Jad091315.dvi

Journal of Alzheimer’s Disease 20 (2010) S85–S94 Astrid Nehlig∗INSERM U 666, Faculty of Medicine, UDS, Strasbourg Cedex, France Abstract. The effects of caffeine on cognition were reviewed based on the large body of literature available on the topic. Caffeine
does not usually affect performance in learning and memory tasks, although caffeine may occasionally have facilitatory or
inhibitory effects on memory and learning. Caffeine facilitates learning in tasks in which information is presented passively; in
tasks in which material is learned intentionally, caffeine has no effect. Caffeine facilitates performance in tasks involving working
memory to a limited extent, but hinders performance in tasks that heavily depend on working memory, and caffeine appears
to rather improve memory performance under suboptimal alertness conditions. Most studies, however, found improvements in
reaction time. The ingestion of caffeine does not seem to affect long-term memory. At low doses, caffeine improves hedonic tone
and reduces anxiety, while at high doses, there is an increase in tense arousal, including anxiety, nervousness, jitteriness. The
larger improvement of performance in fatigued subjects confirms that caffeine is a mild stimulant. Caffeine has also been reported
to prevent cognitive decline in healthy subjects but the results of the studies are heterogeneous, some finding no age-related effect
while others reported effects only in one sex and mainly in the oldest population. In conclusion, it appears that caffeine cannot be
considered a ‘pure’ cognitive enhancer. The indirect action of caffeine on arousal, mood and concentration contributes in large
part to its cognitive enhancing properties.
Keywords: Alertness, attention, caffeine, cognition, learning, memory, mood, performance INTRODUCTION
different modalities (visual, auditory, and tactile). Ex-ecutive functions involve reasoning, planning, evalua- Cognitive processes can be natural or artificial, con- tion, strategic thinking, etc. Language concerns–verbal scious or unconscious. The concept of cognition (from expression, vocabulary, fluency and language compre- the Latin word cognoscere, “to know” or “to recog- hension, and finally, psychomotor functions–are relat- nize”) refers to a capacity for information processing, ed to programming and motor execution. Moreover, all applying knowledge, and changing preferences. Cog- the functions detailed above are influenced by various nition is a complex notion that involves at least memo- factors, such as mood (being happy or sad), the lev- ry, attention, executive functions, perception, language, el of arousal reflecting alertness and energy, physical and psychomotor functions. Each of these individual aspects are themselves complex entities. Indeed, mem- The objectives of the present document are to review ory includes encoding, storage, and retrieval of infor- the components of cognition that can be influenced by mation and can be divided into short-term, long-term caffeine, under either acute or chronic dosage. These and working memory. Attention can be selective, di- aspects have been the subject of numerous studies over vided, or sustained, and perception includes several the past four decades. In all these studies, some biases stages of processing to achieve object recognition in can be found. It must first be reminded that everyoneconsumes caffeine in the form of drinks (coffee, tea,mate, hot chocolate, soft drinks, energy drinks), food ∗Correspondence to: Astrid Nehlig, PhD, INSERM U 666, Fac- (mainly chocolate), or medication. Therefore, it is very ulty of Medicine, UDS, 11, rue Humann, 67085 Strasbourg Cedex,France. Tel.: +33 368 85 32 43; Fax: +33 368 85 32 56; E-mail: difficult to control for the intrinsic effects of caffeine, since groups of non-exposed true control subjects with ISSN 1387-2877/10/$27.50  2010 – IOS Press and the authors. All rights reserved A. Nehlig / Is Caffeine a Cognitive Enhancer? zero exposure almost do not exist. Second, most studies fluence word learning across 2–6 presentations [6–12].
considering the acute effect of caffeine have imposed a Nevertheless, high to moderate users recall more words period of abstinence of caffeine before testing, typically than low users, particularly at the beginning of the lists, overnight. This led to a debate of whether or not the reflecting a potential beneficial effect of habitual caf- effects of caffeine are true effects or partly due to the feine consumption on learning [7,10,13,14].
reversal of more or less prolonged abstinence [1].
