Mental effort affects vigilance enduringly: after-effects in EEG and behavior.

Vigilance is assumed to decline with sustained task performance. The EEG-effects during performance on mental tasks, however, cannot be ascribed indisputably to vigilance decline per se. During task performance itself, effects of information processing and vigilance decline may be confounded. In this study, effects of sustained mental effort were studied in the absence of specific information processing, after sustained information processing had taken place, namely after an effortful 70-min intelligence test. Vigilance was determined by means of EEG-measures in a rest condition. Furthermore, behavioral performance was assessed on two different tasks, the traditional Clock test and the SART. After mental effort, theta power in the EEG and errors on the SART were increased. Beta2 power, however, also appeared enhanced. We conclude that sustained mental effort produces an enduring decrease in vigilance, but that some active processing is enhanced at the same time. A second study replicated the EEG-results after mental effort.     

Self‐awakening improves alertness in the morning and during the day after partial sleep deprivation

The ability to awaken at a predetermined time without an alarm is known as self‐awakening. Self‐awakening improves morning alertness by eliminating sleep inertia; however, the effects of self‐awakening on daytime alertness and alertness that has deteriorated as a result of sleep loss are unknown. The aim of this study was to determine the effects of self‐awakening on both morning and daytime alertness after partial sleep deprivation. Fifteen healthy males without the habit of self‐awakening participated in a cross‐over trial including forced awakening and self‐awakening conditions. In each condition, participants' sleep was restricted to 5 h per night in their homes for 4 consecutive days. They completed a psychomotor vigilance task and subjective ratings of sleepiness immediately upon awakening each morning. On the fourth day, participants completed subjective ratings of sleepiness, a psychomotor vigilance task and sleep latency tests in the laboratory seven times at 1‐h intervals during the day. The response speed on the psychomotor vigilance task, in the morning and during the day, was higher in the self‐awakening than the forced awakening condition. Our results showed that self‐awakening improved alertness (assessed by response speeds) by reducing sleep inertia and alleviated daytime sleepiness heightened by partial sleep deprivation.     

Diurnal spectral EEG fluctuations in narcoleptic patients during rest and reaction time tasks

Narcolepsy is associated with lowered vigilance. Diurnal variation in vigilance appears altered, but the exact nature of this change is unclear. It was hypothesized that the homeostatic sleep drive is increased in narcolepsy. Decreased levels of vigilance are reflected in low frequency band power in the electroencephalogram (EEG), so these frequencies were expected to be increased in the narcolepsy group. Furthermore, it was expected that low frequency power should increase over the day. Narcoleptic patients and healthy controls participated (36 participants in total); they were not allowed to take medication or naps on the experimental day. EEG was measured at 9:00, 11:00, 13:00, 15:00, and 17:00 hours, during rest and during reaction time tasks. In the narcolepsy group, alpha power was lower at rest at all times. Delta and theta power during rest and task performance increased steadily over the day in this group, from 11:00 hours onwards. Additionally, in the narcolepsy group beta1 and beta2 power during rest appeared increased at the end of the day. The effects in the lower frequency bands strongly suggest that vigilance is low at all times. The progressive increase in low frequency power indicates that the sleep drive is enhanced. It is not clear whether this pattern reflects an extreme state of low vigilance, or a pathological brain condition. The effects in the higher frequencies suggest that narcoleptic patients may make an effort to counteract their low vigilance level.     

Detecting deteriorated vigilance using percentage of eyelid closure time during behavioral maintenance of wakefulness tests

The present study tested the hypothesis of an additive interaction between intrinsic, extraneous and germane cognitive load, by manipulating factors of mental workload assumed to have a specific effect on either type of cognitive load. The study of cognitive load factors and their interaction is essential if we are to improve workers' wellbeing and safety at work. High cognitive load requires the individual to allocate extra resources to entering information. It is thought that this demand for extra resources may reduce processing efficiency and performance. The present study tested the effects of three factors thought to act on either cognitive load type, i.e. task difficulty, time pressure and alertness in a working memory task. Results revealed additive effects of task difficulty and time pressure, and a modulation by alertness on behavioral, subjective and psychophysiological workload measures. Mental overload can be the result of a combination of task-related components, but its occurrence may also depend on subject-related characteristics, including alertness. Solutions designed to reduce incidents and accidents at work should consider work organization in addition to task constraints in so far that both these factors may interfere with mental workload.     

