| Summary: | This thesis examines how adult ADHD alters the processing of information in three task unrelated domains; distractor interference, incidental information processing and time perception, and how this is affected by the level of perceptual load in the attended task. An enhanced susceptibility to irrelevant distraction is a core clinical characteristic of ADHD. Despite this, previously established laboratory measures of distraction have failed to consistently report evidence for excessive distractibility in ADHD. Therefore, this thesis starts by investigating whether a recently developed measure of distraction (Forster & Lavie, 2008), which uses entirely task irrelevant distractors, is sensitive to the type of distraction experienced by patients with ADHD (Chapter 2). A wealth of empirical evidence has shown that high perceptual load reduces task irrelevant distraction in the neurotypical population (see Lavie, 2010). Importantly, this thesis examines whether high perceptual load could also reduce task irrelevant distraction (Chapter 2) and task irrelevant incidental information processing (Chapter 3) in this high distractible clinical group. Another form of task unrelated information processing is the perception of time. Previous research suggests that there is a time perception deficit in children with ADHD. However, it is unclear whether such a deficit is retained in adulthood, and whether time perception will be differentially affected by attention in those with ADHD (vs. controls). Therefore, I first established the effect of perceptual load on time perception (Chapter 4) and then compared the effects between adults with a clinical diagnosis of ADHD and age and IQ equated controls (Chapter 5). The attention tasks used were either visual search or rapid streams presented at fixation. Perceptual load was varied by either changing the similarity of the non-target visual search items, or by requesting participants to search for either a feature target (low load) or a conjunction target (high load). In chapters 2-3 an irrelevant distractor image was presented and either interference effects (Chapter 2) or its priming and recognition effects (Chapter 3) were assessed. In chapters 4-5 participants were asked to perform a visual task while also attempting to monitor the passage of time. The results establish that the novel method of distraction developed by Forster and Lavie (2008) is sensitive to the heightened distraction reported in ADHD patients (Chapter 2). Most importantly, it was found that high perceptual load was equally as effective at reducing task irrelevant distraction in adults with ADHD as it was for controls (Chapter 2), and that this effect generalises to incidental task irrelevant information (Chapter 3). In relation to the third task unrelated information processing domain assessed in this thesis, it was established that high perceptual load reduces the perceived length of time intervals (Chapter 4), and that no deficit in time perception or differential effect of load on timing was found for the adult ADHD group (vs. controls) (Chapter 5). The findings from this thesis demonstrate the importance of considering perceptual load and distractor type in studies of task unrelated distraction, incidental information processing and time perception, in those with and without ADHD.
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