Introduction
The use of cannabis is often associated with memory challenges in the short term and long term basis. Studies continue to be carried out to determine if chronic cannabis use affects long-term memory by examining variety of cognitive functions. This paper reviews related literature on memory dysfunction in cannabis users (Parath, 2009).
The literature review includes studies that looked at memory function in cannabis users of chronic intoxication period (Austin, 2010). Specifically, it examines studies in working memory and verbal episodic memory. In addition, they have continued to deduce evidence indicating impaired encoding, storage, manipulation and retrieval systems in long-term cannabis users (Allhoff, 2010).
Cannabis, Brain, and Memory
Cannabis is extracted from the plant Cannabis sativa. Usually, Cannabis is taken in the form of dried leaves and female flower heads, or the resin secreted by these. This drug can be eaten but is more usually smoked in the form of cannabis cigarette, or joint, often mixed with tobacco, or in pipe.
Cannabis is an illicit drug that is commonly consumed in Europe and approximately 10% of adults aged 16-59 years in the UK used it in the year 2000 (Solowij, 1998). Majority of cannabis users attribute short memory problems as the most prevalent and this forms part of the reason many of them seek help to quit or reduce its consumption.
Scientific literature in general avers memory impairment as often cited in relation to cannabis use (Wilson et al., 2002). Cannabis use has risen to become the most widely used drug in the developed world over the years. The memory function, in general, has been studied in acute administration studies of long-term users of cannabis to humans and animals, and in long-term studies of cannabis users (Pope et al., 2002).
Cannabinoid System and Memory
Endogenous Cannabinoid system is directly involved in the necessary functions of memory. This is because cannabinoid receptors happen in high density in brain areas critically involved in memory functions. Profoundly, cannabinoid affects synaptic plasticity underlying learning and memory, disrupting long-term potential in hippocampus (Martin-Santos, 2010).
Cannabinoid receptors are metabotropic receptors which are the most common in the brain and are involved in multiple physiological and behavioural events. They are found on pre-synaptic terminals in locations concerned in cognition, especially learning and memory, critically in hippocampus, prefrontal cortex, anterior cingulate, basal ganglia and cerebellum (Miller, 2010).
Endogenous cannabinoid system guides the flow of information in the brain through retrograded signalling, modulating inhibitory and excitatory neurotransmitter release critical for synaptic plasticity, depolarisation-induced suppression of inhibition or excitation, long-term potentiation, and hence learning, memory and other higher cognitive functions (Kanayama et al., 2004).
Structural Brain Changes Related with Chronic Cannabis Use
There is lack of concrete lack evidence in most undertaken indicating that structural brain alterations in cannabis users. There are no global or regional alterations in brain tissue volume or composition in some recent studies (Ries, 2009). Other studies have discovered grey and white matter density alterations globally or in para-hippocampal areas.
Utilizing more sensitive measures and assessing cannabis consumers with greater exposure to cannabis than previous research, critical reduction of hippocampus and amygdale volumes in long-term cannabis users have recently been reported (Lyketsos et al., 1999).
Hippocampus volume reduction was related to dose, correlating with current daily dose, and cumulatively. Only excessive daily doses over long period of time, will lead to structural changes.
Another critical factor may be the age of onset of cannabis use. This has a devastating impact to the brain, specifically cannabis consumers’ start at the early stages of neurodevelopment (Allhoff, 2010). Evidence adduced recently of reduced neuronal and axonal integrity in the dorsolateral prefrontal cortex represented by magnetic resonance spectroscopic markers of metabolism (Solowij, 2009).
Changes related to dose were found in anterior cingulate and globus pallidum, but not in hippocampus. Solid evidence for dose-related cumulative neuronal damage, neuronal and synaptic density. Since functional dysfunction is likely to precede major structural changes in the brain or to show concomitant to more minor neural alterations, this presents good reason to think that long-term effects of use of cannabis on memory function (Wilson et al., 2002).
A Review of Related Literature
Effects of Cannabis Use on Cognitive Ability in the Long-Term
Cannabis has the ability to exert prominent effects on the central nervous system. In the central nervous system, cannabis acts on an endogenous cannabinoid system that is concerned with regulation of mood, memory, emotion, attention, and other cognitive functions (Hall, 2009).
Cannabinoid receptors play a significant role in memory storage and retrieval processes. Discoveries from human and animal research reveal that prolonged use of cannabis changes the functioning of the cannabinoid system of the brain. However, this does not lead to serious impairment (Solowij, 2002).
Observation for structural brain impairment for in humans following long-term cannabis use has not been sustained generally. Some current research has discovered no global or regional alterations in brain tissue volumes. Other studies have, however, shown grey and white matter density changes worldwide undertaken to date lack evidence of changes in structural brain in cannabis users or in para-hippocampal areas (Wilson et al., 2002).
A recent study that used unique techniques of measurement to indicate that frequent but relatively short term use of cannabis creates neither structural brain abnormalities nor global or regional alterations in the brain tissue volume or composition that are assessable by magnetic resonance imaging (MRI) (Pope et al., 2002).
