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Impact of top-down control during mental fatigue
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a v a i l a b l e a t w w w. s c i e n c e d i r e c t . c o m
w w w. e l s e v i e r. c o m / l o c a t e / b r a i n r e s
Research Report
Impact of top-down control during mental fatigue Monicque M. Lorist⁎ Department of Experimental and Work Psychology, University of Groningen, The Netherlands BCN-NeuroImaging Center, University Medical Center Groningen, University of Groningen, The Netherlands
A R T I C LE I N FO
AB S T R A C T
Article history:
The influence of mental fatigue, as induced by time on task, on top-down control involved in
Accepted 12 July 2008
planning goal-directed behavior and conflict resolution was examined, using an S1–S2
Available online 25 July 2008
paradigm. S2 stimuli consisted of compatible and incompatible stimuli, placing dissimilar demands on automatic and controlled processes involved in conflict solving. Information
Keywords:
provided by explicit cues (S1) affected brain activity elicited during the S1–S2 interval. P2 and
Mental fatigue
CNV effects were more pronounced if advance information was relevant for subsequent
Top-down control
behavior (hand cue) than after a cue providing information about stimulus features of S2
Planning
(color cue). Brain activity elicited by cue information was significantly attenuated with time
Event-related potential
on task. The behavioral results showed that advance information facilitated processing of
Behavior
S2; reactions were faster if cue information was valid, especially in the hand cue condition. In this condition invalid information led to significant costs in the form of increased error rates, as well. Performance efficiency deteriorated with time on task and differences between validly and invalidly cued stimuli became smaller. Concerning the time course of the behavioral effect it seems that top-down processes indexed by the CNV are the most likely candidate to underlie the performance effects of mental fatigue. Time on task effects on the cue-P2 were unrelated to observed behavioral effects. These results showed that the influence of advance information on information processing diminished with increasing mental fatigue. No evidence was found that mental fatigue had differential effects on controlled and automatic processes involved in conflict solving. © 2008 Elsevier B.V. All rights reserved.
1.
Introduction
In order to facilitate goals to be reached in the near future, it is crucial that humans plan and prepare their behavior. These planning and preparation processes strongly rely on the availability of information from our ever-changing external and internal environment. Human observers are, for example, better at detecting relevant information when they know in advance something about specific features of that informa-
tion. Moreover, if relevant advance information is provided, responses can be made faster and more accurately. Of course, facilitation of behavioral goals depends crucially on our ability to process available information and to use it to bias the processing of information (Corbetta and Shulman, 2002). Therefore, it is important to realize that although information can be provided, people might not always be able to use this information to select and prepare appropriate goal-directed actions, even if task demands indicate a need for
⁎ Department of Experimental and Work Psychology, University of Groningen, Grote Kruisstraat 2/1, 9712 TS Groningen, The Netherlands. Fax: +31 50 363 6304. E-mail address: [email protected]. 0006-8993/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.brainres.2008.07.053
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increased cognitive control to achieve specific goals. ‘What can humans do with knowledge’ is an important question; however, ‘What do they actually do with available knowledge’ seems to be an even more challenging question that will be addressed in the present study. In previous studies we observed that available information was no longer used effectively to reconfigure a task set in advance if subjects became mentally fatigued (Lorist et al., 2000; Boksem et al., 2005). Mental fatigue refers to changes in the psycho-physiological state that people experience during and following the course of prolonged periods of demanding cognitive activity requiring sustained mental efficiency. It is generally associated with increased difficulties in maintaining adequate levels of performance efficiency. Mental fatigue was induced by 2 h of continuous task performance. Effects of mental fatigue observed by Lorist et al. (2000) and Boksem et al. (2005) on preparation processes are in agreement with findings of Bartlett, who, already in 1943, noticed that among the operations most vulnerable to mental fatigue are topdown processes involved in the co-ordination and the accurate timing of activities. As a consequence of reduced top-down modulation of behavior, actions are more likely to ‘escape’ control, which would lead to an increased inattentiveness and proneness for errors, which is typical for fatigued people (Van der Linden et al., 2003). Planning and preparation processes not only facilitate information processing, they can lead to costs, as well. If, for example, advance information is not valid, then the activated action has to be terminated and replaced by correct behavior. This process will usually result in relatively slow responses, which are error prone (Kornblum et al., 1990). In addition, if subjects reach a fully prepared state, it is necessary to maintain that state until a response has to be given, this maintenance requires costs in the form of the investment of additional effort (Niemi and Näätänen, 1981). In the previously mentioned study (Lorist et al., 2000) we found evidence that subjects also had more difficulties maintaining the prepared state if they became mentally fatigued. Since mental fatigue can lead to sub-optimal functioning or even human error which can have serious negative consequences in daily life, an important question is what we can do to prevent fatigue-related performance deteriorations. The findings of Lorist et al. (2000) were observed using a task switching paradigm in which no explicit cues were provided about task requirements during task performance. Subjects were informed at the beginning of the task that they had to alternate between tasks on every second trial. As a consequence, task set reconfigurations had to be initiated endogenously on each trial during the 2 h of task performance. It can be argued that fatigue effects might be reduced if more explicit information about subsequent goal-directed behavior will be provided. In the present study we therefore focused on the effects of mental fatigue on planning and goal-directed behavior under conditions in which advance information was presented prior to each stimulus by an explicit cue. In the study of Boksem et al. (2005) explicit cues were used, as well, providing information that could be used to direct attention to the spatial position of the subsequent stimulus. In the present study, we used an S1–S2 paradigm, in which the S1 or cue
provided information about either processing- or response requirements of the S2 stimulus that occurred after a distinct time interval, and signaled to the subject to make a response (Tecce, 1972). The relevance of cue information for subsequent behavior differed between both cue types; the cue either provided information that could be used to guide the action that had to be executed in response to upcoming stimuli (‘hand’ cue), or the cue contained information about specific stimulus features of S2 (color of the target letter; ‘color’ cue), which was less relevant for subsequent behavior. We hypothesized that if indeed mental fatigue has an effect on planning and preparation processes, this effect would be more pronounced in the hand cue condition than in the color cue condition. Behavior measures were complemented with eventrelated potential (ERP) measurements, to examine brain activity elicited by processing of advance information. An ERP component related to planning and preparation processes that are classically induced using the S1–S2 paradigm is the contingent negative variation (CNV; Brunia, 1993; Tecce, 1972; Walter et al., 1964). The CNV develops during the anticipatory interval between S1 and the S2 stimulus, and originates from several generators, which can be differentially activated as a function of task requirements. The early phase of the CNV, for example, was found to be maximal at mid-line frontal electrodes, and comprises an orienting response to the cue stimulus. This CNV component was found to be modulated by time on task and by properties of the cue stimulus (Rohrbaugh et al., 1976; Rohrbaugh and Gaillard, 1983). The phase of the CNV that immediately precedes S2, was found to be maximal at the vertex, and has been assumed to indicate preparation of the response and to reflect activity of the hand-motor areas (Rohrbaugh and Gaillard, 1983), although, this component might contain a non-motor component, as well (e.g., Van Boxtel and Brunia, 1994a, 1994b), reflecting cognitive control mechanisms involved in top-down modulation of more basic cognitive functions. In this study we will investigate whether the effects of mental fatigue are due to less efficient planning and preparation processes as indexed by the CNV. The second issue examined in the present study is related to the observation that fatigued subjects have a problem in the engagement of top-down control, while more automatic processes seem to be relatively unaffected by mental fatigue (e.g., Boksem et al., 2005). To gain further evidence for this, we used S2 stimuli that consisted of a central target letter that was either surrounded by compatible flanker letters or by incompatible flankers. These two stimulus types have been found place dissimilar demands on top-down mechanisms that play a role in conflict solving. More specific, differences between compatible and incompatible stimuli are generally attributed to interference between the outcome of two different processing routes, the so-called “dual-route hypothesis” (Kornblum et al., 1990; Ridderinkhof, 1997). It has been assumed that relevant information is processed via a controlled route, activating a response according to instruction. Via the second, automatic route distracting information activates the second response. In case of incompatible flanker letters, this second response will be different from the response to the central target letter, eliciting a conflict. Controlled processing and processes involved in resolving
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existing conflicts take time, which might explain why subjects in general respond slower on incompatible trials compared to compatible trials. If indeed top-down processes are vulnerable to mental fatigue to a greater extent than automatic processes, they will be executed less efficiently than automatic processes with increasing mental fatigue. Given that stimulus processing relies on both top-down and bottom-up processes, it is hypothesized that the influence of the latter will become more salient with increasing mental fatigue and as a result distracting information might interfere more strongly with time on task. In summary, the purpose of the present study was to investigate the effects of mental fatigue, as induced by time on task, on processes involved in planning and preparation. Therefore, an S1–S2 paradigm was used, in which the relevance of information provided by S1 for subsequent behavior was varied. Moreover, we sought to determine whether mental fatigue has differential effects on top-down mechanisms and more automatic processes involved in conflict solving. Conflict was induced by using S2 stimuli containing incompatible information, requiring controlled suppression of irrelevant information, in addition to S2 stimuli that contained non-conflicting or compatible information.
2.
Results
2.1.
Subjective data
As expected, levels of general activation were lower at the end of task performance (M=9.9, SD=3.4) than levels measured at the start of the experimental session (M=13.8, SD=2.6; F(1,14)=21.25, pb .001). In addition, subjects scored higher on the tiredness subscale of the AD-ACL (F(1,14) =27.88), pb .001) after 2 h of continuous task performance (M=10.3 (SD=2.3) and M=15.6 (SD=3.4), for levels measured at the start and end of the experimental session, respectively). With time on task subjects also reported increased feelings of resistance against continuation of task performance (F(3.5, 41.99)=45.12, pb .001); scores ranged from very little (M=0.8, SD=1.0) at the start of the experimental session to very strong (M=7.9, SD=2.7) at the end of task performance.
2.2.
