Shared mechanisms for representing the sides of the visual world and the sides of objects: Evidence from a localization deficit following parietal brain damage.
23.301, Saturday, May 11, 8:30 am - 12:30 pm, Royal Ballroom 6-8
Zheng Ma1, Jose Nino2, Jonathan Flombaum1, Michael McCloskey2; 1Department of Psychological and Brain Sciences, The Johns Hopkins University, 2Department of Cognitive Science, The Johns Hopkins University
We report results from a single patient, JKI, with extensive right parietal damage, and some left parietal damage. JKI is severely impaired in localizing visual stimuli presented in the left hemifield, perceptually displacing them toward the vertical meridian. For example, a stimulus presented at an eccentricity of 9.8° is typically localized by JKI to an eccentricity of only 1.4° or 2.8°. JKI also shows some impairment in detecting and identifying LVF stimuli, and this impairment increases in severity with stimulus eccentricity. In contrast, detection, identification, and localization of RVF stimuli are virtually intact. We investigated JKIÂ’s ability to detect, identify, and localize features within objects. We presented rectangular objects with features (circular or triangular notches) that could appear on the left or right side of the object. When objects were presented in the left hemifield, JKI was impaired in detecting the notch features, identifying their shapes, and localizing them to the right or left side of the object. Holding constant the location of a feature in the visual field, his performance was significantly worse for features on the left side of an object than for those on the right. These results demonstrate an object-based deficit similar to JKI's impairment for whole objects; that is, JKI shows similar impairment in processing features on the left side of an object, and whole LVF stimuli. Remarkably, the object-based impairment was also apparent for object stimuli presented in the otherwise intact RVF. When localizing features within objects in the RVF, JKI made the same type of error as for LVF stimuli: He systematically reported notches on the left side of an object as appearing on the right. These results suggest that shared mechanisms are involved in representing the sides of the visual world and the sides of objects, irrespective of absolute position.