Noxious and innocuous cold discrimination in humans: evidence for separate afferent channels

Pain,68 (1996)33-43 @ 1996InternationalAssociationfor the Studyof Pain. 0304-3959/96/ $15.00

33

PAIN 3180

Noxious and innocuouscold discriminationin humans: evidencefor separateafferentchannels Chao-Chen Chen, Pierre Rainville and M. Catherine Bushnell* Centrede Rechercheen SciencesNeurologiques,et FacultkdeM4decineDentaire,Universikfde Montrtal,Montrial, Qu4bec,H3C3J7 (Canada) (Received16November1995,revisedversionreceived12April 1996,accepted8 May 1996)

The present study evaluatedthe abilityof humansto discriminatetemperaturedecreasesin the noxiousand innocuous Summary cold range.‘IVOgroupsof five subjectsdetectedchangesin cold stimuliappliedto the maxillaryface.For five subjects,adaptingtemperatures of 22°, 16°, 6° and O°Cwere used, and thresholdsfor detectingtemperaturedecreaseswere determinedusing an adaptivepsychophysicalparadigm.Visualanaloguescale(VAS)ratingsof cold and pain sensationwere also recordedat 5-reinintervalsthroughouteach session.A second group of five subjectsperformeda similardetectiontask, but in this case using classical psychophysicaltechniques (methodof constant stimuli) and adaptingtemperaturesof 22°, 16°, 10° and 6“C. These subjeetsdescribedthe quality of the detected changein sensation,in additionto rating overallcold and pain sensationthroughoutthe session.Detectionthresholdswere 0.27°, 0.48°, 4.8°, 8,0° and >1O.O”Cfor baselinesof 22°, 16°, 10°,6° and O“C,respectively,indicatingthat discriminationwas better in the innocuous cool (22° and 16°C)than in the noxiousand near-noxiouscold (lO-O°C)range(P< 0.05).Tonicadaptingtemperaturesof 22° and 16°C werealwaysrated as cool but not painful,whereasadaptingtemperaturesof 10°and 6° weresometimesand O°Cusuallyrated as painful. Phaaictemperaturedecreasesfrom 22° and 16°Calwaysproducedcoolingsensations,whereasdecreasesfrom baselinesof 10° and 6°C producedprimarily sensationsof painful and non-painfulprickle.These data suggestthat differentafferentchannels mediate cool and noxiouscold perceptionand add supportto the hypothesisthatnoxiouscold sensationis mediatedby subdermalnociceptors. Keywords: Pain;Temperature;Thermal;Cold;Cool;Psychophysics

Introduction Various evidence suggests that the sensations of innocuous cool and noxious cold are mediated by different primary afferent fibers and may be processed differently within the central nervous system. The primary sensation evoked when the skin is cooled to temperatures as low as about 15–20°C is that of ‘cool’ (Croze and Duclaux 1978; Stevens 1979; Greenspan et al. 1993). However, when the temperature is reduced further, a painful sensation arises that is frequently described as ‘pricking’, ‘burning’, or ‘aching’ Wolf and Hardy 1941; Kunkle 1949; Chery-Croze and Duclaux 1980; Chery-Croze 1983; Yarrtitsky and Ochoa 1990; Morin et al. 1994). Recordings from primary afferent fibers in primates show the existence of a group of fibers with steady state discharges to temperatures between about 15° and 40°C, The relation* Grrrvspondingauthor:M.C. BushneIl,Dept. Anesthesiology, McGill

University, 687 PineAve.,Rm,F3.01,Montresd, QuebecH3AIA1, Canada.Tel.:(1)514-398-3493; Fax:(1)514-398-8241; E-mail:bushnelle@medcor. mcgill.ca PII S0304-3959(96)03 180-6

ship between temperature and firing frequency is described by an inverted U-shaped function, with maximum discharges between 20° and 35°C (Darian-Smith et al. 1973; Dubner et al. 1975; Dykes 1975; Beitel et al. 1977; Kenshalo and Duclaux 1977), corresponding to the temperature range perceived by humans as ‘cool’. A few studies have identified other primary afferents that respond best to skin cooling in the noxious range in primate (Georgopoulos 1976; Lamotte and Thalhammer 1982),cat (Saumet et rd. 1985),rat (Simone and Kajander 1993) and dog (Iriuchijima and Zotterman 1961), although many of these fibers also respond to noxious mechanical and/or heat stimuli. Results of several studies suggest that many primary afferents responding to innocuous cool have their receptors located supertlcially in cutaneous tissue (Hensel et al. 1951, 1974). In contrast, there is some evidence suggesting that the receptors for noxious cold are preferentially located on vein walls (Fruhstorfer and Lindblom 1983; Kreh et al. 1984; Klement and Arndt 1992). The differential representation of cool and noxious cold appears to be maintained at the level of the spinal cord. Various studies show cool-specific neurons in the dorsal horn of primates (Iggo and .,. Rarnsey 1974; Burton 1975), cats (Dos-