Behavioral data1
Subjects received a cue, either containing advance information about relevant response characteristics of the impending target or less relevant information about perceptual characteristics of the impending target letter2. Subsequently, subjects received a stimulus, which required a response. Subjects reacted faster on validly cued stimuli than on stimuli in which the cue provided invalid information, even if the cue contained information that was less relevant for subsequent behavior (cue validity: F(1,11) = 25.21, p b .001; validity in the hand cue condition: F(1,11) = 16.54, p = .002; 1
RT data of three subjects contained one or two missing data points and were rejected from part of the statistical analyses. 2 No differences were observed between the different hand cues (F(1,11) = .74, n.s.) or the color cues that were used (F(1,14) = .56, n.s.).
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validity in the color cue condition: F(1,14) = 16.69, p = .001). For the validly cued stimuli, reaction times were faster in the condition in which subjects received behavior-relevant information (413 ms, SD = 91) than in the condition in which the cue provided information about irrelevant target features (473 ms, SD = 73; cue type × cue validity: F(1,11) = 9.42, p = .011; cue type in the valid cue condition: F(1,14) = 18.40, p b .001). For the invalidly cued stimulus condition, RTs were not dependent on cue type (M = 501 ms (SD = 78) and M = 485 ms (SD = 77) for handand color cues, respectively: F(1,11) = .86, n.s.). The observed differences between validly and invalidly cued stimuli became smaller with time on task (time on task × cue validity: F(2.63,28.97) = 5.39, p = .006). This effect was mainly due to an increase in RTs from interval 3 to 4 in valid hand cue trials (see Fig. 1; time on task × cue type × validity: F(2.79, 30.72) = 3.82, p = .022). The proportion of errors was lowest in the valid hand cue condition (M = .06, SD = .04), but trials in this condition which were invalidly cued elicited the highest number of errors (M = .28, SD = .18; see Fig. 1; cue type: F(1,14) = 8.92, p = .010; cue validity: F(1,14) = 17.35, p = .001; cue type × cue validity: F(1,14) = 19.36, p = .001; validity in hand cue condition: F(1,14) = 18.41, p = .001). In the color cue condition the proportion of errors was similar for validly and invalidly cued trials (M = .11, SD = .04; F(1,14) = .02, n.s.). The proportion of errors was not influenced by time on task (F(3,12) = 3.34, n.s.). In approximately 2% (SD = 1.45) of the trials subjects did not provide a response to S2. No significant differences were observed between experimental conditions, concerning these misses. As expected, mean correct RT was shorter (M= 452 ms, SD= 74) and proportion errors was lower (M=.10, SD= .05) for trials in which flanker letters were compatible with relevant stimulus characteristics than for incompatible stimuli (M = 484 ms, SD = 75; (F(1,11) = 36.16, p b .001 and M = .17, SD = .06; F(1,14) = 79.59, p b .001, for RT and proportion of errors, respectively). These compatibility effects were not affected by time on task (time on task × compatibility; F(2.48,27.25) = .5, n.s. and F (2.81,39.39) = 1.89, n.s., for RTs and proportion of errors, respectively) or by cue type (F(1,11) = .82, n.s. and F(1,14) = 4.17, n.s., for RTs and proportion of errors, respectively). Since invalid color cues might prime the irrelevant flankers in the incompatible condition, one might expect that in this condition interference from the flanker letters would be more. pronounced than in the valid cue condition. However, no significant cue validity × compatibility effect was elicited in the color cue condition (F(1,14) = .46, n.s. and F(1,14) = 3.39, n.s., for RT and error data, respectively), indicating that cue validity had a similar effect in compatible and incompatible trials.
2.3.
Erps elicited by the cue (s1)
The cue stimulus elicited an ERP characterized by a pattern of negative and positive deflections, followed by a gradual increasing slow negative wave, which was maximal at frontocentral sites (Fig. 2). Differences between hand and color cues became significant around 200 ms after presentation of the cue. This early difference took the form of a more pronounced P2, with a Cz maximum, in the condition in which hand information was provided, compared to the condition in which color
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to the ERP elicited in the color cue condition (Fig. 2; cue type × electrode, 700–1000 ms latency area: F(1.40–1.50, 19.59– 21.12) = 4.13–8.64, all p's ≤ .044). For both cue conditions the negative shift became smaller with time on task (Fig. 3; time on task, 700–1000 ms area: F(2.31–2.95, 32.36–41.30) = 3.15–4.57, all p's ≤ .047). Contrary to the early time on task effect, this ‘late’ effect of time on task was due to a change in ERP activity from the third to the fourth time on task interval, that is, during the first 1.5 h of task performance subjects showed a relative steady slow negative wave (F(1.38–1.74, 19.34– 24.40) ≤ 1.29, all p's n.s.). In sum, cue-locked ERPs indicated that advance information provided by a cue stimulus, modulated brain activity before an imperative stimulus was presented. These changes
Fig. 1 – Average reaction times (RTs: upper panel) and proportion of errors (lower panel) for the different time on task intervals, superimposed, for the valid cued trials (solid lines) and invalid cued trials (dashed lines) in the hand cue (circles) and color cue (squares) conditions.
information was provided. Subsequently, the ERP remained more positive in the hand cue condition until 700 ms post-cue compared to the color cue condition, especially at the parietal electrode (cue type, 200–700 ms area: F(1,14) = 7.73–47.34, all p's ≤ .015; cue type × electrode, 200–700 ms area: F(1.34–3.00, 18.76–42.00) = 7.91–19.52, all p's ≤ .002). This ‘early’ effect related to cue type disappeared from the first to the second 30-min interval (Figs. 3 and 4 ). For both cue types, the ERPs became less positive between the first and second time on task interval, between 100 and 600 ms post-cue, especially at fronto-central sites (time on task, 100 and 600 ms interval: F(1.78–3.00, 24.86– 42.00) = 4.39–15.86, all p's ≤ .019; time on task × electrode, 200– 600 ms interval: F(2.80–3.76, 39.26–52.61) = 2.95–4.32, all p's ≤ .047). However, this negative shift elicited by time on task was more pronounced and started earlier in the hand cue condition (between 100 and 200 ms after the presentation of the cue) than in the color cue condition (between 200 and 300 ms after the presentation of the cue; cue type × time on task, 100–400 ms interval: F(1.87–3, 26.14–42.00) = 3.41–4.09, all p's ≤ .036). During the last three time on task intervals ERPs elicited by hand- and color cues did not differ in the 100–600 ms interval (F(1.64–2, 23.35–28) ≤ 1.74, all p's n.s.). Between 700 and 1000 ms post-cue a slow negative wave with a fronto-central maximum was elicited, which was, again, more pronounced for the hand cue condition compared
Fig. 2 – Average cue-locked ERP waveforms recorded from Fz, Cz and Pz, superimposed for the hand cue condition (dashed line) and color cue condition (solid line). The ERPs are averaged across time on task intervals.
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Fig. 3 – Average cue-locked ERP waveforms recorded from Fz and Cz for the color (left panel) and hand cue conditions (right panel), superimposed for the four time on task intervals.
in brain activity were more pronounced if advance information was relevant for subsequent goal-directed behavior than if information about task irrelevant target features was provided. Time on task had a two-fold effect, the early modification (between 100–600 ms after presentation of the cue) of the ERP due to cue type disappeared already after 30 min of task performance. The negative shift in the 700– 1000 ms interval lasted until the third time on task interval, after which it disappeared, as well.
2.4.
Erps elicited by s2
Subjects received a cue stimulus, containing advance information that could be used to guide the action required in response to S2 or about the color of the upcoming target letter, which was valid on 80% of the trials; therefore in 20% of the trials the stimulus contained information that was not predicted by the cue. Although information presented by the color cue was less relevant for subsequent behavior, in this condition an early effect of cue validity was found between 100 and 150 ms post-stimulus, reflecting a more negative going N1 for invalidly cued stimuli compared to validly cued stimuli (Fig. 5; cue type × cue validity: F(1,14) = 6.59, p = .022; validity in the color cue condition: F(1,14) = 8.07, p = .013). In the hand cue condition, no effect of cue validity was observed in this latency area (hand cue condition: F(1,14) = 1.46, n.s.). However, in this condition an effect of validity was observed between 400 and 700 ms post-stimulus, which was most pronounced at Pz (Fig. 5). In this latency area, the ERP elicited in the invalid hand cue condition was more positive compared to the ERP observed in the valid condition (cue type × cue validity, 400–
700 ms interval: F(1,14) = 4.71–13.05, all p's ≤ .003; cue type × cue validity × electrode, 500–700 ms interval: F(1.60–2.65, 22.43– 37.10) = 4.37–9.21, all p's ≤ .032). In the color cue condition no differences between validly and invalidly cued stimuli were observed in this later latency area. The effects of validity of advance information on S2-locked brain activity remained similar during the 2 h of continuous task performance. The target letter was surrounded by flanker letters, which should have been ignored by the subject. Nevertheless, effects of flanker identity became visible in the ERP around 150 ms after the presentation of the stimulus (Fig. 6). In the 150– 200 ms area the ERP observed in the incompatible condition was more positive going in comparison with the ERP elicited in the compatible condition. This effect was followed by a more
Fig. 4 – Average cue-locked ERP waveforms recorded from Cz, superimposed for the hand and color cue conditions, for the first and last time on task intervals.
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performance, that is, decreased levels of energy were reported while levels of tiredness increased. Moreover, subjects developed an aversion against continuation of task performance with time on task, an important characteristic of mental fatigue according to Holding (1983) and Hockey (1997).
3.1.
