Effects of Dietary Caffeine on Topographic EEG after ...

Original Paper Neuropsychobiology 2007;56:197–207 DOI: 10.1159/000120625

Received: August 5, 2007 Accepted after revision: December 5, 2007 Published online: March 13, 2008

Effects of Dietary Caffeine on Topographic EEG after Controlling for Withdrawal and Withdrawal Reversal Michael A. Keane Jack E. James Michael J. Hogan National University of Ireland, Galway, Ireland

Key Words Caffeine ⴢ Electroencephalogram ⴢ Mood ⴢ Performance ⴢ Withdrawal ⴢ Withdrawal reversal ⴢ Tolerance

Abstract Background/Aims: Despite several decades of research into the effects of caffeine on EEG, few consistent findings have emerged. Notwithstanding the likelihood that differences in methodology may explain some of the inconsistency, confidence in the published findings is undermined by the failure in previous studies to control for the effects of caffeine withdrawal and withdrawal reversal. Methods: Participants (n = 22) alternated weekly between ingesting placebo and caffeine (1.75 mg/kg) 3 times daily for 4 consecutive weeks. EEG activity was measured at 32 sites during eyes closed, eyes open, and performance of a vigilance task. Results: Caffeine was found to have few and modest effects on EEG in the theta and alpha bandwidths, and no effects in the delta and beta bandwidths. Evidence was found of withdrawal, withdrawal reversal, and tolerance in relation to observed increases in theta power during task performance; withdrawal and withdrawal reversal in relation to increases in alpha power during all three behavioural conditions (eyes closed, eyes open, and task performance), and withdrawal-induced adverse effects in relation to aspects of subjective mood. Conclusion: The finding of similar increases in theta power following caffeine challenge and acute caffeine withdrawal

© 2008 S. Karger AG, Basel 0302–282X/07/0564–0197$23.50/0 Fax +41 61 306 12 34 E-Mail [email protected] www.karger.com

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casts doubt on whether caffeine may be viewed as having direct stimulant effects. Results could suggest that change in drug state, whether in the form of acute caffeine withdrawal or challenge, may be disruptive to electrophysiological activity in the brain. Copyright © 2008 S. Karger AG, Basel

Caffeine is the most widely consumed psychoactive substance in history, and usage is frequently accompanied by beliefs about putative ‘stimulant’ psychopharmacological effects [1, 2]. One prominent set of beliefs is that the stimulant effects of caffeine enhance human performance and mood, but recent findings show these beliefs to be unsustainable [3]. The crux of the matter is that a large number of empirical studies, conducted over several decades, contained a fundamental flaw arising from the uncritical adoption of the standard placebo-controlled drug trial [4, 5]. Recent studies designed to control for confounding due to caffeine withdrawal and withdrawal reversal show that the effects of caffeine on performance and mood, widely perceived to be net beneficial psychostimulant effects, are almost wholly attributable to reversal of adverse withdrawal effects associated with short periods of abstinence from the drug [3]. In much the same way that withdrawal reversal has traditionally been ignored in relation to the effects of caffeine on performance and mood, withdrawal reversal continues to be Jack E. James, PhD School of Psychology National University of Ireland Galway (Ireland) Tel. +353 91 493 101, Fax +353 91 521 355, E-Mail [email protected]

ignored in research concerned with the effects of caffeine on electroencephalogram (EEG) spectral power [6]. Although researchers have long used topographical EEG to investigate the effects of caffeine, a consistent account of effects has yet to emerge. In the delta bandwidth, broadly defined as 0–4 Hz, caffeine has been reported to increase [7], decrease [8–11], and to have no effect [12–16] on EEG power within that band. In the theta bandwidth, broadly defined as 4–8 Hz, caffeine has been reported to decrease theta power [8, 10, 11] and to have no effect [7, 12–16]. Several studies have reported caffeine-induced decreases in the alpha bandwidth, broadly defined as 8– 12 Hz [8–10, 15–19], but an increase has also been reported [11], as has no effect [7, 12, 13]. Effects on power in the beta bandwidth, broadly defined as 12–30 Hz, appear equally inconsistent, with reports of increases [11, 18], decreases [8, 10, 19], and no effect [7, 12, 13, 15, 16]. Inconsistencies in findings could be due to a host of methodological differences between studies. Caffeine dose has varied widely from 75 mg [7] to 400 mg [8, 16, 18]. There has also been variability in the definition of bandwidths, and in the selection of electrode sites, ranging from one electrode at one site [14] to the reporting of average effects of multiple electrodes sited over the whole scalp [16]. In addition, the experimental conditions under which EEG measurements have been taken have varied, including eyes closed [7, 15], eyes open [16], and performance of various tasks [13, 14]. Notwithstanding methodological differences and inconsistencies in results, there is further reason to question reported findings. This is because, as mentioned above, researchers have ignored the problems of caffeine withdrawal and withdrawal reversal. In humans, sleepiness, lethargy, and headache are confirmed common symptoms of caffeine withdrawal that are reversed when caffeine is re-ingested [20–31]. Cessation of as little as 100 mg (one cup of coffee) per day, and possibly considerably less [32, 33], can produce symptoms. These are generally noticed within about 12–16 h, peak at around 24–48 h, abate within 3–5 days, and only infrequently extend for up to a week [2, 21, 34, 35]. Moreover, recent studies show that decreases in psychomotor performance (not necessarily discernible to the individual) are detectable after as little as 6–8 h since caffeine was last ingested [36–38]. The importance of taking account of the processes of withdrawal and withdrawal reversal is highlighted by recent findings suggesting that EEG may be affected by caffeine withdrawal, with the further suggestion that these effects may be reversed when caffeine is re-ingested [39, 40]. Consequently, there is a pressing need for the effects 198

