A comparison of the effects of transcranial direct current stimulation and caffeine on vigilance and cognitive performance during extended wakefulness.

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A comparison of the effects of transcranial direct current stimulation and caffeine on vigilance and cognitive performance during extended wakefulness. McIntire,LK
Title A comparison of the effects of transcranial direct current stimulation and caffeine on vigilance and cognitive performance during extended wakefulness.
Authors LK McIntire,RA McKinley,C Goodyear,J Nelson
Journal Brain stimulation
Issue 4
Issn 1876-4754
Isbn
Doi 10.1016/j.brs.2014.04.008
PMID 25047826
Volume 7
Pages 499-507
Keywords
Website [[Website::[1]]]
Publication Year

Abstract


Sleep deprivation from extended duty hours is a common complaint for many occupations. Caffeine is one of the most common countermeasures used to combat fatigue. However, the benefits of caffeine decline over time and with chronic use. Our objective was to evaluate the efficacy of anodal transcranial direct current stimulation (tDCS) applied to the pre-frontal cortex at 2 mA for 30 min to remediate the effects of sleep deprivation and to compare the behavioral effects of tDCS with those of caffeine. Three groups of 10 participants each received either active tDCS with placebo gum, caffeine gum with sham tDCS, or sham tDCS with placebo gum during 30 h of extended wakefulness. Our results show that tDCS prevented a decrement in vigilance and led to better subjective ratings for fatigue, drowsiness, energy, and composite mood compared to caffeine and control in sleep-deprived individuals. Both the tDCS and caffeine produced similar improvements in latencies on a short-term memory task and faster reaction times in a psychomotor task when compared to the placebo group. Interestingly, changes in accuracy for the tDCS group were not correlated to changes in mood; whereas, there was a relationship for the caffeine and sham groups. Our data suggest that tDCS could be a useful fatigue countermeasure and may be more beneficial than caffeine since boosts in performance and mood last several hours.


Acronyms

Acronyms
CNS => central nervous system
DMS => Delayed Matching-To-Sample
EEG => electroencephalographic
LCD => liquid crystal display
LTD => long term depression
LTP => long term potentiation
POMS => Profile of Mood States
PVT => Psychomotor Vigilance Task
TMS => transcranial magnetic stimulation
VAS => Visual Analog Scale
WAM => Wrist Activity Monitor
t = 2.60, p= .013 => (t = 2.60, p= .013) and sham
tDCS => transcranial direct current stimulation

Results

  • significant main effect of ``group Mackworth task metric of accuracy (F(2, 27) = 8.50, p< .001) found.
  • post-hoc t-tests showed that the tDCS group performed significantly better (averaging sessions 0400, 0600, 0800) than both the sham (t = -4.64, p< .001) caffeine groups (t = -2.84, p= .006).
  • sham and caffeine groups were not statistically significant.

  • Mackworth task metric of accuracy showed significant group and session interaction (F(2, 54) = 3.70, p= .010).
  • t-tests showed that the tDCS group performed significantly better than both the sham (t = -4.50, p< .001) caffeine (t = -2.35, p= .034) groups at 0400 caffeine and sham group were not significant from one (t = -1.79, p= .057); however, Cohen's d 0.84.
  • value above .8 effect that detected larger n-size.
  • At 0600 comparisons between the groups did not achieve statistical significance.
  • However, at 0800 performance tDCS group significantly better than the sham (t = -4.79, p< .001) caffeine group (t = -3.95, p< .001).
  • effects are shown in Figure 2, top left quadrant.

Delayed Matching to Sample

  • significant main effect of ``group found Latency in Delayed Matching to task (F(2,27) = 3.71, p= .038).
  • t-tests showed that the caffeine group performed significantly better than the sham group (t = 2.28, p= .015).
  • test between sham and tDCS groups not reach statistical significance (t = 1.85, p= .056); Cohen's d .87.
  • caffeine and tDCS groups not significantly different p= .552.
  • Based on test results Figure 2, concluded that the caffeine and tDCS groups similar, cannot conclude that tDCS showed improvement over the sham group.
  • High Cohen's d in

  • comparison between the tDCS and sham groups suggests cannot rule out effect.
  • effects are shown in Figure 2, top right quadrant.