In incidental learning, subjects are not told that they Caffeine absorption from the gastrointestinal tract is will be tested for memory. In this paradigm, it is consid- rapid and reaches 99% in humans in about 45 min after ered that the experimenter is able to gain more knowl- ingestion. Pharmacokinetics are comparable after oral edge over information processing activities at the time or i.v. administration of caffeine in humans and ani- of learning, since subjects cannot use additional pro- mals. The hydrophobic properties of caffeine allow its cessing activities (rehearsal, memory aids) to improve passage through all biological membranes, including performance. In one study, performed on 7414 sub- the blood-brain barrier. Peak plasma caffeine concen- jects over 18 with daily coffee intakes ranging from trations are reached between 15 and 120 min after oral zero to over 7 cups per day, subjects performed dose- ingestion in humans, and most studies reported here dependently better and older people appeared more sus- were performed between 45 and 120 min after caffeine ceptible to the performance-improving effects of caf- ingestion. Caffeine half-life ranges from 2.5 to 4.5 h in feine [15]. In another incidental learning paradigm, 300 students, aged 19–24 years, were tested 30 min af-ter 1–4 m/kg caffeine on different encoding processes.
Subjects were non-drinkers or occasional low coffee CAFFEINE AND INCIDENTAL VERSUS
drinkers and were told to abstain from caffeinated bev- INTENTIONAL LEARNING
erages for 10 h before testing. They were categorizedby impulsivity level, knowing that a key characteristic In learning and memory tasks, there are several com- of impulsive behavior is inappropriate attention to ir- ponents which include the variable tested but also many relevant information and susceptibility to distraction in other parameters such as the level of vigilance and at- working memory [16]. In this test, caffeine facilitated tention, the speed of reaction time, and mood. It has acquisition and recall in high-impulsive subjects after been repeatedly demonstrated that caffeine decreases rhyming acquisition, but hindered it after semantic ac- reaction time, increases vigilance and attention, and has quisition. Caffeine did not reliably influence recall in positive effects on mood (at the doses used in most stud- ies that will be considered here). The effects of caffeine In summary, caffeine facilitates learning in tasks in on learning were tested in different types of situations.
which information is presented passively; in tasks in In some experiments, the subjects were aware that they which material is learned intentionally, caffeine has no would be tested for learning, while in others they were not. The paradigms used also differed with the studies:in some tests some cue for recall was provided, whilein other ones it was free recall from lists of words.
CAFFEINE AND MEMORY
In paired-associate learning, subjects are usually giv- en word pairs of a high or low degree of semantic asso- Human memory can be divided into short-term ciation (e.g., tree-apple; car-sea). Subjects have some (sometimes called working) memory, and long-term time to learn the word pairs and then are given the first memory. Working memory stores information over word of each pair and have to recall the second one.
brief intervals of time during which further process- This situation is a learning task that is different from ing can be performed (e.g., recognition). Only limit- free recall since the experimenter is giving the cue for ed amounts of information can be stored in this work- recall. Caffeine does not affect paired-associate learn- ing memory of which we are aware. Working memory ing performance when recall is assessed immediate- provides the ability to maintain and manipulate infor- ly [3–5], nor when tested after a delay of 20–30 min [4].
mation in the process of guiding and executing com- In serial or intentional learning, the effect of caffeine plex cognitive tasks. It can be described as a multi- has been assessed on the recall performance of words component system guided by an executive component from lists (immediate or after up to 20 min). Caffeine consisting of a number of processes that provide atten- (100–400 mg, or 3–6 mg/kg) does not significantly in- tional control over other components of working mem- A. Nehlig / Is Caffeine a Cognitive Enhancer? ory and other cognitive abilities. Two of the subcompo- Caffeine was found to either (1) exert no effects on nents are domain specific, providing the ability to hold free recall performance in sixteen studies [7–9,11,21– phonological and visuospatial information in separate 32]; (2) improve recall in six studies [33–38]; or (3) stores. A third subcomponent enables the integration of impair recall in three studies [8,12,39].
information into complex multi-modal representations In one study, females performed better than males linking working memory to long-term memory [18].