Neuropsychobiology 2007;56:197–207

of caffeine on EEG to be characterised in studies that include effective controls for withdrawal and withdrawal reversal. Adopting an experimental design that controls for withdrawal and withdrawal reversal, as previously shown in studies of the effects caffeine on performance [3, 23], mood [23, 41], and blood pressure [42], the present study aimed to help clarify the effects of dietary caffeine on topographical EEG. In addition, task performance and mood were examined to facilitate comparison with previous studies. To our knowledge, this study is the first to examine the acute and chronic effects of dietary caffeine on EEG in habitual consumers, while controlling for the potentially confounding effects of caffeine withdrawal and withdrawal reversal. Method Participants A screening questionnaire (which included questions about caffeine and alcohol consumption, cigarette smoking, medication use, and general health) was used to recruit 22 healthy volunteers (5 male, 17 female) from the university student population. Potential participants were contacted by telephone, and details of the study were given verbally. At a subsequent face-to-face meeting, printed details were provided. Participation earned course credits, and the consent form which participants signed emphasised their right to withdraw from the study at any time without penalty. All participants reported daily consumption of caffeine, with estimated intake ranging from 159 to 665 mg caffeine per day (mean = 277 mg or the approximate equivalent of 2–4 cups of coffee). Smokers were excluded as smoking significantly decreases the elimination half-life of caffeine [43]. Participants ranged in age from 17 to 44 years (mean of 19 years), and had a mean body mass index (kg/m2) of 23.1 for females and 20.3 for males. Study Design A double-blind, placebo-controlled design was used in which all participants took part in all four consecutive 1-week phases of the experiment. Identical gelatin capsules were used containing either maize starch alone (placebo) or caffeine plus starch filler, and were assigned double-blind. At the beginning of each week, participants were given a week’s supply of capsules to be taken over the ensuing 7 days plus instructions regarding when to take each capsule. Ingestion times were the same for each day except the 7th day of each week, when participants ingested the first capsule for that day after arrival at the laboratory. Thus, EEG measurements in the laboratory were timed to begin when peak plasma caffeine concentration was reached (i.e. approximately 50 min after ingestion). During this post-ingestion period, participants completed questionnaires and were prepared for EEG recording. By taking account of the chronic effects of dietary caffeine generally ignored in previous studies [6], the design employed in the present study extends the core features of the traditional drugchallenge paradigm with its attendant strengths of double blinding and placebo control. The extended paradigm and versions of it have been employed in various studies of caffeine-induced effects

Keane/James/Hogan

Table 1. Summary of a double-blind placebo-controlled crossover protocol incorporating alternating periods of ‘long-term’ caffeine

exposure and abstinence1 Week

Run-in days (days 1–6)

‘Challenge’ Condition Effects revealed by challenge (day 7)

1 2 3 4

placebo placebo caffeine caffeine

placebo caffeine placebo caffeine

PP PC CP CC

sustained abstinence, caffeine ‘washout’; serves as a caffeine-free baseline acute challenge; when compared to PP and CC, reveals the presence of tolerance acute abstinence; when compared to PP and CC, reveals the presence of withdrawal habitual use; when compared to PP, reveals the net effects of usual consumption

PP = Placebo ingested for 6 consecutive days followed by 1 day of placebo challenge; PC = 6 days of placebo followed by 1 day of caffeine challenge; CP = 6 days of caffeine followed by 1 day of placebo challenge; CC = 6 days of caffeine followed by 1 day of caffeine challenge. 1 Design originally described by James and Keane [6] and James [23, 42, 44].