Inter-modal and intra-subject consistency in EEG correlates of vigilance

Occipital EEG was monitored continuously for 20 subjects who took part in an auditory vigilance task and for 20 in a visual vigilance task. Ten subjects were common to both experiments. The two tasks were virtually identical. EEG samples were taken for each of 250 signal presentations. Automatic low frequency analysis was performed on 4 frequency bands between 2.0–19.5 cps. There were 5 types of signal in the task and each signal was followed by a rest period. The wanted signal was 4 consecutive odd and different digits. The other signals contained either 3 odd and 1 even digit, 2 odd and 2 even digits, 1 odd and 3 even digits or 4 even digits. Since the 5 signal types varied in their approximation to the wanted signal they were ranked on that criterion for their arousal value. There were a number of statistically significant findings as follows. A decreasing monotonic trend is demonstrated for EEG abundance across signal types, that is, EEG abundance is inversely proportional to the arousal value of the signal for both the auditory and the visual tasks. Subjects common to the 2 tasks show consistency of individual differences in the 2 EEG trends across the tasks. EEG alpha and beta abundance provide strong evidence for individual differences in the magnitude of abundance under different stimulus conditions. Subjects with faster median reaction times demonstrate a stronger EEG trend than those with slow reaction times. However, reaction time is not related to either pre-, during or post-signal presentation EEG. A decreasing monotonic trend is demonstrated for post-test subjective ratings of alertness for each signal type and also for the associated rest periods in both the auditory and the visual task. The trend in the subjective ratings for signals agrees with the trend in EEG abundance. Subjects are consistent in their ratings across the 2 tasks.     

A higher illuminance induces alertness even during office hours: findings on subjective measures, task performance and heart rate measures

Nocturnal white light exposure has shown marked results on subjective and objective indicators of alertness, vitality and mood, yet effects of white light during daytime and under usual office work conditions have not been investigated extensively. The current study employed a mixed-group design (N=32), testing effects of two illuminance levels (200lx or 1000lx at eye level, 4000K) during one hour of morning versus afternoon exposure. In four repeated blocks, subjective reports, objective performance and physiological arousal were measured. Results showed effects of illuminance on subjective alertness and vitality, sustained attention in tasks, and heart rate and heart rate variability. Participants felt less sleepy and more energetic in the high versus the low lighting condition, had shorter reaction times on the psychomotor vigilance task and increased physiological arousal. Effects of illuminance on the subjective measures, as well as those on heart rate were not dependent on time of day or duration of exposure. Performance effects were most pronounced in the morning sessions and towards the end of the one-hour exposure period. The effect on heart rate variability was also most pronounced at the end of the one-hour exposure. The results demonstrate that even under normal, i.e., neither sleep nor light deprived conditions, more intense light can improve feelings of alertness and vitality, as well as objective performance and physiological arousal.     

Effects of performance feedback on cardiovascular reactivity and frontal EEG asymmetry

The process of goal-setting may be captured by psychophysiological variables, such as cardiovascular reactivity (representative of effort mobilisation) and frontal EEG asymmetry (motivational disposition). The current study exposed 32 participants to false performance feedback in order to manipulate goal-setting and mental effort investment. Participants performed five consecutive blocks of the n-back task and received false performance feedback. One group received repeated positive feedback (i.e. performance steadily improved over the five blocks) whilst a second group were exposed to repeated negative feedback (i.e. performance deterioration over five blocks). Blood pressure, power in the mid-frequency and high-frequency component of Heart Rate Variability (HRV), heart rate, frontal EEG asymmetry and subjective self-assessment data were collected. Sustained and repeated positive feedback led to increased systolic blood pressure reactivity and a suppression of the 0.1 Hz component of HRV. Increased relative left hemisphere activation was observed at F3/F4 and FC1/FC2 over successive task blocks in the presence of feedback regardless of positive or negative direction. It is argued that upward goal adjustment accounted for the psychophysiological changes observed in the positive feedback condition.     