Psychomotor Vigilance Task

  • PVT, found significant affect ``group on Mean Reaction Time (F(2, 27) = 7.03, p= .004).
  • t-tests showed that the caffeine tDCS group performed significantly better than the sham group (t = 2.88, p= .002 t= 2.77, p= .005, ).
  • There was not a significant difference between the caffeine and tDCS group (t = -0.34, p= .865).
  • Effect shown in Figure 2, right quadrant.
  • not main effect found PVT Lapses (F(2, 27) = 3.03, p= .065); Cohen's d between sham and caffeine group 1.22 sham and tDCS group .78.
  • believe larger n-size effect.

Subjective Questionnaires

  • found significant main effect of ``group on Drowsiness from side-effects questionnaire (F(2, 27) = 4.90, p= .015).
  • tDCS group was significantly less drowsy than the sham group (t = 2.73, p= .004).
  • comparison between the tDCS group and caffeine group as well as comparison between caffeine and sham group were not significant.

  • created composite score for the VAS by adding Alert/Able to Concentrate, , Feel Confident, .
  • then subtracted Anxious, Irritable, Jittery/Nervous, from total.
  • significant main effect group (F(2,27) = 3.04, p= .064) not found.
  • Cohen's d difference between the sham and tDCS groups 1.30 (t = -2.76, p= .020).
  • believe larger number participants comparison significant cannot state results reported herein.

  • significant interactions side effects questionnaire found (Figure 3).
  • First, Drowsiness significant interaction group and session (F(2, 54) = 3.46, p= .014).
  • t-tests showed that at 0400 both the caffeine (t = 3.22, p= .003) tDCS group (t = 2.04, p = .025) reported feeling significantly less drowsy than the sham group.
  • drowsiness ratings for the tDCS and caffeine group were not statistically different at 0400.
  • At 0600, none groups drowsiness ratings significantly different from one .
  • At 0800, tDCS group reported less than caffeine (t = 2.59, p= .031) sham (t = 2.94, p= .005) groups.
  • There was not a significant difference between the caffeine and sham group at this session.
  • second interaction from side effects questionnaire Fatigue (F(2, 54) = 2.70, p= .040).
  • t-tests showed no significant difference between the groups at 0400 but at 0600 tDCS group feeling less than both the caffeine (t = 2.60, p= .013) and sham (t = 2.60, p= .013) groups.
  • not significant difference between the sham and caffeine groups during this session.
  • Again at 0800, tDCS group subjective ratings for fatigue significantly lower than ratings for the sham (t = 2.46, p= .032) caffeine (t = 2.40, p= .015 groups).
  • again no significant difference between the sham and caffeine group during this session.
  • significant interaction in side

  • effects questionnaire Composite Score (F(2, 54) = 3.68, p= .010).
  • All thirty questions added to number.
  • t-tests showed there was a significant difference at 0400 between sham and caffeine group (t = -1.95, p= .047).
  • At 0600 there was a significant difference between tDCS and caffeine group (t = -1.96, p= .042).
  • Finally, at 0800 significant difference in composite score between tDCS and caffeine group (t = -2.51, p= .013).
  • comparison between the tDCS and sham group at 0800 not significant (t =-2.29, p= .061); however, effect size as measured by Cohen's d 1.26.

  • Partial correlations controlling for subject were determined between Accuracy on the Mackworth Clock Test all other variables separately for each group using values at 0200, 0400, 0600, 0800.
  • Table 2 displays any partial correlation with p .01.

  • tDCS group did not significant partial correlation any subjective questionnaire variable.
  • Significant correlations found both sham and caffeine groups.
  • Changes in Accuracy related to subjective mood in all variables in table caffeine group.
  • Most variables significantly correlated in sham group except for Difficulty Staying Awake Drugged Feeling on side effects questionnaire.

