Agnosia for scenes in topographagnosia

Topographagnosia is most commonly attributed to an agnosia for landmarks. In order to define the nature of this agnosia, we studied a patient with isolated topographic disorientation (TD) after a stroke in the right medial occipitotemporal region. The patient got lost in familiar environments but could readily read and draw maps, describe familiar routes, and provide correct directions. He had normal perceptual test performance and met criteria for topographagnosia rather than for other forms of topographic disorientation. Two ecologically valid route tests assessed the nature of his agnosia. On a familiar route, he could recognize major landmarks. He could not, however, recognize route configurations made up of combinations of visual features each lacking individual distinctiveness. On a test of route learning, he learned landmarks that differed in minor details and could use them to orient himself along a route. He had difficulty, however, recognizing and learning scenes lacking salient landmarks. This agnosia for scenes was worse for semantically-related environments, but improved with semantic knowledge such as street names. In addition, the patient lacked overt prosopagnosia but tended toward semantic errors in the recognition of famous faces. Together these findings suggest that this patient's inability to recognize a route resulted from an inability of intact perceptual units for scenes, composed of specific visual configurations of individually indefinite features, from accessing stored representations.