Diurnal variations in the waking EEG: comparisons with sleep latencies and subjective alertness

Daytime measures of sleep latency and subjective alertness do not correlate with one another, suggesting that they assess different aspects of alertness. In addition, their typical diurnal variations show very different time courses. Quantitative analysis of the waking electroencephalogram (EEG) has been proposed as an objective measure of alertness, but it is not clear how it compares with other measures. In this study, the waking EEG was measured in the daytime to determine the presence of diurnal variations in the activity of standard frequency bands and to compare these variations with the temporal patterns typical of sleep propensity and subjective alertness. Alertness was evaluated in four men and 12 women, aged 19-33 y. Assessments were conducted every 2 h, from 10.00 to 24.00, in the following order: a visual analogue scale of alertness, a waking EEG recording and a sleep latency test. The waking EEG was recorded with eyes open. For each recording session, 32-60 s of artefact-free signals were selected from the C3/A2 derivation, then subjected to amplitude spectral analysis. Four EEG frequency bands showed significant diurnal variations: delta, theta, sigma and beta1. None of these variations showed a significant correlation with the temporal patterns of sleep latencies or subjective alertness. At the individual level, however, theta band activity increased when subjective alertness decreased, suggesting that the theta band can be used to monitor variations in alertness in a given individual, even at the moderate levels of sleepiness experienced during the daytime.     

Task-dependent differences in subjective fatigue scores

The aim of the present study was to evaluate time-on-task effects on subjective fatigue in two different tasks of varying monotony during night-time testing (20:00 to 4:00 hours) in a sleep deprivation intervention. The experiment included eight test runs separated by breaks of approximately 20 min. Twenty healthy volunteers performed a driving simulator and the Mackworth clock vigilance task in four of the test runs each. Sequence of tasks was varied across subjects. Before and after each task, subjective sleepiness was assessed by means of the Karolinska sleepiness scale and subjective fatigue was rated on the Samn-Perelli checklist. Fatigue and sleepiness significantly increased over the course of the night. Both tasks led to an increase in fatigue and sleepiness across test runs. However, this time-on-task effect was larger in the vigilance than in the driving simulator task. It is important to note that fatigue and sleepiness in one test run were not influenced by the task performed in the preceding test run, that is there were no cross-over effects. The results suggest that time-on-task effects superimpose circadian and sleep-related factors affecting fatigue. They depend on the monotony of the task and can be quantified by means of a design including separate test runs divided by breaks.     

Combined assessment of attentional reserve and cognitive‐motor effort under various levels of challenge with a dry EEG system

A novel ERP approach was proposed to index variations in mental workload, particularly in attentional reserve, which is complementary to EEG spectral content thought to reflect mental effort. To our knowledge, no study has assessed mental effort and attentional reserve simultaneously in EEG gel‐based and, importantly, dry systems, which are particularly well suited for real‐world settings. Therefore, by systematically considering ERP, EEG spectral, and importantly the combination of both, this study examined if a small set of dry EEG electrodes could detect changes in both spectral and ERP metrics to assess the mental workload under various challenges with a similar fidelity to their gel‐based counterparts in a laboratory setting. By employing both EEG gel‐based and dry systems, the ERP and spectral markers were computed while participants executed a visuomotor task under three levels of challenge. For both EEG systems, more challenging levels of difficulty were associated with concomitant changes in ERP amplitude, and spectral power reflected a reduction of the attentional reserve and an increase in cognitive‐motor effort, respectively. Those variations in attentional reserve and cognitive‐motor effort collectively indexed mental workload with nearly identical fidelity for both gel‐based and dry EEG systems. These findings promise to assess the mental workload in situations where the use of dry EEG systems could be advantageously employed to examine human cognitive‐motor performance.     

