Perceptions of control over work: psychophysiological responses to self-paced and externally-paced...

Ž .International Journal of Psychophysiology 25 1997 211]220

Perceptions of control over work: psychophysiologicalresponses to self-paced and externally-paced tasks in an

adult population sample

Andrew SteptoeU , Olga Evans, George FieldmanDepartment of Psychology, St. George’s Hospital Medical School, Uni®ersity of London, London, UK

Received 30 June 1996; revised 31 October 1996; accepted 1 November 1996

Abstract

This experiment tested the hypothesis that perceptions of control over work pace would modulate cardiovascularŽ .reactions to mental stress tests. One hundred and thirty two adults aged 30]65 years 64 men and 68 women were

randomized to self-paced and externally-paced task conditions. The self-paced group carried out visual matrixproblem solving and mirror tracing tasks at their own pace. Work pace requirements were imposed on theexternally-paced group, with performance requirements being set to equate those in the self-paced condition. This

Ž .was done to equate work demands. Blood pressure assessed with the Finapres , heart rate, cardiac baroreceptorreflex sensitivity, skin conductance, respiration rate and tidal volume were monitored. Behavioural performance ofthe mirror tracing task was comparable in the two conditions, but the externally-paced group attempted moreproblems and made more errors on the visual matrix task. Systolic blood pressure reactions to mirror drawing were

Ž .greater in the external than self-paced conditions mean increases of 27.5"16.1 vs. 23.1"18.5 mmHg, P-0.025 ,Ž .and electrodermal responsivity was also heightened in the externally-paced group P-0.05 . No differences were

recorded in diastolic blood pressure, heart rate, baroreflex sensitivity or respiratory parameters. Results are discussedin relation to the literature on control and physiological stress responsivity, and their implications for understandingthe health consequences of lack of control at work are considered. Q 1997 Elsevier Science B.V.

Keywords: Work stress; Control; Blood pressure; Work pace

1. Introduction

There is growing concern about job stress inthe work force, reflected in the fact that theInternational Labour Office is devoting a subs-

U Corresponding author. fax: q44 181 7672741.

tantial section of its forthcoming Encyclopaedia ofOccupational Health and Safety to the health im-plications of psychosocial and organizational fac-

Ž .tors at work Stellman, in press . One of the mostinfluential developments over recent years hasbeen the demandrcontrol model of job strain

Ž .formulated by Karasek and Theorell 1990 . Theypostulate that job strain and subsequent health

0167-8760r97r$17.00 Q 1997 Elsevier Science B.V. All rights reserved.Ž .P I I S 0 1 6 7 - 8 7 6 0 9 6 0 0 7 4 7 - 7

( )A. Steptoe et al. r International Journal of Psychophysiology 25 1997 211]220212

risks are greatest among people who face highdemands at work coupled with low control overhow the work is carried out. One reason thismodel has proved attractive is that associationswith cardiovascular disease have been established

Žby several independent research groups Schnall. Žet al., 1994 . The data are not all consistent e.g.

.Hlatky et al., 1995 , and the model has also beencriticised for inadequate specification of the con-

Žcepts of demand and control e.g. Wall et al.,.1996 . Recent studies indicate that the control

dimension may be particularly important for car-diovascular risk. A 25-year follow-up of middle-aged men in the US showed that lack of controlwas associated with coronary heart disease afteradjusting for risk factors such as age, blood pres-sure, cholesterol, smoking, alcohol consumptionand family history, but that no links with job

Ž .demand were apparent Alterman et al., 1994 . Acase-control study of cardiac mortality in Swedenshowed independent effects for job control but

Ž .not demands Johnson et al., 1996 , while lack ofcontrol has also been associated with extended

Žabsence from work through sickness North et al.,.1996 .

