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ORIGINAL INVESTIGATION
Differential responsiveness to caffeine and perceived effects
of caffeine in moderate and high regular caffeine consumers
A. S. Attwood &S. Higgs &P. Terry
Received: 13 July 2006 /Accepted: 6 November 2006 / Published online: 29 November 2006
#Springer-Verlag 2006
Abstract
Rationale Individual differences in responsiveness to caf-
feine occur even within a caffeine-consuming population,
but the factors that mediate differential responsiveness
remain unclear.
Objectives To compare caffeine’s effects on performance
and mood in a group of high vs moderate consumers of
caffeine and to examine the potential role of subjective
awareness of the effects of caffeine in mediating any
differential responsiveness.
Materials and methods Two groups of regular caffeine
consumers (<200 mg/day and >200 mg/day) attended two
sessions at which mood and cognitive functions were
measured before and 30 min after consumption of 400-mg
caffeine or placebo in a capsule. Cognitive tests included
visual information processing, match-to-sample visual
search (MTS) and simple and choice reaction times. Post-
session questionnaires asked participants to describe any
perceived effect of capsule consumption.
Results High consumers, but not moderate consumers,
demonstrated significantly faster simple and choice reac-
tion times after caffeine relative to placebo. These effects
were not attributable to obvious group differences in
withdrawal or tolerance because there were no group
differences in baseline mood or in reports of negative
affect after caffeine. Instead, the high consumers were more
likely to report experiencing positive effects of caffeine,
whereas the moderate consumers were more likely to report
no effect.
Conclusions The sensitivity of caffeine consumers to the
mood- and performance-enhancing effects of caffeine is
related to their levels of habitual intake. High caffeine
consumers are more likely than moderate consumers to
perceive broadly positive effects of caffeine, and this may
contribute to their levels of use.
Keywords Caffeine .Reaction time .
Psychomotor performance .Reinforcement
Introduction
The mood-enhancing effects of caffeine have been well
documented (e.g. see Smith 2002 for review). However,
there is evidence to suggest that not all habitual caffeine
consumers report such effects of caffeine. Evans and
Griffiths (1992) stratified a group of regular caffeine
consumers into caffeine “choosers”and “non-choosers”
based on a caffeine-vs-placebo choice procedure. In tests of
the subjective effects elicited by caffeine, the “choosers”
reported more positive effects of caffeine on mood (e.g.
increased vigor, friendliness, energy) in comparison with
the “non-choosers”who tended to report negative subjec-
tive effects (e.g. tension, anxiety, jitteriness). The observa-
tion that the “choosers”tended to show negative subjective
effects of placebo (e.g. headache and fatigue) relative to
“non-choosers”indicates that the “choosers”might have
experienced greater caffeine withdrawal symptoms com-
pared with the “non-choosers”, and this influenced their
responding. This is consistent with the observation that the
facilitatory effects of caffeine are often observed in
deprived consumers of caffeine, but not in non-consumers
Psychopharmacology (2007) 190:469–477
DOI 10.1007/s00213-006-0643-5
A. S. Attwood (*):S. Higgs :P. Terry
School of Psychology, University of Birmingham,
Edgbaston,
Birmingham B15 2TT, UK
e-mail: asa894@bham.ac.uk
(James and Rogers 2005; Rogers et al. 2003; Smit and
Rogers 2002). However, it is also possible that the caffeine
“choosers”had developed tolerance to the aversive,
anxiety-related effects of caffeine that occur at high doses
(Jacobson and Thurman-Lacey 1992), which differentiated
their responses from those of “non-choosers”.
Smit and Rogers (2000) have also argued that levels of
daily caffeine intake may influence the effects of caffeine
on cognitive performance. They reported that positive
effects of caffeine on cognitive performance were more
robust in a group of high caffeine consumers (>200 mg/day)
relative to a group of low consumers (<100 mg/day).
