Knowing where and knowing what: a double dissociation.

We report a double dissociation between visuo-spatial abilities and semantic knowledge (knowledge of the names and attributes of objects and people), in two brain-injured people with longstanding stable impairments, using a wide range of tests to explore the extent of the dissociation, MU, who has bilateral lesions of occipito-parietal cortex, shows severe spatial disorientation with relatively well-preserved semantic knowledge. He is contrasted with JBR, who has bilateral temporal lobe damage and shows severe semantic problems and no impairment on visuo-spatial tasks. Our findings thus demonstrate a double dissociation between the performance of semantic and spatial tasks by MU and JBR. This pattern is consistent with Ungerleider and Mishkin's (1982) neurophysiological hypothesis of separable cortical visual pathways; one which is specialised for spatial perception and follows a dorsal route from occipital to parietal lobes, and the other following a more ventral route from occipital to temporal lobes, whose target is semantic information needed in specifying what an object is.