on performance, mood, cardiovascular reactivity, and sleep [23, 41, 42, 44–46]. The paradigm includes four consecutive 1-week periods (table 1), with a strictly prescribed and biologically verified regimen of caffeine intake or abstinence for every day of each week. For caffeine consumption phases, the typical population pattern of caffeine use has been simulated by operationally defining average dietary use as the ingestion of caffeine 1.75 mg/kg of body weight (the approximate equivalent of 1.0–1.5 cups of coffee) taken 3 times daily at 09.00, 11.00, and 15.00. Previous research has shown that with few exceptions caffeine tolerance plateaus [47, 48] and withdrawal effects abate [34, 49] within 3–5 days of regular intake. Advantages of the new paradigm include being able to examine and compare the separate acute and chronic effects of caffeine in the one experiment, while also controlling for the effects of caffeine withdrawal, withdrawal reversal, and tolerance. Participants were instructed not to drink any caffeinated beverages throughout the 4 weeks of study, and were provided with decaffeinated coffee and tea to facilitate adherence to instructions. As summarised in table 1, the four 1-week conditions alternated between caffeine and caffeine abstinence in counterbalanced order. One of the four conditions represented ‘long-term’ abstinence (or caffeine ‘washout’), and involved placebo being ingested for 6 days followed by placebo ‘challenge’ on the 7th day (PP condition). The PP condition served as a caffeine-free baseline for comparison purposes with the other conditions of the experiment. Placebo for 6 days followed by caffeine challenge on the 7th day (PC) provided the basis for assessing the acute effects of caffeine unconfounded by caffeine withdrawal. Caffeine for 6 days followed by placebo challenge on the 7th (CP) provided the basis for assessing the acute withdrawal effects of caffeine. Finally, caffeine for six days followed by caffeine challenge (CC) provided the basis for assessing the acute effects of caffeine in the context of habitual use (table 1). Direct testing for the presence of tolerance, withdrawal, and withdrawal reversal can be achieved by systematically comparing the PC and CP conditions to PP and CC, thereby obtaining a true measure of the net effects of habitual caffeine consumption.

liva at 17.00 for every day of the study. Alkaysi et al. [50] have shown that caffeine concentrations in blood and saliva are highly correlated, and saliva was chosen as the preferred matrix in the present and previous studies [41, 45, 46] because of ease of collection. Caffeine levels generally rise progressively throughout the day due to intermittent consumption of caffeine beverages, and plateau in the late afternoon and early evening after the last beverage of the day has been consumed. Consequently, the best single-sample estimate of systemic caffeine level is generally obtainable in the late afternoon [51]. In addition to afternoon samples, participants were asked to provide a saliva sample 10 min after arriving at the laboratory, before they took their first capsule for that day, thereby providing a check on compliance with the caffeine/placebo regimen at each laboratory visit. Samples were stored at –20 ° C until assayed using reversed-phase high performance liquid chromatography (HPLC).

Compliance with the Caffeine Regimen To check compliance with the prescribed caffeine/abstinence regimen, participants were asked to supply a 5-ml sample of sa-

Electroencephalogram On the 7th day of each week (challenge day), participants attended a 2-hour laboratory session. Participants were seated in a comfortable chair for the duration of recording, with support provided for the head and neck to minimise movement artefact. Using a commercially available cap (EASY CAP EC40; EASYCAP GmbH, Herrsching-Breitbrunn, Germany) and in accordance with the international 10-20 system, EEG activity was recorded for a baseline of 5 min eyes closed and 5 min eyes open as well as during two 15-min performance tasks (outlined below) at 32 sites, as follows: Fp1, Fp2, F3, F4, C3, C4, P3, P4, O1, O2, F7, F8, T7, T8, P7, P8, Fz, Cz, Pz, FC1, FC2, CP1, CP2, FC5, FC6, CP5, CP6, TP9, TP10, PO9, Iz and PO10. With the electrode cap in place, each electrode site was prepared by abrading the skin and bridging the gap between electrode and scalp with a chloride-free abrasive electrolyte gel. Impedances were assessed with BrainVision Recorder (v. 1.03.0001) software (Brain Products, GmbH, München, Germany) and were kept below 8 k⍀. Silver/silver-chloride (Ag/ AgCl) recessed ring electrodes were used. All electrode cables were individually shielded (ActiShieldTM) and all channels were amplified against the average of all connected inputs. A QuickAmp 40 EEG amplifier (Brain Products) was used in conjunction with BrainVision Recorder and BrainVision Analyser (v. 1.05.0003) software (Brain Products). Sampling rate was 1,000 Hz, and fre-