However, because all participants were overnight withdrawn
from caffeine, it was not possible to determine whether this
was due to a greater negative impact of withdrawal in the
high consumers or to differences in sensitivity to the effects
of caffeine. In a similar vein, previous research from our
laboratory has highlighted individual differences in the
effects of caffeine on cognition and mood in a group of
regular caffeine consumers: Not all participants who re-
ceived a 400-mg dose of caffeine showed significant
improvements in responding (Attwood 2006). Interestingly,
it has also been shown that individuals consuming high
levels of caffeine (>350 mg/day) demonstrate an atten-
tional bias for caffeine-related words, whereas moderate
consumers (100–250 mg/day) and non-consumers do not
(Yeomans et al. 2005). Moreover, Yeomans et al. (2005)
found that attentional bias in the high consumers correlated
both with reported levels of habitual caffeine intake and
caffeine craving.
Hence, although there is evidence to suggest that
individual differences in habitual consumption of caffeine
influence the response to caffeine, the factors that mediate
this differential responsiveness remain unclear. The aim of
the present study was, firstly, to examine the effect of
habitual consumption of caffeine on both cognitive perfor-
mance and conventional mood measures. It was predicted
that only the high consumers would demonstrate significant
improvements in cognitive performance after caffeine.
Secondly, our aim was to distinguish between several
possible explanations for this differential responsiveness
by taking extensive measures of the effects of caffeine on
mood in both moderate and high consumers and asking
participants about their perceptions of any effects experi-
enced at the end of the test.
The actual and perceived effects of a 400-mg dose of
caffeine and placebo were measured in moderate- and high
caffeine consumers. All participants were regular tea and/or
coffee drinkers, and a cut-off consumption criterion of
200 mg/day of caffeine (Smit and Rogers 2000) was used to
distinguish the moderate (<200 mg/day) from high
(>200 mg/day) consumer groups. Because, in our labora-
tory, a dose of 400 mg of caffeine (but not 250 mg) has
produced facilitatory effects on cognition and mood
compared with placebo (Attwood 2006) and significant
improvements in performance and arousal have previously
been demonstrated after ingestion of 400 mg of caffeine
(e.g. Flaten et al. 2003;Loke1988; Jacobson and Edgley
1987), this dose was selected as being likely to produce
robust effects on performance. Furthermore, because we
wanted to test whether differential tolerance might
underlie any differences in responding between moderate
andhighconsumergroups,wechosearelativelyhighdose
of caffeine that has been shown previously to induce some
aversive, anxiety-related effects (Childs and de Wit 2006;
Jacobson and Thurman-Lacey 1992).
The cognitive test battery used to measure performance
comprised tasks that have been shown to be sensitive to the
effects of caffeine. Measures of both simple reaction time
(SRT) and choice reaction time (CRT) were taken; many
studies have reported decreased reaction times over a wide
range of caffeine doses (e.g. Brice and Smith 2002; Smit
and Rogers 2000; Smith et al. 1994a,b; Lieberman et al.
1987; Roache and Griffiths 1987). In addition, a rapid
visual information processing task (RVIP) and pattern
detection task were chosen because caffeine has been
shown to improve both speed and accuracy in these tests
(Yeomans et al. 2002; Smit and Rogers 2000; Durlach
1998; Warburton 1995; Frewer and Lader 1991). Visual
analogue scales (VAS) were used to measure subjective
mood effects and were adapted from a questionnaire that
has detected significant effects of caffeine on mood
(Richardson et al. 1995).
Materials and methods
Participants
Forty-nine students and staff from the University of
Birmingham, UK (18 male; mean age 23.2 years, range
18–41 years) responded to poster advertisements to take
part in a study examining the cognitive effects of over-the-
counter drugs in return for cash or course credits.
Participants were screened by means of an e-mailed intake
questionnaire to ensure that all were non-smoking, habitual
caffeine consumers. Three participants were excluded due
to not turning up for a scheduled session, and one
participant was excluded due to being unable to swallow
the capsule; thus, 45 participants were included in the
analyses. The participants were separated into two groups
(moderate vs high consumers) whose daily caffeine intake
was less than or greater than 200 mg/day, respectively (see
Table 1). To minimize expectancy effects, participants were
informed that they could receive one of several possible
substances (paracetamol, caffeine, ibuprofen or aspirin).