An important issue that is not yet resolved isthe mechanism responsible for these effects. Ithas been argued that heightened psychophysio-logical stress responsivity is induced by high de-mandrlow control work, and may increase car-

Ž .diac risk Steptoe, 1993 . Some studies have shownthat ambulatory blood pressure at work is ele-vated among individuals in high strain jobsŽ . Ž .Schnall et al., 1994 , while Steptoe et al. 1995found that a combination of perceived job strainand heightened cardiovascular responses to stan-dard laboratory mental stress tests predicted ele-vated blood pressure at work in otherwise nor-motensive people.

A valuable way of investigating the mechanismslinking job characteristics and cardiovascular dis-ease risk is to carry out laboratory studies inwhich the salient elements can be modelled ex-

Ž .perimentally Steptoe, 1996 . The relationshipŽbetween job demands conceptualised in terms of

.task difficulty or work load , and physiologicalresponsivity has been examined in a number of

Žstudies e.g. Carroll et al., 1986; Mulder and

.Mulder, 1987 . Evidence related to control overwork is less extensive. The most direct tests ofwork controllability have compared psychophysio-

Ž .logical responses to self-paced controllable andŽ .externally-paced uncontrollable tasks performed

at the same rate or level of task demand. BohlinŽ .et al. 1986 compared self and externally-paced

mental arithmetic, and found that at comparablework levels, lower levels of stress and irritationwere reported in the self-paced than externally-paced conditions. There were also larger in-creases in diastolic blood pressure in the exter-nally-paced group, but no differences in systolicpressure, heart rate or urinary excretion of cate-cholamines.

We have previously reported two studies com-paring cardiovascular responses to self-paced and

Ž .externally-paced tasks Steptoe et al., 1993a,b .Two tasks were investigated, and perceived con-trol over pace was manipulated in both cases. Thevisual matrix task involved the solution of prob-lems presented on a computer. The self-pacedgroup carried out the task at their own preferredpace. Demands across experimental conditionswere equated by setting the maximum time ofexposure for each problem in the externally-pacedgroup to the average response time of the self-paced group; thus the difference between condi-tions lay in control over the rate of task presenta-tion, not in rate per se. The other task involvedmirror tracing, and the manipulation of perceivedcontrol was carried out at the instructional level.Again, self-paced subjects carried out the task attheir own rate. Externally-paced subjects wereasked to complete the amount of mirror tracingthat matched the mean achieved by the self-pacedgroup, and were given time markers to allowthem to judge whether their work rates wereappropriate. It was predicted that both these ma-nipulations would induce a relative lack of controlover work pace in the externally-paced condition,and that cardiovascular responses would be corre-spondingly enhanced. In the first study, we foundthat men aged 55]65 years showed larger in-creases in systolic and diastolic blood pressureand heart rate during tasks in the externally-paced

Ž .than self-paced conditions Steptoe et al., 1993a .The second study involved female students aged

( )A. Steptoe et al. r International Journal of Psychophysiology 25 1997 211]220 213

19]27 years, and replicated these differences inblood pressure and heart rate response, whilealso showing enhanced suppression of cardiacbaroreceptor reflex sensitivity in the externally-

Ž .paced group Steptoe et al., 1993b .The present study was designed to replicate

and extend the results of these investigations intwo ways. Firstly, we wished to assess the differ-ences in physiological response to controllableand uncontrollable tasks in a broad populationsample of adult men and women. If the pheno-menon of heightened cardiovascular responsivityto uncontrollable work is of general relevance tothe impact of job strain, then it is necessary toshow effects in the adult working population morewidely. Secondly, we altered the presentation ofthe visual matrix task so that the punishing ef-fects of failure were less salient. Previously, everytime an error was made in either group, an aver-sive tone sequence was sounded. The externally-paced subjects produced more errors than did theself-paced individuals, since they evidently feltpressured by the time constraints even thoughtime available for task solution was adequate.The externally-paced group therefore experi-enced these aversive tones more frequently thanthe self-paced group. It can be argued that theauditory signal may have acted as a punishingstimulus that might itself have elicited increasesin cardiovascular activity. In the present study,the visual matrix task was presented in a silentmode so as to assess whether differences in car-diovascular response during self and externally-paced performance were maintained. The manip-ulation of perceived pace control for the mirrortracing task was the same as that used in previousstudies, so as to ensure comparability.