34

trovsky and Hellon 1978; Davies et al. 1983; Craig and Hunsley 1991; Craig and Serrano 1994), and rats (Davies et al. 1985; Mokha 1993) that respond similarly to cool-specific primary afferents. In contrast, it is the multireceptive nociceptive cells (i.e. neurons that respond to more than one stimulus modality, such as mechanical and thermal) that respond best to noxious skin cooling (Christensen and Perl 1970; Kenshalo et al. 1982; Craig and Bushnell 1994; Craig and Serrano 1994). Much less is known about the responses of thakunic and cortical cells to skin cooling, but human lesion data showing cool thermanesthesia with enhanced cold pain after lesions of the thrdamus or cerebral cortex (Dejerine and Roussy 1906; Breuer et al. 1981; Bogousslavsky et al. 1988; Boivie et al. 1989; Leijon et al. 1989) suggest that different neuronal groups might underlie these perceptions. The identification of neurons responding to cool but not noxious cold in the thalamus of humans (Lenz et al. 1993), monkeys (Poulos and Benjamin 1968; Burton et al. 1970; Bushnell et al. 1993; Craig et al. 1994), cats (Landgren 1960; Auen et al. 1980), and rats (Hellon 1973; Jahns 1975; Werner et al. 1981), as well as neurons responding specifically to noxious cold in the monkey thalamus (Bushnell and Craig 1993; Craig et al. 1994) and rat cortex (Backonja and Miletic 1991), supports this distinction. Psychophysical examination of liminal perceptual ability can be used as a tool for evaluating differential processing of sensory information in the brain. For example, it has been shown that both humans and monkeys discriminate intensities of noxious heat better than intensities of innocuous warmth (Handwerker et al. 1982; Bushnell et al. 1983; Robinson et al. 1983), consistent with findings that different primary afferent fibers signal noxious and innocuous heat (Lamotte and Campbell 1978; Gybels et rd. 1979; Sumino and Dubner 1981; Yamitsky and Ochoa 1991; Yarnitsky et al. 1992). These psychophysical data suggest that the differential processing is preserved at the cortical level, where cognitive evaluations are made. In the cold range, psychophysical studies have evaluated human discrimination of the intensity of innocuous cool stimuli (Johnson et al. 1973;MoIinari et al. 1976; Rozsa et al. 1985), but such data do not exist for the discrimination of painful cold. The present study compares humans’ cutaneous temperature discrimination for a range of innocuous cool and noxious cold stimuli applied to the face. We hypothesize that if the differential treatment of innocuous cool and noxious cold seen for primary afferents is preserved in the brain, subjects’ ability to detect cooling pulses from adapting temperatures above approximately 15°C will differ from that for colder temperatures. A preliminary report of these data has been presented (Chen et al. 1994). Methods Two groupsof experimentalsubjectswere used to evaluatethe sensations evoked by different levels of cutaneouscold, as well as their

,~’-’”

j

4

f

PREss

RESPONSEWINOOW

Fig.1.Schematicrepresentation of stimulusandrvsponsesequenceduring coolingdetectiontrisf. Skin was adapted to a specified temperature between 0° and 22”C (Tl). The subjectinitiateda triafby pressinga lever. After4-7 SCC,the thcrmodetemperaturedecreasedat 5°C/secto a specifiedlevel, and returned to baseline when the subject released the lever or afier a specifiedtime (Fm). CorYectresponseswere thOSe OCcurring between F~n and Fmax(see dashed line). These values were determined from reaction times to suprathreshold stimuli during pilotexperiments. abilityto detect coolingsteps frominnocuousand noxiouscold adapting skintemperatures,

Subjects Elevensubjectsrecmitedfrom studentsand staff at the Universityof Montrealparticipated in this experiment.Six subjects (five male, one female,aged 28-55 years)participatedin Experiment1 and five subjects (threemale,two female,aged 2444 years) participatedin Experiment2. All participantsgave informedconsentacknowledgingthat the methods to be used and the risks involvedwere clearly explained and that they understoodthat they were free to withdrawfrom the experimentat any time without prejudice. All procedures were approved by the Human EthicsCommitteeof the Universityof Montrerdand were in accordance with the Declarationof HumsnRights,Helsinki 1975.One male subject in Experiment1 was withdrawnafter experiencinga mild locafizedvesicationof the epidermisat the site of cold stimulation,so that analysesof eachexperimentare basedon five subjects.