470 Psychopharmacology (2007) 190:469–477
Experimental design
Each participant attended two sessions (caffeine and
placebo). Inter-session intervals did not fall below 24 h or
exceed 14 days. All testing took place between 1200 and
1800 h, and participants attended their sessions at approx-
imately the same time of day. Test times were balanced
between consumer groups. Capsule administration was
double blind, and approximately equal groups received
placebo first vs caffeine first (14 moderate and 9 high
consumers had placebo first, and 10 moderate and 12 high
consumers had caffeine first). Before each session, partic-
ipants were asked to abstain from all psychoactive
substances from 2300 h the previous night. To promote
compliance, a reminder e-mail was sent the day before
each session, a saliva assay (which was not analysed) was
taken on arrival, and a form confirming abstinence was
signed.
Capsule administration
Caffeine (400 mg; Sigma-Aldrich, Poole, UK) was admin-
istered in a gelatin capsule (Shionogi Qualicaps, size-00,
Madrid, Spain) with water. The placebo capsule contained
arrowroot (Supercook, Leeds, UK).
Cognitive tasks
The cognitive tasks were from the Cambridge Neuropsy-
chological Test Automated Battery (CANTAB; Cambridge
Cognition, Cambridge, UK). To keep the length of the test
session to a reasonable duration, baseline testing on each
session comprised the simple reaction time and choice
reaction time tasks only.
Simple reaction time The simple reaction time task com-
prised two experimental blocks, each of 15 trials. The
stimulus was a small yellow spot, which was presented for
250 ms inside a white circle that remained centrally on
screen throughout the test. Each trial began with the
participants depressing the press-pad key until the yellow
spot appeared, at which point they were required to touch
the screen (inside the white circle) as quickly as possible.
The next trial did not begin until the participants had once
again depressed the press-pad key.
Choice reaction time The choice reaction time task com-
prised four experimental blocks (15 trials each) in which
five white circles were arranged around a central point on
the screen. The white circles were fixed on screen
throughout the test. For each trial, the yellow spot appeared
in one of these five circles, and participants had to touch
this circle as quickly as possible.
For both reaction time tests, the inter-trial interval varied
randomly between 750 and 2,250 ms to reduce anticipatory
responding. Decision and movement latencies were mea-
sured separately. Both tasks were preceded by an unan-
alysed practice block of ten trials.
Match-to-sample visual search An abstract pattern (target)
appeared centrally on the computer screen. After a 1.5-s
interval, distracter patterns appeared in boxes surrounding
this central pattern. Within the array of distracters, one
pattern was an identical match to the central target. To
initiate each trial, a press-pad key was depressed until the
identical pattern was identified. At this point, the partic-
ipants were required to move their hands from the press pad
to touch the identical pattern on screen. If the response was
incorrect (indicated by a red cross on the screen),
participants were asked to continue choosing from the
distracter patterns until they had chosen the correct one.
There were three levels of difficulty that differed according
to the number of distracters: two (level 1), four (level 2) or
eight (level 3). Reaction duration, movement duration and
errors were recorded. The main task was preceded by a
practice block of three trials (one per level), but these data
were not analysed.
Rapid visual information processing A series of single
digits appeared sequentially on the computer screen, and
participants were required to respond, via a press pad,
whenever they identified one of three three-digit number
strings (3-5-7; 2-4-6; 4-6-8). Each digit was displayed for
600 ms before being replaced with the next. The task ran
for six consecutive blocks, each comprising nine target
strings, although the data from the first block were not
analysed (total task time, 6 min). A practice version of
2 min also preceded each test run.
Mood measures Participants completed a pen-and-paper
mood questionnaire on arrival and also immediately before
and after completion of the computer test battery (approx-
imately 30 and 55 min post-capsule ingestion). Seventeen
Table 1 Characteristics of the two participant groups: moderate
consumers (intake <200 mg/day) and high consumers (intake
>200 mg/day) of caffeine
Moderate
consumers (N=24)
High consumers
(N=21)
Gender (m:f) 9:15 9:12
Mean daily caffeine
consumption (mg)
150.7 (44.2) 304.8 (76.4)
Body mass index (BMI) 22.3 (3.2) 21.8 (3.3)
Age (years) 23.7 (4.3) 22.6 (5.8)
Standard deviations are given in parentheses.