These issues were investigated in the context ofa study concerning the influence of age and gen-

Žder on cardiovascular stress responsivity Steptoe.et al., 1996 . We hypothesised that effects related

to control would emerge in blood pressure andheart rate, and might be sustained by differencesin baroreceptor reflex regulation of cardiovascu-lar function. Respiration patterns and electroder-mal activity were also assessed so as to determinewhether control had an impact solely on cardio-vascular parameters, or else generalised to other

physiological measures. Physical fitness was as-sessed to ensure comparability in the self-pacedand externally-paced conditions, since differencesin cardiovascular stress responses related to fit-

Žness have been described in some studies Crews.and Landers, 1987 .

2. Methods

2.1. Subjects

Participants aged 30]65 years were recruited inbatches from patient lists held in three localHealth Centres. After exclusion of people knownto be suffering from major systemic disease ordisability, names were selected at random, andthe first 72 men and 72 women who agreed to

Ž .participate were included. Fifty-four 27.5% re-fused to participate, but the response rate did notdiffer across age or gender groups. The first 35subjects were allocated to the self-paced condi-tion in order to generate targets for the exter-nally-paced group. Subsequently, subjects wereallocated randomly. Twelve of the 144 subjectswere later omitted from analysis after it was re-vealed that they were taking medication, or be-cause they had been told in the past they hadhigh blood pressure. The final sample consisted of64 men and 68 women, with 32 men and 34women in each experimental condition.

2.2. Apparatus

During the psychophysiological stress session,beat-to-beat systolic and diastolic blood pressureand heart rate were transduced continuously with

Ža Finapres model V TNO, Biomedical Instru-.mentation, Amsterdam, NL . The Finapres finger

cuff was positioned around the middle phalanx ofthe middle finger of the non-dominant hand, andan arm rest was adjusted in height so that the cuffrested 10 cm below the base of the sternum.Signals were digitized using a 1401 Laboratory

Ž .Interface Cambridge Electronic Design Limitedand subsequently processed by computer. Respi-ration was monitored non-invasively from thoracicand abdominal excursions using the Respitrace

Ž .system Respitrace Limited calibrated against a


Ž .wedge bellows spirometer Vitalograph Limitedand digitized using the 1401 Interface. Elec-trodermal activity was measured using AgrAgClelectrodes attached to the thenar and hypothenareminences of the non-dominant hand, andrecorded using a Grass 7D Polygraph. Skin resis-tance responses exceeding 2 kV were counted foreach minute from the polygraph record to indexelectrodermal responses.

During exercise testing, a Monark 864 cycleŽ .ergometer Monark Crescent AB was used, and

heart rate was monitored by chest electrodes in aŽV5 configuration using a cardiorater CR7 Cardiac

.Recorders Limited . The mirror tracing task wascarried out using a Lafayette mirror drawing ap-

Ž .paratus Lafayette Instruments Corp .

2.3. Procedure

Participants attended the laboratory on threeoccasions. The first session involved familiariza-tion with the apparatus, collection of demo-graphic information and aerobic fitness testing.Participants exercised on the bicycle ergometer atworkloads increasing by 25 W every 3 min untilheart rates attained 67]75% of estimated maxi-mum, or until exhaustion. Heart rates at the end

Ž .of 3 min work at 50 W ns130 and 75 WŽ .ns92 were analyzed.

