Heart Rate Variability Study

Source: http://www.novadesk.com/ergonomicsArticlesDetail.asp?ID=8

[Citation: Ankrum, D.R. and Suzuki, K. (1997). Heart rate variability in eye-level and low monitor conditions. Design of Computing Systems, (Eds. G. Salvendy, M.J. Smith, R.J. Koubek). 21A, 571-574. Amsterdam: Elsevier]

Dennis R. Ankrum and Kaoru Suzuki

Human Factors Research, Nova Solutions, 10007 San Luis Trail, Austin, TX 78733-1253 USA Ankrum@aol.com)

College of Engineering, Hosei University, 3-7-2 Kajinocho, Koganei-city, Tokyo 184 Japan (suzuki@ergo.is.hosei.ac.jp)

Introduction

In recent years the evaluation of mental strain has become a concern in the field of human factors. Methods to evaluate the mental strain caused by computer-based tasks may be classified into two categories: those based on subjective measures (e.g. questionnaires) and those based on objective measures (e.g. physiological tests).

Heart Rate Variability

In the latter category, the measurement of Heart Rate Variability (HRV) has been considered to be very effective [1]. One of the influences on heart rate variability is the interaction between the sympathetic and parasympathetic components of the autonomic nervous system. The autonomic nervous system functions to help maintain a stable internal environment. For example, it functions in temperature regulation, digestion, respiration, blood circulation, and heartbeat.

In general, the sympathetic nervous system stimulates those activities which are most dramatically expressed and mobilized in preparation for stressful situations. It also increases mental activity. The parasympathetic nervous system stimulates those activities that are associated with the conservation and restoration of energy. It is active when we are at rest. The sympathetic nervous system acts to increase the rate and force of the heart beat and increase blood pressure, while the parasympathetic nervous system acts to reduce them. The two systems work together to achieve a balance and are not antagonistic.

Our daily activities, including VDT operation, driving, etc, require the sympathetic nervous system's activity to be high. However, excessive mental strain suppresses it. In many cases, that reduces one's performance.

The HRV-index is a method based on an octave-band analysis with an R-wave detector to measure heart rate variability for the evaluation of mental strain [2]. Because the heart rate of a seated person (not engaged in strenuous physical work) is mainly affected by the activities of the autonomic nervous system, the HRV index reflects that activity. A lower HRV-index represents a condition of less mental strain.

Monitor Placement

The benefits of lower monitor placement have been documented. As gaze angle tilts downward, the resting point of vergence moves inward [3], which reduces stress on the extraocular muscles. The ability to accommodate improves [4], and reports of headaches [5], eye strain [5], and fatigue [6] decrease. Because less of the cornea is exposed to the atmosphere, the risk of dry eye syndrome is reduced [7]. Lower monitor placement also allows a wider range of comfortable neck postures [8] and reduces musculoskeletal discomfort [8, 9]. This paper reports on a study comparing Heart Rate Variability at low and high monitor placements.

Figure 1. An example of 5x5 display format type.

Method

The subjects were 5 emmetropic students ranging in age from 20 to 21 years of age. All had at least 2 years of computer experience. There were four experimental conditions: two levels of difficulty - easy and hard, and two gaze angles - high and low.

The experimental task was a forced, information-retrieval task in which subjects located a specific set of three nonsense letters and entered its location on a 10-key pad [10]. The number of rows and columns was fixed at 5 (see Figure 1). The order of presentation was varied. Subjects performed the task until they successfully completed 10 sets. A set consisted of 8 successive correct answers, with each incorrect response resulting in restarting of a set.

The level of difficulty was determined by the length of time allowed for the choice. The time limits were individually determined on the basis of each subject's performance during a trial period in which the response time for 20 trials, performed without a time limit, was measured. The time limits (easy and difficult) were determined by the average and standard deviation of the 20 trials. Approximately 30 minutes were required for each condition.

Two monitor positions were employed: 15 degrees (high condition) and 36 degrees (low condition) below the horizontal line of sight to the center of the screen. The viewing distance was 87 cm to the center of the screen in both conditions. Figure 2 shows the workstation layout.

Results

Figure 2. The workstation layout for high and low monitor conditions.

The HRV-index was calculated from R-R intervals (the interval between the R-waves of the ECG signal, appearing once per heartbeat) shown on an Electrocardiogram. The range is from 1 to 4. Lower values indicate greater activity of the sympathetic nervous system.

Figure 3 shows the mean and 95% confidence intervals of HRV indices under each of the 4 conditions. The index was lower in the low monitor condition for both the easy and difficult tasks. While there were no task differences found in the low monitor condition, the easy task resulted in a higher HRV-index when the monitor was positioned high.

Figure 3. HRV indices for the easy and difficult tasks at both high and low monitor placements. Error bars represent 95% confidence intervals.

Discussion

In the high monitor condition, the difficult task resulted in a lower HRV-index. Because lower values indicate that the sympathetic nervous system is dominant, this would be expected. However, for both easy and difficult tasks the HRV-index was lower in the low monitor condition. If the lower HRV-index represents increased activity of the sympathetic nervous system, this suggests that a low monitor position would result in a condition of lower mental strain.

References

Suzuki, K., and Hayashi, Y. (1993). On a simple and effective method to analyze Heart Rate Variability, Advances in Human Factors/Ergonomics 19A, pp. 914-919.

Suzuki, K. (1995). A method based on an octave-band analysis with an R-wave detector to analyze heart rate variability for evaluation of mental strain. The Japanese Journal of Ergonomics, 31, 379. (in Japanese).

Heuer, H., and Owens, D. (1989). Vertical gaze direction and the resting posture of the eyes. Perception, 18, 363-377.

Ripple, P. (1952). Variation in accommodation in vertical directions of gaze. American Journal of Ophthalmology, 35, 1630-1634

Tyrrell, R., and Leibowitz, H. (1990). The relation of vergence effort to reports of visual fatigue following prolonged near work. Human Factors, 32, 341-357.

Owens, D.A., and Wolf-Kelly, K. (1987). Near work, visual fatigue, and variations of oculomotor tonus. Investigative Ophthalmology and Visual Science, 28, 743-749.

Tsubota, K., and Nakamori, K. (1993). Dry eyes and video display terminals. New England Journal of Medicine, 328, 584.

Ankrum, D. R. and Nemeth, K.J. (1995). Posture, Comfort and Monitor Placement. Ergonomics in Design, p. 7-9.

Lie, I. and Fosterwald, K.I. (1995). VDT- work with different gaze inclinations. In A. Grieco, G. Molteni, B. Piccoli and E. Occhipinti (eds.). Work With Display Units '94. Amsterdam: Elsevier

Bishu, R., Batra, S., Cochran, D., and Riley, M. (1988). Effects of complexity on information processing. Proceedings of International Ergonomics Association, pp. 621-623.