heart rate variability in a binary choice reaction task: an evaluation of some scoring methods

Actu Psychologica 36 (1972) 239-251; 0 North-Holland Publishing Company Not to be reproduced in any form without written permission from the publisher

HEART RATE VARIABILITY IN A BINARY CHOICE REACTION

TASK: AN EVALUATION OF SOME SCORING METHODS1

G. MULDER and W. R. E. H. MULDER-HAJONIDES VAN DER MEULEN

Department of Experimental Psychology, University of Groningen, Groningen, The Netherlands

ABSTRACT

Several scoring methods of heart rate variability have been investigated in a binary choice task. It was concluded that in this type of task the number of waves in the cardiotachogram and the sum of absolute differences between successive R-R intervals divided by the number of waves are the best indicators of the task levels (20, 30,4O, 50 and 60 binary choices per min). A detailed analysis of these scoring methods in different types of information processing tasks is recommended.

1. INTRODUCTION

In the last few years much psychophysiological research has been directed towards the relationship between cognitive activity and physiological responses. Among the physiological responses the heart rate variability (HV), also called sinus arrhythmia, has been proposed (KALSBEEK and ETTEMA, 1963, 1964, 1965). HV is the variability in time between successive R-tops of the cardiogram. Kalsbeek and Ettema observed that during a mental task this variability diminished. In other words: the heart rate (HR) pattern, or cardiotachogram, became more regular. There were no important changes in the level of HR. Kalsbeek and Ettema used a handscoring method in assessing HV. Loos (1968) found also a diminished HV during a mental task. He used the variance of the R-R-intervals as indicator of HV. Kalsbeek and Ettema expressed HV in percentages of rest value; Loos used a quotient: variance of R-R-intervals in rest divided by variance of R-R-intervals during the task. BLITZ et al. (1970) found that HR and HV both differentiated between levels of mental load, but HR appeared to be a better indicator. These authors used the S/N index, proposed by the Institute of Medical Physics, T.N.O. (Utrecht), S being the sum of positive differences between successive R-R intervals and N being the number of fluctuations

1 This experiment was done at the Laboratory of Ergonomic Psychology (T.N.O.), Amsterdam.

239

240 G. MULDER AND W. R. E. H. MULDER-HAJONIDES VAN DER MEULEN

above a certain threshold in the cardiotachogram. These investigators did not express the HV in percentages of rest value.

In all these investigations a paced choice reaction task was used. However, the HV was measured in different ways. Comparison of the results of these respective investigators is not possible because of differences in scoring methods.

The aim of this investigation is to evaluate some scoring methods of HV in a paced binary choice task. The load in this task was increased by increasing the number of binary choices per minute. Also the S-R compatibility was varied. For one group of subjects the task was compatible and for the other one incompatible. It is assumed that in the incompatible case the load on the information processing mechanisms is higher. The number of choices, however, was the same for both groups.

2. METHOD

2.1. Subjects

Twenty subjects, both men and women, at the University of Amster- dam were used as Ss. They were paid.

2.2. Task conditions and training

S was seated at a table in the same room where E monitored stimulus presentation and recording equipment. Auditory stimuli were presented over earphones. In the incompatible case (10 Ss) S had to respond to low (250 Hz) and high (2000 Hz) tones which were presented binaurally. Responses were made by pressing a left foot pedal when a low tone occurred and by pressing a right foot pedal when a high tone occurred.

In the compatible case (10 Ss) S had to respond to a tone (2000 HZ) in the left ear by pressing the left foot pedal and to a tone (2000 Hz) in the right ear by pressing a right foot pedal. In both conditions the two signals were presented in random order each with a probability of 0.50. There were five task conditions, consisting of 20, 30, 40, 50 and 60 signals per minute respectively. The chosen task conditions were within the limits of capacity of Ss. The time between the successive signals was equally spaced. The presentation time, however, was a function of the number of signals per time unit: The higher this number, the shorter the presentation time (the presentation time is approximately (60 sec)/x, where x is the number of signals per minute. The Ahrend-Van Gogh Binary Choice Generator (Ahrend-Van Gogh N.V., Medical Instru- ments, Amsterdam) was used). The task duration was 4 min. Each

HEART RATE VARIABILITY 241

task was preceded by a rest period of 4 min. There were two experi-

mental days. On each day the five task conditions were presented in

random order. A period of two days separated training and experiment.

The training took place on 5 successive days during sessions of one hour.

A standardized training scheme was used (BLITZ et al., 1970).