The mental stress testing session was carriedout several days later at the same time in theafternoon. The participant was seated in an arm-chair facing a computer monitor. Following in-strumentation and calibration, the subject sat qui-etly for 15 min. A 5-min baseline trial of bloodpressure, heart rate, respiration and electroder-mal monitoring was then recorded. Subjects ratedhow tense they felt on a numerical scale ranging

Ž . Ž .from 1 low to 10 high . The visual matrix prob-lem solving task was then administered. This has

Žbeen described in detail elsewhere Steptoe et al.,.1993b . A series of visual matrices of varying

difficulty based on Raven’s Progressive MatricesŽ .Raven, 1938 were presented in random order ona computer monitor for a 5-min period. The sub-ject was required to solve each item by complet-ing the matrix with one of the six or eight choicesprovided, pressing the appropriate key on a

numeric keypad. A vertical time gauge was dis-played on the right of the screen to indicate thetime remaining for solution of the problem. Atintervals, messages appeared at the bottom of thescreen to encourage participants to respond, andevery time a response was made, a new problemwas presented. The time taken by participants inthe self-paced condition to respond to each ma-trix was recorded and averaged. Subjects in theexternally-paced condition were exposed to eachmatrix for a period 1 s longer than the medianresponse time for the self-paced condition whenthat median was -10 s, and 2 s longer when themedian was )10 s. The time gauge was set atthis interval in the externally-paced condition,

Ž .and at a very long interval )15 min in theself-paced condition. Both groups were told to tryto solve each problem as quickly as they could butwith as few errors as possible, and that accuracyshould take priority over speed. Monitoring ofphysiological activity continued throughout the5-min trial, and was followed by a 7-min recoveryperiod. Subjective ratings of tension and stressŽwhere 1 s not at all stressed and 5 s highly

.stressed were obtained immediately after the task,with a further tension rating at the end of therecovery period. Participants also rated the de-gree of control they felt over the pace of the taskŽfrom 1space completely under control to 9space

.completely out of control , and the extent to whichthey felt that their performance was affected by

Žtime pressure from 1sperformance ®ery muchimpro®ed to 9sperformance made ®ery much

.worse .There followed a 5-min break from physiologi-

cal recording before the mirror tracing task wasdescribed. In this task, the subject was required touse a metal stylus to trace the outline of a starthat could only be seen in mirror image. Eachtime the stylus came off the star, a mistake wasregistered and a loud click was emitted by theapparatus. The number of complete circuits ofthe star during the 5-min task period wasrecorded, together with the number of errors.Subjects in the self-paced condition worked attheir own rate, and were told that accuracy shouldtake priority over speed. The number of circuitscompleted in the self-paced condition was aver-


aged. Participants in the externally-paced condi-tion were given the number of circuits completedby the self-paced group as a target, and were alsotold that accuracy should take priority over speed.To allow participants to gauge their performance,a tone sounded at time intervals corresponding totarget circuit completion times. As with the visualmatrix task, the performance phase was followedby a 7-min recovery period, and ratings wereobtained of tension, stressfulness, and percep-tions of control.

The third assessment session was included sothat a fasting blood sample could be drawn forlipid analysis. These data are not relevant to this

Žpaper, and are described elsewhere Steptoe et.al., 1996 .

2.4. Data reduction and analysis

Blood pressure, heart rate, respiration rate andtidal volume were averaged over the baseline, andthe performance and recovery phases of the vi-sual matrix and mirror tracing tasks, resulting infive trials for analysis. Electrodermal responseswere counted over the same periods. Equipmentproblems led to a loss of blood pressure datafrom parts of the study from a number of partici-pants, so systolic blood pressure analyses arebased on 123 and diastolic blood pressure on 122individuals. Data were complete for heart rateand respiratory variables. However, electrodermalresponses were disrupted during mirror tracing in38 individuals, so these analyses are based on 94subjects.

Baroreceptor reflex sensitivity was estimatedusing the spontaneous sequence technique as de-

Ž .scribed by Steptoe and Vogele 1990 . Scanning¨of systolic blood pressure and pulse intervalrecords was carried out by computer, and se-quences were detected in which systolic pressureincreased or decreased by at least 1 mmHg forthree or more consecutive cycles. Each value waspaired with the pulse interval of the subsequentcycle. If the systolic blood pressure sequence wasaccompanied by an appropriate pulse interval

Žchange progressive lengthening of pulse intervalof at least 4 ms coupled with blood pressureincreases, and shortening of pulse interval with

.blood pressure decreases , a baroreflex sequencewas identified. Sequences were included only ifthe correlation coefficient was )0.80. The corre-lation coefficients and intercepts were calculated,and the slope or sensitivity of the reflex was

Ž .expressed as change in pulse interval in ms permmHg change in systolic blood pressure.