Processing of cue information
The ability to process and integrate advance information is crucial for adequate planning and preparation of behavior. In this study we found that information provided by explicit cues presented on each trial did indeed affect brain activity, however, with increasing mental fatigue the effects of advance information diminished. Moreover, the size of the effects depended on the relevance of the information provided by the cue. First, we observed that hand cueselicited a more pronounced P2 component, which can be regarded as an index of activity engaged in selection of relevant information (Wijers et al.,
Fig. 5 – Average stimulus-locked ERP waveforms recorded from Pz, superimposed for valid and invalid-cue conditions for the hand and color cue conditions separately. The ERPs are averaged across time on task intervals. Inlet: enlargement of 80–150 ms interval.
pronounced negativity for incompatible stimuli starting around 250 ms. The P3 component elicited by incompatible stimuli was found to be delayed compared to the P3 in response to compatible stimuli (compatibility in the 150–200, 300–400 and 450–750 ms latency areas: F(1,14) ≥ 5.63, all p's ≤ .033; compatibility × electrode, 150–400 and 550–750 ms areas: F(1.17–2.37, 16.40–37.35) = 3.89–13.34, all p's ≤ .046). With time on task a significant reduction of the negative deflection in the 100–150 ms area was observed in the color cue condition for compatible trials only (time on task × cue type × compatibility: F(2.88,40.31) = 4.43, p = .010). In summary, S2-locked ERPs showed that validity of advance information had differential effects on brain activity dependent on the relevance of advance information. Color cues modulated early components in the ERP (i.e. N1 area), while hand cues had effects in later intervals (i.e. P3 area). Validity effects were not modulated by time on task. Subjects were not able to ignore irrelevant information, compatibility effects were observed already 150 ms after the presentation of S2.
3.
Discussion
We examined the influence of mental fatigue, as induced by time on task, on top-down control involved in planning goaldirected behavior and conflict resolution. In accordance with previous findings (e.g., Lorist et al., 2000, Boksem et al., 2005) our fatigue manipulation seemed successful; subjects developed feelings of mental fatigue after 2 h of continuous task
Fig. 6 – Averaged stimulus-locked ERPs recorded from Fz, Cz and Pz, superimposed for the compatible (solid lines) and incompatible condition (dashed lines).
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1989; Kenemans et al., 1993; Luck and Hillyard, 1994; O'Donnell et al., 1997). Because the physical characteristics of the hand and color cue stimuli are highly similar in this study, the differential effects due to cue type are less likely to be due to physical characteristics of the cue. It seems plausible that this P2 effect is predominantly based on the relevance of specific information provided by the cue, that is, the enlarged positive shift evoked by hand cues compared to the color cues might be related to modulation of specialized brain areas involved in the preparation for goal-directed action. This interpretation is in line with findings of Slagter et al. (2005); in their study a similar positivity was elicited by cue stimuli providing spatial (location) or non-spatial (color) information. They argued that this P2 effect reflects preparation processes related to associating cue information to the functional properties of this information. The domain-independency of this effect in the study of Slagter et al. supports the importance of relevance of cue information; both location and color information was relevant for subsequent behavior in their study. In the present study, the functional significance of the hand cue was larger for subsequent behavior than the significance of color information, which is reflected in a more pronounced P2 observed in the hand cue condition compared to the color cue condition. The P2 effect was significantly diminished after 30 min of task performance in the present study, and differences related to cue type also disappeared after this period of time, implying that cue information is less efficiently associated with relevant stimulus features with increasing mental fatigue. The second effect elicited by the presentation of advance information concerned a modulation of the CNV observed between 700–1000 ms after the presentation of the cue stimulus. The CNV was most pronounced for the hand cue condition and had a maximum at fronto-central electrode sites. Different processes underlying the CNV have been discerned (e.g., Van Boxtel, 1994; Van Boxtel and Böcker, 2004; Van Boxtel and Brunia, 1994a; Verleger et al., 2000); a negative wave with a central maximum, reflecting preparation for the motor response to be executed after S2, and a frontal component, reflecting cognitive control mechanisms related to implementation of goal representations. In the present study, advance information in the hand cue condition allowed specific preparation of subsequent goal-directed actions, while the color cue provided advance information about irrelevant target features of the upcoming stimulus. The more pronounced negativity in the hand cue condition as compared to the negativity elicited after a color cue was delivered indicated that the observed effect should be attributed, at least partly, to motor related preparatory activation. However, it should be noticed that the negative shift was not restricted to the hand cue condition, in the color cue condition a CNV was observed, as well. The CNV elicited by the color cue stimulus, and probably part of the negativity elicited by the hand cue stimulus, more likely reflect the ‘frontal’ CNV component, indexing cognitive control mechanisms, involved in top-down modulation of more basic cognitive functions involved in future goal-directed behavior. The size of the CNV remained stable during 1.5 h of continuous performance; thereafter the negative wave became significantly smaller for both the hand and color cue
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conditions. The effect of time on task on the CNV seems to reflect a reduction in top-down modulation of cognitive functions with increasing mental fatigue, which is in accordance with previous findings (Lorist et al., 2000; Van der Linden et al., 2003; Boksem et al., 2005) indicating that topdown processes are indeed vulnerable to mental fatigue. What is striking in the observed effects of mental fatigue on cue-related brain activity is the time course of these effects. Cue-related modulations of the P2 disappeared already after half an hour of task performance, while the CNV effect remained significant until the third time on task interval, that is, during 1.5 h of task performance. It seems that with increasing mental fatigue advance information has less impact on human information processing, but more important: some processing mechanisms seem to be more at risk of becoming deteriorated due to mental fatigue than others.
3.2.
Influence of advance information on processing of s2
We found that advance information affected brain activity. However, did the information provided by the cue facilitate processing of S2, or in other words: did subjects use available information? Behavioral results indicated that advance information was indeed processed adequately and facilitated the processing of S2. A significant RT benefit of 88 ms in validly cued trials compared to invalidly cued trials was observed if relevant motor information was delivered in advance. Although information provided by the color cue was less relevant for subsequent goal-directed behavior, a RT benefit of 12 ms was observed in this condition. The behavioral benefits in the hand cue condition were accompanied by relative large costs in case cue information was incorrect; the proportion of incorrect responses was significantly higher after invalid hand cues compared to invalid color cues. The larger proportion of errors in the invalid hand cue condition was accompanied by an enhancement of P3 amplitude. The higher P3 amplitude might be related to the lower event probability of invalidly cued stimulus, but alternatively, it might be a reflection of additional demands placed on the information processing system in the invalid condition, necessary to adjust the incorrectly prepared motor system. Subjects did use relevant advance information to prepare for subsequent goal-directed actions. A major issue in this study concerned the question what happens with top-down control when subjects become mentally fatigued, do they still use advance information to prepare their behavior? In accordance with the subjective measures, the behavioral measures indicated that performance efficiency deteriorated with time on task, indicating subjects indeed became mentally fatigued during prolonged task performance. It is important to note that speed was not replaced by accuracy; the proportion of incorrect responses remained stable during the 2 h of performance. In other words, subjects adopted a uniform accuracy criterion during the experimental session. Changes in performance can therefore not be explained by changes in the speed-accuracy trade-off. In addition to the influence of mental fatigue on subjective and behavioral measures, brain activity elicited by cue information was significantly attenuated with time on task,
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which strongly suggests that cue effectiveness depended on the state of the subject. Comparing the effects of time on task on RT and cue-related brain activity, it becomes clear that the decrease in CNV magnitude paralleled the time course of the RT effects; both effects became most pronounced after 120 min of task performance. It seems that top-down processes indexed by the CNV are the most likely candidate to underlie the performance effects of mental fatigue. No clear match between effects of time on task on the cue-elicited P2 component and performance were observed. The P2 was argued to reflect top-down processes involved in the selection of relevant information. It can be argued that due to practice information selection becomes more automatic, and task demands will decrease with time on task. P2 amplitude has indeed been related to cognitive processing demands (Freunberger et al., 2007). However, if these effects reflect learning one might expect beneficial effects on performance, which were absent in the present study. The reduction in P2 amplitude after the first half hour, in the absence of clear behavioral effects in this time on task interval remains puzzling. The effects of time on task were most pronounced in the hand cue condition, that is, in the condition which benefited most from advance information during the first part of the experimental session. In the color cue condition, advance information cannot be used to prepare a specific response; however, cue information might be helpful in identifying the target letter by tuning the perceptual system. The more negative going N1 component for invalidly cued stimuli compared to validly cued stimuli in the color cue condition seems to be in agreement with this interpretation; attention directed incorrectly toward specific stimulus characteristics by the invalid cue had to be re-directed to relevant stimulus features producing the enhanced N1 component (Luck et al., 1994; Wright et al., 1995). In summary, the present findings support the interpretation of cue-related brain activity in terms of preparation processes, facilitating goal-directed behavior. With increasing mental fatigue, however, the influence of relevant advance information became less evident. Subjects do not seem to use available information anymore, resulting in less efficient behavior.
3.3.