Psychopharmacology (2007) 190:469–477 471
adjectives were rated on 100-mm VAS (adapted from
Rogers et al. 1995) from 0 (not at all) to 100 mm
(extremely): “Friendly”,“Headache”,“Alert”,“Cheerful”,
“Drowsy”,“Anxious”,“Energetic”,“Angry”,“Muddled”,
“Calm”,“Tired”,“Dejected”,“Tense”,“Clear-headed”,
“Relaxed”,“Thirsty”and “Jittery”. Participants were told
to consider each line as the extreme spectrum of that
emotion and to transect the line appropriately to show how
they were feeling at that specific moment.
Post-session questionnaires The post-session questionnaire
asked participants to report any perceived effects of the
capsule. First, they were asked to tick “yes”or “no”to the
question “Did you feel any effect of the capsule?”If they
ticked “yes”, space was provided for participants to
describe these effects. For analysis, these responses were
classified as either positive, negative or no effect. The
positive responses mentioned were alertness, cheerfulness,
increased well-being, decreased drowsiness/tiredness and
decreased headache. The negative responses mentioned
were anxiety, tension, jitteriness, deflated mood, headache
and nausea. “No effect”was recorded when participants
ticked “no”to the first part of the question.
Procedure
On arrival, participants confirmed overnight abstinence
from any psychoactive substances and provided a saliva
sample. A baseline mood questionnaire and practice on the
SRT and CRT (two blocks each) were completed immedi-
ately before capsule ingestion. An interval of 30 min then
elapsed to allow for drug absorption, during which time the
participants were asked to sit quietly (reading material, e.g.
current periodicals, was provided). After this interval, the
experimenter returned to administer the tests in the
following order: second mood questionnaire, RVIP, MTS,
SRT, CRT, third mood questionnaire. Finally, participants
completed the post-session questionnaire.
Statistical analyses
Statistical analyses were conducted using Statistical Pack-
age for the Social Sciences (SPSS; version 11.5) for
Windows. For SRT and CRT movement duration data,
reaction times below 100 ms or more than three standard
deviations above an individual’s mean score were deemed
outliers and removed. There were no differences in the
mood changes observed over 30–50 min vs the changes
over 0–50 min, so only the changes from baseline to 50 min
(i.e. the whole test session) are presented.
Independent t-tests examined between-group differences
at baseline for reaction time performance and mood.
Gender and order of treatment (capsule to be consumed)
were included as blocking factors in these analyses but did
not influence the outcome.
Two-way analyses of variance (ANOVAs) with group
(high consumer, moderate consumer) and drug (caffeine,
placebo) as between- and within-subject factors, respec-
tively, were conducted on data from the cognitive tasks.
For SRT, CRT and MTS, reaction times were separated
into two scores: reaction duration (i.e. time between
presentation of distracters and movement from the press
pad) and movement duration (time between movement
from the press pad key to touching the correct pattern on
screen). The number of errors was also analysed for the
RVIP and MTS tasks. A priori planned comparisons were
conducted to analyse the effect of capsule on mood and
performance scores in the high and moderate consumer
groups. Because baseline scores were found to correlate
with the test measures, baseline score was included as a
covariate where possible. Two participants were omitted
from the analyses of the SRT and CRT experimental trials
due to corrupt data.
For the mood ratings, a principle components analysis
was performed using varimax rotation. Analysis of the 17
mood items revealed five factors with eigenvalues greater
than 1 that accounted for 69% of the variance. Items with
loadings greater than 0.5 were retained on each factor
unless the same items had a loading greater than 0.4 on
another factor. The five factors were: “Tense negative
mood”(items loaded onto this factor were: Tense, Jittery,
Angry, Dejected), “Sleepiness/hydration”(items loaded
onto this factor were: Drowsy, Tired, Thirsty), “General
positive mood”(items loaded onto this factor were:
Energetic, Cheerful, Friendly) “Mental clarity”(items
loaded onto this factor were: Alert, Clear-headed, Mud-
dled) and “Mental repose”(items loaded onto this factor
were: Calm, Relaxed). Participant’s scores for each factor
were calculated by adding the scores of all the items
loading onto the factor. Two ratings did not load clearly
onto any factor (anxiety and headache) and were
analysed separately. A two-way ANOVA was conducted
on each mood factor with consumer group as a between-
subjects factor. This analysis reduced the number
comparisons, thus reducing the likelihood of a Type II
error due to conservative corrections for multiple com-
parisons (Bonferroni correction).