Group differences in background characteris-tics, task performance and subjective ratings of

Ž .control were analysed using t-tests two-tailed .Task appraisal ratings were analysed with groupŽ .self-paced, externally-paced as the between-sub-

Ž .ject factor, and task visual matrix, mirror tracingas the within-subject factor. Subjective and physi-ological stress responses were analyzed usinganalysis of variance and covariance, with groupŽ .self-paced, externally-paced as the between-sub-

Ž .ject factor, task visual matrix, mirror tracing andŽ .phase of task performance, recovery as within-

subject factors, and baseline as the covariate. TheGreenhouse-Geisser correction was applied whereappropriate, and probabilities have been adjustedto reflect this.

Additional analyses were carried out using ageand sex as between-subject factors. However, nei-ther of these factors interacted with the self-pacedand externally-paced grouping, and the presentresults concentrate on the manipulation of taskcontrol.

3. Results

The background characteristics of participantsin this study are summarised in Table 1. Therewere no significant differences between self-pacedand externally-paced groups on any demographicvariables, nor in terms of health behaviours suchas smoking and alcohol consumption, family his-tory of cardiovascular disease, or physical fitness.In all these respects, the two groups were wellmatched.

3.1. Task performance and appraisal

The measures of task performance and subjec-tive ratings of control are shown in Table 2. Theexternally-paced group attempted significantlymore problems in the visual matrix task than the


Table 1Ž . Ž .Background characteristics of participants. Means standard deviations and percentages 95% C.I.

Self-paced Externally-paced

Menrwomen 32r34 32r34Ž . Ž .Age 48.2 13.2 49.0 13.2Ž . Ž .Body mass index 24.5 4.6 24.6 4.3

Ž .Social class %Ž . Ž .IrII 56.5 41.1, 71.1 59.6 44.3, 73.6Ž . Ž .III]V 43.5 28.9, 58.9 40.4 26.4, 55.7

Ž .Marital status %Ž . Ž .Marriedrcohabiting 59.1 46.3, 71.1 68.8 55.9, 79.8Ž . Ž .Singlerdivorcedrwidowed 40.9 28.9, 53.7 31.2 20.2, 44.1

Ž .Work status %Ž . Ž .Employed 81.8 70.4, 90.2 76.2 63.8, 86.0Ž . Ž .Retired 7.6 2.5, 16.8 11.1 4.6, 21.6Ž . Ž .Unemployedrnot employed 10.6 4.9, 20.6 12.7 11.3, 32.2

Ž . Ž . Ž .Current smokers % 33.3 22.2, 46.0 20.3 11.3, 32.2Ž .Alcohol consumption %

Ž . Ž .Regular drinkers 30.3 19.6, 42.9 25.0 15.0, 37.4Ž .Parental history of heart disease %

Ž . Ž .Fathers 45.9 33.1, 59.1 43.1 29.3, 57.8Ž . Ž .Mothers 30.0 18.8, 43.2 37.3 24.1, 51.9

Physical fitnessŽ .Heart rate bpm at

Ž . Ž . Ž .50 W ns130 106.7 14.3 109.7 16.7Ž . Ž . Ž .75 W ns92 116.6 16.8 119.0 18.7

Ž .self-paced group, t 129 s7.42, P-0.001, butŽ .their performance was also less accurate, t 129

s2.16, P-0.05. There was no significant differ-ence in the performance of the mirror tracing

task, either in terms of circuits completed orerror rate.