Stimulationprocedures For both experiments,cold stimuli were delivered to the maxillary face by a contact thcrmode.The thermal surface was a 1cm2 Pekier element with an attached thermocoupleto provide precise feed-back controlledstimuli(ML1”C).The rate of tempcratvmchangewas constant at 50/seethroughoutthe experiments.Duringboth experiments,subjects were seated with the head supportedby a chin rest. The thermodewas positionedjust above the naaolabial fold, approximately 1cm from midline.

Experiment1 protocol An adaptivepsychophysicalprocedurewas used to assess the ability of subjectsto detect small decreasesin skin temperaturefrom innocuous and noxiouscold adaptingtemperatures.Based on data from pilot studies, two innocuouscool (22° and 16”C)and two noxious/near-noxious cold baselines (6° and O°C)were used. Testing was conductedduring four sessions,two on each of two subsequentdays. One session of noxious/near-noxious(0° or 6“C)andone sessionof innocuous(16°or 22”C) adaptingtemperaturewere presentedon each day with a pause of 1020 minbetweenthe twosessions.Theorderof adaptingtemperatureswas counterbalancedacrosssubjectsand acrossdays.

35

TABLEI IN EXPERIMENT2 ATSFOREACHADAPTING TEMPERATURE Adaptingtemp.(“C)

AT(°C)

22

0.1,0.2,0.5, 0.1,0.5,1.0, 1.0,2.0,5.0, 1.0,2.0,5.0,

16 10 6

Finaltemp.(“C) 1.0 2.0 10.0 10.0

21.9,21.8,21.5,21.0 15.9,15.5,15.0,14.0 9.0, 8.0, 5.0, 0.0 5.0, 4.0, 1.0,-4.0

Thesubjects’taskwasto initiateatrial bypressingaleverandtoreleasetheleverwhentheyperceivedachangein sensationevokedbythe thermode.For each baseline temperature,the thermodewas left on the face for 5 minbeforebeginningthe experiment. The subject then performed 50 practice trials, in which easily detectable suprathreshold stimuluschanges(ATs)were presented.To facilitate learningduringthe practice trials, subjects viewed an oscilloscopeshowingwhen the temperaturechangesoccurred.Immediatelyfollowingthe practice trials, 50 trials of data were collected, using variable ATs and no supplemental cues. Duringdata collection,MSadaptivealgorithmwas utilizedin whicha target performancelevel was identified, and the AT for each trial was determined based on the subject’s performance on previous trials

22°c

(Duncanand Miron 1990).Twoadaptivesequenceswere alternated,with target performancesof 80% and 60%, respectively. In general, with a target performanceof 80%, the AT was reduced if the subject detected more than 80% of the stimuli and increased if he detected fewer than 80%.Initial ATs for each baseline were determinedfrom pilot data and werechosento minimizethe task difficultyat the beginningof the testing sequence(AT= 4°C for baselinesof 22° and 16”and AT= 8°C for baselinesof 0° and 6°C).In orderto reducethe subjects’use of temporalcues for detectingthe coolingramps, temperaturedecreases occurredat variable times, rangingfrom 4 to 7 sec after trial initiation. Only responses occurnrrgwithin a 2-see window were considered as correct, and MS auditory signal indicated successful detection of the stimulus change (Fig. 1).

Experiment2 protocol Based on results of Experiment1, the cold discriminationtask was modifiedfor a second group of five subjects. Since the use of a O“C baselinein Experiment1resultedin a mild skin irritationin two subjects, oneof whomdiscontinuedthe experiment,baselinesof 22°, 16°, 10°and 6°C were used in Experiment2. Further,instead of employinga&ptive psychophysicalmethods,the classical method of constant stimuli was employed,in whichfour AT values were chosen for each adaptingtem-

B

16°C

10.

6.

6

4

2

0 5

10

15

20

25

30

5

D

6°C

10

15

20

25

30

10

15

20

25

( 30

O“c

lo-

10

9-

8

6-

7-

62

5-

o~o

4-I 0 TRIAL

5

TRIAL

Fig. 2. The performanceof one subject(H5)in the adaptiveprocedure,for baselinetemperaturesof 22°C(A), 16°C(B), 6°C (C), and O“C(D). Initial AT was 4°C for baselinesof 22° and 16”C,whereasinitial AT was 8°Cfor baselinesof 6° and O°C.Opencirclesrepresentcorrectdetectionand filled circles representlate responses,i.e. the subject’sfaihm to detect the temperaturechange.On alternatetrials of each testingsession,target performancewas 80% correct(shownhere).Forthe otherhalf of the trials,the targetperformancewas60Y0correct(not shown).

