Recent work in Alzheimer's disease (AD) and semantic dementia (SD) has reported a double dissociation in AD and SD on tests of visual discrimination, with poor performance on spatial tests in AD and impaired face discrimination in SD. This pattern has been attributed to the different patterns of atrophy seen in the medial temporal lobe (MTL) in these two neurodegenerative conditions. To investigate whether this functional distinction would extend to another task that employed different types of spatial and object stimuli, two groups of AD and SD patients were assessed on a simple test involving discriminations between blended stimuli. While neither group showed impairment when asked to discriminate objects and colour patches, the SD patients showed a selective deficit in the discrimination of faces whereas the AD patients had significant difficulties discriminating landscapes. These findings extend existing theoretical accounts of MTL function, and challenge current concepts of cognitive impairment in dementia.