Caffeine affects cognitive functioning in a number of ways. We will consider vigilance performance, information processing studies, memory, selective attention, and complex cognitive functioning.
One aspect of cognitive performance in which there is considerable consistency in the literature as to the effects of caffeine is vigilance. First, caffeine has consistently been shown to increase arousal level during prolonged vigilance tasks.91-94 Second, caffeine improves performance on these tasks. An example is a study conducted by Lieberman and his colleagues,91 who used the Wilkenson Vigilance Test.91 Above a white-noise background, subjects were presented with a 400 ms tone every two seconds for 1 hour. Targets were 70 ms tones that were randomly interspersed among the longer tones, and subjects responded by pressing a key on a keyboard. Hits, misses, and false alarms were recorded. The investigators found that low (32 mg) to moderate (256 mg) doses of caffeine increased the number of correct hits without affecting the error rate. Similar results were reported in a study in which subjects received a single, timed-release dose (600 mg) of caffeine. The caffeine mimicked the benefits of sleep. Sleep-deprived subjects taking the caffeine performed the same on tests of vigilance and alertness as those who received a normal night's sleep.95 96 Caffeine has also been shown to increase the metabolic rates of sleep-deprived subjects to their predeprived levels.97 A number of other studies have also shown that caffeine improves vigilance performance98-100 on both auditory98-104 and visual88,104 tasks. Conversely, decrements in performance have been attributed to low levels of arousal,100 while higher levels improve performance on sustained attention tasks.14,15,105,106
Although some earlier work suggested negative effects on cognitive functioning in children, a meta-analysis found no supportive evidence.107 In fact, children exhibit improved functioning on both auditory vigilance108 and sustained attention108 tasks when caffeine is given. Another study showed that these beneficial effects are seen not only in younger children, but also in adolescents between the ages of 11 and 15.109 In addition, moderate doses (200 mg) decrease self-reported feelings of boredom, possibly providing a partial explanation for the positive effect of caffeine in vigilance and sustained attention tasks.110 These are the same results reported in a study of middle-aged subjects.111 More generally, a recent review showed that caffeine improved vigilance performance in 14 of 17 studies of adolescents.112
Despite the apparent overall positive effect of caffeine on vigilance, there is evidence to suggest that there may be a limitation on the size of the arousal increment (induced by caffeine, intense stimulation, or both) that will produce improvement in vigilance studies.111,113 If that limit is exceeded, performance may approach asymptote or deteriorate. Frewer and Lader114 found that low doses of caffeine improved vigilance performance, while high doses initially impaired it. After 2 hours, however, these high doses also improved vigilance. In another study, the senior author and colleagues preselected subjects high and low in habitual caffeine ingestion. Using double-blind controls, they were given either caffeine (4.0 mg/kg) or a placebo and exposed to 93 dB background white noise or no noise. A visual vigilance task consisted of 3 blocks of ten 1-min trials with targets appearing on trials 1, 3, 4, 7, and 10 of each block. Stimuli were angled lines appearing on a computer screen; targets were vertical lines. Results showed that caffeine decreased target response times (improved performance) when there was no white noise, but increased response times when the 93 dB noise was present.115 In addition, subjects with histories of habitually high levels of caffeine use exhibited relative decrements in vigilance performance.
The results of these two studies and others provide further support for the theoretical inverted U-shaped arousal function. In the first case,114 high doses of caffeine pushed subjects over the top of the curve and hence impaired vigilance performance. As time passed and the subject became more accustomed to the experimental situation, arousal returned to more optimal levels and performance improved. In the second study,115 caffeine combined with moderately high intensity situational stimulation (white noise) to drive arousal up beyond the point of transmarginal inhibition and, again, decrease performance.
