ABSTRACT
Caffeine is said to have arousing effects (e.g., Zwyghuizen-Doorenbos et al., 1990), although its effects on perceptual and cognitive functions are not always consistent. For example, the influence of caffeine on short-term memory search processes is occasionally positive (Anderson and Revelle, 1983), sometimes negative (Anderson et al., 1989; Erikson et al., 1985), but mostly no effect has been obtained at all (Clubley et al., 1979; Mitchell et al., 1974; Erikson et al., 1985; Loke et al., 1985, 1988). One reason for this lack of consistent results is the influence of person bound factors such as boredom, fatigue, motivation and morningness-eveningness and factors in the environment like noise and odours. In general, effects of caffeine appear to be most pronounced in subjects who are in suboptimal conditions as for example when tired (Dews, 1984; Weiss and Laties, 1962; Lorist et al., 1994a). Since many experimental designs in drug research usually do not take account of these factors, it is impossible to disentangle their influences from the desired experimental effects (Gaillard, 1988).