in a recall task after 300 mg caffeine. This effect may Long-term memory contains large amounts of infor- be specific to the task used (digit memory task) and mation stored for longer periods that can extend to life- to different lateralization in both genders [38]. Gender time. All information stored within the human brain for also influenced the effects of caffeine (2 and 4 mg/kg) a period exceeding two minutes is usually considered on the recall performance of lists of words presented to involve long-term memory [19]. We are not aware of at a fast or slow rate. Caffeine inhibited females’ recall this information until it is activated and becomes part at the slow rate but not at the fast rate and had no of working memory. Long-term and working memo- effect in males. The data were not influenced by the ries can be deliberately accessed during task perfor- daily caffeine consumption. Thus, caffeine may impair mance, they become then explicit memories. Implicit the efficiency with which females rehearse information memories are memory representations which cannot be in working memory [39], reflecting possible variation with the level of circulating estrogens.
Most experimental studies concerning the potential In focused and divided selective attention tasks, sub- effect of caffeine on memory have focused only on lim- jects are instructed to detect the appearance of a memo- ited simple measurements of newly established episod- ry item in relevant positions among items displayed on ic memory, i.e. acquisition, short-term storage, and re- a screen [40–42]. Increased reaction times with increas- trieval of related or unrelated word lists. Some excep- ing task load suggest that subjects apply serial, limited tions concern a few studies measuring (i) paired asso- capacity searches, in which each memory item is com- ciated learning where the rate of acquisition of two un- pared sequentially with all other items. An increase of related items is assessed, (ii) spatial memory related to the number of targets to be memorised or of the items the location of items, or (iii) semantic memory where presented on the display results in search processes access to stored representations of factual or lexical of longer duration and more pronounced negativity of information is the variable of interest. A few studies considered working memory, assessing the ability to A 3 mg/kg caffeine dose decreased reaction times in retain and manipulate the information store in a tran- a low display load but not in a high display load condi- sient space, for example solving arithmetic problems.
tion [42]. This differential effect indicates that caffeine In most of these studies, the effects of caffeine were facilitates performance only in simple or moderatelycomplex tasks. In more complex tasks, caffeine may not really robust and were frequently entangled by in- either have no effect or even impair performance [44, teraction with other variables [20]. Therefore, in the 45]. Thus, performance tested with caffeine improves following part of this review we will focus mainly on as long as energetic supplies increase up to a certain level, beyond which it may deteriorate.
In summary, caffeine does not seem to consistent- ly improve immediate free recall of words, letters anddigits. Caffeine facilitates performance in tasks involv- In immediate free recall tasks, subjects are presented ing working memory to a limited extent, but hinders a list of unrelated words, exceeding their memory span, performance in tasks that heavily depend on working and then asked to recall as many words as possible in any order. Words placed at the beginning and end of The discrepancies among the studies may originate the list are recalled better than words in the middle in the fact that the studies vary from one another in of the list, producing so-called primacy and recency the memory assessment method (recall or recognition), effects, respectively. This recall pattern is referred to time frame (immediate versus delayed), sex and age of as the serial position effect. It is usually hypothesized subjects. One possible explanation lies in the inverted that the more recent words are retained in the primary, U-shaped arousal-performance function [46]. Support- working memory, while the earlier ones are retained in ing this conceptualization, Kaplan and colleagues [47] found that low doses of caffeine enhance working mem- A. Nehlig / Is Caffeine a Cognitive Enhancer? ory performance, while higher doses decrease it. Other CAFFEINE, ALERTNESS AND MOOD
studies reported that caffeine facilitates low-difficultyperformance on low-load memory tasks and impairs it It is well-known that caffeine ingestion leads to dose- on high-load tasks [48,49]. This could be due to in- dependent increased energetic arousal. At low doses, creased arousal induced by high load tasks, which, in caffeine improves hedonic tone and reduces anxiety, the presence of caffeine, could produce over-arousal while at high doses there is an increase in tense arousal, leading to a decrement in memory performance. How- including anxiety, nervousness, and jitteriness. Caf- ever, most studies did not control for memory load. In feine improves concentration and help to focus mainly addition, arousal increments produced by novel stimuli by eliminating distractors (for review see [2,54–60]).