Compatibility
Subjects performed a conflict task, in which irrelevant information was presented together with relevant information. Although subjects were instructed to respond to the centrally located target letter and to ignore adjacent irrelevant information, RTs were found to be faster in the compatible condition than in the incompatible condition in which irrelevant information contained features associated with the incorrect response. The faster reactions to compatible stimuli were accompanied by fewer errors, indicating that the increase in speed was not due to a decrease in accuracy. ERP modulations related to the compatibility manipulation took the form of an enhanced P2 component in the incompatible condition compared to compatible condition, which was most pronounced at frontal sites. The P2 has been found to reflect filtering mechanisms engaged in identification of relevant information (Wijers et al., 1989; Kenemans et al.,
1993; Luck and Hillyard, 1994; O'Donnell et al., 1997). The P2 enlargement observed in incompatible stimuli compared to compatible stimuli might reflect this additional selection activity engaged more strongly in incompatible task condition, in which a correct localization of the target letter is of crucial importance in order to provide a correct response. In addition, in incompatible trials an enhancement of the fronto-central N2 component was observed. The N2 component has been implicated in the detection of response conflict, that is, the amplitude is increased on high-conflict trials (Kopp et al., 1996; Bekker et al., 2004; Yeung and Cohen, 2006), which conforms with the findings in the present study. The P3 component was found to be dependent on the identity of flanker letters; if flanker letters were incompatible with the target letter the P3 was delayed compared to the compatible condition. P3 latency was found previously to increase with difficulty of discrimination and identification of relevant stimulus features (Donchin and Coles, 1988), which seems to confirm that resolving the conflict between incompatible target and flanker information in the present study indeed takes time, producing a delay in stimulus processing. This interpretation is in line with the stimulus evaluation view of the P3. Alternatively, Verleger (1997) reviewed evidence that in a flanker task a delay in P3 latency might be affected by response selection, as well. In the incompatible task condition different response tendencies are activated, causing response competition which might be reflected in a delay in P3 latency. Although it has been reported that the effects of time on task are most pronounced in tasks placing high demands on top-down processing, we did not observe a differential effect of time on task on compatible and incompatible trials, supposed to place dissimilar demands on control mechanisms playing a role in conflict solving. These findings are in agreement with those of Boksem et al. (2005), who did not observe effects of time on task in a modulated Simon task, in which response conflict was manipulated by presenting stimuli either at the spatially corresponding side to the response side or at the opposite side. An important issue in the study of mental fatigue concerns the relationship between fatigue and factors such as motivation and boredom. It has, for example, been argued that the effects of mental fatigue are, at least in part, due to a lack of motivation (Chaudhuri and Behan, 2000). Boksem et al. (2006) indeed showed that fatigued subjects who were motivated could once again control their actions adequately. However, mentally fatigued, but motivated subjects were unable to improve performance in terms of both speed and accuracy; instead they were found to improve on one measure by sacrificing the other. This implies that fatigue is more than an effort/reward imbalance influencing motivation; it involves adaptive strategies to keep performance at an acceptable level under adverse internal circumstances. This finding is consistent with Grier et al. (2003) who found that performance deterioration in vigilance tasks were not related to mindlessness or underarousal, but these deteriorations were better characterized by limitations in ‘effortful attention’. In conclusion, the results of this study indicated that the availability of relevant advance information modulated processing of S2. Moreover, performance was faster and more accurate if this information was valid, while invalid
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information led to costs in the form of increased error rates. Planning and preparation processes, reflected in behavioral measures and brain activity, were clearly diminished by mental fatigue, induced by time on task. No evidence was found that controlled and automatic processes involved in conflict solving were differentially affected by mental fatigue, that is, no evidence was found for stronger interference of distracting information with time on task.
subscales: energy (general activation), tiredness (deactivationsleep), tension (high activation) and calmness (general deactivation). During task performance, subjects indicated the amount of resistance they experienced to continue task performance on a simple rating scale, employing verbal statements as anchors. Scores varied from 0 (not at all) to more than 10 (maximal).
4.4.
4.
Experimental procedures
4.1.
Participants
Fifteen right-handed healthy young women (mean age = 21.1 (SD = 1.8)) participated in this experiment. All reported to be non-smokers, having normal sleep patterns, and neither did work night shifts nor used prescription medication. They all had normal or corrected-to-normal vision. Written informed consent was collected prior to the study. Subjects received monetary payment for their participation.
4.2.
Stimuli
Stimuli were presented in the centre of the visual field on a color monitor positioned at 80 cm from the subject's eyes. On each trial, the participants were presented with a row of three uppercase letters, the central letter was the target letter and the remaining letters were the flanker letters. The participants were instructed to make a left-hand response if the central letter was an H and a right-hand response if the central letter was an S. The letters remained on the screen until a response was made or in case no response was made the letters disappeared after 1200 ms. The letters were presented in red or in green on a dark background. On half of the trials the flanker letters had the same identity (HHH or SSS; compatible) and color as the target letter. In the other half of the trials flankers had a different identity (SHS or HSH; incompatible) and color than the target letter. The different stimulus categories were presented, with equal probability, in random order. A cue, appearing 1000 ms before the letters, was presented for 150 ms, providing advance information either about the response hand (hand cue; 50%; the word ‘links’ (left) or ‘rechts’ (right)) or about the color of the target letter (color cue; 50%; the word ‘rood’ (red) or ‘groen’ (green),). To prevent a fast guess strategy, cues were valid on 80% of the trials. During a trial a fixation mark remained visible on the screen (an asterisk of 0.5 × 0.5 cm). Stimuli were presented 0.5 cm above this fixation mark and a single letter subtended 0.18° by 0.18° of visual angle. Response-cue interval jittered with a range of 900 to 1100 ms.
4.3.
Subjective measurements
The Activation–Deactivation Adjective Check List (AD-ACL; Thayer, 1989) was used for assessments of momentary activation states. This self-rating questionnaire includes 20 activation descriptive adjectives, each of which is rated on a 4-point scale according to how well the adjective describes one's immediate feelings. The adjectives are grouped into four
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Procedure
The experimental sessions started around 1.00 p.m. and lasted 3 h. After the subject arrived at the laboratory the EEG electrodes were applied and the procedure was explained, without giving specific information about the duration of the experimental task. Thereafter, the subject was seated in a dimly illuminated, sound-attenuated, and electrically shielded room. The AD-ACL was completed and a practice block of 80 trials was performed. The subject was instructed to respond as quickly as possible, maintaining a high level of accuracy, and to minimize eye movements and blinking during task performance. After the practice trials, the subject performed the 2-hour experimental block. This block started with a question about the level of resistance the subject felt at that moment against performing the task. This question appeared on the screen every 30 min. After task performance the AD-ACL was filled out for the second time and the subjects were paid.
4.5.
Eeg recordings
EEG was recorded, using Sn electrodes attached to an electrode cap (ElectroCap International), positioned according to the standard 10–10 system (American Electroencephalographic Society, 1994). The electrodes were referenced to electronically linked earlobes. The electro-oculogram (EOG) was recorded bipolarily with Sn electrodes from the outer canthi of both eyes and above and below the left eye. The Ag/AgCl electrode for earthing the subject was placed on the sternum. Electrode impedances were reduced to less than 5 kΩ. The signals were amplified with a time constant of 10 s, low pass filtered at 30 Hz and digitized at a rate of 100 Hz.
4.6.
Data reduction and statistical analyses
Mean reaction times (RTs) were calculated for correct trials. Trials containing premature motor responses (b50 ms) were excluded from analysis. Mean error rate was quantified as the proportion of incorrect responses within each condition, and trials in which no response was made were regarded as misses. For ERP analyses trials with incorrect responses or trials containing amplifier saturation artifacts were excluded from analysis. Ocular correction was performed using the Gratton and Coles method (Gratton et al., 1983). To evaluate the effect of advance information on brain activity, average cue-locked ERPs were computed separately for each mid-line electrode position (Fz, Cz, Pz and Oz) for the different conditions, that is, cue type (hand/color) and time on task interval (4 intervals of 30 min). The averaging epoch started 100 ms prior to cue
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onset and lasted until 1000 ms post-cue. All averages were aligned to a 100 ms pre-cue baseline. For further analysis mean amplitudes were calculated in 10 periods of 100 ms each, from 0 to 1000 ms post-cue. Average S2-locked ERPs were computed separately for each electrode position for the correct trials in each condition (i.e. cue type, time on task interval, cue identity (either left/ right or red/green for hand and color cues, respectively), cue validity (valid/invalid), compatibility (compatible/incompatible), and target letter (H/S)). The averaging epoch started 100 ms prior to stimulus onset and lasted until 1000 ms poststimulus. The averages were aligned to a 100 ms prestimulus baseline. For further analysis mean amplitudes for Fz, Cz, Pz and Oz electrodes were calculated in 15 periods of 50 ms each, from 50 to 800 ms post-stimulus. Behavioral data and mean ERP amplitudes were entered as dependent variables to SPSS. The within-subject factors consisted of time on task (4 intervals of 30 min), cue validity (valid/invalid), cue type (hand/color) and compatibility (compatible/incompatible) for the behavioral analysis, while electrode (Fz, Cz, Pz and Oz) was added as additional factor to the ERP analysis. For the statistical analysis of the data the univariate approach for repeated measures was used, using the ɛ∼⁎-adjustment procedure recommended by Quintana and Maxwell (1994). When the main analysis indicated a significant interaction (α ≤ .05) between factors, follow-up analyses were performed, using Bonferroni adjustments.
Acknowledgments Thanks are due to Joop Clots and Jan Smit for their technical support. Thanks to Berry Weijers and to the anonymous reviewers for their valuable comments on previous versions of the manuscript. REFERENCES
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Freunberger, R., Klimesch, W., Doppelmayr, M., Höller, Y., 2007. Visual P2 component is related to theta phase-locking. Neurosci. Lett. 426, 181–186. Gratton, G., Coles, M.G.H., Donchin, E., 1983. A new method for off-line removal of ocular artefact. Electroencephalogr. Clin. Neurophysiol. 55, 468–484. Grier, R.A., Warm, J.S., Dember, W.N., Matthews, G., Galinski, T.L., 2003. The vigilance decrement reflects limitations in effortful attention, not mindlessness. Hum. Factors 45, 349–359. Hockey, G.R.J., 1997. Compensatory control in the regulation of human performance under stress and high workload: a cognitive energetical framework. Biol. Psychol. 45, 73–93. Holding, D., 1983. Fatigue. In: Hockey, G.R.J. (Ed.), Stress and Fatigue in Human Performance. John Wiley and Sons, Durnham, pp. 145–164. Kenemans, J.L., Kok, A., Smulders, F.T.Y., 1993. Event-related potentials to conjunctions of spatial frequency and orientation as a function of stimulus parameters and response requirements. Electroencephalogr. Clin. Neurophysiol. 88, 51–63. Kopp, B., Rist, F., Mattler, U., 1996. N200 in the flanker task as a neurobehavioral tool for investigating executive control. Psychophysiology 33, 282–294. Kornblum, S., Hasbroucq, T., Osman, A., 1990. Dimensional overlap: cognitive basis for stimulus-compatibility. A model and taxonomy. Psychol. Rev. 97, 253–270. Lorist, M.M., Klein, M., Nieuwenhuis, S., De Jong, R., Mulder, G., Meijman, T.F., 2000. Mental fatigue and task control: planning and preparation. Psychophysiology 37, 614–625. Luck, S.J., Hillyard, S.A., 1994. Electrophysiological correlates of feature analysis during visual search. Psychophysiology 31, 291–308. Luck, S.J., Hillyard, S.A., Mouloua, M., Woldorff, M.G., Clark, V.P., Hawkins, H.L., 1994. Effects of spatial cuing on luminance detectability: psychophysiological and electrophysiological evidence for early selection. Exp. Psychol. Hum. Percept. Perform. 20, 887–904. Niemi, P., Näätänen, R., 1981. Foreperiod and simple reaction time. Psychol. Bull. 89, 133–162. O'Donnell, B.F., Swearer, J.M., Smith, L.T., Hokama, H., McCarley, R.W., 1997. A topographic study of ERPs elicited by visual feature discrimination. Brain Topogr. 10, 133–143. Quintana, S.M., Maxwell, S.E., 1994. A Monte Carlo comparison of seven e-adjustment procedures in repeated measures designs with small sample sizes. J. Educ. Stat. 19, 57–71. Ridderinkhof, K.R., 1997. A dual-route processing architecture for stimulus-response correspondence effects. In: Hommel, B., Prinz, W. (Eds.), Theoretical Issues in Stimulus-Response Compatibility. Elsevier Science, Amsterdam, pp. 119–131. Rohrbaugh, J.W., Gaillard, A.W.K., 1983. Sensory and motor aspects of the CNV. In: Gaillard, A.W.K., Ritter, W. (Eds.), Tutorials in ERP Research: Endogenous Components. North Holland, Amsterdam, pp. 269–310. Rohrbaugh, J.W., Syndulko, K., Lindsley, D.B., 1976. Brain wave components of the contingent negative variation in humans. Science 191, 1055–1057. Slagter, H.A., Kok, A., Mol, N., Kenemans, J.L., 2005. Spatio-temporal dynamics of top-down control: directing attention to location and/or color as revealed by ERPs and source modeling. Cognitive Brain Res. 22, 333–348. Tecce, J.J., 1972. Contingent negative variation (CNV) and psychological processes in man. Psychol. Bull. 77, 73–108. Thayer, R.E., 1989. The Biopsychology of Mood and Arousal. Oxford University Press, New York. Van Boxtel, G.J.M., 1994. Non-motor components of slow brain potentials. Doctoral dissertation, Tilburg University, Tilburg, The Netherlands. Van Boxtel, G.J., Brunia, C.H., 1994a. Motor and non-motor aspects of slow brain potentials. Biol. Psychol. 38, 37–51.