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199

quencies between 0.5 and 30 Hz were used for analysis. Vertical and horizontal electro-oculogram activity were also recorded to control for eye blink artefact. Following offline ocular correction, artefact-free activity around correct positive responses during the task was selected for analysis. For all periods, EEG spectral power was determined for delta (0.5–3.4 Hz), theta (3.5–7.5 Hz), alpha (7.6–12 Hz) and beta (12.1–30 Hz) bandwidths, using the fast Fourier transformation with a Hanning window of 20%. Performance Performance was measured using two modified versions of the sustained attention to response task (SART), consisting of a series of numbers from 1 to 9 presented individually in a random (Random) or fixed order of 1–9 repeated (Fixed) [52]. Participants were required to respond with a spacebar press to every digit except the number 3 (NOGO stimulus). In an effort to minimise disengagement from the Fixed task resulting from the predictability of the stimulus sequence, participants were instructed to respond as soon as they could following the disappearance of the digit from the screen. Stimulus duration was 150 ms with an inter-stimulus interval of 1,000 ms. During the Random SART, the NOGO stimulus appeared on average every 9 s (range of 1–19.4 s). Each participant completed 15 min of both Fixed and Random versions of the task at each laboratory session, with a 1-min rest period between the two, presented in counterbalanced order across participants and weeks. This resulted in a total of 779 stimuli per task, with an average of 86 NOGOs in both tasks. Following the gelling-up procedure and EEG recording for eyes closed and eyes open, information was given about the task and task order (Random/Fixed or Fixed/Random). Participants were instructed to press the spacebar on the keyboard, which brought them to a screen that gave them a 10-second countdown to the beginning of the task. A crosshair in the centre of the screen marked the position above which stimuli appeared. Mood After the performance task, mood was assessed using the bipolar version of the Profile of Mood States (POMS, EdITS, San Diego, Calif., USA). This is a 72-item 4-point (0–3) adjective rating scale that measures subjective mood or affective states along six bipolar dimensions: clear-headed–confused, elated–depressed, confident–unsure, agreeable–hostile, energetic–tired, and composed–anxious. For each dimension, a higher score indicates more positive affect. Drug Guessing Since responses to caffeine may be influenced by consumer expectations [53], participants were asked to guess whether they had placebo or caffeine during the week preceding each laboratory visit and to guess what had been ingested in the laboratory. Following a procedure we have used previously [23, 41, 45, 46], participants were asked whether ‘you think you had’ caffeine or placebo during the 6-day run-in period and during the laboratory session itself. Guesses were recorded, and participants were allowed to revise their guesses as the study progressed. Statistical Analysis For reasons of clarity and brevity, the 32 electrode sites were reduced to nine brain regions as follows: left-frontal (Fp1, F3, F7, Fc5), mid-frontal (Fz, Fc1, Fc2), right-frontal (Fp2, F4, F8, Fc6),

200


left-central (C3, T7, Cp5), mid-central (Cz, Cp1, Cp2), right-central (C4, T8, Cp6), left-posterior (P3, P7, PO9, TP9), mid-posterior (Pz, Iz, O1, O2), and right-posterior (P4, P8, PO10, TP10). All EEG power values are given in microvolts squared (␮V2), and the main statistical analyses involved separate two-way repeatedmeasures ANOVA for each brain region and each bandwidth (delta, theta, alpha, and beta). Using a probability alpha level of 0.05, Greenhouse-Geisser values were used to determine statistical significance. Because preliminary analyses revealed few differential effects of caffeine on EEG power between the two versions of SART (results not shown), data for the two versions of the task were reduced to a single task and incorporated into a three-level period factor (eyes closed, eyes open, task). Hence, the main ANOVA model consisted of a 4 (caffeine: PP, PC, CP, CC) ! 3 (period) repeated-measures analysis. In addition, because experimental sessions were held at three different times (08.30, 11.00, and 13.30), the main two-factor repeated-measures ANOVA was repeated with time of day added as a between-groups factor (i.e. three-factor mixed-model ANOVA). Contingent on a main effect of caffeine on EEG power, post hoc pairwise comparisons were performed in which PP (abstinence) served as baseline against which PC, CP, and CC were tested for significance of difference. This set of post hoc tests included one additional specific test to facilitate comparison between the present study and previous studies by Reeves et al. [40] and Jones et al. [39] that suggested EEG may be sensitive to caffeine withdrawal. Since neither of the two previous studies included a caffeine-free manipulation comparable to the PP condition in the present study, evidence of withdrawal effects in those studies was inferred by comparing an acute caffeine-abstinence condition (comparable to CP in the present study) to ‘usual’ caffeine diet (comparable to CC in the present study). Thus, contingent on a main effect for caffeine, CP was tested against CC, additionally to the set of post hoc tests involving PP as baseline, and Bonferroni correction applied to the entire set of four comparisons (i.e. PP ! PC, PP ! CP, PP ! CC, and CP ! CC). In the event of main effects being found for the period factor, post hoc pairwise comparisons were performed in which eyes closed served as a baseline. Finally, repeated-measures ANOVA, incorporating a single four-level caffeine factor, was used to examine performance and mood data, which when significant was followed by post hoc pairwise comparisons between the four conditions (6 comparisons in all) using t test for related means. As above, Bonferroni correction was applied to these separate sets of post hoc tests. For performance data, total responses for the Fixed and Random SART were initially examined separately and each task divided into four equal time epochs labelled Time 1, 2, 3 and 4, with that factor being incorporated into the ANOVA in order to test for possible changes in performance as a function of time.