There were significant differences between thegroups in ratings of perceptions that the pace of

Table 2Ž .Task performance and appraisal. Means standard deviations


Matrix taskŽ . Ž .Problems attempted 13.7 4.9 19.5 3.8

Ž . Ž . Ž .Correct solutions % 74.1 17.6 67.4 17.5Ratings:

Ž . Ž .Pace under control 4.72 2.6 5.36 2.2Ž . Ž .Performance affected by time pressure 5.83 1.7 6.45 1.8Ž . Ž .Task stress 3.97 1.8 4.53 2.2

Mirror drawing taskŽ . Ž .Circuits completed 3.85 4.0 3.97 2.6Ž . Ž .Number of errors 60.9 61.8 64.7 54.7

Ratings:Ž . Ž .Pace under control 4.72 3.1 5.44 2.8Ž . Ž .Performance affected by time pressure 5.58 1.5 6.12 1.5Ž . Ž .Task stress 5.74 2.2 5.70 2.3


tasks was under personal control, and whetherperformance was affected by time pressure,Ž .F 1,129 s3.93 and 7.21 respectively, P-0.05.

The self-paced group rated the pace of tasks asmore under their control, and stated they wereless impaired by time pressure than the exter-nally-paced group. Neither effect interacted withtask type, indicating that these were general per-ceptions that did not differ across the two tasks.Stress ratings on the other hand showed a main

Ž .effect of task, F 1,129 s67.6, P-0.0001, sinceparticipants rated mirror tracing as more stressfulthan the visual matrix task. The two groups didnot differ in this respect.

In sum, these data indicate that in the visualmatrix task, the externally-paced group traded offaccuracy for speed in comparison with the self-paced subjects. Performance of the mirror tracingtask was well matched. The manipulation of taskcontrol was corroborated by subjective ratings ofcontrol and impairment of performance by timepressure.

3.2. Subjecti®e responses to tasks

The mean tension ratings of the self-paced andexternally-paced groups did not differ at baselineŽ .means 2.30"1.4 and 2.44"1.6 respectively .Analysis of ratings during tasks, using baseline asthe covariate, uncovered significant effects of task

Ž .and phase, F 1,130 s61.8 and 336.9 respectively,P-0.0001, together with group by task and task

Ž .by phase interactions, F 1,130 s4.31 and 68.4respectively, P-0.05. These data are plotted inFig. 1. It is evident that covariance-adjusted ten-sion ratings were higher following performancethan recovery phases of tasks, and that tensionwas also greater overall for the mirror tracing

Fig. 1. Mean covariance-adjusted ratings on the subjectivetension scale in the two experimental groups. MAT: visualmatrix task performance trial; MATr: visual matrix task recov-ery trial; MD: mirror tracing task performance trial; MDr:mirror tracing task recovery trial. Error bars represent stan-dard errors of the mean.

than visual matrix tasks. Separate analysis of rat-ings from the performance phases identified a

Ž .significant group by task interaction, F 1,130 s5.38, P-0.05. As can be seen from Fig. 1, ten-sion ratings were marginally greater in the exter-nal than self-paced groups following the visualmatrix task, while the situation was reversed fol-lowing mirror tracing. The groups did not differin tension ratings during the recovery phases oftasks.

3.3. Physiological responses

The average baseline levels of the physiologicalvariables in this study are shown in Table 3.There were no significant differences betweengroups on any variables except for respirationrate, where higher levels were recorded in the

Ž .externally-paced than self-paced groups, F 1,130s6.95, P-0.05.