* Background Sleep deprivation from extended duty hours is a common complaint many occupations.
* Caffeine one most countermeasures used to fatigue.
* However, benefits caffeine decline over time chronic use.
* Objective objective to evaluate efficacy transcranial direct current stimulation (tDCS) applied to pre-frontal cortex at 2 mA 30 minutes to effects sleep deprivation to compare effects of tDCS those of caffeine.
* Methods Three groups of 10 participants received either tDCS with placebo gum, caffeine gum sham tDCS, sham tDCS with placebo gum during 30 hours extended wakefulness.
* Results results show that tDCS prevented decrement in vigilance led to better subjective ratings for fatigue, drowsiness, energy, composite mood compared to caffeine control in sleep-deprived individuals.
* Both tDCS and caffeine produced similar improvements in latencies on short-term memory task faster reaction times in psychomotor task when compared to placebo group.
* Interestingly, changes in accuracy tDCS group were not correlated to changes in mood; , relationship caffeine and sham groups.
* Conclusion Our data suggests that tDCS fatigue countermeasure more than caffeine since boosts in performance mood last hours.
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Introduction

* Sleep deprivation from extended duty hours is a common complaint in many occupations.
* extended periods wakefulness lead to decrements in mood and performance.
* example, study sleep deprived residents reported higher scores in hostility, anger, fatigue compared to non-sleep deprived counterparts.
* review by Krueger found sleep deprivation resulted in increased reaction times, decreased accuracy, decreased attention, alterations in mood.
* studies relate performance during extended wakefulness to intoxicated.
* instance, Williamson Feyer found after 17 hours wakefulness participants performance equivalent to individual BAC 0.05%, considered illegal to drive car in most countries.
* Unfortunately, many occupations require shifts lasting even longer than .
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* fatigue detrimental problem many occupations; however, unlikely conditions in environments change to allow more time off rest.
* to investigate fatigue countermeasures enhance alertness.
* One method used in in other fields caffeine.
* Due to ease access, become commonplace intervention to counter the effects of fatigue.
* Not only beverages , but rations include gum.
* Caffeine has found to improve performance during inadequate sleep variation.
* example, SEAL trainees given caffeine after 72 hours of sleep deprivation significantly improved vigilance, choice reaction time, self-reported fatigue.
* However, benefits from caffeine decline over time chronic use effects
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* short-lived.
* caffeine increase ability to , not aid in decisions, skill to most other occupations.
* unclear not caffeine has negative effect on mood.
* example, one study found that doses of caffeine (100, 200, 300mg) lead to increase in depression confusion (9), others found doses 100-300mg associated improved mood and only at higher doses (above 400mg) mood deteriorate (10, 11, 12).
* research conflicting affects caffeine on individual.
* to investigate form fatigue countermeasure to enhance alertness performance.
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* used to address disorders such as Parkinson disease, major depressive disorder, schizophrenia, stroke, dementia, chronic pain, etc., expansion research over past decade showing form non-invasive brain stimulation, known as transcranial direct current stimulation (tDCS), in enhancing human performance ( 13 14 reviews).
* technology uses direct passed between electrodes on scalp to modify membrane potential in polarity manner, lowering neuron excitability in region.
* description technologies, design, physics, principles activation, Wagner al. (17).
* Past studies shown that tDCS applied to scalp locations over areas cortex significantly improve abilities such as working memory visuomotor coordination.
* abilities often affected by fatigue.
* research that tDCS improve individuals skills, such as attention.
* example, tDCS accelerated learning target detection in image analysis task.
* Participants received brain stimulation improved
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* performance accuracy 2.5 times over the sham control group.
* In study, found that tDCS human sustained attention by at least two-fold when compared to sham.
* Luber, al. (22) provided some evidence that performance improvements produced from similar technique -- transcranial magnetic stimulation (TMS) -- extend to humans experiencing declines resulting from sustained wakefulness.
* Luber found 5Hz repeated rTMS applied to upper-middle brain region significantly reduced sleep-deprivation induced deficits in reaction times in subjects experiencing 48 hours sustained wakefulness.
* authors discovered stimulation region aides in engagement networks engaged in wakefulness cycles.
* working memory and attention performance to the effects fatigue, this study to extend our results on enhancement by examining the effect of tDCS on performance following period extended wakefulness.
* compared effects to those of caffeine to determine any benefits tDCS in mitigation fatigue above intake caffeine.
* Based on literature, hypothesized that both tDCS and caffeine induce significant improvement in task performance (i.e. reduction of the performance decline) when compared to sham/placebo group.
* tDCS has much greater specificity in terms targeted brain region, expected produce greater performance improvements fewer negative side effects than caffeine.
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* tDCS Stimulator MagStim DC stimulator (Magstim Company Limited; Whitland, UK) used to provide tDCS stimulation.
* battery-powered device controlled
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* microprocessor to ensure at up to 5,000μA.
* safety, monitoring output electrode/tissue impedance included.
* device off impedance becomes greater than 50 k Ω to prevent shocks burns.
* device only (not FDA approved).
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* tDCS Electrodes In place of sponge electrodes delivered Magstim unit, used custom set silver/silver chloride electroencephalographic (EEG) electrodes as described in McKinley, al. (20).
* electrodes shown to more over time, produce lower sensation levels, produce fewer skin reactions when compared to sponge electrodes.
* Both anode cathode consist separate array 5 EEG electrodes as in Figure 1.
* Each electrode diameter 1.6 cm yielding contact area 2.01 cm 2 each electrode.
* array, electrodes 0.75 cm from center 0.1 cm apart as measured from outer edge of the electrode to outer edge of the neighboring electrodes to either side.
* At 2 mA supplied , average
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* tDCS Paradigm stimulation condition, tDCS was applied at 2mA for 30 minutes.
* Sham tDCS was applied at same intensity but only 30 seconds.
* anode applied to scalp location F3 according to 10-20 EEG electrode placement system cathode placed over (i.e. ) bicep.
* Electrodes secured using bandages, connectivity ensured using highly gel (SignaGel, Parker Laboratories, Fairfield, NJ).
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* Wrist Activity Monitor (WAM) Two days prior to data collection, each participant wore wrist activity monitor (WAM; Ambulatory Monitoring, Inc.).
* WAM non-invasive device worn on wrist like wristwatch.
* recorded limb body movements to determine when participant when .
* used to ensure participants received at least 7 hours of sleep in two days prior to data collection.
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* Caffeine StayAlert® gum (MarketRight, Inc., Plano, IL) delivery mechanism used to administer 200 mg caffeine to participants in caffeine group.
* participant received 2 pieces gum (placebo or ) to gum 5 minutes.
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Subjects