Results

Electroencephalogram Table 2 summarises the results of the main two-way repeated-measures ANOVA for each of the four EEG bands at each of nine brain regions. For delta power, there Keane/James/Hogan

Table 2. Summary of F values for 4 ! 3 repeated-measures ANOVA of caffeine (abstinence, acute administration, withdrawal, ha-

bitual use; d.f. = 3, 63), period (eyes closed, eyes open, task; d.f. = 2, 42), and interaction (d.f. = 6, 126) at each of nine brain regions for delta, theta, alpha, and beta EEG power bandwidths Brain region

Delta (0.5–3.4 Hz)

Theta (3.5–7.5 Hz)

Alpha (7.6–12 Hz)

Beta (12.1–30 Hz)

caffeine

period

caffeine ! period

caffeine

period

caffeine ! period

caffeine period

caffeine ! period

caffeine

period

caffeine ! period

LF MF RF LC MC RC LP MP RP

2.25 1.08 2.44 1.10 1.01 1.02 1.21 1.26 1.10

3.60 0.86 4.53* 0.35 0.49 0.58 0.40 0.45 0.70

2.02 1.05 2.03 1.07 1.00 0.99 1.06 1.21 1.07

5.29* 2.23 2.52 1.56 0.44 0.81 1.07 0.88 1.96

14.29*** 18.87*** 6.95* 3.75 1.30 7.35* 16.81* 30.82*** 32.10***

0.41 1.43 2.45* 2.96* 2.24 1.83 2.22 1.41 1.95

3.20* 66.58*** 1.38 79.31*** 1.41 72.25*** 2.76 67.25*** 2.77 112.98*** 4.74** 84.37*** 1.43 57.88*** 1.25 115.59*** 1.28 101.83***

0.81 0.23 1.00 0.18 0.66 1.84 0.69 0.63 0.71

0.71 0.74 0.76 0.81 0.35 0.43 2.48 1.03 0.49

83.89*** 108.19*** 82.34*** 103.76*** 89.01*** 91.98*** 87.04*** 120.05*** 103.79***

0.67 0.75 0.38 1.26 0.27 1.47 0.74 0.33 0.30

LF = Left-frontal; MF = mid-frontal; RF = right-frontal; LC = left-central; MC = mid-central; RC = right-central; LP = left-posterior; MP = mid-posterior; RP = right-posterior. ␩p2 ranges = 0.01–0.18 (caffeine), 0.02–0.85 (period), 0.02–0.13 (caffeine ! period). * p < 0.05; ** p < 0.01; *** p < 0.001.

was no significant caffeine main effect at any brain region. There was a significant period effect at the rightfrontal brain region. However, using eyes closed as baseline, pairwise comparisons of EEG activity did not reach statistical significance for either eyes open or the task after applying Bonferroni correction (p values 1 0.05). Table 2 shows that there was no significant caffeine ! period interaction for delta power at any brain region. There were no effects of time of day in the delta bandwidth (p values 1 0.05). For theta power, a significant main effect for caffeine was found at the left-frontal region (table 2), but none of the post hoc pairwise comparisons reached significance after Bonferroni correction (p values 1 0.05). A significant main effect for period on theta was found at seven of the nine brain regions (table 2). Post hoc comparisons showed that theta power was significantly higher during eyes open compared to eyes closed at the mid-central and mid-posterior regions (fig. 1a), and significantly higher during the task compared to eyes closed at left- and mid-frontal and left-, mid- and right-posterior regions (fig. 1a). Table 2 shows that there was a significant caffeine ! period interaction effect for theta at one of the nine regions, namely, right-frontal. Post hoc testing revealed this to be due to theta power being unaffected by caffeine during eyes closed and eyes open (p values 1 0.05), whereas an effect of caffeine was evident during the task. Spe-

cifically, during the task, PC and CP values for theta power were significantly higher than PP (p values ! 0.05), whereas values for PP and CC theta power did not differ significantly from one another (p 1 0.05). Indeed, on closer examination of the results, it was found that mean PC/CP values for theta power were higher than mean PP/ CC values at all brain regions (fig. 2). Using t test for related means and Bonferroni correction, mean PC/CP values were found to be significantly higher than mean PP/ CC values at right-frontal, left-central, and right-central regions (p values ! 0.05). There were significant main effects of time of day for left-, mid- and right-posterior brain regions in the theta bandwidth. At left-posterior, theta power was lowest at session 1, followed by session 3 and highest at session 2 [F(2, 19) = 6.86, p ! 0.01, ␩p2 = 0.42]. At mid-posterior, theta power was lowest at session 1, higher at session 2 and highest at session 3 [F(2, 19) = 5.79, p ! 0.05, ␩p2 = 0.38]. At right-posterior, power was lowest at session 1, followed by session 3 and highest at session 2 [F(2, 19) = 3.95, p ! 0.05, ␩p2 = 0.29]. At mid-posterior, there was also a period ! time of day interaction [F(4, 38) = 3.53, p ! 0.05, ␩p2 = 0.27]. Here, during eyes closed and eyes open, power was higher at session 2 than at sessions 1 and 3. A significant main effect for caffeine on alpha was found at the left-frontal and right-central brain regions (table 2). Post hoc testing revealed the same pattern of effects at both regions. Specifically, CP alpha power was