Gender and order of capsule administration were
included as blocking factors in the analyses of perfor-
mance and mood; however, their inclusion did not change
an outcome, so these results are not reported. Correlation
analyses also showed that time since having eaten and
time of day of testing did not correlate with the test
measures. All post-hoc multiple comparisons employed
the Bonferonni correction.
472 Psychopharmacology (2007) 190:469–477
Results
Participant characteristics
Some body mass index (BMI) data were missing due to six
participants choosing not to complete the personal details
form in full, therefore the BMI analysis comprised data from
39 participants. The moderate and high consumer groups did
not differ significantly in age (t=0.45; df=43; p>0.05) or
BMI (t=0.44; df =37; p> 0.05). There was a significant
difference between the two groups in daily levels of caffeine
consumption (t=−7.50; df=43; p< 0.0001; see Table 1).
CANTAB test battery
Simple Reaction Time
At baseline, there were no significant differences between
groups on SRT reaction duration or movement duration
(Table 2). There were no significant main effects of caffeine
or group on SRT reaction duration or movement duration.
However, there were significant caffeine-by-group interac-
tions for both SRT reaction duration [F(1, 40)=4.59; p<
0.05] and movement duration [F(1, 40)=4.57; p< 0.05].
Paired t-tests revealed that the high consumers (t=2.80; df =
20; p<0.05), but not the moderate consumers (t= 0.16; df =
21; p>0.05), were significantly faster on SRT reaction
duration after caffeine compared with placebo (Fig. 1a). For
movement duration, the moderate consumers were signifi-
cantly slower after caffeine relative to placebo (t=2.78; df =
21; p<0.05). In contrast, the high consumers were
marginally faster after caffeine, although this difference
was not significant (t= 0.74; df = 20; p> 0.05; Fig. 1a).
Choice Reaction Time
For CRT, there were no significant differences at baseline
between groups for either reaction duration or movement
duration (Table 2). There was a significant main effect of
caffeine for CRT reaction duration [F(1, 40)=8.58; p<0.01],
but no effect of group [F(1, 40)=1.56; p> 0.05] nor a
significant interaction [F(1, 40)=1.12; p>0.05]. Paired
t-tests showed that only the high consumers were signifi-
cantly faster after caffeine compared with placebo (t= 2.78;
df=20; p<0.05; Fig. 1b). For CRT movement durations,
there were no significant main effects of caffeine or group,
nor was there a significant interaction between group and
capsule (Fig. 1b).
MTS and RVIP
There were no significant main effects of capsule or group
on any level of MTS reaction and movement durations (see
Table 3). There was one significant interaction between
group and capsule for level 3 movement duration [F(1, 43)=
4.42; p<0.05]. The group means suggested that the
moderate consumers were slower after caffeine relative to
placebo, with little difference for the high consumers;
however, this difference did not reach significance (t=
−1.84; df=23; p>0.05). There were no other significant
interactions.
There were no errors for level 1 MTS performance;
therefore, only errors for levels 2 and 3 were analysed.
There were no significant main effects or significant
interaction for level 2 errors. There was a significant effect
of caffeine for level 3 errors [F(1, 43)=5.17; p< 0.05] due to
participants making fewer errors after caffeine than placebo.
There was no main effect of group and no significant
interaction (see Table 3). Paired t-tests revealed that the
moderate consumers made significantly fewer errors after
caffeine relative to placebo (t=2.72; df = 23; p< 0.02). After
caffeine, the number of errors made by the moderate
consumers at level 3 was similar to the performance shown
by the higher consumers in both drug conditions.