Several studies have indicated altered brain function and metabolism in humans due to acute and chronic use of cannabis using cerebral blood flow, positron emission tomography, and electroencephalographic methods. In sum, use of cannabis has deleterious effects on memory and attention (Solowij, 2009). Individuals who use cannabis show clear signs of cognitive impairments relative to controls.
More of concern about cannabis is that its use may cause neurological damage resulting in persistent cognitive deficits, but the evidence is currently inconclusive. The evidence, however, does suggest that early use of cannabis may cause long-term cognitive problems (Solowij, 1998).
Chronic Use of Cannabis and Cognitive Dysfunction
Cognitive dysfunctions or impairments, specifically deficits in short-term memory, are reported by many cannabis addicts who seek help to stop using cannabis and are often advanced as one of the main reasons for needing to stop using cannabis (Allhoff, 2010).
However, evidence provided from controlled studies shows that long-term heavy use of cannabis does not appear to produce severe debilitating dysfunction of cognitive function like that produced by chronic heavy alcohol use (Ries, 2009). Nonetheless, there is evidence that long-term or heavy cannabis users exhibit more subtle types of cognitive impairment that are detected in well-controlled studies using sensitive measures (Lyketsos et al., 1999).
Earlier studies of the cognitive effects of chronic cannabis use have elicited major concern that cannabis users may have had poorer cognitive functioning than controls before they commenced to administer cannabis (Sadider, 2010). However, studies from the recent past have looked at this problem by matching users and non-users on estimated premorbid intellectual functioning or on test performance prior to the onset of cannabis use.
These studies have revealed cognitive impairments associated with frequent and long-term use of cannabis (Solowij, 1998). Frequent cannabis consumers were showed impairment in tests assessing verbal expression, mathematics, and memory. Heavy users of cannabis were more susceptible to interference, made more perseverance errors, had poorer recall, and indicated deficient learning compared to light users (Wilson et al., 2002).
Solowij et al., (2002) discovered few dysfunctions when they compared neuropsychological performance of dependent, heavy cannabis users with an average 10 years of regular use to anon-user control group. Chronic cannabis users with a regular use averaging 24 years were discovered with impaired attention and had retarded memory in general with dysfunctional verbal learning.
Both groups of users indicated impaired temporal judgment. Solowij (1998) in a series of earlier studies applied more sensitive measures of brain function to demonstrate attention impairments in short-term users. Solowij et al., (2002) deduced that long term use of cannabis escalated memory impairment.
Specific deficits in verbal learning, memory and attention continue to be the most consistently replicated impairments to cannabis users. These impairments are associated to the period, frequency, and cumulative dose impacts (Pope et al., 2002).
Differential effects of the various parameters of cannabis use such as, frequency, duration and dose, have not been investigated consistently. As a result, studies are still ongoing to determine whether memory impairments should be related or associated to acute, drug dose, and others occurring the brain memory due to long-term cannabis exposure (Solowij et al., 2002).
Studies continue to be conducted to investigate the propensity for recovery of cognitive functioning following cessation of cannabis use. Solowij (1998) discovered partial recovery following median 2 years abstinence in a small group of ex-users performing a selective attention task. However, sensitive brain event-related potential measures continued to indicate impaired information processing that was correlated with the number of years of cannabis use.
Solowij et al., (2002) showed persistent dose-associated decrements in neuro-cognitive performance after 28 days abstinence in heavy young users of mean age 20, 5 years use. According to Pope et al., (2002), verbal and memory deficits persisted in those who had started using cannabis prior to the age of 17 years but not in those who commenced later in life.
The sampled population was between the ages of 30 and 55 years at the time of research. This observation agrees with other observations of adverse effects in that beginning regular cannabis use before versus after the age of 17 years (Wilson, et al., 2000). There is still need for further studies to elucidate the effect of cannabis use in developing brain.
Solowij (2009) reported that hippocampus, prefrontal cortex and cerebellum are main sections of endogenous cannabinoid activity and heavily implicated in the cognitive impairments associated with chronic cannabis use. Lyketsos et al., (1999) were able to report the only large-scale prospective epidemiological study of the effect of cannabis use on cognitive functioning.
They made assessments on cognitive reduction on the Mini-Mental State Examination in 1318 adults over 11.5 years. They deduced no relationship between cannabis use and decline in Mini-Mental state Examination score, and this persisted when adjustments were made for sex, age, education, minority status, and use of alcohol and tobacco. This study concurs with other evidence that cannabis does not produce gross cognitive impairment (Pope et al., 2002).
Memory in Chronic Cannabis Users
A cute administration of cannabis can disrupt the working memory. Animal literature exists that reports impaired working memory following acute and chronic use of cannabinoids, including an impaired delayed matching to sample tasks that resemble lesions or removal of the hippocampus (Azzam, 2010). A growing number of recent literature have continued to study working memory and related functions in chronic cannabis users.
