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Thesis (Ed.M.)--Boston University / PLEASE NOTE: Boston University Libraries did not receive an Authorization To Manage form for this thesis or dissertation. It is therefore not openly accessible, though it may be available by request. If you are the author or principal advisor of this work and would like to request open access for it, please contact us at open-help@bu.edu. Thank you. / 2031-01-01

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The research reported was inspired by the Perfect and Maylor (2000) chapter ‘Rejecting the Dull Hypothesis’. This suggested that cognitive ageing research should not focus purely on whether younger adults outperform older adults on a given task. Hartley, Speer, Jonides, Reuter-Lorez and Smith (2001) showed that older adults do not maintain the dissociability of naming identity, visual identity, and spatial location abilities that is seen in younger adults. Away from the ageing literature, Brown, Forbes and McConnell (2006) demonstrated improvement in visual task performance when the availability of verbal coding was increased. The hypothesis that older adults are less likely to use task specific cognitive mechanisms during short-term visual memory tasks was explored. This was carried out by means of a series of 8 experiments (outlined below), which broadly looked at differences in verbal interference effects on visual task performance, differences in Visual Patterns Task performance based on the availability of verbal encoding, and assessed for age-related differences in interference from an executive task in Visual Patterns Task performance. Data was interpreted through the prism of the Scaffolding Theory of Aging (Park & Reuter-Lorenz, 2009), which suggests that compensatory recruitment is employed both young and older adults in response to extrinsic challenges such as task difficulty, and intrinsic challenges, such as declining performance with age. Experiments 1-3 focused on differential effects of articulatory suppression on visual task performance between young (18-25) and older (60-75) adults. Older adults showed negative effects of suppression in short-term maintenance tasks that were not present in younger adults. Both age groups showed negative effects in a mental image rotation task. This suggested a level of verbal activation in visual tasks for both age groups, but that this activation was more common in older adults. Experiments 4-5 assessed differences in Visual Patterns Task performance between both age-groups depending on the availability of verbal encoding. Younger adults displayed the benefit of available verbal encoding with simultaneous but not sequential presentation of information. Older adults showed a benefit of verbal coding in the simultaneous task if the sequential task featured ordered, not randomised presentation pathways. This suggested that older adult task performance may be affected by all conditions within an experiment, not just the current manipulation condition. Experiments 6-7 demonstrated that older adults’ performance in the simultaneous presentation version of the Visual Patterns Task is affected by the availability of verbal encoding in the first task presented to them. Mean performance on subsequent conditions was higher when ‘high verbal coding’ patterns were seen in the first instance. This was not the case for younger adults. The demonstration of a benefit to performance from the ‘high-verbal coding’ pattern set compared to the ‘low-verbal coding’ set was a marker of higher overall performance across all task conditions for younger adults, but not for the older group. This suggested that even if verbal activation during visual task performance was an occurrence for older adults, it was not necessarily a marker for improved performance. Experiment 8 demonstrated that there were no age-related differences in the level of interference from an executive task (Random Month Generation) on Visual Patterns Task performance. This suggested that older adults do not try to actively recruit executive processes during Visual Patterns Task performance to any greater extent than younger adults do. It is suggested that older adults do use specialised task mechanisms to a lesser extent than younger adults in visual memory task performance. It is likely that this is a passive outcome of a decreased inhibition of verbal coding mechanisms, rather than an active attempt to maintain performance through the recruitment of executive cognitive resources. This is seen by the lack of age-group effects from executive interference tasks.

Research into the nature and content of visual long-term memory has investigated what aspects of its representation may account for the remarkable ability we have to remember large amounts of detailed visual information. One theory proposed is that visual memories are supported by an underlying structure of conceptual knowledge around which visual information is organized. However, findings in memory for visual information learned in a visual search task were not explained by this theory of conceptual support, and a new theory is proposed that incorporates the importance of episodic, task-relevant visual information into the organizational structure of visual memory. The current study examined visual long-term memory organization as evidenced by retroactive interference effects in memory for objects learned in a visual search. Four experiments were conducted to examine the amount of retroactive interference induced based on aspects in which interfering objects were related to learned objects. Specifically, episodically task-relevant information about objects was manipulated between conditions based on search instructions. Aspects of conceptual category, perceptual information (color), and context (object role in search) were examined for their contribution to retroactive interference for learned objects. Findings indicated that when made episodically task-relevant, perceptual, as well as conceptual, information contributed to the organization of visual long-term memory. However, when made episodically non-relevant, perceptual information did not contribute to memory organization, and memory defaulted to conceptual category organization. This finding supports the theory of an episodically defined organizational structure in visual long-term memory that is overlaid upon an underlying conceptual structure.

