The impact of marijuana smoking on memory and cognition



Marijuana is the most common illicit drug that it abused in the world today, with close to over 160 million people reported to be using it. It has both therapeutic and toxic effects on individuals in that it causes alterations in memory and cognitive functions. The responsive cannabinoid in Marijuana is tetyrahydrocannabinol (THC), which binds with the cannabinoid receptors (CB1) to cause impairment to memory and cognitive function. Use of marijuana impairs cognitive functions ranging from motor coordination to an individual’s ability to organise, plan, solve problems, control emotions, and make decisions. It also causes change in behaviour and memory. This paper discusses the impacts that marijuana smoking has on the brain neurodevelopment and cognitive functions.

The Impact of Marijuana Smoking on Memory and Cognition

Among the illicit drugs that are abused in the world today, Marijuana is the most common. It has been reported to be smoked by close to 160 million individuals (NIDA, 2017). The use of marijuana is on the rise and about 3% of the population of people aged between 15-64 years are frequently involved in its use. Marijuana has close to 60 types of cannabinoids and over 600 ingredients that are thought to have toxic and therapeutic effects on individuals. Each of the cannabinoids present in marijuana has a different effect on the behaviour and health of the individual. Of all the cannabinoids present in marijuana, delta 9 tetrahydrocannabinol (THC) is the most responsive and it has effects on the memory and cognitive functions hence at times, it produces psychotic symptoms (NIDA, 2017).  There is a notion that marijuana is inert to the brain, although most of the marijuana users have a perception that it is an innocuous substance that is that can be used for recreation purposes. However, there are concerns that it has effects on the brain and cognitive functioning, where individuals exhibit signs slower reaction time to tasks (Schoeler & Bhattacharyya, 2013). For instance, studies show that marijuana produces mixed effects on cognition, such that even carrying out of simple individual psychomotor tasks becomes hampered. As a result, frequent use of marijuana has been associated with brain impairment and delayed recall, which eventually causes memory disruptions and thus prompting cognitive shortfalls.

The responsive cannabinoid in marijuana (THC) works in a manner that it binds with the universal cannabinoid receptors (CB1) in the brain, hence causing neural changes that will eventually affect cognitive functions. Intoxication of THC impairs cognitive functions ranging from motor coordination to an individual’s ability to organise, plan, solve problems, control emotions, and make decisions, and it also lead to change in behaviour and memory (Filbey et al., 2015). The changes that occur in the brain are long lasting and therefore affect the neural architecture. Although such changes are unclear, studies indicate that there are structural changes that may be observed as a result of using marijuana, in that there are reported cases of decrease in brain volume, especially in regions such as the orbitofrontal cortex, hippocampus, stratum, and the amygdala (Hart et al., 2001).  The binding of THC and CB1 causes shrinkage of cells and damage to DNA strands in the hippocampal region thus causing decrease in brain volume. For instance, a study on the interaction between marijuana usage and schizophrenia found out that use of marijuana caused changes in the shape of the hippocampus in both individuals with and those without schizophrenia (Schoeler & Bhattacharyya, 2013). The hippocampus is always associated with memory and as a result, a smaller volume of hippocampus among users is an indication of poor memory performance, thus reflecting the potential neurotoxic effect of THC.

Presence of THC in the brain decreases brain volume especially in the hippocampus region. A study by Filbey et al. (2015) indicates that there are differences in the association between individual cognitive behaviour and brain volume. For instance, poor memory performance in marijuana users is associated with smaller hippocampal volumes. The anomalies in the hippocampal morphology as a result of marijuana smoking therefore causes impaired cognitive and memory function (Filbey et al., 2014). Chronic usage of marijuana can result in severe decrease in hippocampal volume and therefore reflecting possible neurotoxic effects where there is cell loss in the brain. Studies on treatment of rats with THC demonstrate the effect through reduced length of dendrites and spine density, which indicate that a decrease in volume indicates the morphological changes and alterations that the hippocampal region goes through (Filbey et al., 2014). As a result of the morphological changes in the hippocampus, users tend to show poor memory performance and memory functions. Use of marijuana can equally cause changes in the frontal parietal brain thus impairing recall memory and performance of tasks.