2. Information processing Information-processing theory116 has served as a partial basis for a number of studies of the effects of caffeine on cognitive functioning. Several studies have shown beneficial effects of the drug in rapid information processing (RIP) tasks.117-119 Other work demonstrates that caffeine enhances problem-solving120 and improves logical reasoning,121,122 as well as increasing performance on mental arithmetic tasks.123,124 Studies have also shown that caffeine counteracts the effects of sleep deprivation on spatial capacity98 and abstract reasoning101 tasks and partially reverses age-related deficits in cognitive functioning.125
As an example, Hasenfratz and Battig126 used a rapid information-processing task in which a subject looked for a 3-digit odd or even target on a computer screen that also presented other, nontarget digits. When the subject perceived the target, he or she pressed a response key as quickly as possible. The investigators found that a low dose (150 mg) of caffeine improved performance on the RIP task, while a higher dose (600 mg) impaired performance.
This elucidates the first potential confound in caffeine studies, namely the lack of control for dose level. Warburton120 found that low (75 and 150 mg) doses of caffeine cause significant increases in performance on atten-tional, problem-solving, and memory tasks. Other studies have shown that high doses of caffeine can interfere with performance on complex tasks,127 including the processing of ambiguous information.128 They can also negatively affect scores on such cognitive tasks as spatial abilities in refreshed subjects.101 In other studies, in which dose level was controlled, caffeine has not affected the search/detection domain of information processing129 or reading comprehension at high or low dose levels.130
In addition to dosage variability, some of these differential cognitive results may be a product of baseline or prestimulation arousal level differences. When baseline arousal is already elevated, highly arousing tasks, such as those that are complex or involve intense stimuli, can result in overstimulation. As Hasenfratz and Battig126 note, it may be such excessive levels of stimulation that account for the commission errors seen in the RIP paradigm. Baseline arousal levels therefore need to be taken into account in estimating or calculating overall levels of stimulation and resultant activation.
One important influence on baseline arousal is task difficulty or complexity. Complex information-processing tasks, have been shown to increase arousal levels in otherwise unstimulated subjects.131 The more complex the task, the higher the arousal level and the greater the chance that a given caffeine dosage would contribute to overstimulation. The additive nature of arousal sources (in this case, caffeine and complexity) means that arousal is high when caffeine is present and complexity or difficulty level is substantial. The result is that caffeine impairs performance on complex tasks.127,128
A second factor in prestimulation arousal level appears to be gender. When gender differences in activation have been specifically examined, female subjects have been quite consistently shown to exhibit higher levels of arousal or greater arousability than males. Physiologically, this arousal differential has been reported for both EEG132-134 and EDA76,135 measures. In studies involving mixed-gender groups with no analysis of the gender factor, it is possible that caffeine-induced increases in information processing in males would be counteracted by decrements in females. Depending on the gender balances of the groups, results could be considerably distorted. Stimulation that is highly arousing for males could lead to overarousal in females, whose responses would be dampened as suggested by the curvilinear (inverted U-shaped) relationship between arousal input and arousal output.
Interestingly enough, most of the studies reporting positive effects of caffeine in information-processing tasks have used primarily or exclusively male subjects.117,120,121,123,124,136 In fact, the only study that found positive cognitive effects of caffeine on females employed fatigued women, whose baseline arousal levels may have been lower.119 Conversely, most studies that have shown either detrimental effects98,127 or no effects129,130 of caffeine on information processing have used both males and females in the same study. In fact, the only study with this result that used exclusively male subjects also used moderate (250 mg) doses of caffeine with a difficult (visual Stroop) task. The combined effect of the drug and the task probably produced a degree of arousal high enough to interfere with cognitive performance. Caffeine thus contributed to an overall detrimental effect of arousal.128 Another study, using the same Stroop task and low levels of caffeine, found that caffeine does not impair complex visual processing.137
A different type of study showing detrimental effects used the same caffeine dose on both tired and refreshed subjects.101 The tired subjects were aroused to the point of showing beneficial effects from the drugs. Therefore, the same dose should be expected to overarouse the refreshed subjects and decrease their information processing abilities, as was the case.