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a v a i l a b l e a t w w w. s c i e n c e d i r e c t . c o m
w w w. e l s e v i e r. c o m / l o c a t e / b r a i n r e s
Research Report
Impact of top-down control during mental fatigue Monicque M. Lorist⁎ Department of Experimental and Work Psychology, University of Groningen, The Netherlands BCN-NeuroImaging Center, University Medical Center Groningen, University of Groningen, The Netherlands
A R T I C LE I N FO
AB S T R A C T
Article history:
The influence of mental fatigue, as induced by time on task, on top-down control involved in
Accepted 12 July 2008
planning goal-directed behavior and conflict resolution was examined, using an S1–S2
Available online 25 July 2008
paradigm. S2 stimuli consisted of compatible and incompatible stimuli, placing dissimilar demands on automatic and controlled processes involved in conflict solving. Information
Keywords:
provided by explicit cues (S1) affected brain activity elicited during the S1–S2 interval. P2 and
Mental fatigue
CNV effects were more pronounced if advance information was relevant for subsequent
Top-down control
behavior (hand cue) than after a cue providing information about stimulus features of S2
Planning
(color cue). Brain activity elicited by cue information was significantly attenuated with time
Event-related potential
on task. The behavioral results showed that advance information facilitated processing of
Behavior
S2; reactions were faster if cue information was valid, especially in the hand cue condition. In this condition invalid information led to significant costs in the form of increased error rates, as well. Performance efficiency deteriorated with time on task and differences between validly and invalidly cued stimuli became smaller. Concerning the time course of the behavioral effect it seems that top-down processes indexed by the CNV are the most likely candidate to underlie the performance effects of mental fatigue. Time on task effects on the cue-P2 were unrelated to observed behavioral effects. These results showed that the influence of advance information on information processing diminished with increasing mental fatigue. No evidence was found that mental fatigue had differential effects on controlled and automatic processes involved in conflict solving. © 2008 Elsevier B.V. All rights reserved.
1.
Introduction
In order to facilitate goals to be reached in the near future, it is crucial that humans plan and prepare their behavior. These planning and preparation processes strongly rely on the availability of information from our ever-changing external and internal environment. Human observers are, for example, better at detecting relevant information when they know in advance something about specific features of that informa-
tion. Moreover, if relevant advance information is provided, responses can be made faster and more accurately. Of course, facilitation of behavioral goals depends crucially on our ability to process available information and to use it to bias the processing of information (Corbetta and Shulman, 2002). Therefore, it is important to realize that although information can be provided, people might not always be able to use this information to select and prepare appropriate goal-directed actions, even if task demands indicate a need for
⁎ Department of Experimental and Work Psychology, University of Groningen, Grote Kruisstraat 2/1, 9712 TS Groningen, The Netherlands. Fax: +31 50 363 6304. E-mail address: [email protected]. 0006-8993/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.brainres.2008.07.053
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increased cognitive control to achieve specific goals. ‘What can humans do with knowledge’ is an important question; however, ‘What do they actually do with available knowledge’ seems to be an even more challenging question that will be addressed in the present study. In previous studies we observed that available information was no longer used effectively to reconfigure a task set in advance if subjects became mentally fatigued (Lorist et al., 2000; Boksem et al., 2005). Mental fatigue refers to changes in the psycho-physiological state that people experience during and following the course of prolonged periods of demanding cognitive activity requiring sustained mental efficiency. It is generally associated with increased difficulties in maintaining adequate levels of performance efficiency. Mental fatigue was induced by 2 h of continuous task performance. Effects of mental fatigue observed by Lorist et al. (2000) and Boksem et al. (2005) on preparation processes are in agreement with findings of Bartlett, who, already in 1943, noticed that among the operations most vulnerable to mental fatigue are topdown processes involved in the co-ordination and the accurate timing of activities. As a consequence of reduced top-down modulation of behavior, actions are more likely to ‘escape’ control, which would lead to an increased inattentiveness and proneness for errors, which is typical for fatigued people (Van der Linden et al., 2003). Planning and preparation processes not only facilitate information processing, they can lead to costs, as well. If, for example, advance information is not valid, then the activated action has to be terminated and replaced by correct behavior. This process will usually result in relatively slow responses, which are error prone (Kornblum et al., 1990). In addition, if subjects reach a fully prepared state, it is necessary to maintain that state until a response has to be given, this maintenance requires costs in the form of the investment of additional effort (Niemi and Näätänen, 1981). In the previously mentioned study (Lorist et al., 2000) we found evidence that subjects also had more difficulties maintaining the prepared state if they became mentally fatigued. Since mental fatigue can lead to sub-optimal functioning or even human error which can have serious negative consequences in daily life, an important question is what we can do to prevent fatigue-related performance deteriorations. The findings of Lorist et al. (2000) were observed using a task switching paradigm in which no explicit cues were provided about task requirements during task performance. Subjects were informed at the beginning of the task that they had to alternate between tasks on every second trial. As a consequence, task set reconfigurations had to be initiated endogenously on each trial during the 2 h of task performance. It can be argued that fatigue effects might be reduced if more explicit information about subsequent goal-directed behavior will be provided. In the present study we therefore focused on the effects of mental fatigue on planning and goal-directed behavior under conditions in which advance information was presented prior to each stimulus by an explicit cue. In the study of Boksem et al. (2005) explicit cues were used, as well, providing information that could be used to direct attention to the spatial position of the subsequent stimulus. In the present study, we used an S1–S2 paradigm, in which the S1 or cue
provided information about either processing- or response requirements of the S2 stimulus that occurred after a distinct time interval, and signaled to the subject to make a response (Tecce, 1972). The relevance of cue information for subsequent behavior differed between both cue types; the cue either provided information that could be used to guide the action that had to be executed in response to upcoming stimuli (‘hand’ cue), or the cue contained information about specific stimulus features of S2 (color of the target letter; ‘color’ cue), which was less relevant for subsequent behavior. We hypothesized that if indeed mental fatigue has an effect on planning and preparation processes, this effect would be more pronounced in the hand cue condition than in the color cue condition. Behavior measures were complemented with eventrelated potential (ERP) measurements, to examine brain activity elicited by processing of advance information. An ERP component related to planning and preparation processes that are classically induced using the S1–S2 paradigm is the contingent negative variation (CNV; Brunia, 1993; Tecce, 1972; Walter et al., 1964). The CNV develops during the anticipatory interval between S1 and the S2 stimulus, and originates from several generators, which can be differentially activated as a function of task requirements. The early phase of the CNV, for example, was found to be maximal at mid-line frontal electrodes, and comprises an orienting response to the cue stimulus. This CNV component was found to be modulated by time on task and by properties of the cue stimulus (Rohrbaugh et al., 1976; Rohrbaugh and Gaillard, 1983). The phase of the CNV that immediately precedes S2, was found to be maximal at the vertex, and has been assumed to indicate preparation of the response and to reflect activity of the hand-motor areas (Rohrbaugh and Gaillard, 1983), although, this component might contain a non-motor component, as well (e.g., Van Boxtel and Brunia, 1994a, 1994b), reflecting cognitive control mechanisms involved in top-down modulation of more basic cognitive functions. In this study we will investigate whether the effects of mental fatigue are due to less efficient planning and preparation processes as indexed by the CNV. The second issue examined in the present study is related to the observation that fatigued subjects have a problem in the engagement of top-down control, while more automatic processes seem to be relatively unaffected by mental fatigue (e.g., Boksem et al., 2005). To gain further evidence for this, we used S2 stimuli that consisted of a central target letter that was either surrounded by compatible flanker letters or by incompatible flankers. These two stimulus types have been found place dissimilar demands on top-down mechanisms that play a role in conflict solving. More specific, differences between compatible and incompatible stimuli are generally attributed to interference between the outcome of two different processing routes, the so-called “dual-route hypothesis” (Kornblum et al., 1990; Ridderinkhof, 1997). It has been assumed that relevant information is processed via a controlled route, activating a response according to instruction. Via the second, automatic route distracting information activates the second response. In case of incompatible flanker letters, this second response will be different from the response to the central target letter, eliciting a conflict. Controlled processing and processes involved in resolving
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existing conflicts take time, which might explain why subjects in general respond slower on incompatible trials compared to compatible trials. If indeed top-down processes are vulnerable to mental fatigue to a greater extent than automatic processes, they will be executed less efficiently than automatic processes with increasing mental fatigue. Given that stimulus processing relies on both top-down and bottom-up processes, it is hypothesized that the influence of the latter will become more salient with increasing mental fatigue and as a result distracting information might interfere more strongly with time on task. In summary, the purpose of the present study was to investigate the effects of mental fatigue, as induced by time on task, on processes involved in planning and preparation. Therefore, an S1–S2 paradigm was used, in which the relevance of information provided by S1 for subsequent behavior was varied. Moreover, we sought to determine whether mental fatigue has differential effects on top-down mechanisms and more automatic processes involved in conflict solving. Conflict was induced by using S2 stimuli containing incompatible information, requiring controlled suppression of irrelevant information, in addition to S2 stimuli that contained non-conflicting or compatible information.