There were no significant main effects or significant
interactions for RVIP reaction time or errors. A blocked
analysis was conducted by splitting the data into two
equal blocks of 18 targets. For reaction time, there was a
significant effect of block [F(1, 43)=4.46; p<0.05] and a
Table 2 Mean baseline scores for SRT and CRT reaction and
movement durations (ms) and the seven mood factors (mm) in the
moderate and high consumer groups
Measure Consumer group Baseline mean
SRT reaction duration Moderate 307.3 (35.8)
High 297.8 (41.8)
SRT movement duration Moderate 296.7 (57.1)
High 289.9 (53.0)
CRT reaction duration Moderate 340.6 (53.0)
High 327.6 (46.3)
CRT movement duration Moderate 321.3 (64.5)
High 308.3 (51.7)
Tense negative mood Moderate 65.0 (65.6)
High 65.1 (53.4)
Sleepiness Moderate 120.5 (73.2)
High 138.8 (61.4)
Positive mood Moderate 168.3 (52.9)
High 191.0 (32.2)
Mental clarity Moderate 127.1 (41.7)
High 130.8 (29.5)
Mental repose Moderate 141.3 (34.0)
High 134.8 (30.4)
Anxiety Moderate 20.8 (19.3)
High 26.9 (23.2)
Headache Moderate 19.2 (27.2)
High 10.4 (16.3)
Standard deviation given in parentheses
Psychopharmacology (2007) 190:469–477 473
trend towards a block by capsule interaction [F(1, 43)=2.9;
p=0.097] suggesting that the deterioration in performance
across blocks was most prominent in the placebo condi-
tion. Although the caffeine-by-block-by-group interaction
did not reach significance [F(1, 43)=2.30; p>0.05],
inspection of the means suggests that this deterioration
was counteracted by caffeine in the high but not moderate
consumers.
Subjective effects
There were no significant group differences on any of the
mood factors at baseline (Table 2). Caffeine, relative to
placebo, significantly increased ratings of “tense negative
mood”[F(1, 43)=9.30; p<0.005] and “anxious”
[F(1, 43)=4.68; p<0.05] and decreased ratings of “mental
repose”[F(1, 43)=9.31; p<0.005] and “sleepiness”[F(1,
43)=6.88; p<0.02]. There were no other significant main
effects of caffeine. There were no significant main effects
of consumer group.
There was a near significant caffeine-by-group interac-
tion for ratings of “sleepiness”[F(1, 43)=7.25; p= 0.01]. A
priori t-tests showed that the high consumers, but not the
moderate consumers, reported significant reductions in
“sleepiness”(t=−3.54; df=20; p< 0.003) relative to placebo
(Fig. 2).
a
200
220
240
260
280
300
320
340
SRT RD SRT MD SRT RD SRT MD
Moderate Consumers High Consumers
SRT (ms)
Plac ebo
Caffeine
b
200
220
240
260
280
300
320
340
360
380
CRT RD CRT MD CRT RD CRT MD
Moderate Consumers High Consumers
CRT (ms)
Placebo
Caffeine
*
*
*
Fig. 1 a Mean SRT reaction
(RD) and movement (MD)
durations (ms) for moderate
(<200 mg) and high (>200
mg/day) consumer groups.
bMean CRT reaction (RD) and
movement (MD) durations (ms)
for moderate (<200 mg/day) and
high (>200 mg/day) consumer
groups. In all cases, error bars
represent +1 SEM. Asterisk
Significantly different from
placebo, p<0.05
474 Psychopharmacology (2007) 190:469–477
Post-session questionnaires
There were no differences between groups in terms of the
number of participants reporting negative effects of
caffeine. In contrast, the high consumers were significantly
more likely to report positive consequences of caffeine, and
the moderate consumers were more likely to report “no
effect”[χ
2
(2, N=45)
=7.92; p<0.05; see Table 4].
