Cannabis use alters both motor and cognition-based behaviour in man. Collectively, data strongly indicate acute intoxication to be more effective in disrupting memory than chronic use, probably due to long-term habituation and related changes in brain function. While simple cognitive tasks can be performed normally, the severity of cognitive impairment correlates with task difficulty, and this maybe the direct consequence of deficits in attention and goal-directed learning. Importantly, there are few if any gross motor impairments, even after chronic cannabis smoking over many years.
Acute cannabis intoxication leads to multiple effects, including changes in reaction time and perception. Simple reaction times are recorded such that test subjects have to press a button in response to a tone or light. This merely requires motor execution; such tasks are devoid of complex cognitive processing. Several studies have reported that reaction times increase after marijuana use (Borg et al. 1975; Dornbush et al. 1971), but this has not been confirmed by others (Braden et al. 1974; Evans et al. 1976) despite comparable sample sizes and drug doses. Increasing the complexity of the task (pressing different buttons in response to different stimuli) consistently leads to up to 50% longer reaction times in users relative to controls, and there seems to be a strong correlation between task complexity and cannabis-induced impairment (Clark and Nakashima 1968; Chait and Pierri 1992).
Stronger evidence supports the notion that cannabis use alters perception, such as taste, smell, hearing and vision. In users there are clear problems of colour discrimination (Adams et al. 1976) and identification of figures hidden in pictures (Pearl et al. 1973). Perceptual changes also pertain to time sense, which is generally altered in cannabis users. As they estimate time to pass more slowly than control subjects (Tart 1971; Chait and Pierri 1992), this could explain why they are prepared to take greater risks, for example in driving faster and more dangerously (Bech et al. 1973). Such drug effects are of importance when investigating complex behaviours. Hasty reactions and perceptual deficits could easily explain impairments in memory tasks and need to be excluded.
Despite perceptual effects, it is still possible to identify cannabis-induced memory problems. A type of memory highly sensitive to marijuana intoxication is recognition memory. Typically, test subjects are presented with a series of words. After a delay period, a second series is presented containing some words from the original series, but also some new ones. Cannabis users have no problem identifying the words from the original list, but they often recognise some words that are actually new (Dornbush 1974). Such memory intrusions may reflect problems in distinguishing between relevant and irrelevant words, a hypothesis that is supported by observations on free recall. Here, participants write down as many words as they remember from the original list without being primed. This is a more complex paradigm, and users not only remember fewer words than controls (Dornbush et al. 1971), they also have memory intrusions, inserting words that were not presented (Miller and Cornett 1978).
Determinations of cognitive alterations in chronic marijuana users are more difficult. Classical studies of Jamaican (Bowman and Pihl 1973) and Costa Rican (Satz et al. 1976) subjects did not reveal any cognitive impairment, despite a battery of psychological tests and the fact that chronic users had been smoking more than nine joints per day for more that 10 years. These results were confirmed in a recent report on 1,300 residents in Baltimore that had been followed in a longitudinal study over 11 years. Mini-Mental State Examination was applied to investigate any changes in mental functioning, yet no significant difference was observed between chronic marijuana consumption and controls (Lyketsos et al. 1999).
Cognitive differences were revealed in a study on 1,600 Egyptian prisoners (Soueif 1976). In this study, 16 different measures were recorded, of which 10 revealed impairment in the user group, while 2 showed better performance. However, the selected groups were not well controlled and many of the critiques listed below apply to this investigation. Similarly, deficits in IQ, memory, time estimation and reaction times were reported in several studies performed in India (Wig and Varma 1977; Menhiratta et al. 1978). Finally, investigations on college students with at least twice weekly marijuana consumption revealed deficits in memory formation, specifically deficits in information transfer into long-term memory (Gianut-sos and Litwack 1976; Entin and Glodzung 1973). However, a later study did not confirm these memory impairments (Rochford et al. 1977). More recent studies on cognitive deficits in marijuana users collectively suggest that impairments are (1) predominant for the attentional/executive system related to prefrontal cortex, and (2) increase with the length of cannabis use (Pope and Yurgelun-Todd 1996; Fletcher et al. 1996; Elwan et al. 1997). Such deficits can readily explain impairments in short-term memory, which are frequently reported for cannabis users (Schwartz et al. 1989).
Many of these studies, however, are flawed and do not reveal the true extent to which long-term cannabis use affects human cognition. Especially, early studies from the 1970s and 1980s were conducted on small sample sizes, and it has been calculated that an n = 25 per group is necessary to attain reliable results (Cohen 1990). Moreover, subjects that feel less affected by drug use are more likely to sign up for trials, while those experiencing severe problems may feel less eager to participate, even for pay (Strohmetz et al. 1990). As a consequence, the finding of subtle differences may not be a true reflection of the effects of chronic cannabis use. Psychological testing has seen considerable refinement, and the emergence of novel, increasingly sensitive tasks has helped to reveal differences between long-term marijuana smokers and controls. This suggests that tests used in the original studies, which have not found differences between test and control groups, were insensitive and might have been too simple.
Another critique frequently raised with respect to chronic use is the idea that users may have already been different from non-users prior to ever smoking marijuana. This is a valid point, as one might argue that (1) people of lower IQ may be more prone to drug use and (2) any intellectual difference may have preceded any cannabis smoking habits. Randomised control studies in which non-users are signed up for chronic smoking, however, are ethically difficult to justify. Another potential confounder is the use of multiple drugs. Many marijuana smokers are likely to use other and more drugs than controls (Earleywine and Newcomb 1997). Multi-drug effects can only be assessed in the context of each drug alone. Subjects who meet this criterion do not normally form part of studies. Consequently, multidrug use will make the sample group heterogeneous so that results may not reflect the typical cannabis user.
In contrast, animal research is devoid of many of the above critiques and results are thus not confounded by, for example, polydrug use, low sample sizes, pre-treatment differences, etc. Consequently, the main focus of this chapter rests on such animal models and the effects of acute and chronic cannabis administration on learning, memory, and related brain physiology.
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