Sharp and sleepy: evidence for dissociation between sleep pressure and nocturnal performance

While sleep restriction decreases performance, not all individuals are equal with regard to sensitivity to sleep loss. We tested the hypothesis that performance could be independent of sleep pressure as defined by EEG alpha-theta power. Twenty healthy subjects (10 vulnerable and 10 resistant) underwent sleep deprivation for 25 h. Subjects had to rate their sleepiness (Karolinska Sleepiness Scale) and to perform a 10-min psychomotor vigilance task (PVT) every 2 h (20:00-08:00 hours). Sleep pressure was measured by EEG power spectral analysis (alpha-theta band 6.0-9.0 Hz). Initial performance, EEG spectral power and KSS score were equal in both groups (ANOVA, NS). The performance of vulnerable subjects significantly increased during the night (rANOVA, P < 0.01), whereas resistant subjects globally sustained their performance. Homeostatic pressure and subjective sleepiness significantly increased during the night (rANOVA, P < 0.01) identically in both categories (rANOVA, NS). Resistant subjects sustained their reaction time independently of the increase in homeostatic pressure. The phenotypic determinants of vulnerability to extended wakefulness remain unknown.     

Fluctuation of waking electroencephalogram and subjective alertness during a 25-hour sleep-deprivation episode in young and middle-aged subjects

STUDY OBJECTIVES: To evaluate the effects of a 25-hour sleep-deprivation episode on quantitative waking electroencephalogram (EEG) and subjective alertness in young and middle-aged subjects. DESIGN: A 25-hour constant-routine protocol followed by a daytime recuperative sleep episode. SETTING: Chronobiology laboratory. PARTICIPANTS: Twenty-five normal subjects separated into 2 groups: young (aged 20-39 years) and middle-aged (aged 40-60 years). INTERVENTIONS: None. MEASUREMENTS AND RESULTS: Waking EEGs were recorded every 2 hours and subjective measures of alertness every 30 minutes during the 25-hour sleep-deprivation episode. Overall, results indicated no age-related differences over a 25-hour constant routine in the temporal evolution of subjective alertness and of spectral power in the theta/alpha (4-12 Hz) frequencies of the waking EEG. The middle-aged compared to the young subjects showed a reduced rebound of slow-wave activity in the recovery-sleep episode. While the waking EEG and subjective alertness levels showed strong correlations in both groups, there was no relationship between theta rise during wakefulness and slow-wave activity rebound in the recovery-sleep episode. CONCLUSIONS: These results suggest a dissociation in the middle-aged population between the sensitivity of their alertness level on the one hand and the sensitivity of their sleep on the other to the number of hours of wakefulness. Hence, alertness of young and middle-aged subjects would show the same deterioration with an accumulation of wakefulness (possibly reflecting a similar sleep need), yet the middle-aged subjects would be less able to increase their recuperative sleep intensity following enhanced time awake (reduction in sleep ability).     

Circadian rhythms in human performance and mood under constant conditions

This study explored the relationship between circadian performance rhythms and rhythms in rectal temperature, plasma cortisol, plasma melatonin, subjective alertness and well-being. Seventeen healthy young adults were studied under 36 h of «unmasking» conditions (constant wakeful bedrest, temporal isolation, homogenized «meals») during which rectal temperatures were measured every minute, and plasma cortisol and plasma melatonin measured every 20 min. Hourly subjective ratings of global vigour (alertness) and affect (well-being) were obtained followed by one of two performance batteries. On odd-numbered hours performance (speed and accuracy) of serial search, verbal reasoning and manual dexterity tasks was assessed. On even-numbered hours, performance (% hits, response speed) was measured at a 25–30 min visual vigilance task. Performance of all tasks (except search accuracy) showed a significant time of day variation usually with a nocturnal trough close to the trough in rectal temperature. Performance rhythms appeared not to reliably differ with working memory load. Within subjects, predominantly positive correlations emerged between good performance and higher temperatures and better subjective alertness; predominantly negative correlations between good performance and higher plasma levels of cortisol and melatonin. Temperature and cortisol rhythms correlated with slightly more performance measures (5/7) than did melatonin rhythms (4/7). Global vigour correlated about as well with performance (5/7) as did temperature, and considerably better than global affect (1/7). In conclusion: (1) between-task heterogeneity in circadian performance rhythms appeared to be absent when the sleep/wake cycle was suspended; (2) temperature (positively), cortisol and melatonin (negatively) appeared equally good as circadian correlates of performance, and (3) subjective alertness correlated with performance rhythms as well as (but not better than) body temperature, suggesting that performance rhythms were not directly mediated by rhythms in subjective alertness.     