2.
Results
2.1.
Subjective data
As expected, levels of general activation were lower at the end of task performance (M=9.9, SD=3.4) than levels measured at the start of the experimental session (M=13.8, SD=2.6; F(1,14)=21.25, pb .001). In addition, subjects scored higher on the tiredness subscale of the AD-ACL (F(1,14) =27.88), pb .001) after 2 h of continuous task performance (M=10.3 (SD=2.3) and M=15.6 (SD=3.4), for levels measured at the start and end of the experimental session, respectively). With time on task subjects also reported increased feelings of resistance against continuation of task performance (F(3.5, 41.99)=45.12, pb .001); scores ranged from very little (M=0.8, SD=1.0) at the start of the experimental session to very strong (M=7.9, SD=2.7) at the end of task performance.
2.2.
Behavioral data1
Subjects received a cue, either containing advance information about relevant response characteristics of the impending target or less relevant information about perceptual characteristics of the impending target letter2. Subsequently, subjects received a stimulus, which required a response. Subjects reacted faster on validly cued stimuli than on stimuli in which the cue provided invalid information, even if the cue contained information that was less relevant for subsequent behavior (cue validity: F(1,11) = 25.21, p b .001; validity in the hand cue condition: F(1,11) = 16.54, p = .002; 1
RT data of three subjects contained one or two missing data points and were rejected from part of the statistical analyses. 2 No differences were observed between the different hand cues (F(1,11) = .74, n.s.) or the color cues that were used (F(1,14) = .56, n.s.).
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validity in the color cue condition: F(1,14) = 16.69, p = .001). For the validly cued stimuli, reaction times were faster in the condition in which subjects received behavior-relevant information (413 ms, SD = 91) than in the condition in which the cue provided information about irrelevant target features (473 ms, SD = 73; cue type × cue validity: F(1,11) = 9.42, p = .011; cue type in the valid cue condition: F(1,14) = 18.40, p b .001). For the invalidly cued stimulus condition, RTs were not dependent on cue type (M = 501 ms (SD = 78) and M = 485 ms (SD = 77) for handand color cues, respectively: F(1,11) = .86, n.s.). The observed differences between validly and invalidly cued stimuli became smaller with time on task (time on task × cue validity: F(2.63,28.97) = 5.39, p = .006). This effect was mainly due to an increase in RTs from interval 3 to 4 in valid hand cue trials (see Fig. 1; time on task × cue type × validity: F(2.79, 30.72) = 3.82, p = .022). The proportion of errors was lowest in the valid hand cue condition (M = .06, SD = .04), but trials in this condition which were invalidly cued elicited the highest number of errors (M = .28, SD = .18; see Fig. 1; cue type: F(1,14) = 8.92, p = .010; cue validity: F(1,14) = 17.35, p = .001; cue type × cue validity: F(1,14) = 19.36, p = .001; validity in hand cue condition: F(1,14) = 18.41, p = .001). In the color cue condition the proportion of errors was similar for validly and invalidly cued trials (M = .11, SD = .04; F(1,14) = .02, n.s.). The proportion of errors was not influenced by time on task (F(3,12) = 3.34, n.s.). In approximately 2% (SD = 1.45) of the trials subjects did not provide a response to S2. No significant differences were observed between experimental conditions, concerning these misses. As expected, mean correct RT was shorter (M= 452 ms, SD= 74) and proportion errors was lower (M=.10, SD= .05) for trials in which flanker letters were compatible with relevant stimulus characteristics than for incompatible stimuli (M = 484 ms, SD = 75; (F(1,11) = 36.16, p b .001 and M = .17, SD = .06; F(1,14) = 79.59, p b .001, for RT and proportion of errors, respectively). These compatibility effects were not affected by time on task (time on task × compatibility; F(2.48,27.25) = .5, n.s. and F (2.81,39.39) = 1.89, n.s., for RTs and proportion of errors, respectively) or by cue type (F(1,11) = .82, n.s. and F(1,14) = 4.17, n.s., for RTs and proportion of errors, respectively). Since invalid color cues might prime the irrelevant flankers in the incompatible condition, one might expect that in this condition interference from the flanker letters would be more. pronounced than in the valid cue condition. However, no significant cue validity × compatibility effect was elicited in the color cue condition (F(1,14) = .46, n.s. and F(1,14) = 3.39, n.s., for RT and error data, respectively), indicating that cue validity had a similar effect in compatible and incompatible trials.
2.3.
Erps elicited by the cue (s1)
The cue stimulus elicited an ERP characterized by a pattern of negative and positive deflections, followed by a gradual increasing slow negative wave, which was maximal at frontocentral sites (Fig. 2). Differences between hand and color cues became significant around 200 ms after presentation of the cue. This early difference took the form of a more pronounced P2, with a Cz maximum, in the condition in which hand information was provided, compared to the condition in which color
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to the ERP elicited in the color cue condition (Fig. 2; cue type × electrode, 700–1000 ms latency area: F(1.40–1.50, 19.59– 21.12) = 4.13–8.64, all p's ≤ .044). For both cue conditions the negative shift became smaller with time on task (Fig. 3; time on task, 700–1000 ms area: F(2.31–2.95, 32.36–41.30) = 3.15–4.57, all p's ≤ .047). Contrary to the early time on task effect, this ‘late’ effect of time on task was due to a change in ERP activity from the third to the fourth time on task interval, that is, during the first 1.5 h of task performance subjects showed a relative steady slow negative wave (F(1.38–1.74, 19.34– 24.40) ≤ 1.29, all p's n.s.). In sum, cue-locked ERPs indicated that advance information provided by a cue stimulus, modulated brain activity before an imperative stimulus was presented. These changes
Fig. 1 – Average reaction times (RTs: upper panel) and proportion of errors (lower panel) for the different time on task intervals, superimposed, for the valid cued trials (solid lines) and invalid cued trials (dashed lines) in the hand cue (circles) and color cue (squares) conditions.
information was provided. Subsequently, the ERP remained more positive in the hand cue condition until 700 ms post-cue compared to the color cue condition, especially at the parietal electrode (cue type, 200–700 ms area: F(1,14) = 7.73–47.34, all p's ≤ .015; cue type × electrode, 200–700 ms area: F(1.34–3.00, 18.76–42.00) = 7.91–19.52, all p's ≤ .002). This ‘early’ effect related to cue type disappeared from the first to the second 30-min interval (Figs. 3 and 4 ). For both cue types, the ERPs became less positive between the first and second time on task interval, between 100 and 600 ms post-cue, especially at fronto-central sites (time on task, 100 and 600 ms interval: F(1.78–3.00, 24.86– 42.00) = 4.39–15.86, all p's ≤ .019; time on task × electrode, 200– 600 ms interval: F(2.80–3.76, 39.26–52.61) = 2.95–4.32, all p's ≤ .047). However, this negative shift elicited by time on task was more pronounced and started earlier in the hand cue condition (between 100 and 200 ms after the presentation of the cue) than in the color cue condition (between 200 and 300 ms after the presentation of the cue; cue type × time on task, 100–400 ms interval: F(1.87–3, 26.14–42.00) = 3.41–4.09, all p's ≤ .036). During the last three time on task intervals ERPs elicited by hand- and color cues did not differ in the 100–600 ms interval (F(1.64–2, 23.35–28) ≤ 1.74, all p's n.s.). Between 700 and 1000 ms post-cue a slow negative wave with a fronto-central maximum was elicited, which was, again, more pronounced for the hand cue condition compared
Fig. 2 – Average cue-locked ERP waveforms recorded from Fz, Cz and Pz, superimposed for the hand cue condition (dashed line) and color cue condition (solid line). The ERPs are averaged across time on task intervals.
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Fig. 3 – Average cue-locked ERP waveforms recorded from Fz and Cz for the color (left panel) and hand cue conditions (right panel), superimposed for the four time on task intervals.
in brain activity were more pronounced if advance information was relevant for subsequent goal-directed behavior than if information about task irrelevant target features was provided. Time on task had a two-fold effect, the early modification (between 100–600 ms after presentation of the cue) of the ERP due to cue type disappeared already after 30 min of task performance. The negative shift in the 700– 1000 ms interval lasted until the third time on task interval, after which it disappeared, as well.
2.4.
