The aim of this revised proposal is to further detail the role of the posterior parietal cortex (PPC) in visual short-term memory (VSTM). A number of functional magnetic resonance imaging (fMRI) studies observe bilateral PPC activations during varied VSTM tasks. In contrast, PPC damage is not associated with generalized deficits in VSTM performance. Right PPC damage is known to impair spatial attention while left PPC damage is known to impair processing of numerical magnitude. The conflict between fMRI and patient data suggests several possibilities: VSTM deficits in PPC patients have not been carefully evaluated, or neuroimaging activations are epiphenomenal to VSTM task performance. In preliminary studies, I show that unilateral right PPC patients exhibit general impairment at VSTM for object, spatial and object-spatial VSTM for colors, novel shapes and tools stimulus categories. Left PPC patients do not exhibit general VSTM deficits, performing normally in the VSTM tasks. Bilateral PPC patients with simultanagnosia (the inability to see more than 1 thing at a time) are also globally impaired. Bilateral PPC also demonstrate sensitivity to experimental paradigm and performed worse in old/new VSTM recognition than in comparable recall tasks. The goal of the proposed studies is to investigate the parameters of PPC involvement in VSTM. Unresolved issues are addressed: the mechanism of VSTM impairment (encoding, maintenance, capacity, retrieval), whether recognition paradigms (old/new recognition versus 2 alternative forced-choice) affect bilateral PPC recognition performance, and whether left PPC activitations are due to involvement in processing magnitude. To accomplish this goal, I propose studies examining VSTM using complimentary approaches: neuropsychological studies of unilateral and bilateral parietal patients, as well as functional magnetic resonance imaging (fMRI) with normal subjects.
This set of studies is particularly relevant to the rehabilitation of parietal stroke patients. In the United States, stroke affects approximately 700,000 people annually, is the third leading cause of death, and is the leading cause of serious, long-term disability. Parietal lobe strokes are relatively common and the behavioral consequences on vision and action are devastating. In the proposed studies, VSTM impairments in parietal patients are evaluated. Deficits in VSTM have direct real-world implications relating to cognitive function. These data may provide affordable assessment measures and help guide rehabilitation strategies. ? ? ?
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