Caffeine and EEG


201

μV2 12

Eyes open Task

Theta

8

*

*

*

*

30

*

4

μV2

*

Mean PC/CP

a 0

Mean PP/CC

μV2 0

25

–10 –20

b –30

** * Alpha

* *

* *

*

* *

*

*

* *

*

*

*

*

20

*

μV2 10

*

*

*

*

*

*

0 –10

c –20

*

*

15

*

Beta LF

* MF

* RF

LF

* LC

* MC

*

*

*

RC

LP

MP

* RP

MF

RF

LC

MC

RC

LP

MP

RP

Brain region

Fig. 1. Theta (3.5–7.5 Hz, a), alpha (7.6–12.0 Hz, b), and beta (12.1– 30.0 Hz, c) bandwidths showing mean differences in EEG power (␮V2) during eyes open compared to eyes closed and during performance of a vigilance task compared to eyes closed for each of nine brain regions: LF = left-frontal; MF = mid-frontal; RF = right-frontal; LC = left-central; MC = mid-central; RC = rightcentral; LP = left-posterior; MP = mid-posterior; RP = right-posterior. * p ! 0.05.

Fig. 2. Mean PC/CP and mean PP/CC theta power (␮V2) during

significantly higher than PP at both regions (p values ! 0.05), but none of the other pairwise comparisons was significant (p values 1 0.05). A highly significant main effect of period on alpha was found at all regions (table 2). Post hoc testing revealed a consistent pattern for all brain regions (fig. 1b), with significant decreases in alpha power during eyes open compared to eyes closed and larger decreases during the task. There was no significant caffeine ! period interaction effect on alpha power for any brain region, and there were no time-of-day effects (p values 1 0.05). Table 2 shows that there was no significant caffeine main effect on beta power at any brain region, whereas there was a highly significant period main effect for all regions. Post hoc comparisons revealed significant increases in beta power during eyes open compared to eyes closed at left-, mid- and right-frontal, left-central, and mid- and right-posterior regions (p values ! 0.05; fig. 1c). In addition, figure 1c shows that compared to eyes closed there were significant decreases in beta power during the task at all nine brain regions (p values ! 0.05). There was

no significant caffeine ! period interaction effect on beta power for any brain region, and there were no timeof-day effects (p values 1 0.05).

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the task at all brain regions. PP = 6 days of placebo followed by 1 day of placebo challenge; PC = 6 days of placebo followed by 1 day of caffeine challenge; CP = 6 days of caffeine followed by 1 day of placebo; CC = 6 days of caffeine followed by one day of caffeine. For other abbreviations see figure 1. Pairwise differences significant after Bonferroni correction: * p values ! 0.05.

Performance Performance scores on both versions of the SART were computed by expressing the number of correct GO responses (i.e. true positives: button presses in response to any stimulus not a ‘3’) as a percentage of the total number of responses made (i.e. true and false positives). As such, scoring took account of actual responses only, while ignoring non-response (true and false negatives). A 4 (PP, PC, CP, CC) ! 2 (Random, Fixed) ! 4 (Time 1, Time 2, Time 3, Time 4) repeated-measures ANOVA revealed no significant effects of caffeine [F(3, 63) = 2.43, p 1 0.05, ␩p2 = 0.10], task type [F(1, 21) = 2.40, p 1 0.05, ␩p2 = 0.10], or time epoch [F(3, 63) ! 1]. There were no significant two-way or higher-order interaction effects (p values 1 0.05), and there were no time-of-day effects (p values 1 0.05).

Keane/James/Hogan

Table 3. Means and standard deviations (in parentheses) for salivary caffeine level (␮g/ml)

Drug regimen

Caffeine (1.75 mg/kg 3 times daily) Placebo

Afternoon samples Morning samples (n = 144) (n = 88) days 1–6a day 7b

60

PP

58

PC CP

56

52

3.46 (2.95) 0.14 (1.01)

1.01 (1.58) 0.30 (0.99)

a HPLC assays performed on all afternoon samples from a random selection of 6 participants. b HPLC assays performed on all morning samples from all participants during their four weekly visits to the laboratory.

CC

54

* *

50 48 46 44 42 40 Clear-headed – Elated – confused depressed

Mood A separate one-way ANOVA for caffeine was performed for each of the six POMS dimensions, and a significant effect was found for two, clear-headed–confused [F(3, 63) = 3.44, p ! 0.05, ␩p2 = 0.14] and energetic–tired [F(3, 63) = 3.23, p ! 0.05, ␩p2 = 0.13]. Post hoc testing with Bonferroni correction yielded the same pattern of results for both dimensions. Specifically, as can be seen from figure 3, participants reported being significantly less clearheaded–more confused [t(21) = 3.51, p ! 0.05] and less energetic–more tired [t(21) = 3.50, p ! 0.05] in the CP condition compared to the PC condition. There were no time-of-day effects (p values 1 0.05). Salivary Caffeine Morning samples (day 7) from all 22 participants were analysed (n = 88) as were all afternoon samples (6 per week for 4 weeks) from a random sample of 6 (27%) participants. As a result, a total of 232 samples were analysed, representing approximately 38% of the total of 616 samples collected. Results of HPLC analysis of the collected samples are summarised in table 3. A paired samples t test of all participants’ day-7 samples indicated no significant difference in salivary caffeine levels between caffeine and placebo days [t(43) = –0.03, p 1 0.05]. Similarly, a paired samples t test of the 6 randomly selected afternoon samples for days 1–6 indicated a highly significant difference in salivary caffeine levels between caffeine and placebo days [t(11) = –4.74, p ! 0.01].