Erps elicited by s2
Subjects received a cue stimulus, containing advance information that could be used to guide the action required in response to S2 or about the color of the upcoming target letter, which was valid on 80% of the trials; therefore in 20% of the trials the stimulus contained information that was not predicted by the cue. Although information presented by the color cue was less relevant for subsequent behavior, in this condition an early effect of cue validity was found between 100 and 150 ms post-stimulus, reflecting a more negative going N1 for invalidly cued stimuli compared to validly cued stimuli (Fig. 5; cue type × cue validity: F(1,14) = 6.59, p = .022; validity in the color cue condition: F(1,14) = 8.07, p = .013). In the hand cue condition, no effect of cue validity was observed in this latency area (hand cue condition: F(1,14) = 1.46, n.s.). However, in this condition an effect of validity was observed between 400 and 700 ms post-stimulus, which was most pronounced at Pz (Fig. 5). In this latency area, the ERP elicited in the invalid hand cue condition was more positive compared to the ERP observed in the valid condition (cue type × cue validity, 400–
700 ms interval: F(1,14) = 4.71–13.05, all p's ≤ .003; cue type × cue validity × electrode, 500–700 ms interval: F(1.60–2.65, 22.43– 37.10) = 4.37–9.21, all p's ≤ .032). In the color cue condition no differences between validly and invalidly cued stimuli were observed in this later latency area. The effects of validity of advance information on S2-locked brain activity remained similar during the 2 h of continuous task performance. The target letter was surrounded by flanker letters, which should have been ignored by the subject. Nevertheless, effects of flanker identity became visible in the ERP around 150 ms after the presentation of the stimulus (Fig. 6). In the 150– 200 ms area the ERP observed in the incompatible condition was more positive going in comparison with the ERP elicited in the compatible condition. This effect was followed by a more
Fig. 4 – Average cue-locked ERP waveforms recorded from Cz, superimposed for the hand and color cue conditions, for the first and last time on task intervals.
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performance, that is, decreased levels of energy were reported while levels of tiredness increased. Moreover, subjects developed an aversion against continuation of task performance with time on task, an important characteristic of mental fatigue according to Holding (1983) and Hockey (1997).
3.1.
Processing of cue information
The ability to process and integrate advance information is crucial for adequate planning and preparation of behavior. In this study we found that information provided by explicit cues presented on each trial did indeed affect brain activity, however, with increasing mental fatigue the effects of advance information diminished. Moreover, the size of the effects depended on the relevance of the information provided by the cue. First, we observed that hand cueselicited a more pronounced P2 component, which can be regarded as an index of activity engaged in selection of relevant information (Wijers et al.,
Fig. 5 – Average stimulus-locked ERP waveforms recorded from Pz, superimposed for valid and invalid-cue conditions for the hand and color cue conditions separately. The ERPs are averaged across time on task intervals. Inlet: enlargement of 80–150 ms interval.
pronounced negativity for incompatible stimuli starting around 250 ms. The P3 component elicited by incompatible stimuli was found to be delayed compared to the P3 in response to compatible stimuli (compatibility in the 150–200, 300–400 and 450–750 ms latency areas: F(1,14) ≥ 5.63, all p's ≤ .033; compatibility × electrode, 150–400 and 550–750 ms areas: F(1.17–2.37, 16.40–37.35) = 3.89–13.34, all p's ≤ .046). With time on task a significant reduction of the negative deflection in the 100–150 ms area was observed in the color cue condition for compatible trials only (time on task × cue type × compatibility: F(2.88,40.31) = 4.43, p = .010). In summary, S2-locked ERPs showed that validity of advance information had differential effects on brain activity dependent on the relevance of advance information. Color cues modulated early components in the ERP (i.e. N1 area), while hand cues had effects in later intervals (i.e. P3 area). Validity effects were not modulated by time on task. Subjects were not able to ignore irrelevant information, compatibility effects were observed already 150 ms after the presentation of S2.
3.
Discussion
We examined the influence of mental fatigue, as induced by time on task, on top-down control involved in planning goaldirected behavior and conflict resolution. In accordance with previous findings (e.g., Lorist et al., 2000, Boksem et al., 2005) our fatigue manipulation seemed successful; subjects developed feelings of mental fatigue after 2 h of continuous task
Fig. 6 – Averaged stimulus-locked ERPs recorded from Fz, Cz and Pz, superimposed for the compatible (solid lines) and incompatible condition (dashed lines).
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1989; Kenemans et al., 1993; Luck and Hillyard, 1994; O'Donnell et al., 1997). Because the physical characteristics of the hand and color cue stimuli are highly similar in this study, the differential effects due to cue type are less likely to be due to physical characteristics of the cue. It seems plausible that this P2 effect is predominantly based on the relevance of specific information provided by the cue, that is, the enlarged positive shift evoked by hand cues compared to the color cues might be related to modulation of specialized brain areas involved in the preparation for goal-directed action. This interpretation is in line with findings of Slagter et al. (2005); in their study a similar positivity was elicited by cue stimuli providing spatial (location) or non-spatial (color) information. They argued that this P2 effect reflects preparation processes related to associating cue information to the functional properties of this information. The domain-independency of this effect in the study of Slagter et al. supports the importance of relevance of cue information; both location and color information was relevant for subsequent behavior in their study. In the present study, the functional significance of the hand cue was larger for subsequent behavior than the significance of color information, which is reflected in a more pronounced P2 observed in the hand cue condition compared to the color cue condition. The P2 effect was significantly diminished after 30 min of task performance in the present study, and differences related to cue type also disappeared after this period of time, implying that cue information is less efficiently associated with relevant stimulus features with increasing mental fatigue. The second effect elicited by the presentation of advance information concerned a modulation of the CNV observed between 700–1000 ms after the presentation of the cue stimulus. The CNV was most pronounced for the hand cue condition and had a maximum at fronto-central electrode sites. Different processes underlying the CNV have been discerned (e.g., Van Boxtel, 1994; Van Boxtel and Böcker, 2004; Van Boxtel and Brunia, 1994a; Verleger et al., 2000); a negative wave with a central maximum, reflecting preparation for the motor response to be executed after S2, and a frontal component, reflecting cognitive control mechanisms related to implementation of goal representations. In the present study, advance information in the hand cue condition allowed specific preparation of subsequent goal-directed actions, while the color cue provided advance information about irrelevant target features of the upcoming stimulus. The more pronounced negativity in the hand cue condition as compared to the negativity elicited after a color cue was delivered indicated that the observed effect should be attributed, at least partly, to motor related preparatory activation. However, it should be noticed that the negative shift was not restricted to the hand cue condition, in the color cue condition a CNV was observed, as well. The CNV elicited by the color cue stimulus, and probably part of the negativity elicited by the hand cue stimulus, more likely reflect the ‘frontal’ CNV component, indexing cognitive control mechanisms, involved in top-down modulation of more basic cognitive functions involved in future goal-directed behavior. The size of the CNV remained stable during 1.5 h of continuous performance; thereafter the negative wave became significantly smaller for both the hand and color cue
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conditions. The effect of time on task on the CNV seems to reflect a reduction in top-down modulation of cognitive functions with increasing mental fatigue, which is in accordance with previous findings (Lorist et al., 2000; Van der Linden et al., 2003; Boksem et al., 2005) indicating that topdown processes are indeed vulnerable to mental fatigue. What is striking in the observed effects of mental fatigue on cue-related brain activity is the time course of these effects. Cue-related modulations of the P2 disappeared already after half an hour of task performance, while the CNV effect remained significant until the third time on task interval, that is, during 1.5 h of task performance. It seems that with increasing mental fatigue advance information has less impact on human information processing, but more important: some processing mechanisms seem to be more at risk of becoming deteriorated due to mental fatigue than others.
3.2.
Influence of advance information on processing of s2
We found that advance information affected brain activity. However, did the information provided by the cue facilitate processing of S2, or in other words: did subjects use available information? Behavioral results indicated that advance information was indeed processed adequately and facilitated the processing of S2. A significant RT benefit of 88 ms in validly cued trials compared to invalidly cued trials was observed if relevant motor information was delivered in advance. Although information provided by the color cue was less relevant for subsequent goal-directed behavior, a RT benefit of 12 ms was observed in this condition. The behavioral benefits in the hand cue condition were accompanied by relative large costs in case cue information was incorrect; the proportion of incorrect responses was significantly higher after invalid hand cues compared to invalid color cues. The larger proportion of errors in the invalid hand cue condition was accompanied by an enhancement of P3 amplitude. The higher P3 amplitude might be related to the lower event probability of invalidly cued stimulus, but alternatively, it might be a reflection of additional demands placed on the information processing system in the invalid condition, necessary to adjust the incorrectly prepared motor system. Subjects did use relevant advance information to prepare for subsequent goal-directed actions. A major issue in this study concerned the question what happens with top-down control when subjects become mentally fatigued, do they still use advance information to prepare their behavior? In accordance with the subjective measures, the behavioral measures indicated that performance efficiency deteriorated with time on task, indicating subjects indeed became mentally fatigued during prolonged task performance. It is important to note that speed was not replaced by accuracy; the proportion of incorrect responses remained stable during the 2 h of performance. In other words, subjects adopted a uniform accuracy criterion during the experimental session. Changes in performance can therefore not be explained by changes in the speed-accuracy trade-off. In addition to the influence of mental fatigue on subjective and behavioral measures, brain activity elicited by cue information was significantly attenuated with time on task,
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which strongly suggests that cue effectiveness depended on the state of the subject. Comparing the effects of time on task on RT and cue-related brain activity, it becomes clear that the decrease in CNV magnitude paralleled the time course of the RT effects; both effects became most pronounced after 120 min of task performance. It seems that top-down processes indexed by the CNV are the most likely candidate to underlie the performance effects of mental fatigue. No clear match between effects of time on task on the cue-elicited P2 component and performance were observed. The P2 was argued to reflect top-down processes involved in the selection of relevant information. It can be argued that due to practice information selection becomes more automatic, and task demands will decrease with time on task. P2 amplitude has indeed been related to cognitive processing demands (Freunberger et al., 2007). However, if these effects reflect learning one might expect beneficial effects on performance, which were absent in the present study. The reduction in P2 amplitude after the first half hour, in the absence of clear behavioral effects in this time on task interval remains puzzling. The effects of time on task were most pronounced in the hand cue condition, that is, in the condition which benefited most from advance information during the first part of the experimental session. In the color cue condition, advance information cannot be used to prepare a specific response; however, cue information might be helpful in identifying the target letter by tuning the perceptual system. The more negative going N1 component for invalidly cued stimuli compared to validly cued stimuli in the color cue condition seems to be in agreement with this interpretation; attention directed incorrectly toward specific stimulus characteristics by the invalid cue had to be re-directed to relevant stimulus features producing the enhanced N1 component (Luck et al., 1994; Wright et al., 1995). In summary, the present findings support the interpretation of cue-related brain activity in terms of preparation processes, facilitating goal-directed behavior. With increasing mental fatigue, however, the influence of relevant advance information became less evident. Subjects do not seem to use available information anymore, resulting in less efficient behavior.
