The research in this section addresses two issues. The first concerns the extent to which nasotemporal overlap at the vertical meridian of the human retina produces a representation of the entire fovea in both of the cerebral hemispheres. The second concerns the role of low spatial frequencies in the integration of stimulus elements into global percepts. We investigate the first question with commissurotomy patients and patients with homonymous hemianopia. Commissurotomy patients will be required to compare extrafoveal reference stimuli with target stimuli close to the vertical meridian in their seeing hemifield with targets close to the vertical meridian in their blind hemifield. In both cases, functional nasotemporal overlap should be revealed by an increasing ability of the subjects to make these comparisons as the targets approach the retinal midline. Nasotemporal overlap is assessed for luminance, crude and detailed shape, stereopsis, motion and color information. Special equipment and procedures will be employed to control for eccentric and/or erratic fixation. These experiments should help to resolve the long standing controversy concerning the nature of macular sparing, and should be of relevance to all investigators who make use of lateralized visual stimuli. We address the second issue with normal subjects and right parietal patients who are impaired at global processing. With both these groups, we evaluate the processing of patterns which have both global and local structure, but are devoid of low spatial frequency content. In normals, we search for evidence of global precedence with these stimuli; in patients, we search for global processing deficits. In addition, we assess global precedence in normals using isoluminant stimuli, to evaluate the contribution of the magnocellular system to global processing. With patients, we also obtain contrast sensitivity functions, investigate the processing of suprathreshold compound gratings built from high or low frequency components, and examine the vulnerability of low spatial frequencies to masking. Finally, we compare the perception of ordinary and hipass filtered pictures in our normal and patient groups, and analyze the saccadic scanpaths used to explore these pictures.
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