In addition to their subjective and physiological effects, cannabis and ethanol are also known to affect cognitive function. In a double-blinded placebo controlled study, Manno et al (1971) found that ethanol and marijuana interact synergistically to impair cognitive and psychomotor functions when co-administered. Studies of divided attention in human subjects have demonstrated that acute administration of marijuana and ethanol alone or in combination is sufficient to increase false-positive and false-negative errors in a task that requires simultaneous monitoring of two distinct stimuli (MacAvoy and Marks, 1975; Marks and MacAvoy, 1989). Interestingly, both of these studies reported a cross-tolerance to the effects of acute ethanol on signal sensitivity in a group of cannabis-users when compared to non-users. In addition to attentive processes, acute ethanol and marijuana impaired measures of accuracy and speed on the digit-symbol substitution task as well as word recall, but only acute ethanol impaired performance on a number recognition task (Heishman et al., 1997; Heishman et al., 1988). With regard to the cognitive effects of chronic ethanol and cannabis exposure, one study demonstrated significantly impaired learning in a