Kanayama et al., (2004) examined spatial working memory in long-term heavy cannabis users by using functional magnetic resonance imaging using relatively simple tasks. In this study non-users made non-significant more errors on the task, although few errors in both groups reflected the simplicity of the task and it has been suggested that performance deficits in chronic cannabis users are more likely to be elicited in complex tasks (Kanayama et al., 2004).
In addition, Kanayama et al (2004) studies revealed that cannabis users exhibited widespread brain activation with enhanced activation of areas utilized in spatial working memory tasks. They interpreted their findings in terms of cannabis users experiencing subtle neuro-physiological deficits for which they compensate by working harder and calling upon additional brain regions to meet task requirements (Mack, 2010).
In a study of abstinent adolescents aged 13-18, cannabis and tobacco smokers compared to tobacco-only smokers (Hall, 2009). The group identified functional magnetic resonance imaging evidence of changed neuro-circuitry during the performance of an n-back auditory working memory task in the cannabis group, but only during nicotine withdrawal.
Representative samples were tested twice, once during an ad libitum cigarette smoking condition, and again after 24 hrs abstention from tobacco and cannabis users were in abstinence from cannabis for at least two weeks prior to testing. Cannabis users who abstained from tobacco revealed enhanced task-biased activation, for instance, posterior cortical regions and others (Solowij, 1998).
A study using real world functions approach examined mood and cognitive performance in a sample of workers with and without recent cannabis use, before and after work at the start and end of the working week. There were scanty details regarding cannabis levels in the sample. A verbal reasoning task was used to measure working memory.
Other memory tasks encompassed immediate and delayed free recall and recognition of 20 words presented on a computer screen and a semantic processing task measuring speed of knowledge retrieval from general memory (Pope et al 2002). Poorer performance in verbal reasoning was apparent in cannabis users at the start of the working week and correlated and frequency of cannabis use.
Lacklustre performance in verbal reasoning in delayed recall was found in cannabis users pre-work at the end of the working week and was correlated with duration of cannabis use. Cannabis users also indicated slower response organization and lower alertness than non-users, and slower psychomotor speed toward the end of the week, reflecting a lack of improvement in the speed over the working week in contrast to controls, rather than a progressive slowing by cannabis users (Solowij et al., 2002).
Episodic Verbal Memory
Verbal learning and memory have been the most impaired cognitive functions in the studies of acute cannabis use as well as in chronic cannabis users. Cannabis users experience impairments in cognition in terms of the period of cannabis use and the frequency of cannabis consumption, and lastly, the impact of cumulative dosage.
Studies of acute cannabis use suggest that poorer performance can be seen in immediate and delayed recall of words. Recent studies have replicated dysfunction in learning, recall, and delayed recall, with some evidence of rot. The studies deduced 17-hour abstinent long-term chronic cannabis users recalled fewer words than shorter-term chronic users and non-user controls over learning trials (Roffman, 2009).
Conclusion
Satisfactory evidence has gathered from recent research of cannabis users in the unintoxicated state to conclude heavy cannabis use in the long term is associated with impaired memory function. This implies that impaired memory function goes beyond the period of acute use and is related to a variety of cannabis use parameters.
Studies deduce memory impairments to increase proportionate to frequency, dosage, and cumulative dosage of cannabis administration. However, the exact that lead to memory deficits remain to be determined. A collection of research of cannabis users abstinent for reasonably long durations suggest that dysfunctional memory may persist for some time after acute use (Solowij, 2009).
The overall evidence from the various reviews suggests that the use of cannabis does in a way affect negatively upon the function of memory. Greater deficits in memory may be apparent in tasks that are more complex and among chronic cannabis users. The kind of memory deficits in chronic cannabis users is not different to that observed under acute influence (Roffman, 2009).
Heavy cannabis consumers in the unintoxicated state also indicate impaired immediate, but further delayed free call of verbal information, poor retrieval of information from memory, and difficulties manipulating the contents of the working memory. Memory recognition is inconsistently reported and dysfunctional (Sadider, 2010).
Strategies of organizational nature within memory have not received sufficient research. Limited evidence is available for strategy use in spatial working memory. Several studies found similar dysfunctions in cannabis users in learning, on measures of immediate and delayed recall and to research where other verbal learning tests have been administered to cannabis users (Roffman, 2009).
In sum, there exist a wide range of individual differences in the propensity to create memory impairment associated with long term chronic cannabis use. The effect of multiple interpersonal factors on resilience to and susceptibility to cognitive dysfunction deserves greater attention. Such factors may involve personality and differing genotypes. A perspective to substance use in general may also confer enhanced vulnerability to cannabis related cognitive memory and needs further attention in prospective studies (Allhoff, 2010).
Generally, findings of changed brain activation from imaging studies of cannabis users suggest compensatory procedures activated to ameliorate cognitive deficits. A number of recent advances in techniques are beginning to interrogate pertinent questions; however, the field is still open for continued research. The specific nature of memory deficits in cannabis users has not been comprehensively elucidated. Evidence exists for dysfunctional encoding, storage and retrieval (Roffman, 2009).
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