Loss of visual imagery and loss of visual knowledge--a case study.

As a sequel of a left posterior cerebral artery infarction a patient had severely defective mental imagery of shapes and colours of objects. Imagery of faces, letters, and topological relationships was preserved. The impairment of imagery of object colours was associated with colour agnosia and colour anomia. For colours, there was no difference between performance on tasks calling for imagery of object colours and tasks affording a distinction between correctly and incorrectly coloured objects. For shapes of objects, imagery appeared to be below the level of the patient's knowledge about the visual appearance of objects as manifested by the ability to identify objects and to distinguish correctly from incorrectly drawn pictures. The apparently selective image generation deficit for shapes of objects could be a sequel of loss of knowledge about visual attributes of objects, if superior performance on shape recognition tasks was afforded by perceptual entry level representation which enable a rapid identification of objects but are inaccessible to introspective consciousness.