Year

2010

Degree Name

Doctor of Philosophy

Department

School of Psychology

Abstract

This thesis consists of a literature review followed by four empirical chapters examining the relationship between the chronic use of cannabis and cognitive and brain electrophysiological functioning. In Chapter 2 recent literature is reviewed linking chronic cannabis use with, primarily, memory dysfunction. Deficits in verbal memory processes were noted in particular. It was hypothesised that poorer neural efficiency and use of compensatory mechanisms were associated with chronic cannabis use. Additionally, brain imaging measures sensitive to the often subtle alterations associated with cannabis use, such as the use of psychophysiological recording techniques, were indicated as a likely area of viable future research. Chapter 3 follows with a brief review of the existing psychophysiological literature on the acute and chronic effects of cannabis. From the, quite limited, available literature it was identified that acute and chronic cannabis use are each associated with alterations in both early sensory registration and later cognitive processing-related event-related brain potentials (ERPs).

In the empirical studies, Chapter 4 examines chronic cannabis use effects as they relate to a task of verbal memory performance among 24 cannabis users and 24 matched healthy controls. Poorer delayed free recall was present following a list learning task with alterations noted in the amplitude of ERP components related to memory encoding and consolidation, specifically the subsequent memory effect N4 and late positive component. It is argued that chronic cannabis use disrupted the optimal functioning of neural mechanisms related to verbal memory performance and that novel, albeit less efficient, neural pathways are required by cannabis users to perform verbal memory tasks, albeit to a poorer degree than controls. An additional relationship with a longer history of use among cannabis users being related to improved memory performance was observed. This effect was observed though within the context of overall poorer performance within the cannabis-using group relative to controls. It is hypothesised that this reflects neuroadaptive processes related to the brains of longer-term users attempting to develop optimal neural pathways, during the presence of disruption by continued cannabis use. In practical terms this means a longer history of use is related to a recovery of functioning and/or more efficient neural pathways in comparison to a shorter history of use.

The study in Chapter 5 examines recognition memory performance among 21 cannabis users and 22 controls. Previously seen and unseen words were discretely presented to participants with the effects of simple (more novel words) and more difficult distractors (related words) examined for each group. Evidence was present for increased interference from distractors for cannabis users, in particular on the more difficult distractor condition. Impairment of ERP components related to attention (N1 amplitude) and stimulus recognition processing (N4 and P3 amplitudes in the OLD/NEW effect) were present. As with chapter 4, evidence of processes that may indicate neuroadaptation was present with a longer history of use among cannabis users related to the formation of more efficient neural performance as well as more accurate detection of target stimuli from among distractors.

In Chapter 6 the relationship between chronic cannabis use and inhibitory control was examined using a variant of the Stroop colour naming task. Twenty-one cannabis users and 19 controls were compared on Stroop task performance with ERPs generated to stimulus presentations. Users were observed to have overall slower response times as well as poorer performance for colour naming during instances of incongruent colour naming presentations (e.g. the word “RED” appearing in blue ink). Opposite patterns of brain electrical activity (i.e. amplitude) were observed for cannabis users versus controls at central and parietal sites for a late positive potential, known as the Conflict SP. An earlier age of cannabis use uptake was additionally related to poorer task performance. Findings of this study suggest that chronic users of cannabis may have an impaired inhibitory ability that may manifest as a reduced ability to refrain from future substance misuse, with an earlier age of exposure possibly resulting in increased difficulties.

Lastly, Chapter 7 examines chronic cannabis use effects on a basic task of recognition memory that also examined proactive interference. Twenty cannabis users and 20 controls were administered a variant of the Sternberg paradigm, where several stimuli (consonants) were presented at the same time and followed by a probe stimulus (a single consonant) that they were required to indicate was in the stimulus set or not. Cannabis users showed an attenuated N2 ERP component, previously related to familiarity in recognition memory, at central sites in comparison to controls, with variations in central topography also present. Relationships between a longer history of use and improved behavioural performance among cannabis users and more efficient electrophysiological outcomes were present, suggesting neuroadaptation. This study concluded that complex tasks are required to elicit overt differences in behavioural responding among cannabis users but that electrophysiological measures, such as ERPs are able to detect alterations that may go unnoted otherwise.

The empirical findings of this thesis substantially add to the limited available literature on psychophysiological effects associated with the chronic use of cannabis. In particular, evidence that deficits in overt and neural functioning were present during the unintoxicated state indicate that cannabis use impacts outside the period of acute exposure. Relationships with a longer history of use further indicate that changes associated with chronic cannabis use develop gradually and are enduring. Lastly, findings related to neuroadaptive effects suggest an ability for the brains of cannabis users to compensate for dysfunctional neural pathways. Although there were some limitations related to an inability to completely rule out impacts of residual intoxication, mood disturbances, withdrawal symptoms and tobacco use, this was heavily controlled for or factored in where appropriate. Future research may benefit from taking further measures to control for these potential influences.

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