Discussion
The results indicate that high-caffeine consumers
(>200 mg/day) are more likely than moderate-caffeine
consumers (<200 mg/day) to respond to caffeine. We found
that high, but not moderate consumers, showed significant
benefits of caffeine, relative to placebo on SRT and CRT,
and a reduction in rated sleepiness. Furthermore, the high
consumers were more likely to perceive positive effects of
caffeine, as measured by post-session written reports,
suggesting that level of habitual intake may mediate
variations between people in sensitivity and/or responsive-
ness to the positive effects of caffeine.
There are several possible explanations for the differen-
tial responsiveness of high and moderate consumers to
caffeine. First, the high consumers may experience greater
levels of withdrawal during abstinence, which is reversed
by caffeine (James and Rogers 2005). However, analyses of
baseline cognitive and mood ratings found no significant
differences between high and moderate consumers that
would indicate obvious differences in withdrawal symp-
toms. Griffiths et al. (1990) suggest that measurement of
caffeine withdrawal should involve several different mea-
sures and should take place over different time points,
perhaps implying that more extensive measurement of pre-
session mood may have identified differences in withdrawal
-60
-40
-20
0
20
40
60
Change (mm)
Plac ebo
Caffei ne
Tense Anxious Mental Sleepiness Tense Anxious Mental Sleepiness
Negative Repose Negative Repose
Mood Mood
Moderate Consumers Hi
g
h Consumers
*
Fig. 2 Change (mm) in ratings
of “tense negative mood”,
“anxious”,“mental repose”and
“sleepiness”for the caffeine and
placebo sessions in the moderate
and high caffeine consumer
groups. Asterisk Significantly
different from placebo, p<0.01
Table 3 Match-to-sample mean reaction (RD) and movement (MD) durations (ms) and mean numbers of errors for the two participant groups:
moderate consumers (MC: intake <200 mg/day) and high consumers (HC: intake >200 mg/day) of caffeine
Level Measure Score for
placebo
Score for
caffeine
Level Measure Score for placebo Score for caffeine
MC 1 RD (ms) 1329.9 (623.1) 1176.6 (482.3) HC 1 RD (ms) 1046.5 (295.2) 1095.3 (385.8)
MD (ms) 420.8 (155.6) 406.4 (147.3) MD (ms) 405.5 (90.7) 452.5 (197.1)
2 RD (ms) 2045.7 (801.4) 1801.9 (644.3) 2 RD (ms) 1737.7 (650.9) 1815.8 (628.4)
MD (ms) 513.3 (252.5) 536.8 (245.6) MD (ms) 463.4 (175.0) 496.3 (212.3)
3 RD (ms) 3028.7 (1362.4) 3170.5 (1365.0) 3 RD (ms) 2976.6 (1213.2) 2786.4 (962.9)
MD (ms) 552.9 (504.1) 753.5 (436.6) MD (ms) 526.7 (247.0) 458.5 (147.4)
2 No. of Errors 0.13 (0.34) 0.17 (0.48) 2 No. of Errors 0.24 (0.44) 0.14 (0.36)
3 No. of Errors 1.13 (1.12) 0.58 (0.83)
a
3 No. of Errors 0.57 (0.75) 0.48 (0.93)
Standard deviations are given in parentheses.
a
Significantly different from placebo p<0.05.
Psychopharmacology (2007) 190:469–477 475
symptoms. Considering the sensitivity of the VAS to pick
up post-caffeine mood fluctuations, there is no reason to
expect sensitivity shortcomings with these measures at
baseline, and evidence of a caffeine withdrawal state has
previously been detected using the same items (Rogers et
al. 1995).
Alternatively, the moderate consumers may have expe-
rienced aversive effects of caffeine to which the high
consumers had developed tolerance (Evans and Griffiths
1992). However, the post-consumption mood measures
demonstrated that all participants showed evidence of
increases in tense negative mood and anxiety after caffeine
and a reduction in mental repose, and analysis of the post-
session questionnaire (which examined participants’per-
ceptions of the effects of capsule ingestion) showed that
the two groups did not differ in their reports of negative
effects. This suggests that the group differences were not
due to stronger experiences of aversive effects among the
moderate consumers. In fact, compared with the high
consumers, the moderate consumers were less likely to
report positive effects and more likely to report perceiving
no effect of the capsule. It is possible that the stronger
responses of the high consumers are due to greater
sensitivity to the positive effects of caffeine, and this in
turn might influence their level of caffeine consumption. It
would therefore be informative to assess whether positive
effects of caffeine are still perceived in the absence of
tolerance, e.g. in a group of fully withdrawn high
consumers.