and white noise improved performance on a recall task, In mood ratings, arousal state, i.e., changes in alert- while the further addition of caffeine-induced arousal ness, reaction time and attention have often been includ- decreased performance. The additional arousal gener- ed. When mood per se is considered, a dose-related im- ated by novelty pushed the subjects over the top of the provement in subjective measures of calmness and in- inverted-U curve and so decreased their memory perfor- terest were found after caffeine. This relationship sug- mance [50]. Thus, caffeine appears to rather improve gests that mood improvement may depend on baseline memory performance under conditions that otherwise arousal. Older adults are more sensitive than younger subjects to mood-enhancing effects of caffeine. Mood effects are also influenced by the time of the day, withlargest effects in the late morning. The repeated ad- The effects of caffeine on long-term memory have ministration of 75 mg of caffeine (the equivalent of one not been studied in great detail. In one study, 12 sub- cup of coffee) every 4 h confirmed a pattern of sus- jects (6 males and 6 females) received 5 mg/kg before tained improvement of mood over the day [61]. Highly both the learning and retrieval sessions. Sixteen words fatigued subjects are more likely to experience larg- were incidentally studied during the learning session er subjective mood changes than non- or moderately and memory was evaluated by the number of words fatigued ones. There are also indications of negative correctly recalled at the retrieval session two days later.
mood impact associated with over-arousal (for review Caffeine reliably increased arousal but did not produce any effects on memory [51]. Likewise, long-term mem- Caffeine was also reported to slightly improve vig- ory retrieval functions measured at 30 min after a 15- ilance performance and reaction time in children who word learning test were not affected by 100 mg caffeine are habitual caffeine consumers but caffeine does not in middle aged (45–60 years) and older (60–75 years) consistently improve performance in children with healthy men and women studied in a cross-over de- attention-deficit-hyperactivity-disorder (ADHD) [62].
sign. Subjects were asked to abstain from caffeine in In an animal model of ADHD, the spontaneously hyper- the evening preceding the tests. It is proposed that pos- tensive rat (SHR), pre-training administration of caf- itive effects of cognition may occur at higher dosages feine attenuated the spatial learning deficit [63] and im- (200–250 mg) in this age range [24], particularly for proved performance in an object-recognition task [64].
relatively complex cognitive functions. In elderly peo- At the moment, there are no studies available on the ef- ple, higher caffeine dosages may attenuate age-related fects of caffeine on cognitive abilities in children with arousal decrements, whereas young subjects may be ADHD, most studies were rather interested in whether more susceptible to caffeine-induced overactivation asthey are already operating closely to an optimal arousal caffeine could attenuate symptoms as well as the drug of choice in this pathology, methylphenidate.
The retrieval from the phonological system was stud- ied using phonological priming and tip-of-the-tonguestates in 64 participants of a mean age of 24 years. They CAFFEINE AND ATTENTION
received 200 mg of caffeine or placebo and were tested40 min after intake. Caffeine produced a clear priming The effects of caffeine on attention and psychomo- effect with phonologically related words. When primed tor performance have been extensively reviewed (for with unrelated words, the caffeine subgroup produced review see [20,54,56–59,65]). Not all studies found an increase in tip-of-the-tongue states. This contrasting effects of caffeine on the two variables, although most effect provides evidence that the positive priming effect studies found improvements in reaction time. Like- of caffeine was not the result of increased alertness [53].
wise, when multiple tests were used, not all were af- A. Nehlig / Is Caffeine a Cognitive Enhancer? fected by caffeine but there is clear evidence that caf- situations. Caffeine removes the drop in sustained at- feine can help sustain attention in demanding tasks.
tention after lunch, and is more effective during night There is however, a debate, since most often testing work and prolonged work, and in subjects with upper was performed after a period of abstinence (typically overnight). The extent to which the effects of caffeine After sleep restriction and exposure for 3 weeks to represent an absolute improvement in performance or tea/coffee containing or not containing caffeine, the a restoration of performance degraded by withdraw- administration of 1.2 mg/kg caffeine after overnight al is not clear. A recent study compared overnight- deprivation did not improve cognitive performance in withdrawn consumers and non-consumers of caffeine.
long-term caffeine-deprived participants but prevent- Caffeine generally improved mood and cognitive per- ed further deterioration in overnight-deprived partic- formance in both groups. These effects did not differ ipants [73]. In another study, 21 healthy young men significantly between groups apart from three variables were sleep-deprived for 40 hours twice, one week apart.