36

B

22 ‘c

A

+Hl + + -0&

16‘C

100H2 H3 H4 H5

80-

60

40

1.0

0.1

10

D

6 ‘C

1,0

0.1

10

0‘c

100-

1009 80-

80-

@ V Lu ~

60-

60-

:

40-

40-

20-

20-

00.1

1.0

10

00,1

v

1.0

10

TEMPERATURE CHANGE (“C)

TEMPERATURE CHANGE (“C)

Fig, 3. Percentof ATsdetectedby each subjectin Experiment1 for adaptingtemperaturesof 22° (A), 16°(B), 6° (C), and O°C(D). In the adaptiveproceduremanyATsare presented.Thus,to calculatethe% correct,ATswererankorderedand% correctwasdeterminedfor threerangesof AT (i.e. the smallest, middleand largestthird of ATspresented).The log medianATis plottedon the x-axis.For somesubjectsat adaptingtemperaturesof 6° and O“C,the maximumallowableAT (lO°C)was presentedfor morethan 1/3of the trials. In thesecases,datawerecollapsedintoone or twobins, as shownin (C,D). perature,based on the results of Experiment1 (Table I). To reduce the maximumchance performancefrom 50% to 25170, the variabletime before a stimulus change was expanded to 3–9sec. Other modifications includeda reductionin the length of the practice period from 50 to 10 trials, an eliminationof the 5 min stimulusadaptingperiod, and an extensionof the data collectionperiodto 120trials. Thesechangesallowed us to evaluatepossibletemporalchangesin discriminativeabilitydue to a coldafferentblock.

Stimulusratingprocedures For both experiments,subjects rated ongoingcold intensityat 5 min intervalsthroughoutthe experiment,usinga 100mm VAS,with anchors of ‘Nosensation’and ‘Intolerablepain’ and an intermediatedescriptorof ‘Painthreshold’at 50 mm.At the end of Experiment2, subjectsrdsoused a 100mm VASwith anchorsof ‘Never’and ‘Always’to answereach of the followingquestions: ‘Whenyou detected the stimuluschange,were you respondingto: (1) sensationof cooling;(2) sensationof non-painful tickle or prickle; (3) sensation of painful prickle, shock, burning or other?’ These descriptorswere chosen, based on free verbal reports of pilotsubjects.

Dataanalysis Responseswere categorizedas ‘early’(equivalentto ‘falsealarm’ in signal detectiontheory)if the subject released the lever before the temperaturedecrease, ‘correct’(equivalentto ‘hit’)if he tvleased duringthe responsewindow,and ‘late’(equivalentto ‘miss’)if he failed to release by the end of the 2 sec window.All calculationsof the percentageof ATs detectedwere based on ‘correct’/(’correct’+ ‘late’). Detectionthreshold was determinedusing graphic interpolation,with threshold defined as 75% and 62.5%correct detectionfor Experiments1 and 2, respectively, because of the higher guessing rate in Experiment 1. Non-parametric (Friedmanand Wilcoxonrankingtests) and parametric(ANOVAsand ttests)statisticswereused,accordingto the typeof data analyzed.

Results

Discrimination Fig. 2 shows, for an individual subject of Experiment 1,

37 1001

‘rI Q,

20

T

P

TABLEII MEANPERCENTEARLYRELEASES(FALSEALARMS)

-c--6°C

/-K/

+1O”C

+

+16°C

=&’” Y

-Q-22°C

10

1.0

0,1

TEMPERATURE

CHANGE

(“C)

Fig.4. MeanpercentATsdetectedin Ex~riment 2 for adaptingtemperatures of 22° (diamonds),16” (squares), 10”(triangles)and 6° (circles). N= 5; verticalbars represent*SEM.

the temporal sequence of stimuli and the performance during the adaptive psychophysical procedure. Fig. 2A,B reveals that for baselines of 22° and 16”C, the subject successfully detected the initial ATs. The magnitude of the AT was then progressively decreased by the adaptive algorithm until the subject’s correct detection of the ATs approximated the predestinedperformance target of 8090.For baselines of 22° and 16”C, this resulted in programmed ATs of less than 1°C. Figs. 2C and 2D show that for baselines of 6° and O“C,the subject was unable to detect the 8° AT on 8090of the trials, resulting in an increase in the programmed AT to the 10° maximum. Stimulus-response functions for individual subjects of Experiment 1 are displayed in Fig. 3. For every adapting temperature, subjects almost always detected more stimuli

10-

+

EXP. 1

+

EXP, 2

Experiment

22°C

16°C

10”C

6’C

O“c

1 2

6.5 6.2

4.5 14.7

17.5

11.4 17.3

13.1 -

with larger ATs, indicating that their performance was under stimulus control. Fig. 3A,B shows that all subjects detected ATs of 1°C with at least 80% accuracy for adapting temperatures of 22° and 16°C, whereas Fig. 3C,D shows that most subjects were unable to detect even ATs of IO°C at 8090accuracy from adapting temperatures of 6° and O“C. Fig. 4 shows the average percent correct detection at different ATs for subjects in Experiment 2. At all adapting temperatures subjects’ performance was better with larger ATs. Further, subjects detected smaller ATs with higher adapting temperatures. A comparison of Figs. 3 and 4 reveals that performance was dramatically better at cool baselines (22°C and 16°C) than at extremely cold baselines (lO°C, 6°C, and O“C),irrespective of the psychophysical technique employed.