* Thirty active-duty participants from Wright-Patterson Air Force Base completed this study.
* 22 male 8 female participants average age 29.3+3.4.
* Participants compensated time but any exclusion criteria described in McKinley al. (20).
* 36 individuals , 6 one more study exclusion criteria.
* remaining 30 randomly assigned three groups of 10 individuals tDCS stimulation/placebo caffeine, caffeine/sham tDCS, sham tDCS/placebo caffeine.
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Performance Tasks

* Participants were required to perform three performance tasks at intervals throughout the night.
* required to complete subjective questionnaires throughout the night.
* tasks below.
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* Psychomotor Vigilance Task (PVT) Participants were required to perform PVT during testing.
* PVT -192 ( Monitoring, Inc.; Ardsley, NY) 8 x 4/5 x 2/4 , battery-operated test presentation data capture system records reaction times.
* stimulus was presented on liquid crystal display (LCD) presents number up by milliseconds.
* stimulus was presented up to 1 minute (60,000 msec), allowing participant to respond by using button press thumb.
* Once participant presses microswitch device records reaction time stimulus.
* interval varies randomly from 2 to 12 seconds.
* task 10 minutes in duration.
* PVT requires sustained attention motor responses.
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* Delayed Matching-To-Sample Working Memory Task Delayed Matching-To-Sample (DMS) task working memory task from Cambridge Neuropsychological Assessment Battery (CANTAB).
* CANTAB software package complete battery tasks probe functions.
* DMS task designed to probe matching, delayed memory.
* participant was presented complex arbitrary pattern four quadrants.
* Four patterns then presented either original pattern following delay 0, 4, 12 seconds, where original pattern obscured from view.
* participant was required to then choose one pattern matched .
* Each participant completed two sets 20 randomized trials per session included five , five 0-sec delay, five 4-sec delay, 12-second delay presentations.
* participant select incorrect pattern, x displayed over pattern.
* participant then continue selections until pattern chosen.
* task took 20 minutes to complete.
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* Mackworth Clock Test Sustained Attention task developed according to description task used by Kilpalӓinen, Huttunen, Lohi, Lyytinen.
* task adopted version Mackworth clock test parameters adopted from Teikari on desktop computer.
* participant was presented display 16 hole-like circles against background.
* circles arranged to form clock-like round figure radius 20 cm (7.9 in.)
*
* circle changed from to 0.525 seconds in turn, cycle 8.4 seconds.
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* light moved in pattern by one step, considered stimulus appearance.
* light moving twice distance (i.e., circle) considered critical signal participant was required to respond to signal by pressing spacebar as fast as on keyboard index finger.
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* response defined as correct hit when occurred less than 8 sec after target signal and false alarm reaction occurred outside this time range (+0.1--8.0 sec).
* targets defined as misses.
* task set contained 3,442 stimuli, including 12 targets, takes 30 minutes to complete.
* event rate used Mackworth stimulus series, critical signal event rate varying from 45 sec to 10 min.
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Subjective Questionnaires