(fewer lapses of attention, ratings of alertness, and anx- Subjects had to perform a random generation of 225 iety) where the effects of caffeine were larger in non- numbers at a low and high pace after placebo or 200 mg consumers. These data led to the conclusion that the caffeine. Caffeine preserved simple aspects of cogni- withdrawal hypothesis is not an adequate explanation tive performance but did not prevent the detrimental effects of sleep deprivation on more complex cognitivefunctions [74]. In US Navy trainees subjected to 72 hsleep deprivation, 200 or 300 mg caffeine improved INFLUENCE OF DAYTIME AND FATIGUE
cognitive function in terms of vigilance, learning, andmemory [75]. Finally, 200 mg caffeine given to 36 male Caffeine impaired [67] or did not affect [68] the im- and female psychology students impaired the immedi- mediate reproduction of numbers when testing was per- ate reproduction of spatial relationships of verbal infor- formed during the night. A study including 40 subjects mation after normal sleep, but improved it after sleep over the age of 65 tested immediate and delayed memo- deprivation, indicating a compensation for fatigue [76].
ry in tasks consisting of the recall and recognition of 16 Slow release caffeine tablets were effective for 13 h words from four semantic categories. They abstained in alleviating fatigue-related decrements of vigilance from caffeine for 4 h prior to the session and received a and performance after limited sleep deprivation. The cup of coffee containing 220–270 mg caffeine 30 min 300 mg dose was the most effective [77].
before testing. Caffeine counteracted the decline in In summary, the larger improvement of performance performance from morning to afternoon, which sug- in fatigued subjects confirms that caffeine is a mild gests that time-of-day effects may be mediated by non Likewise, 200 mg caffeine given to 15 well-rested and 15 fatigued students aged 18–25 years led to larg- INFLUENCE OF THE HISTORY OF CAFFEINE
er improvements in performance in fatigued than in CONSUMPTION
well-rested subjects [69]. In subjects fatigued after along testing session, 1.5 mg/kg caffeine improved as- The influence of daily caffeine consumption on cog- pects of cognitive performance such as encoding of new nitive performance and memory was tested in two types information and detection of number of repeats [70].
of situations, either with no addition of caffeine for test- Likewise, immediately after strenuous physical exer- ing or with consumption of a given amount of caffeine cise (1 h time trial on a bicycle ergometer) in 15 trained on top of the regular daily consumption.
athletes of a mean age of 23 years, 150 and 225 mg caf- In caffeine non-deprived young (20–25 years) and feine improved attention, psychomotricity and memo- older subjects (50–65 years), 250 mg caffeine slightly ry [71]. Subjects that consume higher levels of caffeine offset the decline in performance found in the place- are more alert over the working day and have a sig- bo group on a digit span memory test. However, caf- nificantly smaller slowing of reaction time. Caffeine feine did not improve performance in the immediate consumption reduces the risk of frequent/very frequent word recall task in either age group [11]. Caffeine cognitive failures and of accidents at work by about (12.5–100 mg) given after overnight abstinence im- two-fold [72]. Also, beneficial caffeine effects on mood proved cognitive performance in a relatively difficult and performance are more prominent in low arousal and stressful task involving rapid visual information A. Nehlig / Is Caffeine a Cognitive Enhancer? processing and using a high load of working memory during distraction only in simple tasks in young sub- in 11 males and 12 females, aged 18–56 years. The jects, and in more complex tasks requiring sustained effect was more marked in individuals with a high ha- attention in elderly subjects in which the treatment of bitual caffeine intake [78]. In 68 volunteers consuming complex tasks is usually less effective [85]. Finally, a regular daily amount of caffeine, 2 mg/kg caffeine the administration of 250 mg caffeine to young and old improved the speed of encoding new information in a subjects that are moderate caffeine consumers (250– categorical search task [79]. Likewise, in a study com- 600 mg daily) shows that caffeine improves the per- paring 24 non consumers (20 mg/day) to 24 consumers formance in elderly, i.e., the treatment of information of caffeine (217 mg/day), there was no baseline differ- and the speed-accuracy trade-off. Caffeine is able to ence between group performances. Caffeine improved reverse the effects of cognitive aging, making more numeric working memory reaction time and sentence energy resources available in elderly subjects [86].