3. Effects on memory A major focus of cognitive psychology, memory has been studied intensively, and a number of investigations have addressed the effects of caffeine. Unfortunately, results in this important area of mental performance are quite mixed. First, a number of studies have found that caffeine enhances memory performance in several paradigms. It has been shown to improve delayed recall,120 recognition memory,138 semantic memory,89 and verbal memory in general.139 Other studies have shown significant increases in memory performance on both easy140 and difficult110 memory tasks. Females (but not males) in one study showed positive effects of caffeine on word list retention, though only when the words were presented at a slow rate.141
One study demonstrated that caffeine has cholinergic properties that can enhance cognitive functioning. In this investigation, the result was improved performance on short- and long-term memory retrieval, reading speed, and encoding efficiency.142 In another study,143 it was found that caffeine improves memory consolidation. Caffeine has also been shown to counteract the detrimental effects of aging on general memory performance.125
Complicating matters considerably, some studies have gotten opposite or equivocal results for memory tasks. Caffeine can actually decrease immediate word list recall, at least under some circumstances.144145 It has even been shown to amplify the detrimental effects of alcohol on memory.146 Further clouding the picture, other studies have shown that caffeine has no effect on recall.14130147 Some have found no effect on short-term memory with 64 mg of caffeine148 or with 100,149 and no effect on long-term memory with 64 or 128 mg.148 Verbal learning and memory were also unaffected by doses of 125 to 500 mg.150 In addition, no effect was seen on implicit or incidental memory,151 as was also true for delayed recall with 3 and 6 mg/kg152 or with 200, 400, or 600 mg.145 This latter result is somewhat offset by a study in which delayed recall was improved by caffeine, even though immediate recall was not.120 Finally, Linde101 tested both fatigued and nonfatigued subjects on an auditory attention task requiring immediate recall. She found no effects of 150 or 200 mg doses of caffeine.
Given the mixed results in the literature, it is difficult to know just how caffeine does affect memory. To some extent, the differential effects may depend upon the memory assessment method (recall or recognition) and the time frame (immediate or delayed). Gender differences may also cloud the picture, as discussed above. Even when these differences are taken into account, however, unexplained discrepancies remain. One partial explanation may be that the differential effects of caffeine are a function of the subject's memory load. For example, Anderson140 found that caffeine enhanced low load memory tasks but was detrimental in high load tasks. This could be due to the increase in arousal induced by the high load task, which, in the presence of caffeine, could produce overarousal. The drop in arousal output as the subject crossed the peak of the inverted U-shaped function could cause the memory deficits observed in some studies.
An alternative or additional explanation for the mixed results may lie in the differentially arousing effects of other factors present in the experimental situation. One study in our laboratories involved a backward recall task. Each subject received 3 blocks of 4 recall trials each. In a repetitive condition, each block involved one particular stimulus type (letters, digits, or color names). In a novel condition, each block contained one trial of each of the three stimulus types. Using a double-blind procedure, each subject was randomly assigned to receive either 300 mg caffeine or a placebo prior to performing the memory task. Half of each drug group was randomly assigned to a noise condition, involving a constant 80 dB white-noise stimulus, and half to a no-noise control condition. Results showed that both novelty and white noise improved recall performance under placebo and decreased it under caffeine.13 It appears that the additional arousal generated by novelty and white noise served to push caffeine subjects over the top of the inverted-U curve and hence decrease their recall performance. Thus, it may be that caffeine does improve memory performance under conditions that otherwise produce low arousal. However, any condition causing over-arousal, whether or not it is a part of the memory task itself, can yield performance decrements. Confirming this hypothesis, Kaplan153 found dose-dependent caffeine effects on memory. At lower doses (250 mg), subjects showed increased working memory ability. However, at higher doses (500 mg), subjects started to show impaired working memory performance. These results support the hypothesis that memory effects are dependent on both caffeine154 and the level of background arousal. Studies not controlling for these factors may well have confounded results.