3.3.
Compatibility
Subjects performed a conflict task, in which irrelevant information was presented together with relevant information. Although subjects were instructed to respond to the centrally located target letter and to ignore adjacent irrelevant information, RTs were found to be faster in the compatible condition than in the incompatible condition in which irrelevant information contained features associated with the incorrect response. The faster reactions to compatible stimuli were accompanied by fewer errors, indicating that the increase in speed was not due to a decrease in accuracy. ERP modulations related to the compatibility manipulation took the form of an enhanced P2 component in the incompatible condition compared to compatible condition, which was most pronounced at frontal sites. The P2 has been found to reflect filtering mechanisms engaged in identification of relevant information (Wijers et al., 1989; Kenemans et al.,
1993; Luck and Hillyard, 1994; O'Donnell et al., 1997). The P2 enlargement observed in incompatible stimuli compared to compatible stimuli might reflect this additional selection activity engaged more strongly in incompatible task condition, in which a correct localization of the target letter is of crucial importance in order to provide a correct response. In addition, in incompatible trials an enhancement of the fronto-central N2 component was observed. The N2 component has been implicated in the detection of response conflict, that is, the amplitude is increased on high-conflict trials (Kopp et al., 1996; Bekker et al., 2004; Yeung and Cohen, 2006), which conforms with the findings in the present study. The P3 component was found to be dependent on the identity of flanker letters; if flanker letters were incompatible with the target letter the P3 was delayed compared to the compatible condition. P3 latency was found previously to increase with difficulty of discrimination and identification of relevant stimulus features (Donchin and Coles, 1988), which seems to confirm that resolving the conflict between incompatible target and flanker information in the present study indeed takes time, producing a delay in stimulus processing. This interpretation is in line with the stimulus evaluation view of the P3. Alternatively, Verleger (1997) reviewed evidence that in a flanker task a delay in P3 latency might be affected by response selection, as well. In the incompatible task condition different response tendencies are activated, causing response competition which might be reflected in a delay in P3 latency. Although it has been reported that the effects of time on task are most pronounced in tasks placing high demands on top-down processing, we did not observe a differential effect of time on task on compatible and incompatible trials, supposed to place dissimilar demands on control mechanisms playing a role in conflict solving. These findings are in agreement with those of Boksem et al. (2005), who did not observe effects of time on task in a modulated Simon task, in which response conflict was manipulated by presenting stimuli either at the spatially corresponding side to the response side or at the opposite side. An important issue in the study of mental fatigue concerns the relationship between fatigue and factors such as motivation and boredom. It has, for example, been argued that the effects of mental fatigue are, at least in part, due to a lack of motivation (Chaudhuri and Behan, 2000). Boksem et al. (2006) indeed showed that fatigued subjects who were motivated could once again control their actions adequately. However, mentally fatigued, but motivated subjects were unable to improve performance in terms of both speed and accuracy; instead they were found to improve on one measure by sacrificing the other. This implies that fatigue is more than an effort/reward imbalance influencing motivation; it involves adaptive strategies to keep performance at an acceptable level under adverse internal circumstances. This finding is consistent with Grier et al. (2003) who found that performance deterioration in vigilance tasks were not related to mindlessness or underarousal, but these deteriorations were better characterized by limitations in ‘effortful attention’. In conclusion, the results of this study indicated that the availability of relevant advance information modulated processing of S2. Moreover, performance was faster and more accurate if this information was valid, while invalid
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information led to costs in the form of increased error rates. Planning and preparation processes, reflected in behavioral measures and brain activity, were clearly diminished by mental fatigue, induced by time on task. No evidence was found that controlled and automatic processes involved in conflict solving were differentially affected by mental fatigue, that is, no evidence was found for stronger interference of distracting information with time on task.
subscales: energy (general activation), tiredness (deactivationsleep), tension (high activation) and calmness (general deactivation). During task performance, subjects indicated the amount of resistance they experienced to continue task performance on a simple rating scale, employing verbal statements as anchors. Scores varied from 0 (not at all) to more than 10 (maximal).
4.4.
4.
Experimental procedures
4.1.
Participants
Fifteen right-handed healthy young women (mean age = 21.1 (SD = 1.8)) participated in this experiment. All reported to be non-smokers, having normal sleep patterns, and neither did work night shifts nor used prescription medication. They all had normal or corrected-to-normal vision. Written informed consent was collected prior to the study. Subjects received monetary payment for their participation.
4.2.
Stimuli
Stimuli were presented in the centre of the visual field on a color monitor positioned at 80 cm from the subject's eyes. On each trial, the participants were presented with a row of three uppercase letters, the central letter was the target letter and the remaining letters were the flanker letters. The participants were instructed to make a left-hand response if the central letter was an H and a right-hand response if the central letter was an S. The letters remained on the screen until a response was made or in case no response was made the letters disappeared after 1200 ms. The letters were presented in red or in green on a dark background. On half of the trials the flanker letters had the same identity (HHH or SSS; compatible) and color as the target letter. In the other half of the trials flankers had a different identity (SHS or HSH; incompatible) and color than the target letter. The different stimulus categories were presented, with equal probability, in random order. A cue, appearing 1000 ms before the letters, was presented for 150 ms, providing advance information either about the response hand (hand cue; 50%; the word ‘links’ (left) or ‘rechts’ (right)) or about the color of the target letter (color cue; 50%; the word ‘rood’ (red) or ‘groen’ (green),). To prevent a fast guess strategy, cues were valid on 80% of the trials. During a trial a fixation mark remained visible on the screen (an asterisk of 0.5 × 0.5 cm). Stimuli were presented 0.5 cm above this fixation mark and a single letter subtended 0.18° by 0.18° of visual angle. Response-cue interval jittered with a range of 900 to 1100 ms.
4.3.
Subjective measurements
The Activation–Deactivation Adjective Check List (AD-ACL; Thayer, 1989) was used for assessments of momentary activation states. This self-rating questionnaire includes 20 activation descriptive adjectives, each of which is rated on a 4-point scale according to how well the adjective describes one's immediate feelings. The adjectives are grouped into four
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Procedure
The experimental sessions started around 1.00 p.m. and lasted 3 h. After the subject arrived at the laboratory the EEG electrodes were applied and the procedure was explained, without giving specific information about the duration of the experimental task. Thereafter, the subject was seated in a dimly illuminated, sound-attenuated, and electrically shielded room. The AD-ACL was completed and a practice block of 80 trials was performed. The subject was instructed to respond as quickly as possible, maintaining a high level of accuracy, and to minimize eye movements and blinking during task performance. After the practice trials, the subject performed the 2-hour experimental block. This block started with a question about the level of resistance the subject felt at that moment against performing the task. This question appeared on the screen every 30 min. After task performance the AD-ACL was filled out for the second time and the subjects were paid.
4.5.
Eeg recordings
EEG was recorded, using Sn electrodes attached to an electrode cap (ElectroCap International), positioned according to the standard 10–10 system (American Electroencephalographic Society, 1994). The electrodes were referenced to electronically linked earlobes. The electro-oculogram (EOG) was recorded bipolarily with Sn electrodes from the outer canthi of both eyes and above and below the left eye. The Ag/AgCl electrode for earthing the subject was placed on the sternum. Electrode impedances were reduced to less than 5 kΩ. The signals were amplified with a time constant of 10 s, low pass filtered at 30 Hz and digitized at a rate of 100 Hz.
4.6.
Data reduction and statistical analyses
Mean reaction times (RTs) were calculated for correct trials. Trials containing premature motor responses (b50 ms) were excluded from analysis. Mean error rate was quantified as the proportion of incorrect responses within each condition, and trials in which no response was made were regarded as misses. For ERP analyses trials with incorrect responses or trials containing amplifier saturation artifacts were excluded from analysis. Ocular correction was performed using the Gratton and Coles method (Gratton et al., 1983). To evaluate the effect of advance information on brain activity, average cue-locked ERPs were computed separately for each mid-line electrode position (Fz, Cz, Pz and Oz) for the different conditions, that is, cue type (hand/color) and time on task interval (4 intervals of 30 min). The averaging epoch started 100 ms prior to cue
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onset and lasted until 1000 ms post-cue. All averages were aligned to a 100 ms pre-cue baseline. For further analysis mean amplitudes were calculated in 10 periods of 100 ms each, from 0 to 1000 ms post-cue. Average S2-locked ERPs were computed separately for each electrode position for the correct trials in each condition (i.e. cue type, time on task interval, cue identity (either left/ right or red/green for hand and color cues, respectively), cue validity (valid/invalid), compatibility (compatible/incompatible), and target letter (H/S)). The averaging epoch started 100 ms prior to stimulus onset and lasted until 1000 ms poststimulus. The averages were aligned to a 100 ms prestimulus baseline. For further analysis mean amplitudes for Fz, Cz, Pz and Oz electrodes were calculated in 15 periods of 50 ms each, from 50 to 800 ms post-stimulus. Behavioral data and mean ERP amplitudes were entered as dependent variables to SPSS. The within-subject factors consisted of time on task (4 intervals of 30 min), cue validity (valid/invalid), cue type (hand/color) and compatibility (compatible/incompatible) for the behavioral analysis, while electrode (Fz, Cz, Pz and Oz) was added as additional factor to the ERP analysis. For the statistical analysis of the data the univariate approach for repeated measures was used, using the ɛ∼⁎-adjustment procedure recommended by Quintana and Maxwell (1994). When the main analysis indicated a significant interaction (α ≤ .05) between factors, follow-up analyses were performed, using Bonferroni adjustments.
Acknowledgments Thanks are due to Joop Clots and Jan Smit for their technical support. Thanks to Berry Weijers and to the anonymous reviewers for their valuable comments on previous versions of the manuscript. REFERENCES
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