* Profile of Mood States (POMS) Profile Mood States (POMS) 65-item questionnaire measures mood using 6 categories tension-anxiety, depression-dejection, anger-hostility, vigor-activity, fatigue-inertia, confusion-bewilderment.
* Participants rated feelings about item (example items Tense, Vigor, Fatigue) on scale 1 to 5, 1 not at all 5 .
* scores six categories
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* score, added  ; next vigor-activity score subtracted from total to produce composite mood disturbance score.
* analyzed factor score .
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* Visual Analog Scale (VAS) affect measured via Visual Analog Scale (VAS) (27).
* VAS required participants points on different lines correspond to how he/she along specified affect continuum at time at test taken.
* adjectives included in VAS as follows Alert/Able to Concentrate, Anxious, , Feel Confident, Irritable, Jittery/Nervous, Sleepy, Talkative.
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* Side-Effects Questionnaire A side-effects questionnaire administered at end each session.
* Participants responded none , , , severe to 33 items.
* Examples items included , Headache , Drowsiness, Drugged Feeling .
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Procedures

* Using similar methodology in Caldwell, al. (28), participants underwent 30 hours continuous wakefulness.
* Participants randomly assigned to one three groups (n=10 each group).
* Group 1 received anodal tDCS at 2mA for 30 minutes placebo gum, group 2 received sham tDCS with placebo gum, 2 mA for 30 seconds, group 3 received 200mg caffeine in gum form sham tDCS.
* After to participate in study, participants filled out screening questionnaire.
* Two days prior to scheduled experimental trial, participants given activity wrist monitor schedules include minimum seven hours of sleep per night between hours 1100 0600.
* during this time participants received training on all three performance tasks to utilized in study.
* on Sustained Attention task
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* consisted 2 -5 minute practice sessions followed by the full 30 minute task.
* training Delayed Matching-to-Sample task included 2- 5 minute practice session followed by the full task, takes about 20 minutes to complete.
* on PVT consisted participants completing 10 minute task after instructions on the task given.
* Participants to on all three performance tasks to against effects during testing.
* Participants subjective questionnaires at this time.
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* On day experimental trial, participants were required to at 0600 perform activities as .
* to not consume any caffeine central nervous system (CNS)-altering medications/substances on test day.
* Each participant arrived at test facility at 1730 hours.
* WAM data analyzed to ensure sleep amounts maintained.
* Starting at 1800 hours, participants completed one session of the sustained attention task (30 mins), one session of the delayed matching-to-sample task (20 mins), one session of the PVT task (10 mins), filled out POMS-B, VAS, side--effects questionnaire.
* Participants provided short break 45 minutes , where , TV, walk, , video games.
* second session at 2000 hours exactly same as first session.
* procedures repeated two hours.
* testing session took place at 1000 hours on second day (30 hours continuous wakefulness).
* Prior to testing session 8 (occurring at 0400), participants received assigned treatment (i.e. tDCS-placebo gum, sham-placebo gum, or caffeine gum-sham tDCS).
* Participants receiving either or sham stimulation tDCS electrodes 30 minutes prior to stimulation session.
* Caffeine and placebo gum given at 0300 takes hour to reach levels in blood.
* remaining test sessions allowed evaluation effects
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Analysis