Beta band oscillations as a correlate of alertness--changes in aging

Older adults (>60 years) show attentional deficits in comparison to younger people (18-30 years). As beta-band EEG activity has been previously postulated to indicate attentional modulation in the visual system, we searched for possible deficits in beta power in elderly subjects performing an attentional task with spatial differentiation between visual stimuli. We found that in older adults a lower level of beta activity correlated with decreased behavioral performance. As compared to young subjects, older adults expressed decreased activation in beta band during an attentional task, which displayed two different dynamics during the anticipatory period. Those dynamics were accompanied by one of two different behavioral pattern deficits. We hypothesize that one group of elderly participants suffered from difficulty in the activation of attentional processes (alertness deficits), while the other--from difficulty in sustaining those processes (vigilance deficits).     

Mindfulness‐of‐breathing exercise modulates EEG alpha activity during cognitive performance

The present study investigated whether engaging in a mindful breathing exercise would affect EEG oscillatory activity associated with self‐monitoring processes, based on the notion that mindfulness enhances attentional awareness. Participants were assigned to either an audio exercise in mindful breathing or an audio control condition, and then completed a Stroop task while EEG was recorded. The primary EEG measure of interest was error‐related alpha suppression (ERAS), an index of self‐monitoring in which alpha power is reduced, suggesting mental engagement, following errors compared to correct responses. Participants in the mindful‐breathing condition showed increased alpha power during the listening exercise and enhanced ERAS during the subsequent Stroop task. These results indicate enhanced error‐monitoring among those in the mindful‐breathing group.     

Daytime sleepiness, psychomotor performance, waking EEG spectra and evoked potentials in women with severe premenstrual syndrome

We assessed daytime sleepiness using objective and subjective measures in women with severe premenstrual syndrome (PMS) compared with women without significant premenstrual symptoms. Nine women with severe PMS and eight controls (aged 18−40 years) completed a laboratory-based daytime protocol including the maintenance of wakefulness test (MWT), psychomotor vigilance task (PVT), quantitative waking electroencephalogram (EEG), auditory and visual event-related potentials (ERPs), and sleepiness and mood scales during the mid-follicular and late-luteal (premenstrual) phases of the menstrual cycle. In association with increased perceived sleepiness, fatigue and other premenstrual symptoms in the late-luteal phase, women with PMS performed more poorly on the PVT, with increased lapses and slower reaction times ( P  <   0.05), compared with the follicular phase and controls. However, there were no significant group or menstrual phase differences in latency to sleep on the MWT. Waking spectral EEG power and ERP measures also did not differentiate PMS women when symptomatic. Both groups of women displayed increased spectral power in the delta/theta frequencies (2−6 Hz) and fast alpha frequency (11−12 Hz) in the late-luteal phase relative to the follicular phase. Trait-like differences were apparent in that women with PMS had increased beta1 (12−16 Hz) power and smaller P300 amplitude than controls in both menstrual cycle phases. Our findings indicate that women with severe PMS are subjectively sleepy and fatigued, and show psychomotor slowing when symptomatic compared with when they are symptom-free and compared with controls. However, the ability to maintain wakefulness under soporific conditions, spectral properties of waking EEG and cognitive processing do not vary in synchrony with premenstrual symptoms.     

Slow eye movements and subjective estimates of sleepiness predict EEG power changes during sleep deprivation

RATIONALE: The aim of the present study was to assess, intraindividually, the relationship among slow eye movements, electroencephalogram (EEG) power, and subjective measures of sleepiness during a 40-hour sleep deprivation comparing 2 experimental conditions: eyes-open and eyes-closed. METHODS: Nineteen normal subjects participated in a sleep-deprivation protocol with recordings of the waking Cz-A1-2 EEG in 36 sessions at 1-hour intervals starting at 10:00 AM. Each session consisted of a 2-minute eyes-closed period, followed by a 4-minute eyes-open period. Electrooculogram, self-ratings (Karolinska Sleepiness Scale and Visual Analog Scale for Global Vigor), and tympanic temperature were also recorded. RESULTS: Changes in sleepiness and alertness are paralleled by increases in slow eye movements and theta and delta EEG power. The beginning of the rise of delta, theta, and slow eye movement activity corresponded to the nadir of temperature, peaking at 7:00AM. Cross-correlational analyses showed that changes in slow eye movements were strictly phase locked to those in slow-frequency EEG bands and in subjective measures. The comparison of time intervals that were equivalent with respect to circadian phase confirms the effects of the increased sleepiness on slow eye movement activity and on the other measures. The temporal concordance of the different physiologic and subjective measures is also reflected in the individual time courses. Individual and group analyses converged in indicating that slow eye movements can be considered reliable indexes of sleepiness but only in the eyes-closed condition. CONCLUSIONS: Results suggest that subjective and EEG changes associated with higher sleepiness are paralleled by an increase in slow eye movement activity, but this relationship exists almost exclusively with the eyes closed. Hence, its use in practical and operational contexts seems limited.     

