The focus of the thesis is the investigation of the causal nature of the established association between rumination and that ability to resolve interference from task-irrelevant information, and prepotent responses. Rumination is a term used to refer to both unhelpful dwelling on negative moods and depressive symptoms (e.g., Nolen-Hoeksema, 1991), and repetitive intrusive thoughts around the theme of unresolved personal goals (Martin & Tesser, 1996). It has been proposed that rumination occupies working memory resources, thereby depleting cognitive control capabilities necessary for the performance of concurrent effortful tasks (Hartlage, Alloy, Vasquez, & Dykman, 1993; Hertel, 2004; Watkins & Brown, 2002). This model constitutes one possible account of the considerable data demonstrating an association between depressive rumination and deficits on tasks invoking inhibitory processes (Joormann, Yoon, & Gotlib, 2007). An inhibition construct is invoked to account for the empirical observation of interference; however there are few instances where inhibition is unambiguously driving interference (MacLeod, 2007). Moreover, there is evidence that inhibition is not a unitary construct (Friedman & Miyake, 2004). Five experiments manipulated rumination on depressive symptoms and on personal goals in dysphoric and unselected samples in order to test Watkins and Brown’s (2002) hypothesis that state rumination impairs interference control capabilities. The causal impact of state rumination was examined on interference control tasks that implicate different inhibitory sub-types: resistance to proactive interference from positive and negative material (Studies One, Two, and Five), and prepotent response inhibition on a go/no-go paradigm (Studies Three and Four). No evidence was found to support the prediction that state rumination about depressed mood (Studies One and Two) or on-going personal goal discrepancies (Study Five) causes difficulties resolving interference from irrelevant emotional material relative to non-ruminative control conditions in both dysphoric (Studies One and Two) and unselected (Study Five) samples. No evidence was found to support the prediction that state rumination about personal goal discrepancies impairs prepotent response inhibition relative to non-ruminative control conditions (Studies Three and Four). There was some tentative evidence to suggest that ruminating on personal goal discrepancies increased efficiency in holding a single goal active in working memory without reinforcement (Study Four). The implications of these findings for existing models of the causal nature of the relationship between rumination and interference control processes is discussed (Chapter Nine). It is concluded that models proposing a causal impact of state rumination on available working memory capacity are insufficient to fully account for the established association between the trait tendency to ruminate and increased susceptibility to interference from irrelevant material.

Working memory, or the ability to maintain and manipulate information such that it can be used to guide behavior, is known to be severely capacity limited, in most circumstances, to about 3-4 objects. Both infants and adults have the ability to surpass these limits by encoding to-be-remembered items in groups or collections, exploiting statistical regularities or conceptual information to devise more efficient coding schema. Despite progress made toward understanding continuities in working memory, little is known about how changes over development interact with the ability to employ maximally efficient mnemonic data structures. Paper 1 demonstrates that although adults can encode at most three mutually exclusive collections that accrue sequentially over time, they can circumvent this limit when items overlap in features (e.g. red and blue circles and triangles) and statistical regularities are introduced among collections defined by a single visual feature (e.g. most red items are triangular and not circular). Adults' performance suggests they are able to encode items from intersecting collections hierarchically and exploit statistical regularities among collections to reconstruct the numerosities of up to six collections in parallel, exemplifying how efficient coding can radically enhance working memory. Paper 2 demonstrates that young preschoolers can also represent three mutually exclusive collections that accrue in an intermixed fashion over time. Results show that the ability to surpass this capacity limit by hierarchically reorganizing collections and exploiting statistical regularities among them develops between the ages of three and seven. These results are discussed in the context of executive function development. Paper 3 provides evidence that computations of average size and orientation rely on qualitatively different processes with distinct developmental trajectories. Experiment 1 demonstrates that while the presence of additional identical elements in an array detrimentally impacts 6-month-olds' representations of element size, it improves the precision with which infants represent orientation. Experiment 2 demonstrates that performance is not affected when infants' attention is cued to a single item within arrays. These results are discussed in the context of the development of controlled attention. / Psychology

