Studies Used

Studies were used that: (a) examined acute effects of caffeine on aspects of thinking, that is, mental performance; (b) included a placebo condition; and (c) utilized healthy young human subjects (mostly students). These studies had the following characteristics:

  • The self-reported habitual level of daily caffeine consumption was about 200 to 300 mg, in general, which is similar to the contents of two to four cups of coffee (85 mg/cup). These amounts may be somewhat stronger than most people habitually drink at one occasion and also are substantially larger than those amounts typically consumed in beverages (soft drinks), foods (chocolate or cookies), or over-the-counter (OTC) drugs (Lieberman, 1992).
  • In nearly all studies the participants were instructed to abstain from caffeine-containing substances for a certain period of time (usually 10 to 12 h or more) prior to testing, which is similar to the condition when people get out of bed in the morning.
  • In some studies the subjects were also asked to abstain from alcohol and/or to fast for some hours before the experiment. Since most people drink coffee especially during work or other activities later during the day in a nondeprived state, generalizations from laboratory findings to everyday situations should be made with caution.
  • Strikingly, many studies did not assess caffeine levels in saliva or plasma. As a consequence, uncontrolled variations in baseline and/or achieved plasma or saliva caffeine concentrations, due to noncompliance with the abstinence instructions or to differences in caffeine metabolism, may have confounded some of the reported results.
  • Experimental control was often, but certainly not always, exerted over a number of well-known factors associated with interindividual differences in caffeine metabolism, such as smoking, liver disease, and, for females, the use of oral contraceptives and pregnancy.
  • The caffeine given to the subjects was mostly taken orally, as a fixed dose or as a dose of milligrams per kilogram body weight (mg/kg), in powder form (e.g., in gelatin capsules) or dissolved in a drink (e.g., fruit juice or decaffeinated coffee).

About half of the studies used a within-subjects (cross-over) design. The other half employed a between-subjects (independent groups) design. The potential advantage of the former over the latter design is its higher statistical power to detect true caffeine effects by preventing interindividual differences from contributing to the error variance. A disadvantage is the presence of a potential differential carryover effect of caffeine, which confounds estimates of its effects.

Several studies have examined caffeine's effects with a test battery purporting to sample a diversity of mental functions. A problem with this approach is that most tests in these batteries have no history of reliability and validity, which seriously hampers the interpretation of the results.

By comparison, other studies have evaluated caffeine's effects within an information-processing framework, for example Lorist and colleagues who used the Sanders' Additional Factor Method (AFM). This method typically employs single tasks, each representing a solid, theoretically based aspect of mental performance. Systematically manipulating a specific task variable, such as stimulus quality (degraded and intact stimuli) (Lorist, 1998), usually allows more robust and specific interpretations of caffeine's effects.

Although there exists no generally accepted taxonomy of human task performance, to facilitate generalizations of research findings the tasks are ordered, if possible, on the basis of their nature or structure and as similarly as possible to the sequential stages of information processing. Five broad but related aspects of mental functioning are distinguished: (1) sensation and perception, (2) cognition, (3) learning and memory, (4) attention, and (5) mental fatigue.

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