Melbourne School of Psychological Sciences - Theses
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ItemThe temporal dynamics of neural processing of static and moving objects( 2023-01)Transmission and processing of information takes time. In the case of motion perception, the brain could theoretically compensate for these delays by using information about a moving object's past trajectory to predict where the object is located at the present moment. This thesis aimed to explore the neural delays that accumulate during visual processing in humans, and whether these can be compensated during perception of objects moving with a predictable trajectory. In Study 1, we used forward encoding modelling of EEG data to show that, after onset of a simple, static stimulus, the spatial specificity of the neural representation of the stimulus fluctuated over time. This indicates that stimulus processing unfolds in a series of feedforward and feedback sweeps of neural activity through regions of the visual cortex consisting of neurons with varying receptive field sizes. Following this, in Study 2, we again used multivariate analysis of EEG data to investigate the neural response to moving stimuli, to discover whether the brain extrapolates the position of moving objects to compensate for neural delays. We found that objects moving into a position on the screen were represented in that location much earlier than if they were flashed in the same position. This predictive encoding of position served to fully compensate for the delays that accumulate during early stimulus processing. Finally, in Study 3, we investigated the consequences of compensation for delays on perception. In the High-Phi illusion, uncorrelated noise textures are interpreted as motion due to a rotating inducing texture; we show that perception of illusory motion can be explained by extrapolation of the preceding motion. Furthermore, the perceived position of a flashed static object was biased by these predictive signals, and the magnitude of illusory motion jumps and position shifts scaled with the speed of the inducing motion. Overall, these results show that processing even simple visual information takes time, but this time can be compensated when viewing predictable visual motion, leading to changes in neural coding and perception of moving objects.
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ItemThe Cognitive and Neural Mechanisms of Dietary Decision-Making( 2022)Dietary decisions are influential on both physical and mental health. Unhealthy choices can have many negative consequences, including obesity, heart disease, diabetes, and cancer. This thesis investigated the role of tastiness and healthiness in dietary decisions and interventions, with three specific research questions. The first involved neural representations of tastiness and healthiness: more specifically, how these attributes are represented in the brain during the early stages of dietary decisions. The second question was centred on whether health warning labels (HWLs) can reduce sugary drink consumption. Finally, the third question involved how the experience and regulation of incidental emotions impacts dietary decisions. These questions were addressed in the three empirical chapters of this thesis. In Study 1, we investigated neural representations of tastiness and healthiness during the first second of rating food images (Experiment 1) or consumption decisions (Experiment 2). The results showed that fine-grained taste and health ratings could be decoded from electroencephalography data using multivariate pattern analysis. This suggests that during dietary decisions, tastiness and healthiness representations are present in the brain from an early stage, even without explicit instructions to consider them. In Study 2, we investigated whether HWLs encouraged healthier choices regarding sugar-sweetened beverages. In a laboratory-based task, participants viewed HWLs, then indicated their willingness to consume various beverages. The HWLs referred either to consequences of a poor diet (general) or sugary drinks (specific). The results showed that both types of HWLs decreased willingness to consume drinks, compared to a control condition. For general HWLs, this effect was weaker for drinks perceived as healthy, whereas the effect of specific HWLs remained constant regardless of perceived healthiness. Overall, our results suggest that HWLs may be effective at prompting behaviour change, and product-specific messages may reduce consumption of a wider range of drinks. In Study 3, we examined how dietary decisions are influenced by emotions and the regulation of these emotions. Participants completed an online task which involved experiencing and regulating emotions before making hypothetical dietary decisions. The results showed that negative emotions decreased willingness to consume foods, whereas positive emotions led to an increase, particularly for healthy foods. Regulating negative emotions did not change dietary decisions; however, actively increasing and decreasing positive emotions via reappraisal led to respective increases and decreases in desire for food. These results may suggest that people appraise stimuli in a way that matches their emotional state (e.g., feeling positive leads people to view foods as more pleasant). Overall, these findings highlight the importance of both healthiness and tastiness for dietary decision-making, support the effectiveness of HWLs for promoting healthier choices, and deepen our understanding of the effects of incidental emotions and emotion regulation on dietary decisions. This thesis may inform interventions to improve dietary decisions, and provides a strong basis for future studies that aim to evaluate interventions through examining changes to neural representations of taste and health attributes.
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ItemPrediction and neural transmission delays in visual motion processing( 2022)Neural transmission takes time. Although this delay is seemingly insignificant (approximately 70 ms), it complicates the real-time localization of moving objects because the brain has no access to information about where the object is now. In this thesis, we study how prediction, and motion extrapolation in particular, can help overcome these neural transmission delays in visual motion processing. For motion extrapolation mechanisms to do this, they need to be able to drive sensory representations in the absence of sensory information, since sensory information arrives on a delayed time-line. In Study 1, we find that sensory representations corresponding to the anticipated next location on a motion trajectory are pre-activated before the anticipated stimulus onset and the arrival of sensory information. Correspondingly, in Study 2, we find that moving objects are subsequently represented with a shorter latency. To explain these findings, we outline a neural architecture of horizontal and feedback connections inducing pre-activations at anticipated stimulus locations that fits within the current dominant framework of cortical organization. The consequence of such an architecture however is that, when a moving object diverges from its predictable trajectory, the wrong stimulus location has been pre-activated. We show that, faced with a motion prediction violation, the brain indeed briefly represents the object in the anticipated but never presented stimulus location, resulting in a latency disadvantage for the representation of the mispredicted stimulus. In Study 3 we finally demonstrate that the overextrapolation is actively corrected for, preventing the conscious percept of the stimulus in the overextrapolated location.