The segment of the population that is 65 years and older is undergoing rapid growth. With this growth, there is an increased need for speech-language pathologists (SLPs) to treat cognitive-communicative disorders, most of which are age-related disorders secondary to neurologic disease. To do this effectively, SLPs require an understanding of how healthy aging affects cognition, memory and sensory processing. Although the interactions between sensory and cognitive processing are increasingly being investigated, little attention has been paid to the relationship between modality of information presentation and language comprehension in aging. Language comprehension is affected by component processes such as memory span and processing abilities, but to date there has been no systematic investigation of the effect of presentation modality on the processing of linguistic information across a number of tasks ranging in complexity.The primary purpose of this study was to investigate the effects of presentation modality on language processing. The secondary purposes of the study were to determine whether age affected performance on language processing measures, and whether adults can accurately predict if their performance was better in either the auditory or visual presentation condition. The effects of presentation modality were evaluated on four dependent measures: a digit span task, a word span task, a complex span task and a procedural instruction processing task. The participants were 37 older adults and 41 younger adults, matched for years of education and estimated verbal IQ.Three notable findings emerged from the study. First, both young and older adults recalled more information when it was presented in the auditory modality, regardless of the task. Second, age effects were present on all measures. Third, individuals were generally inaccurate in predicting which modality would result in better processing. Implications for the evaluation and treatment of older adults are discussed.

Children with ADHD diagnosis often display working memory deficits, as well as reading and mathematical disabilities. Previous studies have demonstrated that computerized working memory training (WMT) is a promising intervention. The present study aimed at exploring the effects of WMT on working memory, scholastic skills and behavioral symptoms in children with ADHD. Thirty-two children, aged 6 to 11, were randomized to WMT or a control condition. WMT consisted of nine tasks taxing working memory with adaptive difficulty level. All children trained in their homes, with their parents acting as supervisors. Children who completed more than 20 days of training in 5-8 weeks (8 in the WMT condition and 13 in the control condition) were considered compliers. Assessments were conducted before and after intervention. Results indicated that WMT lead to significant gains of verbal working memory and arithmetic skills. More research is needed to further investigate the effects of WMT.

Traumatic brain injury (TBI) is a leading cause of disability globally. Cognitive deficits represent the primary source of on-going disability in this population, yet the mechanisms of these deficits remain poorly understood. Here functional and diffusion-weighted imaging techniques were employed to characterize the mechanisms of neurofunctional change following TBI and their relationship to cognitive function. TBI subjects who had sustained moderate to severe brain injury, demonstrated good functional and neuropsychological recovery, and screened positive for diffuse axonal injury but negative for focal brain lesions were recruited for the project. TBI subjects and matched controls underwent structural, diffusion-weighted and functional MRI. The functional scanning paradigm consisted of a complex working memory task with both load and executive control manipulations. Study one demonstrated augmented functional engagement for TBI subjects relative to healthy controls associated with executive control processing but not maintenance operations within working memory. In study two, multivariate neuroimaging analyses demonstrated that activity within a network of bilateral prefrontal cortex (PFC) and posterior parietal regions was compensatory for task performance in the TBI sample. Functional connectivity analyses revealed that a common network of bilateral PFC regions was active in both groups during working memory performance, although this activity was behaviourally relevant at lower levels of task demand in TBI subjects relative to healthy controls. In study three, diffusion-imaging was used to characterize the impact of diffuse white matter pathology on these neurofunctional changes. Unexpectedly, decreased white matter integrity was not correlated with working memory performance following TBI. However, markers of white matter pathology did inversely correlate with the compensatory functional changes observed previously. These results implicate diffuse white matter pathology as a primary mechanism of functional brain change following TBI. Moreover, reactive neurofunctional changes appear to mediate the impact of diffuse injury following brain trauma, suggesting new avenues for neurorehabilitation in this population.