Studies indicate that marijuana causes severe mental health effects and it is normally dependent on the period that one has been using it. For those who start using it early during their developmental stages such as adolescence, marijuana is thought to cause derailed brain development and may actually induce changes in brain structure and function. According to studies, the memory deficits as a result of marijuana usage in individuals is dependent on frequency of use, duration, and the age at which one starts to use it (Filbey et al., 2014). According to studies the decline in memory function is more pronounced in adolescents who begin indulging in smoking marijuana at a tender age and for adults, it is dependent on frequency of use (Filbey et al., 2014). From a neurobiological perspective, it is shown that continued usage of marijuana and indulging in its use from tender age normally causes profound morphological and functional changes in a developing brain as compared to a mature one (Schoeler & Bhattacharyya, 2013). Therefore, continued use will cause memory impairment in users as it has been demonstrated in animal studies, especially rats where the effect of THC has been studied in controlled experiments. Similar studies have been carried and results indicated that high levels of THC causes impairment between the neuron in the right and left hemispheres of the brain, which means that there is poor internal brain communication in users as compared to non-users of marijuana.

The THC isomers affect neuropsychological parameters such as oculomotor deficit, response time, executive functioning, and verbal memory. Frequent use of marijuana intensely increases cerebral blood flow and as a result of continued exposure, the THC will eventually cause overall reduction in the flow of cerebral blood (Conroy et al., 2015). Studies indicate that the cannabinoid THC induces memory impairment and psychotic-like states in users of marijuana. A study on the impact of cannabis usage in relation to memory and brain function found out that chronic users of marijuana showed signs of altered memory and brain function in form of poor neural efficiency or rather subsequent memory effect that is normally associated with memory lapse (Conroy et al., 2015).

The levels of THC concentration in blood plasma are normally felt within minutes, and considering the fact the cannabinoid is fat soluble, it is stored easily. The effect of the THC can be felt long after one has smoked marijuana since the cannabinoid is fat soluble, which explains why it can be detected even a month after (Schoeler & Bhattacharyya, 2013). However, the psychoactive outcome of marijuana is felt immediately after smoking and intoxication levels reach peak after around 30 minutes and can last several hours. As a result, individuals immediately exhibit changes in executive functioning and disruptions in learning. The central nervous system that forms part of the brain is very sensitive to introduction of THC and as a result, usage of marijuana will contribute to impairing of memory function. The CB1 receptors are in charge of modulating cognitive learning and thus they play an important role in memory function. For this reason, usage of marijuana induces the THC cannabinoid that attaches to the receptors hence bringing about spatial memory deficits (NIDA, 2017). Therefore, the receptors result in THC causing memory deficits that are normally expressed through glutamatergic mechanisms on the GABAergic neurons.

In a study to investigate the sex differences in the impact of marijuana on cognition, Anderson et al. (2010) found out that smoking marijuana actually led to impairment of attention and estimation of time. Individuals in the study were assigned randomly to smoke either a placebo or active marijuana cigarette, and then a neuropsychological test battery was completed to see the extent of the impact. Results indicated that marijuana had the same cognitive effect on both sexes by affecting their estimation of time, having divided selective attention to tasks, and extensively, their cognitive ability and flexibility. Therefore, individuals smoking marijuana are more likely to either over-estimate or underestimate time, and the possibilities of having divided attention are very high, especially for those who use active marijuana that has high concentration of THC (Filbey et al., 2015). In essence, the lapse in time in marijuana users is attributed by the need of more time so that they can cue a process, which eventually results in increased psychological refractory period. Similarly, reaction time will normally increase when a marijuana user switches a task, which thus suggests that there is a decline in cognitive functioning and flexibility among users.

Marijuana use has a varying effect on cognitive function in both males and females. Animal studies where they are exposed to THC indicate that female rats showed more sensitivity to the THC cannabinoids compared to males, which is associated with a lower number of CB1 receptors in cerebral region (Shrivastava, Johnston, & Tsuang, 2011). A similar study on humans to investigate how humans respond to cognitive tasks under the influence of marijuana indicated differences in visuo-spatial memory between females and males. Equally, there are differences between light and heavy marijuana smokers in that the memory function is more impaired in heavy female smokers, who exhibit difficulties in recalling and make more errors in decision making compared to light smokers. A study on the effect of marijuana on cognition in males and females found out that daily use of marijuana impaired cognitive and memory functioning (Conroy et al., 2015). Considering the fact that most of the CB1 receptors are found in the hippocampus region of the brain, it is the most affected by THC cannabinoids resulting from use of marijuana thus causing impairment, which results in dysfunctional cognition (Conroy et al., 2015). Results of the study by Conroy et al. (2015) equally indicate that there are significant differences in difficulties in cognitive function between males and females, depending on the frequency and extent of marijuana usage.