Attention has also been studied in paradigms other than vigilance, and results show enhancement of selective attention by caffeine.120,155 Several studies have shown that caffeine improves performance on selective156 and divided105 attention tasks. For example, Lorist and Snel156 found that 3 mg/kg of caffeine improved response time in a selective attentional task without decreasing accuracy. Similarly, another study showed that this increased attentional ability extends to self-focused processes. Subjects were better able to determine their current physical state after ingestion of 250 mg of caffeine than with placebo.157
Another approach to studying attention involves evoked potential responses. In particular, an EEG response occurring 300 milliseconds after presentation of a novel stimulus (P300 wave) has long been associated with visual158-161 and auditory162-164 attention. Larger P300 waves have been associated with more intensive attention, and smaller P300 waves with decreased attention.159,160
One method of presenting novel stimuli is the oddball paradigm. In this task, the subject has to attend to a certain auditory or visual stimulus. If the task is passive, the subject simply attends to the stimuli. If the task requires active attention, the subject is told to respond to any changes in the stimulus. When a novel stimulus is presented, EEG shows a consistent P300 response.165-169 This response occurs regardless of differences in passive or active attention, although the wave is larger during the active tasks.170,171
Researchers have applied the oddball paradigm to an examination of the effects of caffeine on attention.169 Kawamura169 found that 500 mg of caffeine significantly increased the P300 wave and that its effects lasted 210 minutes. Other work has also shown that caffeine increases the P300 wave.172 This provides further evidence that the drug can enhance attentional focus.
Real-life settings, including work settings, often require more complex and varied cognitive functioning than is the case for many laboratory tasks. One case in point is decision-making, in which multiple types and sources of information are logically combined to yield an informed decision.173 What effect does caffeine have on such complex reasoning processes? While little work has been done in this area, it thus far appears that the drug has both positive and negative effects. On the positive side is the reduction in time required to arrive at a decision.174 175 On the negative side, potential resources may be neglected.176 Streufert and colleagues176 found that managers who drank 400 mg of caffeine or more on a daily basis often arrived at decisions much faster than other managers. However, this high-caffeine group also tended to show a decrease in their utilization of potential resources when making their decisions.176
Research on the effects of caffeine in the workplace is also rather sparse. However, there have been some studies, and it appears that the drug can have positive effects on mental performance. In one study, for example, managerial effectiveness was reduced when employees abstained from caf-feine.177 Another study found that moderate doses of caffeine (6 mg/kg) caused people to work harder because they underestimated how hard they were working. The workers thought they were working at the same speed they worked without caffeine, but they were actually working much faster.178 It is commonly assumed that the ubiquitous office coffee pot is heavily used by workers in order to increase their levels of wakefulness, alertness, and, more generally, arousal.176 There may, however, be a number of additional perceived or actual benefits of work-related caffeine intake.92 Headaches, for example, are often reported in work settings, and one study showed that workers sometimes consume caffeine primarily to relieve them.179 This finding is consistent with the fact that caffeine inhibits adenos-ine, thereby reducing ischemia and other forms of metabolic stress,29 and with its consequent widespread medical use in treating headache.180181
A more common and salient, though more subtle, basis for on-the-job consumption may be boredom. For some people, work is perceived as boring, and boredom is, in part, a function of habituation. The habituation process takes place when there are multiple repetitions of the same stimulus complex or a continuation of that complex over time.182 Habituation is basically a process of physiological and psychological adaptation to stimuli that cease to yield new information.182 While it is an adaptive mechanism, in that it moves noninformative stimuli into the background and permits active attention to focus on new information, it can also have negative effects. In particular, subjects who become highly habituated or overhabituated experience psychological discomfort, fatigue, and boredom.12 Caffeine can partially offset these detrimental effects of repeated stimulation by slowing the rate at which habituation occurs and smoothing the process.13 Thus, workers may consume the drug not only to increase alertness, but also to slow and smooth habituation.183
There are also documented negative effects of the amounts of caffeine often consumed at work.179 Most of these revolve around the stresses commonly perceived to be present in job settings.184 Caffeine has been shown to exacerbate the effects of stress on neuroendocrine responses185 and cardiovascular functioning, particularly blood pressure.4284 123186 It is no surprise, then, that it has these same effects in work settings, predisposing the individual toward strong physiological and psychological responses to work stressors.187 The blood pressure response to work stress appears to be particularly prone to caffeine enhancement.52188 In addition, a study of telemar-keters showed that employees became more psychologically sensitive to job stressors after consuming caffeine.189
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