2.3. Equipment

The ECG was registrated from chest electrodes and fed into a car-

diotachometer.2 A pulse, coincident with the R-wave was derived from the

cardiotachometer and recorded on magnetic tape (tape recorders Sony

TC 350; 9.5 cm/set; wow and flutter are according to specifications

< 0.25 %). The cardiotachogram was monitored during the whole

experiment. The cardiotachogram was also recorded during training

days to accustom the subject to the registration.

2.4. Data analysis

The time between successive pulses (= R-R intervals) was measured

in msec and punched per subject, per condition and per day. The data

were processed further on the X 8 of the Mathematical Centre of the

University of Amsterdam.

The following measures were computed per 4 min.

(1) HR = number of R-R intervals + 1.

(2) HV.

(a) Variance of R-R intervals = c2 = Zyy(Xa -X)2

HR-1

(b) Sum of absolute differences between

successive R-R intervals = SABS = z:“pilIXi -Xi-r\ .

(c) Number of waves in the cardiotachogram = N = 2 Z,

whereZ=~lifY<o OifY>O’

= = (Xi - ~-l)(Xa.l - x4, (i = 3,4,. . .,HR- 1).

(d) SABS/N.

2 The cardiotachometer developed by the Institute of Medical Physics T.N.O. (Utrecht) and manifactured by Roodt (Rijswijk) was used.


(e) Mean square successive differences

Xi = R-R interval i in msec.

(ad b) This parameter - but corrected for linear trend - was used by Loos (1968)

(ad d) SABS/N is rather analogous to the formulae used by the Institute of Medical Physics T.N.O. (Utrecht). However, instead of the sum of positive differences between successive R-R intervals we use the sum of absolute differences. In the definition of N we do not use a threshold.

(ad e) The mean square successive differences is recommended by VON NEUMANN (1941) for time series with a possible trend (BURDICK and SCARBROUGH, 1968).

The obtained parameters HR (1) en HV (2) were analyzed by means of a three-way analysis of variance (WINER, 1962, pp. 319-337). Ab- solute scores were used. Also the intercorrelations between the scoring methods, and the coefficients of stability (day 1 correlated with day 2) were computed. We considered the most sensitive scoring method to be the one which would

(1) discriminate between the compatible and the incompatible task, (2) discriminate between the task frequencies (the discrimination

power is evaluated in terms of the proportion of explained variance (PV) (VERBERK, 1970),

(3) be stable over days and rest periods, (4) show the least inter-subject variance.

3. RESULTS

A general survey of the results of the anova’s is given in table 1 for task conditions and in table 2 for rest periods. Sample intercorrelations matrices for scoring methods are given in table 3. The coefficients of stability are given in table 4.

3.1. Heart rate (HR)

The difference between 20 signals per min and 60 signals per min is 14 beats in 4 min for day 1 en 15 beats for day 2 (the standard deviation is about 20% of the mean value). It appears that only in the case of 60


TABLE 1

Relevant outcomes of the anova’s for task conditions.

HR

Source df F PV

C 1 < 1 0.00 D 1 3.58 0.01 F 4 8.89 0.00 s 18 3509.19 0.88 CxD 1 < 1 0.00 CxF 4 < 1 0.00 DxF 4 8.16 0.01

P F

u= PV

n.s. 1.47 0.01 < 0.10 3.45 0.01 < 0.01 3.12 0.02 < 0.01 12.16 0.62

ns. 1 0.00 n.s. 1 0.00

< 0.01 1.57 0.00

SABS

P F pv P

n.s. < 1 0.00 n.s. < 0.10 < 1 0.00 n.s. < 0.05 4.10 0.00 < 0.05 < 0.01 81.80 0.70 < 0.01

n.s. < 1 0.00 n.s. n.s. < 1 0.00 n.s. ns. < 1 0.00 n.s.

N SABSIN 02

Source df F PV p F pv P F pv P

C 1 < 1 0.00 n.s. < 1 0.00 n.s. 1.25 0.00 n.s. D 1 4.00 0.01 < 0.10 11.52 0.01 < 0.01 < 1 0.00 n.s. F 4 9.73 0.13 < 0.01 16.02 0.04 < 0.01 1.92 0.01 n.s. s 18 34.24 0.61 < 0.01 15.98 0.73 < 0.01 13.15 0.42 < 0.01 Cx D 1 < 1 0.00 n.s. 1.41 0.00 n.s. < 1 0.00 n.s. Cx F 4 < 1 0.00 n.s. 1.06 0.00 n.s. < 1 0.00 n.s. DX F 4 < 1 0.00 n.s. < 1 0.00 n.s. < 1 0.00 n.s.

