This project consists of a series of fourteen related experiments designed to investigate the capacity for visual processing in the neglected hemifield of patients with unilateral visual neglect. We propose to demonstrate that visual information presented to the neglected hemifield is processed to a greater degree than can be demonstrated with probes of signal detection or conscious discrimination of identity or visual form information. Patients with left hemifield neglect and control patients with primary visual system lesions who show hemianopia without evidence of neglect will be challenged with experiments that systematically vary stimulus and response demands to explore patients' access to, and use of visual information. It is hypothesized that processing of basic visual information may be sufficiently intact in the neglect patients to contact semantic representations, even without the elaborative processes that are dependent upon attention or conscious mediation. A demonstration of preserved, but possibly unconscious, visual processing across a broad range of stimuli will clarify the neural mechanisms of neglect and may begin to link the phenomenology of neglect to current neural models of higher order vision. At the clinical level, the tasks proposed here will contribute to the foundation for objective diagnostic measures of neglect and could provide an important clinical marker to distinguish neglect from hemianopia. The processing distinctions anticipated in these experiments would suggest that treatment of neglect could be successful through stimulation of the capacity of early visual processes or through the manipulation of conscious strategies.