Confident – unsure

Agreeable – hostile

Energetic – Composed – tired anxious

Fig. 3. Mean scores (8SE) for PP, PC, CP, and CC for the six di-

mensions of the bipolar POMS measured during each of four weekly laboratory visits. * p ! 0.05.

at each of four laboratory sessions. Overall accuracy of guessing did not improve with practice, as correct guesses were highest in week 1 (21 correct out of a total of 44 guesses) and lowest in week 4 (18 correct out of 44). ␹2 analysis confirmed that the number of correct guesses did not differ significantly from chance whether participants received caffeine or placebo under either run-in or challenge conditions (0.18 6 ␹2 ^ 2.90, 0.09 6 p ^ 0.67).

Discussion

Drug-Guessing Participants reported whether they had received caffeine or placebo on 8 occasions, representing the ‘run-in’ phase for each of 4 weeks plus the ‘challenge’ dose taken

The main aim of the present study was to investigate the effects of caffeine on EEG, while controlling for possible confounding due to caffeine withdrawal and withdrawal reversal. Omnibus-F testing revealed a few modest effects of caffeine on EEG power (table 2), but not all were sufficiently robust to survive post hoc testing with Bonferroni correction. Specifically, the caffeine main effect observed for the theta bandwidth at the left-frontal region did not yield significant post hoc pairwise differences. However, post hoc testing of the caffeine ! period interaction in the theta bandwidth at the right-frontal region revealed that theta power was unaffected by caffeine during eyes closed and eyes open, but was significantly higher during task performance when participants were

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caffeine-challenged (PC condition) and caffeine withdrawn (CP). This trend was more pronounced when mean values for caffeine challenge and withdrawal were compared to the mean values for caffeine abstinence (PP) and dietary use (CC). In addition, evidence of caffeine withdrawal effects was found in the alpha bandwidth at both the left-frontal and right-central brain regions, where power was significantly higher when participants were caffeine-withdrawn than when abstinent (PP condition). No effect of caffeine on EEG power was found in either the delta or beta bandwidths. There is a level of consistency between some of the findings of the present study and those of previous studies of caffeine and EEG. As in the present study, previous reports of caffeine-induced effects have generally involved theta and alpha power bandwidths [54], with few if any effects being reported for delta and beta bandwidths. In addition, just as Reeves et al. [40] and Jones et al. [39] had reported increases in theta power at a number of sites following caffeine withdrawal, withdrawal-induced increases in theta (and alpha) power were also observed in the present study. Nevertheless, the similarities are less than perfect. In the present study, increased theta power was observed only during task performance, whereas a task manipulation was not included in the studies by Reeves et al. [40] and Jones et al. [39]. In addition to increases in theta power, we also observed increases in power in the alpha bandwidth during acute withdrawal (CP condition) compared to abstinence (PP condition) at the left-frontal and right-central brain regions. Here, the effect was somewhat more generalised than for the theta bandwidth in that the increases in alpha power were evident during all three behavioural conditions (eyes closed, eyes open, and task) rather than being restricted to the task as was the case for theta power. The finding of increased theta and alpha power following acute withdrawal relative to abstinence is evidence of withdrawal reversal, and the finding of increased theta power following acute challenge relative to abstinence and dietary use, with no difference between the latter two, is evidence of the development of tolerance. Although it would be tempting to interpret the withdrawal-induced increases in theta and alpha power as biological confirmation of counter-stimulant effects such as lethargy and increased sleepiness, other findings in the present study cast doubt on the adequacy of any such straightforward inference. In particular, increases in theta power were observed following acute caffeine withdrawal and acute caffeine challenge. Indeed, the mean 204