The present data suggest that experiencing positive
effects of caffeine is not necessary for the maintenance of
caffeine use, as the moderate consumers still drink
caffeinated beverages every day. For these moderate
consumers, consumption might be maintained by other
factors, such as the association of the drink with environ-
mental reinforcers such as social interactions or breaks from
work, or perhaps mere exposure to tea or coffee is sufficient
to generate liking (Zajonc 1968) and, consequently, to
maintain consumption. Alternatively, the moderate con-
sumers may have expectations regarding the positive effects
of caffeine that influence their choice of beverage.
We found no significant effects of caffeine on RVIP
speed or accuracy. It is possible that the 6-min RVIP task
used in this study was not long enough to reveal significant
effects of caffeine. It may also be relevant that, in our
version of the task, participants were required to identify
specific strings of numbers; in other versions of the task,
the target is more typically any string of odd or even
numbers (Yeomans et al. 2002; Smit and Rogers 2000).
Nevertheless, in the present study, RVIP scores slowed
across the two time blocks in all conditions except for the
high consumers after caffeine, suggesting that caffeine may
have reduced task fatigue among the high consumers only.
This is consistent with other reports that caffeine can offset
a natural deterioration in performance in tasks requiring
sustained attention (Frewer and Lader 1991).
There were significant effects of caffeine on level 3 of
the MTS task in the moderate consumers: Relative to
placebo, the moderate consumers made fewer errors after
caffeine. In comparison, the high consumers did not show
effects of caffeine on any level of the task. However, the
moderate consumers were also slower after caffeine on
level 3 of this task, and the number of errors made was
reduced only to the level of the high consumers, suggesting
that their increased accuracy on the caffeine session may
have reflected a speed–accuracy trade-off. The moderate
consumers also had slowed CRT movement durations after
caffeine, suggesting that caffeine may have had a detri-
mental effect on motoric control in moderate consumers. It
is possible that this reflects the increased tense/negative
responses reported by all consumers after caffeine that was
not offset by positive effects in the moderate compared to
the high consumers.
These findings have implications for caffeine researchers
who group caffeine consumers together as a single homoge-
nous sample and may explain some of the variability in
reports of the effects of caffeine. Although many studies
have reported significant effects of caffeine on mood and
cognitive performance, some studies have found little or no
effect on these measures (e.g. Watson et al. 2000; Smith et
al. 1994a,b,1997; Bruce et al. 1986; Kuznicki and Turner
1986). Methodological issues have often been cited to
explain these null effects. However, the present data
suggest that not taking account of a consumer’s habitual
level of caffeine consumption may reduce the likelihood of
detecting caffeine effects. In addition, the present results
suggest that data collected from high caffeine consumers
are not necessarily representative of the effects of caffeine
in moderate consumers, and vice versa.
In summary, these findings suggest that caffeine consum-
ers differ in their psychomotor and subjective responses to
caffeine. This may be, at least in part, due to a heightened
responsiveness and perception of the positive effects of the
drug in high consumers, which, in turn, may drive their high
levels of use. Why individuals should differ in their
sensitivity to the positive effects of drugs is not clear, but
Table 4 Numbers of moderate (intake <200 mg/day) and high (intake
>200 mg/day) caffeine consumers who reported positive, negative or
no effects after caffeine
Positive
effects
Negative
effects
No
effect
Moderate consumers 4 9 11
High consumers 11 7 3
476 Psychopharmacology (2007) 190:469–477
evidence suggests that individual variations in metabolism
could be important (e.g. Grant et al. 1983). Further research
is needed to ascertain whether the greater responsiveness of
the high consumers is a consequence of chronic self-
administration of high daily doses of caffeine or whether
their high levels of consumption are driven by a heightened
sensitivity to caffeine’s positive effects.
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