C = compatibility, D = day, F = task frequency, S = subjects.

signals per min the task value is higher than the rest value. This difference is small however. The results of the anova indicate that the slight rise in HR over the task conditions is significant, but PV is negligible. There is a difference between the rest periods, but again PV is negligible. The difference between the successive days is also small (p < 0.10; PV is about 1%). The score does not discriminate between compatibility levels.

3.2. Variance of R-R intervals (~9)

The difference between 20 signals per min and 60 signals per min is 598 mseca in 4 min for day 1 and 153 msecs for day 2 (the standard deviation is about 80% of the mean value). The task values are lower than the rest values. The standard deviations of these values are, however, very high. The results of the anova indicate that the variance is significantly influenced by the task frequencies. The PV is 2 % however. There


TABLE 2

Relevant outcomes of the anova’s for rest conditions.

HR 0s SABS

Source df F PV p F PV p F PV p

C 1 < 1 0.00 n.s. 1.13 0.01 n.s. < 1 0.00 n.s. D 1 3.18 0.01 < 0.10 1.66 0.00 n.s. 2.65 0.01 n.s. R 4 4.44 0.00 < 0.05 1.47 0.01 n.s. 3.18 0.00 < 0.01 S 18 419.32 0.70 < 0.01 1.69 0.11 < 0.10 53.84 0.70 < 0.01 CXD 1 < 1 0.00 n.s. 2.21 0.01 n.s. 1.02 0.00 n.s. CxR 4 8.21 0.01 < 0.01 < 1 0.00 n.s. 4.32 0.01 < 0.01 DxR 4 < 1 0.00 ns. 1.03 0.00 ns. < 1 0.00 n.s.

N SABSIN 02

Source df F PV p F PV p F pv P

C 1 < 1 0.00 n.s. < 1 0.00 n.s. < 1 0.00 n.s. D 1 < 1 0.00 n.s. 2.66 0.01 n.s. 10.48 0.02 < 0.01 R 4 1.55 0.00 n.s. 5.74 0.01 < 0.01 19.48 0.00 < 0.01 s 18 21.31 0.73 < 0.01 49.23 0.74 < 0.01 17.35 0.71 < 0.01 CxD 1 < 1 0.00 n.s. 1.74 0.00 n.s. 8.32 0.01 < 0.01 CxR 4 1.12 0.00 n.s. 2.20 0.00 <O.lO 20.96 0.00 < 0.01 DX R 4 < 1 0.00 ns. < 1 0.00 n.s. < 1 0.00 n.s.

C = compatibility, D = day, R = rest period, S = subjects.

I I I I 01 I I ,,I R 20 I? 30 R 40 R 50 R 60

Fig. 1. Mean HR scores in 4 min, separately for rest (R) and Solid line day 1; dotted line day 2.

task conditions.

is no significant difference between the rest values. The difference between the two successive days is small (p < 0.10; PV = 1%). The score does not discriminate between compatibility levels.


1600-

1700-

1600 -

1500- !

uoo-

1300- i

1200”

t , R 20 R 30 R 40 R 50 R 60

Fig. 2. Mean scores of the variance of R-R intervals in 4 min, separately for rest (R) and task conditions. Solid line day 1; dotted line day 2.

3.3. Sum of absolute d@erences (SABS)

The difference between 20 signals per min and 60 signals per min is 945 msec in four minutes for day 1 and 315 msec for day 2 (the standard deviation is about 35 % of the mean value). In most cases the task values are higher than the test values. The results of the anova indicate that the

246 G. MULDER AND W, R. E. H. MULDER-HAJONIDES VAN DER MEULEN

influence of the task frequencies is significant, but PV is negligible. There is a difference between the rest values, but PV is negligible. There is no difference between the two days. The score does not discriminate between the two levels of compatibility.

3.4. Number of waves in the cardiotachogram (N)

The difference between 20 signals per min and 60 signals per min is 26 waves in 4 min both for day 1 and day 2 (the standard deviation is about 18 % of the mean value). The results of the anova indicate that the rise in number of waves over the task conditions is significant. PV = 13 %. There is no difference between the rest values. The difference between the two days is small (p < 0.10; PV = 1%). The score does not discriminate between the two levels of compatibility.

190 UI

; 180

; 170

; 160 i

R 20 R 30 R 40 R 50 R 60

Fig. 3. Mean number of waves in the cardiotachogram in 4 min, separately for rest (R) and task conditions. Solid line day 1; dotted line day 2.