Some marijuana users are thought to develop psychiatric symptoms and decline in cognitive ability. Studies indicate that users can have cognitive impairments that are similar to cognitive deficits displayed by schizophrenia patients (Brackenbury, Ladd, & Anderson, 2016). The brain impairment as a result of marijuana use has long lasting effect and recovery functions after withdrawal are not that clear (Brackenbury, Ladd, & Anderson, 2016). The association of marijuana with schizophrenia and psychosis is based on the effect it has on cognitive functioning. For instance, in adolescents, use of marijuana affects their intellectual ability and thus the decline in the cognition function leads to manifestation of psychosis or schizophrenia. However, it is only a limited number of users who normally develop psychological issues as a result of smoking marijuana, hence it is not clear whether the rest who do not become symptomatic really experience cognitive decline. Marijuana is considered to be a risk factor leading to psychosis for individuals, considering the fact that it has the ability to affect neurodevelopment, especially in adolescents, and the dysfunction of the end-cannabinoid system can lead to conditions such as schizophrenia (Shrivastava, Johnston, & Tsuang, 2011). However, certain cannabinoid changes can act as neuro-protectants instead of making individuals vulnerable. Studies indicate that there is a diminishing capacity for behavioural monitoring among marijuana users that is a result of the hyperactivity in the right insula and anterior cingulate cortex (ACC) (Shrivastava, Johnston, & Tsuang, 2011). The increase in the levels of activity in the right insula and ACC after smoking shows the correlation in marijuana use and error awareness.

In a study to examine the effects of acute marijuana use on cognitive performance, results indicated that high concentrations of THC produced minimal impacts on the cognitive functions of users (Hart et al., 2001). Use of marijuana increases premature responses and the amount of time required to complete tasks. Their results indicated that psychomotor performance among users was subtly affected, although there was a decrement in performance of tasks (Hart et al., 2001). The increase in premature responses from marijuana users is a result of the concentrated amounts of THC in the brain, which causes inhibitory control. Heavy marijuana users exhibit challenges by inhibiting inappropriate responses thus affecting executive functioning and working memory processes. As a result, marijuana smoking therefore enhances impulsive behaviour in users who demonstrate minimal inhibition of maladaptive responding. Heavy marijuana users are more likely to demonstrate more errors as a result of inhibition compared to light users because of the high concentrations of THC in the brain (Brackenbury, Ladd, & Anderson, 2016). Severity of inhibition control is dependent on the number of years one has been smoking marijuana, and even through the level of impairment is consistently similar in many users, the residue effect is however limited.

Similarly, cognitive functioning is a subset of attention and concentration, and as a result, information processing is very important. A study to investigate information processing among marijuana users indicated that withdrawal had effects on memory and recall time on individuals (Crean, Crane, & Mason, 2011). Results indicated that when chronic users abstained from smoking marijuana, they registered significantly low rates of information processing and that their cognitive functioning normalised after they had a joint. Therefore, withdrawal from marijuana use can hamper information processing among chronic users and thus cognitive functioning, although this can normalize once the individual is intoxicated (Brackenbury, Ladd, & Anderson, 2016). It is thus a common occurrence that users experiencing deficit information processing indulge in marijuana smoking in order to regain their information processing and cognitive abilities.

In conclusion, it is evident that marijuana use actually impairs memory as a result of the THC cannabinoids binding with the CB1 receptors. The effect of marijuana use on brain impairment is dependent on the concentration of THC, frequency of use, duration one has been smoking, and the age at which someone started smoking. It is apparent that chronic users experience more memory and cognitive function deficits. Similarly, marijuana affects development of the brain, and as a result, adolescents are more likely to experience more damage in that the volume of the hippocampus is decreased and the cannabinoid system impaired hence they are more likely to experience difficulties in making decisions, organising, planning, as well as with selective and divided attention. The cannabinoid THC found in marijuana thus disrupts memory function by interfering with information processing in that there is more information reaching consciousness when someone is high than what the brain can store hence affecting the memory and cognitive ability of an individual.

Equally, marijuana is thought to have long term effects and it causes prolonged deficits in working and verbal memory. The changes in brain morphology and function are also things that are likely to persist even long after one has quit smoking. Although the changes in cognitive ability and time estimation vary in males and females, they are nonetheless insignificant. However, females are thought to show slower reaction times to cognitive performance after using marijuana compared to males, which is attributed to the low number of CB1 receptor cells that women have.  The effects of marijuana use can thus be considered to be subtle and the level of brain impairment and effect on cognition tends to be dependent on the intoxication level, which is subject to individuals’ tolerance to the effects and the circumstances of usage. Nevertheless, although marijuana affects performance and cognition, to a regular user, increased levels of intoxication do not affect their performance, but their memory function is rather derailed when they are not intoxicated. It is essential to note that marijuana smoking does not necessarily scramble the brain; rather, it slows it down especially for adolescents. Therefore, more studies should be carried out on the effects of marijuana on the brain’s memory and cognition to help understand the specific changes that occur as a result of smoking marijuana.

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