Detectionthresholds Median detection thresholds for each temperature are shown in Fig. 5. For both experiments, detection threshold varied as a function of adapting temperature (Friedman test, P <0.01 for each experiment). More specifically, subjects had smaller detection thresholds at the two non-painful adapting temperatures (22° and 16°) than at the painful and near-painful temperatures (10°, 6° and 0°) (Wilcoxon signed ranks tests, P e 0.05). There was no statistical difference in the thresholds determined in Experiments 1 and 2 at any of the common temperatures (i.e. 22°, 16° and 6°) (MannWhitney U-tests, P > 0.05).

Falsealarms

~ +1 58 gz w z z

W 0 z w u If u E z ~ B

[

6-

4-

2-

~ 0 0246

8 ADAPTING

10

12

14

TEMPERATURE

16

18

20

22

(°C)

Fig. 5. Mediandiscriminationthresholds,determinedby graphicinterpoIation,for each adaptingtemperaturein Experiments1 and 2. In Experiment 1, maximumchance performancewas 50% correct, so that 75% correct was defined as detection threshold,in Experiment2, maximum chanceperformancewas 25%correct, so that 62.5%correctwas defined as detectionthreshold.Verticalbars represent*interquartilerange.

Table II shows the mean percentage of trials in which subjects released the lever before the stimulus change occurred. These early responses are similar to ‘false alarms’ described in signal detection theory (SDT), in that the subject indicated that a stimulus occurred when in fact it did not. Thus, different rates of early responses suggest a difference in the subject’s willingness to report a stimulus event when he is somewhat uncertain (termed ‘detection criterion’ in SDT). In both experiments, subjects tended to produce more false alarms for temperatures below 10”C than for 16° and 22°, but this tendency was significant only in Experiment 1 (F(3,12) = 5.01, P< 0.02).

Temporalchangesinperformance The number of correct responses during the first and last 10 trials for each subject in Experiment 2 was compared. An ANOVA revealed a significant difference in the number of

38

Experiment1

A 100

■ ❑

---

+O”c +6QC + 16°C

PAINFUL PRICKLE NON PAINFULTICKLE

+22°C

1

~

Td

C3 g

50

Pain threshold

s

325-

0

1

m

1

1

5

10

f

I 0

15

20

25

30

6

Experiment2

B 100

-0-160C +22°C

~ a

’25-

Pain threshold 50-

o] : W

o

16

22

Fig.7. MeanVASratingsfor the changein sensationrelated to temperature decreases from various adapting temperatures. Perceptions of ‘painfulprickle’(black),‘non-painfultickle’ (white),and ‘coolingsettaation’ (hatched) were rated for adapting temperatures of 6°, 10°, 16° and 22°in Exp.2.

+6°C +Io”c

1

~

lb

BASELINETEMPERATURE (“C)

(

r 5

10

15 TIME (rein)

20

25

30

Fig.6. Meanratingsof cold intensityat 5 min intervalsduringExp. 1 (A) and Exp.2 (B). Ratingsfrom0-50 wereassignedto levelsof non-painful cold, a rating of 50 was definedas ‘painthreshold’,end a rating of 100 was definedas ‘intolerablepain’.

correct responses at the beginning and end of the sessions (F(3,12) = 9.13, P <0.01), but there was also an interaction between adapting temperature and this temporal effect (F’(3,12)= 5.25, P c 0.02). Contrast analyses showed no reduction in the number of correct trials at the end of the experiment for adapting temperatures of 22°C (62% versus 64% correct), 16°C (42% versus 58%) and 10° (44% versus 46%). Only when the adapting temperature was 6° was there a significant decrease in the number of correct trials (40% versus 20’%0).