verification accuracy; alertness was also increased, but When considering the relationship between cogni- in general caffeine tended to improve more the perfor- tion and regular coffee/caffeine intake, two Dutch stud- ies considering 1875 and 1376 subjects, respective- In adult subjects, a higher habitual caffeine intake ly, aged 24–81 years found positive effects on cog- with no additional caffeine administration on the day nition, mainly in terms of reaction time and verbal of testing was positively related to better performance memory, but no age-related difference [81,87]. Con- on incidental verbal learning and visuospatial learning versely, a British study concerning 9003 adult sub- tasks [15]. There was also an association between es- jects reported a dose-related trend to improved perfor- timated caffeine intake and performance in a choice mance with higher levels of coffee consumption. High- reaction time task, and in delayed recall of a verbal er overall caffeine consumption (from coffee and tea) word learning task [81]. In the former study, older sub- improved simple and choice reaction time, inciden- jects appeared to benefit more from higher caffeine tal verbal memory and visuo-spatial reasoning. Older intake [15], while this was not the case in the latter people appeared more susceptible to the performance- study [81]. Also, in real life activities, regular caffeine improving effects of caffeine than younger [88]. In the consumption might benefit cognitive functioning in a Rancho Bernardo study comprising 1538 participants, 890 healthy women and 638 healthy men from South-ern California (mean age 73 years), higher lifetimecaffeine consumption appeared beneficial for cognitive CAFFEINE AND AGE-RELATED COGNITIVE
performance. Indeed it was associated with better per- DECLINE IN HEALTHY SUBJECTS
formance in 6/12 tests with a trend on two other cog-nitive tests. Among women aged 80 or more years, Between 20 and 60 years, cognitive functions in lifetime coffee intake was not significantly associated terms of reaction time and rate of perception and treat- with better performance in 11/12 tests. Current caf- ment remain relatively stable. Between 60 and 80 years, feinated coffee intake was associated with improved there is a general slowing down of cognitive function.
performance in two tests, with a trend in a third one.
It was hypothesized that caffeine could in part compen- No relation was found between coffee intake and cog- sate for this decline because of its effects on vigilance, nitive function in men, or between decaffeinated coffee mainly in situations of reduced alertness, as mentioned intake and cognitive function in either sex [89]. These earlier [25,37,72]. In many studies, young and elderly studies gave some support to the notion that habitu- subjects appear to be differently affected by caffeine.
al caffeine consumption may boost to some extent the In two studies, elderly subjects showed improved at- cognitive reserve of the subject. However, these stud- tention, psychomotor performance, and cognitive func- ies were cross-sectional, which makes the causal inter- tioning on caffeine. Caffeine predominantly improved pretation of a relationship between caffeine intake and performance and feeling of well-being, and the elderly performance somewhat speculative. In the longitudinal appeared more sensitive to the objective effects of caf- Three City cohort study including 4197 healthy wom- feine than the young subjects, particularly in offsetting en and 2820 healthy men over 65 years, women con- declining performance over time [83,84]. Likewise, in suming over 3 cups of caffeine daily for over 4 years a study comparing young (18–37 years) and elderly showed less decline in verbal retrieval and visuospatial (60–75 years) receiving an acute dose of 225 mg caf- memory than women consuming one cup or less. The feine, caffeine was reported to improve performance protective effect of caffeine increased with age with a A. Nehlig / Is Caffeine a Cognitive Enhancer? maximal effect in women over 80 years. No relation GENERAL CONCLUSIONS
was found between caffeine intake and cognitive de-cline in men [90]. Another longitudinal study followed Despite the wide variability (1) in the amount of caf- a population of 1376 subjects that were tested 6 years feine given before testing, (2) in the period of caffeine after the first time they enrolled in the Maastricht Ag- abstinence observed before testing, (3) in the baseline ing Study. The subjects were stratified by age from 25 caffeine consumption of the subjects tested, and (4) in to 80 years. In this follow-up, there was no associa- the type of tests used, there was not much heterogeneity tion between lifetime habitual caffeine intake and ver- in the effects of caffeine on learning and memory.