The analysis of systolic blood pressure respon-

Table 3Ž .Physiological activity at baseline. Means standard deviations


Ž . Ž . Ž .Systolic blood pressure mmHg 132.3 21.3 128.2 19.6Ž . Ž . Ž .Diastolic blood pressure mmHg 72.1 13.4 70.6 12.6

Ž . Ž . Ž .Heart rate bpm 69.6 10.7 69.3 10.3Ž . Ž . Ž .Electrodermal responses nrmin 1.41 1.7 1.91 2.1

Ž . Ž . Ž .Respiration rate cpm 15.6 2.9 14.3 3.1Ž . Ž . Ž .Tidal volume ml 356.1 101.2 358.1 109.8

Ž . Ž . Ž .Baroreceptor reflex sensitivity msrmmHg 10.7 5.0 11.2 5.3


ses to tasks showed main effects of task andŽ .phase, F 1,121 s36.9 and 185.5 respectively,

P-0.0001, together with a significant group byŽ .task interaction, F 1,121 s4.52, P-0.05. These

data are summarised in Fig. 2, where it can beseen that blood pressure was greater during themirror tracing than visual matrix tasks. The groupby task interaction was the result of greater sys-tolic pressure responses in the externally-pacedgroup to the mirror tracing but not to the visualmatrix task. The covariance-adjusted increasesfrom baseline to the performance phase of themirror tracing task averaged 27.5"16.1 mmHg inthe externally-paced and 23.1"18.5 mmHg in the

Ž .self-paced conditions P-0.025 . There were nosignificant differences between groups in theanalyses of either diastolic blood pressure or heartrate. In both cases there were significant effectsfor tasks and phase, since diastolic pressure andheart rate increased to a greater extent in themirror tracing than visual matrix task, but thegroups did not differ in these responses.

In the case of electrodermal responses, themain effect for group approached acceptable lev-

Ž .els of significance, F 1,92 s3.32, Ps0.072, asŽ .did the group by phase interaction, F 1,92 s3.21,

Ps0.078. These data are shown in Fig. 2, whereit is apparent that differences between groupswere more prominent in the performance thanrecovery phases of tasks, with more electrodermalresponses in the external than self-paced condi-tions. This interpretation was supported by sepa-rate analyses of the two task phases, that showedsignificant main effects for group in the perfor-

Ž .mance phase, F 1,92 s4.69, P-0.05, but not inthe recovery phases of tasks.

There were no differences between experimen-tal groups in the respiratory responses to tasks.Both respiration rate and tidal volume showedsignificant effects of task and phase, together with

Ž .task by phase interactions, F 1,130 s16.7 to313.8, P - 0.001. Respiration rate and tidalvolume increased during the performance phasesof tasks, reverting towards baseline during recov-ery. However, in neither case did the pattern ofresponse vary across groups.

The baroreflex sensitivity results are shown inFig. 3. These data were analyzed using analysis ofvariance with five trials including the baseline. A

ŽFig. 2. Mean covariance-adjusted systolic blood pressure up-.per panel in mmHg and electrodermal response ratermin

Ž .lower panel in the two experimental groups. MAT: visualmatrix task performance trial; MATr: visual matrix task recov-ery trial; MD: mirror tracing task performance trial; MDr:mirror tracing task recovery trial. Error bars represent stan-dard errors of the mean.

main effect for trial emerged in this analysis,Ž .F 4,448 s28.7, P-0.0001. As can be seen in

Fig. 3, the baroreceptor reflex was suppressedduring the performance phase of both the visualmatrix and the mirror tracing task, returning to-wards baselines during recovery phases. The aver-age decrease in baroreflex sensitivity was 2.03msrmmHg during the visual matrix task, and 2.23msrmmHg during mirror tracing. These effectsdid not interact with experimental group.

4. Discussion

Efforts were made in a study to recruit fromthe general population instead of relying on avolunteer sample of convenience. Participantswere chosen at random from general practicelists, and the refusal rate was relatively low. Dif-ferences in cardiovascular activity and other vari-


Fig. 3. Mean cardiac baroreceptor reflex sensitivity inmsrmmHg in the two experimental groups. BL: baseline trial;MAT: visual matrix task performance trial; MATr: visualmatrix task recovery trial; MD: mirror tracing task perfor-mance trial; MDr: mirror tracing task recovery trial. Errorbars represent standard errors of the mean.

ables were observed across age and gender groups,but since these did not interact with the controlmanipulation, they have been described in a com-