The leit motif of this thesis is that binding of visual features is a process that begins with input of stimulation and ends with the emergence of an object in working memory so that it can be further manipulated for higher cognitive processes. The primary focus was on the binding process from 0 to 2500 ms, with stimuli defined by location, colour, and shape. The initial experiments explored the relative role of topdown and bottom-up factors. Task relevance was compared by asking participants to detect swaps in bindings of two features whilst the third was either unchanged, or made irrelevant by randomization from study to test, in a change detection task. The experiments also studied the differences among the three defining features across experiments where each feature was randomized, whilst the binding between the other two was tested. Results showed that though features were processed to different time scales, they were treated in the same way by Visual Working Memory processes. Relevant features were consolidated and irrelevant features were inhibited. Later experiments confirmed that consolidation was aided by iconic memory and the inhibitory process was primarily a post-perceptual active inhibition.

Visual memory holds in mind details of objects, textures, faces, and scenes. After initial exposure to an image, however, visual memories rapidly degrade because they are transferred from iconic memory, a high-capacity sensory buffer, to working memory, a low-capacity maintenance system. How does visual memory maintenance work? This dissertation builds the argument that the maintenance of short-term visual memories is analogous to the act of breathing: it is a dynamic process with a default behavior that explains much of its usual workings, but which can be observed, overridden, and controlled. Chapter 1 shows how the act of trying to remember more information causes people to forget faster and to remember less ("load-dependent forgetting" and "overreaching"). It then shows how the paradigm of evolution can be applied to the problem of maintenance, with memories competing over a limited memory-supporting commodity, explaining these effects. Chapter 2 presents experiments on metamemory, the ability of people to observe and make decisions about their own memories. The experiments isolate a component of metamemory that monitors a memory's quality as it degrades over time. Chapter 3 connects memory to metamemory, drawing on work from reinforcement learning and decision theory to liken the problem of memory maintenance to that of an agent who sequentially decides what to prioritize in a partially observable mind. / Psychology

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The brain is highly efficient at processing complicated patterns of information, filtering ambiguous input it receives from the senses. Competition between these representations is regulated by multiple mechanisms, together forming a coherent percept of our environment. One approach to regulating incoming sensory information is the recruitment of a canonical neural computation: divisive normalization. Another approach to further steer processing towards behaviorally relevant goals is by means of cognitive influences. In this project I examined the degree to which cognitive processes interact with normalization to shape human visual perception. First, a set of fMRI experiments (Exps 1-3: n=6) examined the hypothesis that attention-driven gain modulation within human visual cortex is dependent on the magnitude of normalization. Leveraging the fact that normalization is modulated by similarity of visual features, my results illustrated that attentional modulation of BOLD responses is larger when visual cortex is put under stronger normalization. These results suggest that the degree to which a subpopulation exhibits normalization plays a role in dictating its potential for attentional benefits. Second, I examined how attention modulates visual population responses (n=7). Neurons within visual cortex exhibit a well-characterized relationship between stimulus intensity and the neural response, known as a contrast response function. While animal and psychophysical studies suggest that attention improves visual processing by multiplicatively increasing the gain of the contrast response, human fMRI studies instead report additive attentional effects, which act independently of stimulus contrast. Consistent with prior work, I demonstrated using a fMRI model-based analysis that attentional modulation indeed appears additive within early visual cortex. Lastly, I examined whether perceptual and memory representations are distinct from one another (Exp 1: n=12, Exp 2: n=10). A prevailing theory posits that the retention of visual memories involves maintenance of information within visual cortices. I tested the degree to which representations in visual memory undergo contrast normalization, by leveraging a classic demonstration: center- surround suppression. I obtained psychophysical measurements of perceived contrast and found robust normalization in perception, yet no signature of normalization occurring between visual memory representations. Taken together, this work helps unravel the underlying neural mechanisms by which cognitive influences shape visual perception.

Yes / Human memory is content addressable—i.e., contents of the memory can be accessed using partial information about the bound features of a stored item. In this study, we used a cross-feature cuing technique to examine how the human visual system encodes, binds, and retains information about multiple stimulus features within a set of moving objects. We sought to characterize the roles of three different features (position, color, and direction of motion, the latter two of which are processed preferentially within the ventral and dorsal visual streams, respectively) in the construction and maintenance of object representations. We investigated the extent to which these features are bound together across the following processing stages: during stimulus encoding, sensory (iconic) memory, and visual shortterm memory. Whereas all features examined here can serve as cues for addressing content, their effectiveness shows asymmetries and varies according to cue–report pairings and the stage of information processing and storage. Position-based indexing theories predict that position should be more effective as a cue compared to other features. While we found a privileged role for position as a cue at the stimulus-encoding stage, position was not the privileged cue at the sensory and visual short-term memory stages. Instead, the pattern that emerged from our findings is one that mirrors the parallel processing streams in the visual system. This stream-specific binding and cuing effectiveness manifests itself in all three stages of information processing examined here. Finally, we find that the Leaky Flask model proposed in our previous study is applicable to all three features.