bal memory performance [91]. On the other hand, a Caffeine does not usually affect performance in recent prospective study concerned the 10-year cogni- learning and memory tasks. Occasionally, caffeine ef- tive decline in 676 healthy men born between 1900 and fects on memory and learning, facilitatory or inhibito- 1920 from three European countries (Finland, Italy and ry, were found. These effects were rather the result of The Netherlands). Men who consumed coffee had a complex interactions with dose, subject, and task vari- 10-year cognitive decline of 4%. Non-consumers had ables. They may result from effects on encoding, or an additional decline of 4.7%. The authors reported an attention devoted to the information, rather than being inverse J-shaped curve between the number of cups of direct and specific effects on the storage or retrieval of coffee consumed and the extent of cognitive decline, information in short-term and working memory. Caf- with the least cognitive decline for 3 cups coffee daily feine can apparently improve performance directly over (2%). This decline was 4.3 times significantly smaller a wide variety of mental tasks, and indirectly by reduc- than in non-consumers [92]. Finally, the most recent ing decrements in performance under suboptimal alert- cohort study included 923 healthy adults from Scotland ness conditions. The efficacy of caffeine under states belonging to the Lothian Birth Control 1936 Study, in of reduced alertness is quite consistent. Many studies which the IQ of the children was assessed at 11 years.
on the elderly population reported a preventive effect Cognitive function was assessed at 70 years using tests of caffeine on cognitive decline. However, the results measuring general cognitive ability, speed of informa- of the different studies are still heterogeneous and de- tion processing, and memory. In age- and sex-adjusted serve further attention. It may be that in the elderly models, the authors found associations between total population, higher caffeine dosages are necessary to at- caffeine intake (coffee, tea, and total dietary caffeine)and general cognitive ability and memory. After ad- tenuate age-related arousal decrements, whereas young justment for age 11 IQ and social class, a robust posi- subjects may more easily respond to lower doses while tive association remained between drinking ground cof- reaching overactivation at high caffeine dosages. This fee (filter or espresso) and performance at the Nation- still needs to be clarified. The ingestion of caffeine does al Adult Reading Test and the Wechsler Test of Adult not seem to affect long-term memory.
Reading. No gender effects were observed, conversely Two general mechanisms may account for most of to several previous studies. The latter study provides the observed effects of caffeine on performance: (1) only limited evidence that caffeine consumption pro- an indirect, non specific ‘arousal’ or ‘processing re- tects against cognitive decline. It seems that childhood sources’ factor, presumably explaining why the effects IQ and factors such as social class, rather than caffeine of caffeine are generally most pronounced when task intake, would drive the association with cognition in performance is sustained or degraded under subopti- mal conditions; and (2) a more direct and specific In conclusion, it appears that the effects of caffeine ‘perceptual-motor’ speed or efficiency factor that may on cognitive decline still deserve further attention and explain why, under optimal conditions, some aspects research in order to clarify why some studies found of human performance and information processing, in effects only in one sex, and to better define the nature particular those related to sensation, perception, motor of the association between caffeine consumption and preparation, and execution, are more sensitive to caf- the potential prevention of cognitive decline during ag- feine effects than those related to cognition, memory, ing. Other data are available on the potential preven- tive effects of caffeine in Parkinson’s and Alzheimer’s Thus, caffeine apparently cannot be considered a disease. The reader can refer to chapters by Cunha, ‘pure’ cognitive enhancer. Its indirect action on arousal, Arendash, Lunet and Costa in the same issue of the mood and concentration contributes in large part to its A. Nehlig / Is Caffeine a Cognitive Enhancer? DISCLOSURE STATEMENT
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