EEG Asymmetry During Covert Mental Activity

Twenty subjects selected for (overt) task-induced EEG asymmetry performed 17 different covert mental tasks while EEG was recorded from right and left temporoparietal leads. After each task, subjects rated their experiences on four scales: (covert) verbalization, visual imagery, affect, and concentration. Zero-order and semipartial correlations between integrated EEG measures and subjective ratings showed that verbalization and concentration were systematically related to EEG asymmetry. The results suggest that verbal processes may be a more important influence on temporoparietal EEG asymmetry than are visual imagery processes, at least during purely covert tasks. The results also demonstrate that EEG asymmetry reflects lateralized cognitive processes occurring in the absence of external stimuli or overt responses.     

Instantaneous changes in heart rate regulation due to mental load in simulated office work

The cardiac regulation effects of a mental task added to regular office work are described. More insight into the time evolution during the different tasks is created by using time–frequency analysis (TFA). Continuous wavelet transformation was applied to create time series of instantaneous power and frequency in specified frequency bands (LF 0.04–0.15 Hz; HF 0.15–0.4 Hz), in addition to the traditional linear heart rate variability (HRV) parameters. In a laboratory environment, 43 subjects underwent a protocol with three active conditions: a clicking task with low mental load and a clicking task with high mental load (mental arithmetic) performed twice, each followed by a rest condition. The heart rate and measures related to vagal modulation could differentiate the active conditions from the rest condition, meaning that HRV is sensitive to any change in mental or physical state. Differences between physical and mental stress were observed and a higher load in the combined task was observed. Mental stress decreased HF power and caused a shift toward a higher instantaneous frequency in the HF band. TFA revealed habituation to the mental load within the task (after 3 min) and between the two tasks with mental load. In conclusion, the use of TFA in this type of analysis is important as it reveals extra information. The addition of a mental load to a physical task elicited further effect on HRV parameters related to autonomic cardiac modulation.     

Psychophysiological investigation of vigilance decrement: Boredom or cognitive fatigue?

The vigilance decrement has been described as a slowing in reaction times or an increase in error rates as an effect of time-on-task during tedious monitoring tasks. This decrement has been alternatively ascribed to either withdrawal of the supervisory attentional system, due to underarousal caused by the insufficient workload, or to a decreased attentional capacity and thus the impossibility to sustain mental effort. Furthermore, it has previously been reported that controlled processing is the locus of the vigilance decrement. This study aimed at answering three questions, to better define sustained attention. First, is endogenous attention more vulnerable to time-on-task than exogenous attention? Second, do measures of autonomic arousal provide evidence to support the underload vs overload hypothesis? And third, do these measures show a different effect for endogenous and exogenous attention? We applied a cued (valid vs invalid) conjunction search task, and ECG and respiration recordings were used to compute sympathetic (normalized low frequency power) and parasympathetic tone (respiratory sinus arrhythmia, RSA). Behavioural results showed a dual effect of time-on-task: the usually described vigilance decrement, expressed as increased reaction times (RTs) after 30 min for both conditions; and a higher cost in RTs after invalid cues for the endogenous condition only, appearing after 60 min. Physiological results clearly support the underload hypothesis to subtend the vigilance decrement, since heart period and RSA increased over time-on-task. There was no physiological difference between the endogenous and exogenous conditions. Subjective experience of participants was more compatible with boredom than with high mental effort.