- Title
- Visual processing, visual attention and their neural correlates in early-onset cannabis users
- Creator
- Mikulskaya, Elena
- Relation
- University of Newcastle Research Higher Degree Thesis
- Resource Type
- thesis
- Date
- 2016
- Description
- Research Doctorate - Doctor of Philosophy (PhD)
- Description
- Cannabis use worldwide has been increasing steadily over the past decade. The current trend towards legalization of cannabis for medical purposes and/or recreational use increases the potential to expose more and younger people to cannabis. Early age of onset has been linked to visual and attentional deficits in adulthood suggested to arise from blunted dopaminergic functioning. Dopamine is a neurotransmitter, critically important for efficient visual and attentional functioning in addition to general cognitive and motor behaviour. The overall aim of this thesis was to investigate in depth low level visual processing (Phase 1) and visual attention and underlying neural processing (Phase 2) in early-onset cannabis users. Static and dynamic contrast thresholds were examined in Experiments 1 and 2 and motion discrimination thresholds were examined in Experiment 3 under normal and low luminance conditions. An index of brain dopamine production was also collected and compared in Experiment 4. Static but not dynamic contrast detection thresholds were significantly higher in the cannabis group (21 participants: 18 of whom commenced cannabis use before the age of 17) compared to the control group (20 participants) under mesopic levels of luminance. Both static and dynamic contrast thresholds were positively correlated with frequency of use in the cannabis group. Discrimination of direction of translational and radial motion was examined in Experiment 3 under normal and mesopic luminance and at two levels of stimuli contrast: 12 and 24%. Independently of luminance level and motion type, coherence thresholds were significantly higher in the cannabis group at 24% stimuli contrast compared to the control group and in particular the thresholds were higher for radial compared to translational motion, although the differences failed to reach statistical significance. Again, the thresholds in the cannabis group were higher with more frequent cannabis use. We also found significantly lower spontaneous Eye Blink Rate (EBR) in the cannabis group compared to the control group in Experiment 4. We argued that lower dopamine may underlie the higher thresholds shown by the cannabis group across all visual tasks in Phase 1. In Phase 2 of this thesis behavioural data and EEG recordings were collected from 53 participants in four experiments. ERPs and gamma band activity were extracted from EEG recordings and compared between the control group (29 participants) and the cannabis group (24 participants: all commenced cannabis use before age 17). In Experiment 5 we investigated the neural correlates (ERPs and gamma band activity) of radial motion processing to further investigate the results of Experiment 3. As expected reduced ERP amplitudes to motion stimuli were shown by the cannabis group compared to the control group. Motion stimuli were also employed in the oddball task in Experiment 6 in which we aimed to investigate the neural correlates of selective attention. The cannabis group showed significant reductions in P3b amplitude in response to target stimuli, overall reduced amplitude and delayed latency of P3b, and increased connectivity in the gamma range to target and standard stimuli. Increased connectivity for both types of stimuli was argued to compensate for reduced attentional resource allocation, as indexed by reduced P3b amplitudes in the cannabis group. Cannabis related and neutral words were used in a word recognition task in Experiment 7, with target words presented briefly, then presented paired with a distractor for recognition. Cannabis and neutral pictures used as primes followed by cannabis and neutral words in a lexical decision task were employed in Experiment 8. We found some evidence for cannabis biased attention in the cannabis group, as indexed by differences in ERP components and the connectivity patterns in gamma band activity. Overall, in both experiments, the cannabis group showed increased processing effort toward both cannabis and neutral stimuli. The current thesis has demonstrated that visual processing, attention and underlying brain activity are altered by cannabis use with early age of onset. Consistent with previous research, selective attention and its neural correlates differed in respect to stimuli identity, cannabis or neutral, in early-onset cannabis users. It has also been shown that cannabis related stimuli, presented at subliminal levels, influenced attention and subsequent processing of cannabis and neutral stimuli in the cannabis group. Future investigations could examine in depth the effects of early-onset cannabis use on visual processing and attention, and whether and how it is modulated by concomitant reductions in dopamine.
- Subject
- contrast sensitivity; motion processing; oddball; word recognition; masked semantic priming; early-onset cannabis use; ERPs; gamma oscillations; eye blink rate
- Identifier
- http://hdl.handle.net/1959.13/1312742
- Identifier
- uon:22456
- Rights
- Copyright 2016 Elena Mikulskaya
- Language
- eng
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