* treatment conditions started at 0400 session, session occurring at 0200 last time point at all 30 participants same.
* One-way ANOVAs group as the factor (levels sham, caffeine, tDCS) conducted data at 0200.
* F-tests not reveal any significant differences among groups all test and variables, thus 0200 session used as baseline comparisons validity 3 groups at 0200 similar.
* design ANOVA used to compare groups change from 0200 group between factor session factor (levels 0400, 0600, 0800).
* Due to evidence performance improvements at 1000 caused by session due to effects (e.g. 30, 31, 32), 1000 time point excluded from analysis.
* significant interaction found between group and session, one-way ANOVA performed at each session group as the factor.
* Post-hoc comparisons group used two-tailed t-tests error from ANOVA.
* All comparisons used per-comparison level .05.
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Discussion

* This study examined the effects anodal tDCS applied to the dorsolateral prefrontal cortex on attention, working memory, psychomotor performance when in state fatigue caused by sleep deprivation.
* caffeine most intervention used to counter the effects of fatigue, effects of tDCS compared to the effects caffeine to provide more basis comparison.
* Our results suggest
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* tDCS not only larger transient effect on sustained attention (vigilance) than caffeine, but effects that remain at least 6 hours when compared to less than 2 hours effect caffeine.
* reported 10 minutes anodal tDCS applied to the dorsolateral prefrontal cortex ``vigilance decrement at least 30 minutes , expected at least some transient effect in this experiment.
* However, uncertain duration of aftereffect.
* studies found that stimulation lasting as little as 9 13min produced significant after effects in excitability 30 90min, .
* Our data suggest that 30 minutes stimulation produces aftereffects lasting at least 6 hours.
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* improvement in vigilance performance tDCS accompanied by lower subjective ratings for fatigue drowsiness.
* Thus, not only participants perform better, but less tired than counterparts given placebo or caffeine interventions.
* According to the responses from the VAS questionnaire, tDCS group reported more than receiving sham/placebo.
* composite score for the side effects questionnaires better mood states in tDCS group when compared to the responses from the caffeine group.
* supported by fact VAS composite score approaching significance trend toward difference between sham and tDCS.
* correlations changes in accuracy on the Mackworth Clock Test tDCS group were not related to changes in subjective mood state, , highly related sham and caffeine groups.
* means subjective mood role in attention.
* In fact, researchers found moods improved performance on attention-demanding tasks.
* that tDCS caused alterations in mood used to diseases like major depressive disorder.
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* However, research showing there is no difference in mood between sham and stimulation volunteers.
* found that tDCS not alter mood but offset negative effects on mood when stressor, such as fatigue.
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* When examining working memory performance in delayed matching to sample (DMS) task scores, data suggest that both tDCS and caffeine interventions at improving response times (i.e. reducing increase in response time resulting from sleep loss), but not score/accuracy.
* differences in the means not significant between groups at 0400 when interventions applied, differences in response times found two hours later ( mean change from baseline each group Sham = 712ms, caffeine = -64ms, tDCS = 35ms).
* In fact, mean change from baseline (averaging across 0400, 0600, 0800 sessions) in response time at least 326% less tDCS and caffeine groups when compared to sham.
* Luber al., (22) found significant improvements in DMS response times but not accuracy when applying non-invasive brain stimulation to sleep deprived participants.
* In study, Luber used different method stimulation known as transcranial magnetic stimulation (TMS) loci stimulation over regions cortex rather than .
* Only stimulation site showed significant improvement, difference more than reported .
* differences in the stimulation paradigms, such as tDCS vs. TMS, different performance tasks, etc., paradigm to produce larger effect on working memory performance.
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* Declines in psychomotor performance that in sleep deprived individuals by caffeine and tDCS.
* Despite name, PVT is not test
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* vigilance.
* According to Parasuraman, to test vigilance, task 18
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* target stimulus non-target stimulus.
* PVT occurring target devoid non-target stimulus, response time test arousal.
* arousal highly influenced by level of fatigue, PVT test in sleep deprivation studies.
* results PVT are not to Mackworth Clock test.
* Caffeine and tDCS reduced reaction times when compared to sham, but there was no difference between the two interventions.
* no effects either tDCS or caffeine on other psychomotor performance metrics including number lapses false starts.
* Thus both treatments improve speed response without improving accuracy.
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* findings caffeine has beneficial effects on vigilance, attention, working memory, subjective sleepiness ratings, results reported herein suggest that tDCS produces similar results, effects on vigilance performance are far more profound.
* difficult to dosages tDCS and caffeine, 2X larger transient effect at least 3X longer effect duration single dose tDCS when compared to dose of caffeine (200mg; twice dosage in cup coffee).
* high ( 600mg) dosages caffeine have reported to produce longer durations of effect, but dosages trigger caffeine intoxication often accompanied by declines in mood, insomnia, nervousness, hallucinations, complaints.
* in more doses (100mg-300mg) effects short-lived as caffeine quickly removed from brain (1-3 hours depending on dosage) caffeine becomes less at higher levels of fatigue.
* chronic use results in reduction effectiveness.
* Our data suggest that tDCS alternative to effects of fatigue given more
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* at improving vigilance, effect lasts longer, associated improvements in mood side effects.