Traumatic brain injury (TBI) is a leading cause of disability globally. Cognitive deficits represent the primary source of on-going disability in this population, yet the mechanisms of these deficits remain poorly understood. Here functional and diffusion-weighted imaging techniques were employed to characterize the mechanisms of neurofunctional change following TBI and their relationship to cognitive function. TBI subjects who had sustained moderate to severe brain injury, demonstrated good functional and neuropsychological recovery, and screened positive for diffuse axonal injury but negative for focal brain lesions were recruited for the project. TBI subjects and matched controls underwent structural, diffusion-weighted and functional MRI. The functional scanning paradigm consisted of a complex working memory task with both load and executive control manipulations. Study one demonstrated augmented functional engagement for TBI subjects relative to healthy controls associated with executive control processing but not maintenance operations within working memory. In study two, multivariate neuroimaging analyses demonstrated that activity within a network of bilateral prefrontal cortex (PFC) and posterior parietal regions was compensatory for task performance in the TBI sample. Functional connectivity analyses revealed that a common network of bilateral PFC regions was active in both groups during working memory performance, although this activity was behaviourally relevant at lower levels of task demand in TBI subjects relative to healthy controls. In study three, diffusion-imaging was used to characterize the impact of diffuse white matter pathology on these neurofunctional changes. Unexpectedly, decreased white matter integrity was not correlated with working memory performance following TBI. However, markers of white matter pathology did inversely correlate with the compensatory functional changes observed previously. These results implicate diffuse white matter pathology as a primary mechanism of functional brain change following TBI. Moreover, reactive neurofunctional changes appear to mediate the impact of diffuse injury following brain trauma, suggesting new avenues for neurorehabilitation in this population.

Reading comprehension (RC) is a complex, dynamic process by which readers interact with text to construct meaning. It relies on word-level decoding and vocabulary skills, text-level skills such as inference, and general-purpose cognitive processes such as working memory (WM). Bridging inferences, which connect different parts of text to maintain semantic coherence, are necessary for comprehension. WM is thought to provide the mental workspace that allows readers to comprehend text, including making bridging inferences. This dissertation includes two studies that investigate related but unique questions regarding inference-making, WM, and RC in school aged children. The first study demonstrated that bridging inference making increased with age and was affected by text distance such that inferences across larger chunks of text were more difficult to make than those between adjacent sentences. Bridging inferences were also affected by knowledge domain such that affective inferences based on character goals, desires, or motivations were made correctly less often than were concrete inferences based on concrete, causal information. Semantic updating, an aspect of WM that involves efficiently revising the contents of WM, predicted variance in the far-concrete inferencing condition. Semantic reactivation, an aspect of WM that involves bringing previously processed information back into WM, predicted variance in the far inferencing conditions regardless of knowledge domain. The second study investigated the word-level and text-level skills and general purpose cognitive processes that support performance on three different standardized RC measures. Semantic updating and semantic reactivation predicted variance on the RC tool considered to measure mental model building (WJIII-PC, Woodcock-Johnson-III passage comprehension subtest). Semantic reactivation also predicted variance on the RC tool considered to measure the ability to build and then reflect upon the mental model (WIAT-II-RC, Wechsler Individual Test of Achievement reading comprehension subtest). A measure of verbal WM predicted performance on one RC measure in the presence of word-level and text-level skills but only when the hypothesized components of WM (updating and reactivation) were not included in the model. Additionally, bridging inference making uniquely predicted performance on all three RC measures. The findings suggest readers coordinate different cognitive processes depending on the comprehension task. / Social Sciences and Humanities Research Council of Canada (SSHRC)

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The neural signature and cognitive architecture of how a visual stimulus reaches conscious awareness have not been conclusively determined. Recently, we have reported an ERP activity corresponding with a period of perceptual persistence which has been typically reported in working memory research, known as the contralateral delay activity (CDA). Specifically, we reported how the CDA can be used to track how on object fades in out of consciousness. Here, we report on the behaviour of the CDA when the type of item in this paradigm is manipulated between face, animal and object stimuli. We recorded EEG activity while participants viewed a bilateral Shape-from-Motion display and analyzed where and how large a CDA is observed. Our findings show that the CDA is generalizable to different item types fading from awareness, with no differences between item conditions. This work supports the content-invariant involvement of working memory processes in sustaining conscious awareness.

The neural signature and cognitive architecture of how a visual stimulus reaches conscious awareness have not been conclusively determined. Recently, we have reported an ERP activity corresponding with a period of perceptual persistence which has been typically reported in working memory research, known as the contralateral delay activity (CDA). Specifically, we reported how the CDA can be used to track how on object fades in out of consciousness. Here, we report on the behaviour of the CDA when the type of item in this paradigm is manipulated between face, animal and object stimuli. We recorded EEG activity while participants viewed a bilateral Shape-from-Motion display and analyzed where and how large a CDA is observed. Our findings show that the CDA is generalizable to different item types fading from awareness, with no differences between item conditions. This work supports the content-invariant involvement of working memory processes in sustaining conscious awareness.