3.5. SABS/N

The difference between 20 signals per min and 60 signals per min is 11 msec in 4 min for day 1 and 9 msec in four minutes for day 2. The task values are lower than the rest values. The only exception is 20 signals per min on day 1. The results of the anova indicate that SABS/N is significantly influenced by the task frequencies, but PV is only 4%. There is a difference between the rest conditions, but the effect is small (PV = 1%). There is also a difference between the successive days. This effect is also small (1%). The score does not discriminate between the two levels of compatibility.


20- _.___I R 20 R 30 R GO R 50 R 60

Fig. 4. Mean SABSIN scores in 4 min, separately for rest (R) and task conditions. Solid line day 1; dotted line day 2.

3.6. Mean square successive diferences (01)

The difference between 20 signals per min and 60 signals per min is 468 msec2 in 4 min for day 1 and 146 msec2 for day 2 (The standard deviation is about 90% of the mean value.) With the exception of 60 signals per min on day 1 and 40, 50 and 60 signals per min on day 2 the task values are lower than the rest values. The results of the anova indicate that the task frequencies have no significant influence on the mean square successive differences. The difference between the rest values is significant, but PV is about 0%. There is no difference between the successive days. The score does not discriminate between the two levels of compatibility.

600

t- --l---l ---L-.1-1-_. LL-.

R 20 R 30 R 40 R 50 R 60

Fig. 5. Mean scores of D2 in 4 min, separately for rest (R) and task conditions. Solid line day 1; dotted line day 2.


3.7. Intercorrelation of scoring methods

All the matrices show the same tendencies as the ones shown in table 3.

HR is negatively correlated with most of the HV scoring methods. The

TABLE 3

Intercorrelation matrices for the scoring methods.

Task 60 sign/min, day 2.

HR 02 SABS N SABSJN D=

HR 1.00

u2 - 0.596 1.00

SABS - 0.455 0.683 1.00

N 0.832 - 0.428 - 0.337 1.00 SABSjN - 0.675 0.791 0.929 - 0.623 1.00

02 - 0.645 0.802 0.879 -. 0.476 0.915 1.00

HR

Rest 6, day 2

u2 SABS N SABSIN D2

HR 1 .oo

02 - 0.383 1.00

SABS - 0.406 0.837 1.00

N 0.576 -0.267 - 0.303 1 .oo

SABSjN - 0.512 0.770 0.954 - 0.482 1.00 02 * -0.568 0.733 0.926 - 0.297 0.943 1.00

correlation between HR and SABS/N ranges from - 0.512 to - 0.762.

The correlation between HR and N is positive and ranges from 0.526

to 0.832. The correlation is highest in the condition 60 signals per min,

namely 0.763 (day 1) and 0.832 (day 2). The correlation between D2

and SABS/N is high. The correlation between SABS and N is negative

and ranges from - 0.99 (20 signals, day 2) to - 0.527 (20 signals, day 1).

3.8. Coeficients of stability

In table 4 the coefficients of stability of the different scores are given

(day 1 correlated with day 2), both for rest and task values. For the task

values the highest coefficients of stability are HR (average correlation

= 0.823); SABS/N (average correlation = 0.579) and N (average cor-

relation = 0.581). The lowest stability for the score N is 0.040 in the

condition 20 signals per min. The other correlations are much higher.


TABLE 4

Coefficients of stability.

20

Tasks

30 40 50 60

HR 0.722 0.860 0.883 0.814 0.836

& 0.520 0.545 0.764 0.447 0.307

SABS 0.499 0.653 0.593 0.503 0.507

N 0.040 0.611 0.793 0.654 0.809

SAKS/N 0.451 0.672 0.650 0.594 0.528

D2 0.806 0.451 0.491 0.321 0.532

Rests 1 2 3 4 5 6

HR 0.852 0.788 0.844 0.837 0.866 0.833

u2 0.391 0.450 0.702 0.319 0.005 0.485

SABS 0.706 0.354 0.575 0.502 0.482 0.602

N 0.621 0.363 0.747 0.656 0.531 0.447

SABSIN 0.740 0.547 0.638 0.484 0.444 0.573

D2 0.673 0.478 0.709 0.260 0.383 0.444

For the rest values the highest coefficients of stability are HR (average

correlation = 0.835), SABS/N ( average correlation 0.570) and N (ave-

rage correlation 0.560).

4. DISCUSSION

(1) Surveying our results we may conclude that in this paced binary

choice reaction task the number of waves (N) in the cardiotachogram

and the sum of the absolute differences divided by N (SABS/N) are the

best indicators of the task levels. The scores do not discriminate between

the two levels of compatability. It is of course possible that the difference

between them is not high at all. The equipment did not make it possible

to assess the degree of compatibility in terms of reaction times.