Stimulusratings Subjects’ ratings of cold intensity at 5 min intervals throughout Experiments 1 and 2 areshown in Fig. 6. In general, cooler temperatures were rated as colder (Exp. 1: F(3,12) = 9.71, P< 0.01; EXP. 2: F(3,12) = 6.02, P= 0.01), but only O°C was rated as painful (Fig. 6A). For both experiments, there was no significant difference in the rating of

22° and 16° (P= 0.93 and P = 0.90, respectively). Similarly, 10° and 6° were not rated differently (Fig. 6B, P = 0.77). In contrast, 6° and 10° were both rated as significantly colder than 16° (Fig. 6A,B, respectively; P< 0.05). Further, although 0° tended to be rated colder than 6° (Fig. 6B), this effect did not reach significance (P= 0.10). In Experiment 1, for which subjects’ skin was adapted to each temperature for at least 5 rein, there was no significant change in sensation across the 15 min of data collection. However, in Experiment 2, for which data collection began almost immediately after placing the thermode on the skin, the cold ratings continued to increase slightly but consistently throughout 25 min of data collection (F(5,20) = 6.54, P< 0.01). 1.5

46°C *

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+

16°C

+22°C g 03 ~ 0 Lu w

1.0 Id

I% z < w

z

0.5

I ‘p‘‘“ o

20

40

60

60

100

% CORRECT

Fig. 8. Meanresponsespeed(l/latency) and percentof stimulidetectedat each of the four ATs used with each adapting temperature in Exp, 2 (see TableI for ATs).

39

A

TI =22 3-

B

H8

3-

r = -0.46

p<0.031

p<0,001

0

8

2~ >

0

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$? w

T1 = 16 H8 r = -0.44

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T1 = 6 H8

3-

0

0,

0.4

0.2

D

H8

0

0; 00

~

1 1.0

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0 2-

0.8

r = -0.17

r = +0.47

p = 0.25

p.0.002 2

4

6

8

( 10

TEMPERATURE CHANGE (“C)

or

7

0

4

2

6

8

10

TEMPERATURE CHANGE (“C)

Fig. 9. Latencydistributionof correctresponsesas a functionof AT for subjectH8 at adaptingtemperaturesof 22”(A), 16°(B), 10°(C) and 6° (D). The lines showthe linearleastsquaresregressionandrindicates Pearson’scorrelationcoefficient.

Subjects in Experiment 2 were asked to des+be the quality of the change in sensation to which they responded at adapting temperatures from 22° to 6° (see Methods). Despite the finding that all of these adapting temperatures were rated as cold and none as painful (see Fig. 6), the detected change in sensation from baselines of 6° and 10° was described on visual analog scales as evoking ‘painful prickle’, ‘nonpainful tickle’, as well as a cooling sensation (Fig. 7). The subjects’ perception of a decrease in temperature from baselines of 10° and 6° contrasts sharply with their perception of such decreases from baselines of 16° and 22°, which evoked almost exclusively a cooling sensation (Fig. 7). An ANOVA and associated contrast analyses revealed a significantly greater report of cooling sensations with temperature decreases from baselines of 22° and 16° than from baselines of 10° and 6° (ANOVA: F(3,12) = 11.69, P c 0.01; contrast analyses: 22° versus 16° (P= 0.18); 16° versus 10° (P= 0.02); 10° versus 6° (P= 0.91)). There was no effect of stimulus order (P= 0.85), so that the same results accrued when subjects received the noxious stimuli before or after the innocuous stimuli.

Detectionspeed Fig, 8 shows the mean detection speed (i.e. I/latency) plotted against percent of stimuli detected for the four ATs of each adapting temperature in Exp. 2. Adapting temperature had a significant influence on response speed (F(3,12) = 44.9, P< 0.001), with temperatures of 6° and 10° having slower speeds than adapting temperatures of 16° and 22° (contrast analyses: P c 0,001). There was no overall significant effect of level of AT on respon6e speed (P= 0.07), but there was a significant interaction between adapting temperature and AT level (F(9,36) = 3.35, P < 0.01), probably reflecting the observation that response latency decreased with larger ATs at 22° and 16°, but not at 10° and 6°. Fig. 9 shows, for an individual subjec6 the distribution of response latencies for correct responses as a function of AT, The distribution of latencies at 22° is the least variable, and the negative correlation between response latency and AT is greatest at 22° and disappears completely at 6°. The same general tendency was found across all subjects of both experiments, as shown in Table III. Subjects almost always