* Importantly, not known chronic use tDCS leads to declines in effectiveness higher dosages produce longer durations of effect.
* note, effects to extend beyond duration data recording session in this experiment.
* experiments continue recording measurements in performance more than 6 hours after tDCS is applied to examine total duration effectiveness.
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* mechanisms action cannot from data collected in this experiment, theories to explain the effects of tDCS caffeine on performance in (sleep deprived) state.
* Increasing levels of fatigue associated greater concentrations adenosine in brain, byproduct metabolism.
* As adenosine builds up binds to receptors (A1, A2), activity declines through blockage neurotransmitters such as dopamine glutamate (41, 42).
* accompanied by greater levels subjective sleepiness.
* According to Davis, al. (42), caffeine delays reduces effects of fatigue at least in part by blocking adenosine A2 receptors.
* in turn prevents adenosine from blocking dopamine limits reduction in brain activity buildup adenosine.
* Caffeine causes release epinephrine results in high blood pressure, increases in heart rate, blood flow.
* tDCS has found to block adenosine A1 receptors in rabbits, suggesting mechanisms similar.
* However, distinguishing feature tDCS difference in duration of effect.
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* aftereffects tDCS are believed to different mechanisms than effects.
* After-effects tDCS are believed to be caused by effects that long term potentiation (LTP) long term depression (LTD).
* Much existing evidence hypothesis points 20
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* to changes in plasticity associated modulation via NMDA receptors responsible plastic changes.
* Ruohonen Karhu postulate aftereffects in part to excitation glial cells.
* little objective evidence to support theory, help how behavioral effects past stimulation treatment.
* stimulation glial cells mediate slow changes in neurotransmitter release cause long-lasting (minutes hours) increased activity often after tDCS (47).
* Ohn, al. (48) found duration of aftereffects relationship duration stimulation .
* In fact, data longer durations tDCS produce larger much longer behavioral effects.
* Given tDCS treatment lasted 30 minutes, lasting aftereffects expected .
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* performance results from this experiment provide some insight rationale observations, particularly with regard to vigilance.
* decline in vigilance performance over time often to as ``vigilance decrement characterized by reduction of the detection rate critical targets over time.
* well-known phenomenon (e.g. 50, 51), causes not understood.
* two competing theories attempt to source vigilance decrement arousal theory resource theory.
* Arousal theory contends vigilance performance decrements are caused by decline in arousal.
* operator loses interest, becomes , loses focus on the task at hand due to monotonous nature.
* Resource theory posits declines in performance are caused by depletion cognitive resources (i.e. supply ``resources cannot up demand).
* As ``resources continue to decline, decrease in
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* results work examining the effect of tDCS on vigilance to support resource theory over arousal theory.
* found increase in target (i.e. signal) detection not accompanied by increase in false alarms.
* interventions increase arousal generate significant changes in response bias characterized by both increase in target detection increase in false alarm rate, not in our previous experiment.
* In experiment, performance improvements in the PVT when comparing sham and tDCS groups.
* Given PVT test general arousal, provides evidence that tDCS does some effect on arousal.
* Importantly, tDCS-induced improvements in the PVT equivalent to produced caffeine, arousal-amplifying substance.
* Examining vigilance performance results however, change in arousal little effect on vigilance.
* false alarm rates in Mackworth Clock Test (i.e. vigilance task) increased as function time all three groups, there were no differences between the groups.
* Hence, tDCS again not produce larger response bias to sham but yield improvement in target/signal detection, just as in our previous experiment.
* evidence again supports resource theory as lack change in response bias factor beyond general arousal.
* effects of tDCS on vigilance performance were far more profound than those of caffeine.
* vigilance performance improvements result increased arousal, vigilance performance results similar between tDCS and caffeine groups.
*
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* pieces evidence suggest arousal theory not to explain the effects of tDCS on vigilance performance.
* Taken , we conclude that tDCS effect on both arousal and modulation cognitive resources, but effects on arousal less preventing vigilance decrement.
* Based on evidence collected to date, vigilance decrement driven by decline in resources rather than decline in arousal that tDCS attenuating resource decrement.
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* findings suggest that both tDCS and caffeine effects on vigilance, working memory, psychomotor performance during periods sleep-deprivation induced fatigue, effect on vigilance.
* results show that tDCS enhances vigilance to greater extent much longer period time when compared to caffeine.
* tDCS-induced performance benefits improvements in mood including reductions in drowsiness fatigue.
* improvements in feelings drowsiness found caffeine group during session after caffeine administered, but effects short-lived, lasting only 2 hours less.
* Caffeine not accompanied by improvements in fatigue mood.
* Both interventions similar effect on working memory psychomotor performance that to last at least until 0800 session.
* first data to suggest that tDCS may advantages over caffeine in remediation fatigue symptoms.
* Given findings, we conclude that tDCS examined as intervention fatigue.
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* to Dr. Lynn Caldwell guidance throughout process project.
* From design to drafts paper helped  ;
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* could not without experience, knowledge, expertise.
* Finally, thanks to research team.
* Ben Steinhauer, Kathleen Griffin, Jenni Jurcsisn; working any hour day to help collect data this study.
* could not without .
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Table/Figure Legends

