Stress, subjective experience and cognitive performance during exposure to noise and vibration
Introduction
In many environments workers are exposed daily to factors such as noise, vibration, and stress that can adversely affect performance. In some cases, carrying out specific tasks may be made more difficult because the unwanted environmental stimulus directly interferes with the mechanics of performing the tasks. For instance, the sound being emitted from a piece of equipment may be so loud that the worker cannot hear an important signal. In other cases, however, performance may be negatively affected because the unwanted environmental stimulus taxes the worker's mental capacity. Both cases can negatively affect performance and may lead to a higher risk for injury or accidents.
Noise has been shown to have an effect on cognitive resources. Among the many areas that have been investigated, studies have demonstrated a negative influence on a focused attention task (Smith, 1991), on performance in a search and memory task with high memory load (Smith & Miles, 1987) and performance in a task measuring the detection of repeated numbers (Smith, 1988). These studies employed exposures to a continuous free field noise generated in the frequencies of 125–4000 Hz and played at 78, 75 dB(A) and 85 dB(C) respectively.
Traffic noise has been found to impair performance in a search and memory task (Hygge, Boman & Enmarker, 2003) as well as in short-term memory and in a mental arithmetic task (Belojevic, Öhrström & Rylander, 1992). Tafalla and Evans (1997) studied noise exposure on performance during high and low cognitive effort. Effects were found on reaction time performance in an arithmetic task during noise exposure when effort was low.
The relationship between noise exposure and the performed task does not seem to be simple. Banbury, Macken, Tremblay and Jones (2001) concluded that the type of noise and cognitive task used have differential influences on performance. The performance degradation depends largely on the character of the noise more than the level and that sound seems to have obligatory access to memory.
However, in applied situations, it is more often the case that there are many unwanted or unrelated environmental stimuli occurring at the same time as a task is being performed. A common type of exposure is the combination of noise and whole body vibration that occurs when operating motorized vehicles or heavy machinery. As discussed above, noise can clearly affect mental processing, but there are even indications that vibration may also influence mental performance.
Whole-body vibration has been demonstrated to negatively impact cognitive performance in a mental arithmetic task (Sandover & Champion, 1984) and in a short-term memory task (Sherwood & Griffin, 1990) and learning task (Sherwood & Griffin, 1992). There are even indications that occupational exposure to whole-body vibration has negative long term effects for psychomotor functioning (Schneider & Wall, 1989) and mood (Abbate et al., 2004).
Effects have been reported in studies when combining noise and vibration. Physiologically, combined stimuli have been found to have greater effects than single stimuli on palmar sweating (Sakakibara et al., 1989), temporary threshold shifts (Seidel et al. (1990), Seidel et al. (1988); Manninen (1983), Manninen (1984)) and genotoxicity (Silva, Carothers, Castelo Branco, Dias, & Boavida (1999a), Silva, Carothers, Castelo Branco, Dias, & Boavida (1999b)). Performance on tracking performance while exposed to combined stimuli was thoroughly investigated during the 1970s. The primary finding was that while a noise of 100–105 dB(A) when combined with vibration resulted in less adverse effects on tracking than with vibration alone (Grether et al., 1971; Grether, Harris, Ohlbaum, Sampson, & Guignard, 1972; Sommer & Harris, 1973), noise of 110dB(A) combined with vibration resulted in greater effects than either stimulus alone.
Using a complex counting task, Harris and Shoenberger (1980) showed that adding vibration to a 65 dB(A) sound stimulus adversely affected performance while the same vibration stimulus added to a 100 dB(A) had relatively no effect. However in reviewing the results from studies done during the 1960s and 70's, Harris and Shoenberger (1980) noted that effects on cognitive or intellectual tasks of combining stimuli are not always present and that no clear patterns had emerged.
A more consistent picture, however, has emerged in terms of subjective experience. Ljungberg, Neely and Lundström (2004) found that while performance in a short-term memory task was not affected, subjective ratings of difficulty and annoyance were significantly higher during combined exposure than when the stimuli were presented apart or not at all. Increased ratings of annoyance when combining noise and vibration stimuli have been reported by others (Lundström, Landström, & Kjellberg, 1990; Seidel et al. (1990), Seidel et al. (1988)).
Thus, an individual may experience the task to be performed under such conditions as more difficult than when exposed to either one or the other type of stimulus even if it may not be reflected in performance measures. The purpose of the present set of experiments is to investigate whether a measure of physiological stress would reflect the elevated levels of subjective difficulty and stress. In both of the following experiments, cortisol samples were collected during a protocol similar to that employed in our previous study. Concentration levels of cortisol in saliva have been shown to be sensitive to acute stressors in daily activities (Van Eck, Berkhof, Nicolson & Sulon, 1996; Zeier, 1994; Smyth, Ockenfels, Porter, Kirschbaum, Hellhammer & Stone, 1998; Kugler, Reintjes, Tewes & Schedlowski, 1996), including noise (Persson Waye, Bengtsson, Rylander, Hucklebridge, Evans & Clow, 2002).
Section snippets
Experiment 1
The aim of the experiment is to test the hypothesis that exposure to noise and whole-body vibration (separately and in combination) while performing cognitive tasks will lead to increased stress. Physiological stress will be measured by comparing cortisol levels in saliva samples collected pre- and post-exposure and subjective stress through ratings. Due to the fact that the menstruation cycle influences cortisol levels and that the experimental design requires multiple measurements on multiple
Participants
Two groups were selected using a self-reported high or low sensitivity to noise exposure. A total of 201 Noise Sensitivity Questionnaires were distributed to men attending courses at Umeå University. 134 questionnaire were returned, and the mean score for the whole group was 68 points (SD=13.4) ranging from 36 to 98. Using the lower and upper 25 percentiles and then including only those who had indicated on their questionnaire that they would be interested in participating in a study, a Low
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