Neglect and Neural Mechanisms of Magnitude Estimation

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The goal of this research is to add a quantitative element to contemporary theories of spatial neglect and to combine disparate theories into one model. Theoretical development can lead to improved treatment. Neglect affects a large number of stroke patients each year, and current treatments are of limited benefit. Contemporary theories of neglect view the disorder in terms of mental representations of space, spatial attention, and spatially- directed movements. They cannot address a number of recent empiric observations related to magnitude estimation -- misperceiving size, loudness, weight, and the like. We propose that mental representations of stimulus intensity are constructed by neural systems in the brain, in addition to mental representations of space, and that both types of representations are altered in neglect. Further, we propose that representations of stimulus intensity are developed in accordance with the psychophysical principle of ratio scaling, which is the basis for magnitude estimation. Representations of stimulus intensity add the quantitative element that is missing in contemporary theories of neglect. We anticipate this new concept will significantly advance theories concerning neglect and lead to new directions in research. It is also directly applicable to treatment, e.g., rehabilitating representations of stimulus intensity through direct training in ratio scaling. Six experiments are proposed. Subjects will be patients with right and left hemisphere lesions . following ischemic stroke who are selected for (1) clinical signs of neglect and (2) damage to a specific, neural system associated with neglect. Non-patient control subjects will be matched for age, gender, and race. Experiment I tests the hypothesis that representations of stimulus intensity are altered in multiple sensory modalities among patients with neglect. Experiment II tests the hypothesis that attention binds representations of space and stimulus intensity together. Experiment III tests the hypothesis that representation of stimulus intensity are constructed in each cerebral hemisphere. Experiments IV-VI examine alternative explanations for altered ratio scaling among patients with neglect. Finally, we propose to develop and test a mathematical model of ratio scaling during the course of this proposal.