Effects of mental fatigue on attention: An ERP study

Abstract

The effects of mental fatigue on attention were assessed. Subjects performed a visual attention task for 3 h without rest. Subjective levels of fatigue, performance measures and EEG were recorded. Subjective fatigue ratings, as well as theta and lower-alpha EEG band power increased, suggesting that the 3 h of task performance resulted in an increase in fatigue. Reaction times, misses and false alarms increased with time on task, indicating decreased performance efficiency in fatigued subjects. Subjects were unable to inhibit automatic shifting of attention to irrelevant stimuli, reflected by a larger negativity in the N1 latency range for irrelevant, compared to relevant stimuli. This difference in negativity was unaffected by time on task. However, N1 and N2b amplitude did change with time on task: N1 amplitude decreased, and the difference in N2b amplitude between relevant and irrelevant stimuli (larger N2b amplitude evoked by relevant stimuli) decreased with time on task. The results indicate a dissociation in the effects of mental fatigue on goal-directed (top–down) and stimulus-driven (bottom–up) attention: mental fatigue results in a reduction in goal-directed attention, leaving subjects performing in a more stimulus-driven fashion.

Introduction

Mental fatigue refers to the effects that people may experience after or during prolonged periods of cognitive activity. In this sense, it is a very common phenomenon in everyday modern life. Still, very little is known about the psychophysiological mechanisms underlying mental fatigue. Here, we will try to gain some insight in the mechanisms that are central to mental fatigue and in the cognitive functions that are most affected by mental fatigue.

When people become fatigued, they usually report difficulties in concentrating and focusing their attention on the tasks they are required to perform. For example, Bartlett [1], in his studies in which pilots were required to fly a simulator for extended periods of time, reported that lapses in attention happened with increasing frequency and that operators became more easily distracted. Similarly, Brown [3] noted that the main time on task effect in driving is a progressive withdrawal of attention from road and traffic demands, which, as expected, had adverse consequences on task performance. These results suggest that attention is specifically affected by mental fatigue.

Attention is a key feature of dynamic human behavior: it allows us to (i) bias the processing of incoming information [7], [37], [42] so that we can focus on the information that is relevant for achieving the current goals and (ii) to actively ignore irrelevant information that might potentially interfere with those goals.

In the present study, we will examine how mental fatigue affects these attentional processes. Therefore, we had our subjects perform a visual attention task [40] continuously for 3 h, without rest. Subjects were presented with stimulus displays that consisted of two letters at four possible locations (Fig. 1). They were to respond when a target letter appeared at one of the locations that was cued as being relevant. Subjects had to focus their attention on the cued relevant positions and had to ignore stimuli presented on the irrelevant positions. In this way, we were able to detect changes in performance on a task that places high demands on the attentional system, while subjects become more and more fatigued. In addition, by using electroencephalogram (EEG) and event-related potential (ERP) measures, we were able to examine the physiological changes related to fatigue and attention.

Studies on the topic of attention have shown that ERP components reliably reflect the differential processing of attended and unattended information [50]. By recording ERPs to attended and unattended stimuli, direct evidence can be obtained about the level of processing attained by these stimuli. The most consistent finding is a modulation of the posterior P1 (peaking between 100 and 160 ms after stimulus presentation) and N1 (160–210 ms) components by attention (e.g., [8], [44], [49]). When a particular location is attended, the exogenous P1 and N1 waves elicited by stimuli at that location are enlarged [15], [33], [34], an effect that has been interpreted as a sign of attentional modulation of sensory processing in the visual pathways [36]. This has been viewed as a representation of a ‘sensory gain’ mechanism [17]: as a result of biasing the information processing system, the responsivity to stimuli presented at attended locations is amplified, and further processing of these stimuli will therefore be enhanced.

A later component, starting at approximately 200–250 ms post stimulus, consisting of negativity at central electrodes, with a maximum at Cz, has been labeled the N2b component. This ERP component has been found to reflect the further processing of relevant information (i.e. stimuli that require a response) [25], [40], [48]. Selective modulation of these attention-related ERP components by the induction of mental fatigue would provide strong evidence that attentional processes are indeed affected by mental fatigue.

One of the most common findings of EEG studies is a shift from fast, low amplitude waves to slow, high amplitude waves when the level of alertness drops. More specifically, under decreased arousal levels, there is a progressive increase in low-frequency theta and alpha activity [23], [24], [39], probably reflecting a decrease in cortical activation [5], [26]. Therefore, the amount of alpha and theta power provides an adequate index of the level of fatigue that subjects experience. When subjects become fatigued, we would expect the level of arousal to drop, and this would be reflected by an increase in alpha and theta power.

In addition to this objective measure of fatigue, we obtained an indication of the subjective level of fatigue that the subjects were experiencing at that moment. According to Holding [19] and Hockey [18], aversion to further investment of effort in task performance is central to mental fatigue. Therefore, we presented subjects with a visual analog scale on which they could indicate the level of aversion they felt regarding task performance (after Borg [2]), on multiple occasions during the experiment.

In summary, we predict that mental fatigue results in an increase in subjective ratings of the level of fatigue and a shift to slow, high amplitude waves in the EEG. In addition, we predict a selective modulation of ERP components known to be related to selective attention. A deterioration of selective attention would lead to a decreased ability of subjects to focus their attention on task-relevant items and an increased distractability by irrelevant information. This would result in an increase in the number of missed targets and an increase in false alarms with time on task.