus)display a more rapid attenuation than other regions(thalamus,SI,SII,SMA,superior temporal gyrus,cerebellum).No similar decrement of signal is observed forthe four stimuli during the35–41ЊC(non-noxious)para-digm(Fig.4).Beyond attenuation of signal within the experimentalrun for46ЊC,modulation of the signal was also observedwithin each stimulus epoch,namely,the signal responsefor the first and second stimuli appears to display abiphasic response consisting of an early peak followed by asecond peak.Only the second peak of the pair is observedfollowing the third and fourth stimuli.The initial peak ofthe pair is significantly attenuated after the first twostimuli.This biphasic signal change was present in allregions activated,and in both groups.Intriguingly,KSmaps of CNS activation for the second(smaller)peak showno difference in significance,or location across the fourstimuli.However,there are significant differences whenactivation for the first peak is compared with activation ofthe second peak.We have separately observed this phenom-ena in two other distinct data sets.Figure4shows KS maps and time courses for averagedrepresentative ROIs activated in group1(thalamus andfrontal gyrus)by the41ЊC and46ЊC stimuli.Note the signalattenuation(habituation)for the third and fourth heatstimuli in the46ЊC group.The decreasing blood oxygen-ation level-dependent(BOLD)response contrasts with thepersistence of subjective pain defined in off-line experi-ments.fMRI Activation of CNS Circuitry by41°C and46°CHeat StimulationGroup1(35–46ЊC)Figure5shows a mosaic of KS maps superimposed onaveraged structural scans.A priori regions,as defined inthe methods section,were generally activated at significantlevels(P aprioriϽ5.0ϫ10Ϫ4,KS statistics).Table1lists the regions displayed in Fig.5,along with volumes of activa-tion and percent signal change.Activation was present incortical[SI,SII,supplementary motor area(SMA),frontalcortex,insula,and anterior and posterior cingulate gyrus],as well as in subcortical(thalamus)regions.Some struc-tures showed bilateral activation(insula and SII),withonly one side achieving statistical significance(contralat-eral SII,ipsilateral insula).One notable a priori region,namely,the periaqueductal gray of the midbrain,did notmeet our threshold for significance(PϽ6.6ϫ10Ϫ4).Unex-pected areas of activation that met the threshold forpostFIG.3.On-line physiological responses to41°C and46°C during fMRI acquisition.Overall movement dis-placement(in mm)of the brain during fMRI acquisition(left panel)and respiratory and heart rate(per min)(right panel)for one subject during the46°C experi-ment.Horizontal bars represent the periods when the46°C stimulus was on.1048Becerra et al.Table 1Regions Positively Activated by 46°C Stimulation (Group 1)†AnatomyBrain areaTalairach coordinates Volume (cm 3)P value%Change (mean ϮSD)1ϩ2vs.3ϩ4S-I M-L A-P Average Peak A prioriFrontal gyrus (mid)101225450.78.4ϫ10Ϫ50.81Ϯ0.29 1.24Ϯ0.34*Insula 3Ϫ34150.18.4ϫ10Ϫ50.78Ϯ0.270.93Ϯ0.23*(0)(40)(18)(1.4ϫ10Ϫ3)Cingulate gyrus (ant.)2431012 1.98.4ϫ10Ϫ50.55Ϯ0.23 1.02Ϯ0.31*Cingulate gyrus (post.)2437Ϫ3Ϫ39 1.2 3.4ϫ10Ϫ50.97Ϯ0.31 1.36Ϯ0.37*Thalamus69Ϫ210.1 1.4ϫ10Ϫ50.44Ϯ0.200.47Ϯ0.32Post-central gyrus (SI)14043Ϫ240.2 4.5ϫ10Ϫ4 1.57Ϯ0.51 1.67Ϯ0.53Parietal lobe (SII)402159Ϫ240.1 1.3ϫ10Ϫ4 1.05Ϯ0.31 1.66Ϯ0.53(40)(6)(Ϫ46)(Ϫ24)(9.8ϫ10Ϫ4)SMA 658Ϫ3Ϫ120.1 3.0ϫ10Ϫ40.47Ϯ0.340.57Ϯ0.44Post hocFrontal gyrus (medial)840Ϫ37270.4 2.2ϫ10Ϫ50.93Ϯ0.28 1.03Ϯ0.36*Temporal gyrus (sup.)223Ϫ68Ϫ240.1 3.4ϫ10Ϫ5 1.01Ϯ0.31 1.57Ϯ0.5015Ϫ59Ϫ420.3 3.4ϫ10Ϫ50.87Ϯ0.30 1.11Ϯ0.37Temporal gyrus (mid)21356Ϫ48 1.1 1.4ϫ10Ϫ5 1.12Ϯ0.39 1.16Ϯ0.46*Pre-central gyrus 618Ϫ5930.1 5.4ϫ10Ϫ50.86Ϯ0.25 1.13Ϯ0.46*Cerebellum**Ϫ25Ϫ15Ϫ572.82.0ϫ10Ϫ40.88Ϯ0.301.05Ϯ0.45†Structure according to the nomenclature of Talairach and Tournoux (17);Talairach coordinates according to Talairach.The P value indicates the minimum for each cluster of voxels determined by KS maps activated.Regions needed to meet a minimum volume constraint of 81mm 3to be considered activated,with each voxel meeting a threshold of P Ͻ10Ϫ3.In ‘‘1ϩ2vs.3ϩ4’’relative activation of the first 2vs.the last 2stimuli is measured.P Ͻ5ϫ10Ϫ4marked by *.Note that the cerebellum,marked by **,was not included in the a priori regions because we did not image the full structure.Where activation was noted in bilateral structures,but met the statistical threshold for only one side,the subthreshold activation is still listed,but inparentheses.FIG.4.Signal attenuation.Temporal variation of %signal change for the 35–41°C (left side)and 35–46°C (right side)experiments (group 1).Top two time courses display the average of all activated areas.The horizontal bars represent the period when the stimuli were on.Data coincident with the temperature plateau were used in the analysis.There is signal attenuation for then oxious thermal stimulus (35–46°C)not seen in the non-noxious stimulus (35–41°C).fMRI of Thermal Pain1049hoc analysis (P post hoc Ͻ5.0ϫ10Ϫ5,KS statistics),are also listed in Table 1and displayed in Fig.5.Detailed analysis in the thalamus showed activation anterior to the posterior commisure,in the region of the dorsomedial and the ventroposterior nuclei.Figure 6shows the similarity of the activation in the thalamus across ourtwo fMRI groups with regard to both Talairach coordinates and signal intensity change.Group 1(35–41ЊC)Table 2shows regions of activation by a 41ЊC stimulus,based on post hoc analysis of areas found in Table 1.ItFIG.5.Statistical map (KS)of regions of activation following a noxious thermal stimulus (group 1).KS map of the first two stimuli shows pseudocolor activation on averaged structural images across six subjects following a 46°C relative to a 35°C stimulus.A painful stimulation produced significant (P Ͻ5.0ϫ10Ϫ4)activation in the a priori regions shown in white labels.Areas activated below the posthoc threshold are labeled in orange.Loci activated are defined in Table 1.GFm,middle frontal gyrus;Ins,insula;aCG,(pCG),anterior (posterior)cingulate gyrus;Th,thalamus;SI (SII),primary (secondary)somatosensory cortex;Ce,cerebellum.Blue,5ϫ10Ϫ4;yellow,5ϫ10Ϫ5.FIG.6.Detail of activation in the contralateral thalamus following a 46°C stimulus in groups 1and 2.The figure shows coronal (top),sagittal (middle),and axial (bot-tom)views of activation in the thalamus in group 1(left)and group 2(right)compared with the Talairach atlas (center).The Talairach maps (from ref.17,with permission)are taken from the corresponding coordinates of activation.Note that the subtha-lamic region of activation is very similar in both groups.cm,cen-tromedial nucleus;dm,dorsome-dial nucleus;ld,dorsolateral nucleus;lp,posterolateral nucleus;P ,pulvinar;va,antero-ventral nucleus;vpl,ventralpos-terolateral nucleus;vpm,ventro-posteromedial nucleus.1050Becerra et al.。