Ž .panion paper Steptoe et al., 1996 .The experimental manipulation was intended

to mimic conditions of controllable and uncon-trollable work pace. We used a between-subjectdesign, and attempted to match the absolute paceof work demands in the self and externally-pacedconditions. As in our previous studies, the re-sponse to these demands in the externally-pacedcondition was to attempt more problems and make

Ž .more errors Steptoe et al., 1993a . Bohlin et al.Ž .1986 also showed that at a comparable workpace, arithmetic task performance was poorer un-der external pacing. Subjective appraisal of tasksindicated greater awareness of time pressure inthe externally than self-paced conditions. For bothtasks, the self-paced group considered their paceof work to be under personal control to a greaterextent, and their performance to be less affectedby time pressure than did the externally-pacedgroup. However, the two task conditions were

Ž .rated as equally stressful Table 2 .We predicted that externally-paced task admin-

istration would elicit greater blood pressure andheart rate responses than self-paced tasks. Thishypothesis was only partly supported. There wereno differences in cardiovascular response to thevisual matrix task. But during the mirror tracingtask, systolic blood pressure responses were

heightened in the externally-paced condition, anddifferences in electrodermal responsivity were also

Ž .recorded Fig. 2 . Unlike our previous findingsŽ .Steptoe et al., 1993b , we did not observe anydifference in baroreflex response in the two con-ditions. The cardiac baroreceptor reflex is one ofthe short-term regulatory mechanisms through

Žwhich blood pressure stability is maintained Eck-.berg and Sleight, 1992 . Suppression of the sensi-

tivity of the cardiac baroreflex is characteristic ofconditions of behavioural activation, and wasobserved during tasks in this study. However, thelack of difference between experimental groupssuggests that the greater systolic blood pressureresponse to mirror tracing in the externally-pacedcondition was not mediated by central inhibitionof baroreflex regulation.

The finding concerning differences in systolicblood pressure and electrodermal responses tothe mirror tracing task is consistent with previousresults. The difference is striking in that controlover task pace was not set through direct timeconstraints, but by an instructional manipulation.Informing participants about the level of perfor-mance they might achieve was evidently sufficientto induce perceptions of time pressure and corre-sponding increases in physiological activation.This phenomenon is important in that it mightthrow light on the role of perceived rather thanactual lack of control over work.

The failure of the manipulation of pace controlof the visual matrix task was unexpected. Be-havioural performance was impaired in the exter-nally-paced group, and they rated the task as lessunder their control than did the self-paced group.However, changes in the presentation of the taskfrom previous studies may have led to a diminu-tion in differences between the two experimental

Ž .conditions. Steptoe et al. 1993a found that dif-ferences in ratings of the perceived controllabilityof work pace averaged more than one standarddeviation on the 9-point rating scale. In the pre-sent study, the difference was less than half of

Ž .one standard deviation Table 2 . The reducedsalience of the pace control manipulation mayhave eliminated differences in cardiovascular re-sponsivity. It should also be noted that unlike themirror tracing task, the visual matrix task did not


involve an explicit comparison of performancewith other individuals. It may be that a greaterdegree of social evaluation was induced by mirrortracing, and that this stimulated larger differencesin physiological response.

The modelling of job control or decision lati-tude in the laboratory is an important step to-wards understanding the processes through whichjob stress may increase cardiovascular disease risk.Other paradigms have been used, such as themanipulation of the degree of choice over what

Ž .tasks are performed Hutt and Weidner, 1993 .Manipulation of control over work pace may beparticularly relevant to repetitive jobs that arecontrolled by automated systems or by other peo-ple’s choices. If heightened physiological reactiv-ity is a mechanism through which job strain in-fluences disease risk, further psychophysiologicalinvestigations of the problem should be encour-aged.

Acknowledgements

This study was supported by the British HeartFoundation. We are grateful to Drs John Cum-berbatch, Philip Whitehead, Patrick Bower andtheir colleagues for their assistance in recruitingparticipants.

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