40

TABLEIII PEARSONCORRELATIONOFRESPONSELATENCYANDAT

HI H2 H3 H4 H5

H6 H7 H8 H9 H1O

Adaptingtemp.22°C

Adaptingtemp. 16°C

Adaptingtemp.6°C

Adaptingtemp.O“C

r

r

r

P

r

0.195

0.293

0.216 0.462 0.157

0.439 0,009” 0,390

-0.287 -0.145 -0.229 –0.064 0.185

P 0,081 0,43 0.18 0.705 0,366

P

-0.344 -0.31 -0.35 –0,024 –0.281

0.043” 0.074 0.062 0.881 0.092

0.247 -0.331 -0.214 0.185

P 0.159 0.153 0.328 0.366 —

Adaptingtemp.22°C

Adaptingtemp. 16°C

Adaptingtemp. IOaC

Adaptingtemp.6°C

r

P

r

P

r

r

0.004” <0,001* <0.001* <0.001* <0.001*

-0.16 –0.34 -0.44 -0,15 -0.05

0.17 0,001* <0,001* 0.27 0,73

–0.31 -0.49 -0.46 -0.47 -0.58

0.36 -0.31 -0.17 0.11 0.12

P 0.012* 0.019* 0.25 0.47 0.43

-0.08 –0.38 0.47 0.29 -0.07

P 0.64 0.006” 0.002” 0.21 0.64

– indicatesno correlation,becausecorrectresponsesonlyobservedat oneAT

showed a negative correlation (Pearson’s r) between latency and AT for adapting temperatures of 22° and 16°, although these negative correlations were only significant in 8 of 20 cases. In contrast, this consistent relation is absent for adapting temperatures of 10° and below, with about half the subjects producing positive r’s and half negative r’s.

Discussion Our results show that subjects detect smaller temperature decreases from adapting temperatures of 22°C and 16°C than from adapting temperatures of 10”C and below. In addition to detecting smaller temperature differences in the clearly innocuous cool range, subjects’ response latencies were less variable, suggesting that they may have responded to a more consistent, distinct sensation. This interpretation is supported by the finding that temperature changes from adapting temperatures of 22°C and 16°C were almost always perceived as ‘cold’, whereas those from adapting temperatures of 10”C and below were perceived sometimes as ‘non-painfultickle’, sometimes as ‘painful prickle’, and sometimes as ‘cold’. The only adapting temperature reliably perceived as painful was O°C. However, there was a non-linear relationship between perceived cold and adapting temperature, with 6°C and IO°C being rated as significantly colder than 16°C and 22”C. Several explanations could be invoked to account for the striking difference in subjects’ ability to discriminate cooling shifts from the higher and lower adapting temperatures. One possibility is that discriminability is not in fact different, but that subjects used a more conservative criterion for reporting a change in sensation for the lower than for the higher temperature range. However, a stricter response criterion would also lead to fewer ‘false alarms’, i.e. responses before the temperature decrease actually occurred. In reality, the oppo-

site occurred; subjects produced more false alarms for the lower than for the higher adapting temperatures. Another explanation for the poor discriminative ability at low adapting temperatures might be that we produced a cold conduction block of the fibers. At an adapting temperature of O“C,there was a change in perceived pain and cold throughout the session (see Fig. 6), supporting the argument that the receptors may have become desensitized. However, several observations indicate that most of the difference in discrimination cannot be accounted for by a cold conduction block. First, the perceived cold did not decrease across the session for any adapting temperature other than O“C. Further, discriminative performance was worse for lower than for higher adapting temperatures at the beginning as well as at the end of the sessions, and for the adapting temperature of 10”C, discriminative performance did not worsen between the first and last 10 trials. Thus, at least for the 10”C adapting temperature, there is no indication of a cold conduction block. ‘Ilese findings are supported by physiological studies showing that skin must be cooled to below IO°C to produce a conduction block of myelinated fibers and that C fibers are not blocked until the skin temperature is lowered to 3°C (Franz and Iggo 1968; Kunesch et al. 1987). A differential location of receptors for the higher and lower temperature ranges could also explain the differences in discriminability. Several studies suggest that the receptors for noxious cold are located primarily along vein walls, whereas those for cool are located in cutaneous tissue (Hensel et al. 1951, 1974; Fruhstorfer and Lindblom 1983; Klement and Arndt 1992). If this hypothesis is correct, then cold-induced local changes in blood flow caused by vasodiIation or constriction (Folkow et al 1963) could lead to a less reliable stimulation of vessel wall receptors than of cutaneous receptors with sudden skin cooling. The slower response speeds (increased latencies), increased response variability,

41

and lack of relationship between AT and response latency we observed for lower adapting temperatures lend support to such an hypothesis. A final explanation for the observed differences in cooling discrimination is that the upper and lower cold ranges are processed differently in the central nervous system. As discussed in the Introduction and elaborated below, there is a growing body of evidence suggesting that skin cooling to temperatures above 15°C activates a different configuration of neuronal activity than does cooling to temperatures below

