https://www.ncbi.nlm.nih.gov/pubmed/28614161
https://www.ncbi.nlm.nih.gov/pubmed/28560818
https://www.ncbi.nlm.nih.gov/pubmed/28414051
https://www.ncbi.nlm.nih.gov/pubmed/28262852
https://www.ncbi.nlm.nih.gov/pubmed/28219487
https://www.ncbi.nlm.nih.gov/pubmed/18358595
https://www.ncbi.nlm.nih.gov/pubmed/19630711
https://www.ncbi.nlm.nih.gov/pubmed/28498718
https://www.ncbi.nlm.nih.gov/pubmed/28364340
The daily use of cannabis over years and decades appears to produce persistent impairments in memory and cognition, especially when cannabis use begins in adolescence (Meier et al., 2012; Volkow et al., 2014a). The neurobiology of the cannabinoid system suggests that these effects may arise because chronic THC use reduces the number of CB1 receptors (i.e. “down-regulates” these receptors) in brain regions that are involved in memory and cognition (Iversen, 2012) Experimental studies suggest that animals exposed to THC during puberty may be more susceptible to these effects of cannabis (Schneider, 2012).
Brain imaging studies comparing school students who are regular long-term cannabis users and non-using students typically find poorer cognitive performance and large decreases in perfusion in the former using SPECT scans (Mena et al., 2013). These changes could partially explain the lower educational attainment and lower grades among chronic cannabis users (Volkow et al., 2014a) and are discussed in more detail in section 6.1.2
Magnetic resonance imaging (MRI) studies have found structural differences between the brains of chronic adult cannabis users and the brains of non-using controls. Changes can be seen in the grey/white matter, in global brain measures (Batalla et al., 2013), and in connectivity (Lopez-Larson, Rogowska & Yurgelun-Todd, 2015). Structural brain abnormalities are seen in CB1-rich areas involved in cognitive functions. In addition, reduced hippocampal volume has been found in neuroimaging studies (Ashtari et al., 2011; Cousijn et al., 2012; Matochik et al., 2005; Yücel et al., 2008). In some studies these reductions persist after abstinence (Ashtari et al., 2011) and have been associated with impaired memory (Lorenzetti et al., 2015). Neuroimaging studies have also found reduced volumes in the amygdala, the cerebellum and frontal cortex in chronic cannabis users (Batalla et al., 2013; Yücel et al., 2008). In a large study population (1574 participants), in which cortical thickness was measured by MRI, an association was found between cannabis use in early adolescence and reduced cortical thickness in male participants with a high polygenic risk score. Adults who have smoked cannabis since adolescence show reduced neuronal connectivity in the prefrontal areas responsible for executive functioning and inhibitory control and in the subcortical networks that are responsible for habits and routines (Volkow et al., 2014a). The precuneous -a node involved in integration of various brain functions such as awareness and alertness – is particularly affected in frequent cannabis users. Long-term cannabis use is hazardous to the white matter of the developing brain, with evidence of axon connectivity damage in three fibre tracts: the hippocampus (right fimbria), the splenium of the corpus callosum, and commissural fibres (which connect the two halves of the cerebral hemispheres). Damage was higher with younger age of onset of regular cannabis use (Volkow et al., 2014a).
The fimbria is a part of the hippocampus involved in learning and memory (Zalesky et al., 2012). These findings are consistent with the observation that impaired memory is a common complaint among cannabis users seeking treatment (Hall, 2015). Recovery of hippocampal connectivity after long-term abstinence has been reported (Yücel et al., 2016). Atypical orbitofrontal functional connectivity patterns were observed in attentional/executive, motor and reward networks in adolescents with heavy cannabis use. These anomalies may be reflected in suboptimal decision-making capacity and increased impulsivity (Lopez-Larson, Rogowska & Yurgelun-Todd, 2015). Chronic cannabis use has also been shown to reduce the brain’s capacity to synthesize or release dopamine (Bloomfield et al., 2014), which could explain why cannabis users have higher scores on negative emotionality (Volkow et al., 2014b).
https://www.ncbi.nlm.nih.gov/pubmed/21321675
THis study ^ is a good one
not all the studies i linked are relevant