Neural Mechanisms of Mnemonic Precision
Ester, Edward F.
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Ester, Edward F.
Working memory (WM) enables the storage of information in a state that can be rapidly accessed and updated. This system is a core component of higher cognitive function - individual differences in WM ability are strongly predictive of general intelligence (IQ) and scholastic achievement (e.g., SAT scores), and WM ability is compromised in many psychiatric (e.g., schizophrenia) and neurological (e.g., Parkinson's) disorders. Thus, there is a strong motivation to understand the basic properties of this system. Recent studies suggest that WM ability is determined by two independent factors: the number of items an individual can store and the precision with which representations can be maintained. Significant progress has been made in developing neural measures that are sensitive to the number of items stored in WM. For example, electrophysiological and neuroimaging studies have demonstrated that activity in posterior parietal cortex is directly modulated by the number of items stored in WM and reaches a plateau at the same set size where individual memory capacity is exceeded. However, comparably little is known regarding the neural mechanisms that enable the storage of high-fidelity information in WM. This dissertation describes two experiments that evaluate so-called sensory-recruitment models of WM, where the storage of highfidelity information in WM is mediated by sustained activity in sensory cortices that encode memoranda. In Chapter II, functional magnetic resonance imaging (fMRI) and multivoxel pattern analysis were used to demonstrate that sustained patterns of activiation observed in striate cortex discriminate specific feature attribute(s) (e.g., orientation) that an observer is holding in WM. In Chapter III, I show that these patterns of activation can be observed in regions of visual cortex that are not retinotopically mapped to the spatial location of a remembered stimulus and suggest that this spatially global recruitment of visual cortex enhances memory precision by facilitating robust population coding of the stored information. Together, these results provide strong support for so-called sensory recruitment models of WM, where the storage of fine visual details is mediated by sustained activity in sensory cortices that encode information. This dissertation includes previously published and co-authored material.