15”C. Several aspects of our findings, as well as other data, indicate a clear difference between the neural processing of the higher and lower cold temperature ranges. First, in our study subjects rated adapting temperatures of 22°C and 16°C similarly, and adapting temperatures of IO°C and 6°C similarly, but there was a large significant difference between the cold ratings for 16°C and 10”C. Second, temperature decreases from adapting temperatures of 22°C and 16°C produced almost exclusively a cooling sensation, with no feeling of tickling or prickling. In contrast, temperature decreases from adapting temperatures of 10”C and below evoked most prominently a sensation of non-painful tickle and sometimes that of painful prickle. The sensation of cooling was much less than for temperature decreases at higher adapting temperatures. This abrupt perceptual shift between 16° and 10”C corresponds with the point at which we observed a 10-fold increase in the subjects’ thermal discrimination thresholds. The change in perception between 16°C and IO°C corresponds well with two thermal channels observed in the dorsal horn of cats and monkeys (Kenshalo et al. 1982; Craig and Kniffki 1985; Craig and Bushnell 1994; Craig and Serrano 1994). One population of cold sensitive cells, observed primarily in lamina I, responds with increasing frequency to temperature decreases between 35°C and 15”C. At lower temperatures, this channel continues to be active, but does not show increasing responses to decreasing temperatures, i.e. the stimulus-response function reaches a plateau (Craig and Bushnell 1994). The observed activity of these specific cold-sensitive cells suggests that they would respond to small temperature decreases at 22°C and 16”C, but not at temperatures of IO°C and below. Thus, the activity of these cells could account for both the cold sensation observed for adapting temperatures down to O°C and the worsened cold discrimination at lower temperatures. A category of multireceptive nociceptive cells found in lamina I of the dorsal horn responds to noxious heat, noxious pinch and cold below about 27°C (heat-pinch-cold or HPC cells) (Craig and Bushnell 1994; Craig and Serrano 1994). Although these cells should be responsive throughout the range of stimuli presented in the current study, at temperatures below 15”C, the activity of HPC cells surpasses that of cold-specific cells (Craig and Bushnell 1994). Further, tonic HPC cell activity continues to increase with lower temperatures, with a phasic response to cooling steps. It is the activity of these modality non-specific cells that might produce the tickling or prickling

sensation observed in our study for temperature decreases at adapting temperatures of 10”C and below. Another category of dorsal-horn wide-dynamic-range cells, located principally in lamina V, has also been observed to respond to temperatures below 15”C,as well as to mechanical and noxious heat stimuli. Their activity could also contribute to the observed sensations (Kenshalo et al. 1982). Our psychophysical data also support the idea that distinct classes of cold sensitive primary afferents converge at the dorsal horn to produce a cold channel and a non-specific nociceptive channel that lead to distinct but sometimes concurrent sensations of cold and pain. In the primate, the majority of cold sensitive primary afferents that have been reported show the most static activity between 20°C and 30”C and produce a phasic discharge to rapid temperature shifts as low as 15°C (Dubner et al. 1975; Dykes 1975; Kenshalo and Duclaux 1977). Lamotte and Thalhammer (1982) have reported another category of high threshold cold sensitive primary afferents (HCRS), that respond to temperatures below 27”C, with increasing responsiveness to temperatures as low as 5°C (the lowest tested). The convergence of these separate categories of cold sensitive primary afferents onto dorsalhorn cold-specific cells could account for our psychophysical finding that 10° and 6° was perceived as colder than 16° or 22”C. The tickling and prickling sensations produced by cooling shifts below 10”C could result from another convergence. The activity of primate mechanothennal nociceptors (Georgopoulos 1976; Lamotte and Thalhammer 1982), which respond to skin cooling below 15°C, could lead to the cooling responses of dorsal-horn WDR cells, and the convergence of HCR and mechanothermal nociceptive activity could account for lamina I HPC responses. Ourobservation of betterdiscrimination for coolingshifts in the clearly innocuous cool range than in the range of noxious and near-noxious cold contrasts sharply with psychophysical data on heat discrimination. Both humans and monkeys discriminate smaller differences in the intensity of noxious heat than of innocuous warmth (Bushnell et al. 1983; Handwerker et al. 1982). A teleological explanation for this distinction might be that a quick withdrawal from a potentially tissue-damaging heat stimulus, such as fire, would be enhanced by a highly discriminative neural channel for coding noxious heat intensity. In contrast, the exposure of skin to temperatures below 15°C need only invoke a slower behavioral response of seeking shelter. Nevertheless, a highly discriminative cool sensing system in the range between skin temperature and environmental temperature would aid the organism in its somatosensory exploration, since differential conductibility of various substances (i.e. rocks, wood, dirt, etc.) evokes different sensations of cool within this range.

Acknowledgements We thank Drs. Gary Duncan and Bud Craig for their helpful comments on previous versions of this manuscript. We

42

are also grateful to Claude Gauthier for his assistance in graphic arts and to Francine B61angerfor her technical assistance throughout the project. This study was supported by the Medical Research Council of Canada. Dr. Chen was supported by the Qu6bec FCAR and Mr. Rainville by the Medical Research Council of Canada.

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