The project will test the hypothesis that there are two separate attention processes mediated by posterior association cortex: One which conjoins visual features in the process of object recognition, an a second which selects objects for action. In addition the studies will investigate whether these two processes are independent and, if so, identify the neural substrate underlying each. Most of the research will involve experiments that test the attentional abilities of patients with lesions of posterior association cortex. One series of experiments will investigate patients with clinical extinction to determine the degree to which simple visual features are processed in the unattended field in comparison to the ipsilesional field. It is predicted that processing of simple features in the contralesional field will be comparable to that for features in the ipsilesional field, and that the deficit in extinction will reflect primarily a failure to transfer the percept to the action system. A second series of studies will test patients without clinical extinction in a task to examine illusory conjunctions, a phenomenon in which information from different objects is erroneously combined. It is predicted that these patients will show a deficit in feature integration in the contralesional field.

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National Institute of Mental Health (NIMH)
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Perception and Cognition Review Committee (PEC)
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University of California Davis
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Prinzmetal, William (2005) Location perception: the X-Files parable. Percept Psychophys 67:48-71
Machado, Liana; Rafal, Robert D (2004) Control of fixation and saccades during an anti-saccade task: an investigation in humans with chronic lesions of oculomotor cortex. Exp Brain Res 156:55-63
Machado, Liana; Rafal, Robert D (2004) Control of fixation and saccades in humans with chronic lesions of oculomotor cortex. Neuropsychology 18:115-23
Machado, Liana; Franz, Elizabeth (2004) Effects of unilateral distractors: a comparison of eye movement and key press responses. Q J Exp Psychol A 57:1173-90
Prinzmetal, W; Beck, D M (2001) The tilt-constancy theory of visual illusions. J Exp Psychol Hum Percept Perform 27:206-17
Machado, L; Rafal, R (2000) Control of eye movement reflexes. Exp Brain Res 135:73-80
Machado, L; Rafal, R D (2000) Strategic control over saccadic eye movements: studies of the fixation offset effect. Percept Psychophys 62:1236-42
Diedrichsen, J; Ivry, R B; Cohen, A et al. (2000) Asymmetries in a unilateral flanker task depend on the direction of the response: the role of attentional shift and perceptual grouping. J Exp Psychol Hum Percept Perform 26:113-26
Henik, A; Ro, T; Merrill, D et al. (1999) Interactions between color and word processing in a flanker task. J Exp Psychol Hum Percept Perform 25:198-209
Machado, L; Rafal, R D (1999) Ipsilesional line bisection bias in patients with chronic parietal lesions. Neuroreport 10:3143-8

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