PC/CP theta values were higher than the mean PP/CC values at all brain regions in absolute terms (fig. 2), with the differences proving to be statistically significant at three of the nine regions. Two of the regions at which these differences in theta power were significant (rightfrontal and right-central) have been implicated in inhibitory control [55, 56], an activity involved in the performance of the SART. The other region at which a significant difference was found (left-central) has been associated with silent verbalisation [57], a strategy participants may employ when searching for the target NOGO stimulus while monitoring successive stimulus presentations. Present results suggest that these functions may be sensitive to the disruptive effects of acute changes in caffeine status, whether the changed state involves acute challenge or acute withdrawal of the drug. Because previous studies had not included a condition comparable to the PC condition used in the present study, there was no opportunity in those studies to compare the effects of acute challenge and acute withdrawal. That is, while the notion that caffeine is a ‘stimulant’ seems to offer a degree of theoretical coherence to the finding that caffeine withdrawal increases theta and alpha power, this apparent coherence is undermined by the finding in the present study that similar increases in theta power were observed for both acute withdrawal and challenge conditions. If anything, the results for theta power could suggest that relative to the comparatively stable states of caffeine abstinence and dietary use, change in drug state, whether in the form of acute withdrawal or acute challenge, is disruptive to electrophysiological activity in the brain. In contrast to the relatively few and modest effects observed for caffeine, high levels of significance and consistency were found for the effects of the behavioural manipulations on EEG activity, especially in the alpha and beta bandwidths. Alpha power decreased significantly for eyes open relative to eyes closed at all brain regions, and decreased further during task performance at all regions (fig. 1b). These results are consistent with the generally accepted understanding regarding activity in the alpha bandwidth such that alpha power decreases during eyes open relative to eyes closed, and is inversely associated with cognitive activity [58]. Also, consistent with general understanding [58], we observed effects in the beta bandwidth where power increased significantly during eyes open relative to eyes closed at six of the nine brain regions and decreased significantly during task performance relative to eyes closed at all brain regions (fig. 1c). Importantly, the overall findings for the behavKeane/James/Hogan

ioural manipulations help to validate the findings for caffeine. That is, the finding of few and modest caffeine effects cannot be dismissed as merely reflecting measurement insensitivity, when the simple behavioural manipulations of eyes closed, eyes open, and performance of a repetitive task produced many pronounced and robust EEG effects. A substantial body of evidence now exists indicating that improvements in human performance, long held to be primary effects of dietary caffeine, are more readily attributable to reversal of withdrawal effects that accompany brief abstinence [3]. Whereas a number of previous studies found decrements in performance associated with caffeine withdrawal and reversal of performance decrements when caffeine is re-ingested [23, 38, 45, 46, 59], no such withdrawal and reversal of withdrawal effects were observed during task performance in the present study. The absence of caffeine-induced performance effects in the present study could have been due to the presence of ceiling effects. When selecting a task for use in the present study, high priority was placed on obtaining measurable change in EEG activity, while also having a task that would be responsive to caffeine. The SART task appeared to satisfy these dual objectives. The task had been employed successfully in previous EEG research [52] and its simple repetitive nature recommended its use on the grounds that previous studies have shown such tasks to be generally more responsive to caffeine than tasks involving complex higher-level cognitive processes [3]. The presence of ceiling effects is evidenced by participants consistently achieving high levels of accuracy, ranging from 89 to 100% across all conditions. Thus, while the task was reliable in producing changes in EEG activity, task performance was not responsive to the caffeine manipulations. As with performance, a substantial body of evidence now exists indicating that caffeine-induced improvements in subjective mood are also mostly attributable to reversal of withdrawal effects that accompany brief abstinence [3]. In this regard, the findings for mood in the present study were broadly consistent with findings from recent previous studies that controlled for withdrawal and withdrawal reversal [23, 38, 41, 59]. Mood tended towards being most positive in the acute caffeine condition and most negative in the withdrawal condition, and this pattern reached statistical significance for the dimensions of clear-headed–confused and energetic–tired (fig. 3). There was no evidence of improved mood during dietary use relative to abstinence.

Caffeine and EEG

Conclusion

Caffeine was found to have few and modest effects on EEG in the theta and alpha bandwidths, and no effects in the delta and beta bandwidths. Findings were most robust in relation to increases in theta and alpha power following caffeine withdrawal. However, whereas theta power increased in response to caffeine withdrawal, the finding that theta power increased similarly in response to caffeine challenge and acute withdrawal does not lend itself to straightforward inferences about caffeine having direct stimulant effects on electrophysiological activity in the brain. Rather, the findings are suggestive of a perturbation or disruption to ‘normal’ brain activity by both caffeine challenge and withdrawal relative to activity during caffeine abstinence and dietary use (when the processes of withdrawal, withdrawal reversal, and tolerance have abated or stabilised). Nevertheless, the overall effects of caffeine on EEG activity were modest, as evidenced by the comparatively pronounced effects of the behavioural manipulations, especially in the alpha and beta bandwidths. As this study represents the first published attempt to examine the effects of caffeine on EEG while controlling for withdrawal, withdrawal reversal, and tolerance, it is reasonable to conclude that further studies are needed, employing similar controls, to clarify the extent to which the EEG effects of caffeine, though apparently modest, can be reliably characterised. Acknowledgements The authors gratefully acknowledge the programming assistance of Dr. Ian Stewart, the technical advice of Drs. Mark Elliott, Richard Roche, and Paul Dockree and the technical support provided by Mr. Declan Coogan.

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