* Table 1.
* Testing Schedule; all three performance tasks (Mackworth Clock Test, Delayed Matchto-Sample, PVT) in same order testing session.
* Subjective questionnaires completed after tests.
* Participants then given break period.
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* Figure 2.
* Performance Tasks Mean Change from 0200 (Baseline).
* first data point session (1800) included reference.
* Caffeine given at 0315 (requires 1 hour to ); tDCS was applied at 0400.
* Changes in performance measured subject averaged across groups (n=10).
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* Table 2.
* Using values at 0200, 0400, 0600, 0800, Pearson partial correlations controlling for subject were determined between Mackworth Accuracy (%) all other variables, separately for each group.
* Only significant partial correlations included in table.
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Tables

Table 2

Sham Caffeine tDCS
Variable r p r p r p
SE-Difficulty Staying Awake -0.36 0.0439 -0.66 0.0001 0.37 0.0410
SE-Drugged Feeling 0.04 0.8124 -0.64 0.0001 0.23 0.2171
SE-Fatigue -0.41 0.0228 -0.54 0.0017 0.08 0.6682
SE-Drowsiness -0.47 0.0071 -0.58 0.0006 -0.08 0.6801
SE-Trouble Staying Awake -0.61 0.0003 -0.55 0.0012 0.06 0.7588
SE-Composite -0.61 0.0003 -0.60 0.0004 0.02 0.9325
VAS-Alert/Able to Concentrate 0.72 0.0001 0.52 0.0029 0.14 0.4445
VAS-Sleepy -0.60 0.0004 -0.39 0.0296 -0.03 0.8633
VAS-Energetic 0.55 0.0014 0.46 0.0098 -0.01 0.9413
VAS-Composite 0.57 0.0008 0.38 0.0333 0.07 0.7261
POMS-Fatigue/Inertia -0.60 0.0004 -0.50 0.0046 0.11 0.5522
POMS-Confusion/Bewilderment -0.46 0.0099 -0.53 0.0021 0.04 0.8633
POMS-Composite -0.57 0.0008 -0.47 0.0078 0.08 0.6506