In this experiment we used absolute scores. Many investigators relate

task scores to rest scores by expressing the task scores as a percentage

of the rest scores. The mean value of the score N remains almost at the

same level in the different rest periods. This is not the case with the

variance of the R-R intervals. This latter value can therefore be better

expressed as a percentage of the preceding rest value.

(2) The increment in the number of waves remains to be explained.


Loos (1968) analyzed the HV in rest periods and task conditions in the time and frequency domains (autocorrelations and power spectral densities). His conclusions can be summarized as follows: The variance of the R-R intervals is composed mainly of the following frequencies :

(1) a basic frequency (between 0.07 and 0.13 Hz); (2) a respiration frequency (between 0.20 and 0.35 Hz); and in

task conditions; (3) a task frequency (0.10 Hz; 0.13 Hz; 0.20 Hz; 0.40 Hz for one

decision per IO”; 7.5”; 5” and 2.5” respectively); (4) there is also a high amount of power around 0 Hz, indicating

non-stationarity. The influence of a paced choice reaction task is a decrease of the total

power in the spectrum, the appearance of the task frequency and a displacement of the respiration frequency to higher frequencies. Con- cerning the scores N and the variance we may tentatively hypothesize that N reflects the basic frequency, the respiration frequency as well as the task frequency. The variance is a reflection of the total power in the spectrum. A verification of this hypothesis is possible by comparing these scores with spectral densities (MULDER et al., forthcoming). The validity of HV as an indicator of information processing load must be further explored by using other types of information processing tasks (WIEGERSMA and MULDER, 1971).

(3) There are large differences between individuals, especially on the scores SABS, 02 and 02. This is one of the reasons why these scores do not discriminate between the task conditions. The physiological causes of these high inter-individual differences are not clear. In a pilot study the possibility was explored to differentiate between psychiatric patients and normal test subjects on basis of their HR and HV scores. SABS/N. 02, u2 did discriminate between these categories. The score N discri- minated only between rest and task conditions (MULDER and OFFER- HAUS, forthcoming).

(Accepted February 16, 1972.)

REFERENCES

BLITZ, P. S. J. HO~GSTRATEN and G. MULDER, 1970. Mental load, heart rate and

heart rate variability. Psychologische Forschung 33, 277-288.

BLJRDICK, A. J. and J. T. SCARBROUGH, 1968. Heart rate and heart rate variability:

An attempt to clarify. Perceptual and Motor Skills 26, 1047-1053.


KALSBEEK, J. W. H. and J. H. ETTEMA, 1963. Scored regularity of the heart rate pattern and the measurement of perceptual or mental load. Ergonomics 6.

KALSBEEK, J. W. H. and J. H. E~TEMA, 1964. Physiological andpsychological evaluation of distraction stress. Proceedings of 2nd International Ergonomics Asso- ciation Congress, Dortmund.

KALSBEEK, J. W. H. and J. H. EITEMA, 1965. Sinus arrhythmia and the measurement of mental load. Communication at the London Conference of the British Psychological Society, December.

Loos, F. A., 1968. Onderzoek naar zintuigeluke-mentale belasting. Intern rapport. Laboratorim voor Milieu en Bedrijfshygiene, Hoogovens, IJmuiden.

MULDER, G., W. R. E. H. MULDER-HAJONIDES VAN DER MEULEN and J. H. A. VAN DER SIUAN, 1971. Heart rate variability in the time and frequency domain. Heymans Bulletin, Instituut voor Algemene Psychologie, Groningen.

MULDER, G. and R. E. OFFERHAUS, Heart rate variability as a diagnosticum inpsychiatry. To be published.

VON NEUMANN, J., R. H. KENT, H. R. BELLINSON and B. I. HART, 1941. The mean square successive differences. Annual Mathematical Statistics 12, 153-162.

VERBERK, A. J. A., 1970. Variantie analyse in de gedragswetenschappen in het licht van andere multivariate analyse technieken. Thesis, State University of Groningen.

WIEGERSMA, S. and G. MULDER, 1971. Stimulus and response uncertainty, heart rate variability and respiration frequency. Heymans Bulletin, Instituut voor Algemene Psychologie, Groningen.

WINER, B. J., 1962. Statistical principles in experimental design. New York: McGraw- Hill Book Company.

heart rate variability in a binary choice reaction task: an evaluation of some scoring methods

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