Posterior cortical atrophy (PCA) is a progressive neurodegenerative condition most commonly associated with the histopathological features of Alzheimer's disease (AD). Individuals with PCA exhibit a progressive decline in higher visual object and space processing and other posterior cortical functions. However the extent to which these visuoperceptual and visuospatial processing problems are underpinned by basic visual processing deficits is unknown because elementary visual processes (e.g. form, colour, motion, location) have not been examined previously. This study investigated visual deficits in 21 patients with a clinical diagnosis of probable Alzheimer's disease (AD) who also met behavioural criteria for PCA. Basic visual processing skills were examined in detail, including assessments of form detection, form discrimination, form coherence, motion coherence, colour discrimination and point localisation. The relationship between these six basic visual processes was assessed, and their association with general cognitive function, episodic memory, dominant parietal function (calculation, spelling), space perception (dot counting, number location) and object perception (canonical and non-canonical view apperception) was investigated. 100% of participants showed impairment in at least one aspect of basic visual processing, but form, colour, motion and location processing were not uniformly affected (see Table). Higher order object and space perception deficits were associated with different patterns of basic visual impairment. However, with relevance to theories of network-based disease progression between non-contiguous brain areas in AD, basic visual processing did not correlate with performance on non-visual tasks (e.g. spelling, calculation) thought to depend upon cortical regions of equivalent proximity. Furthermore, the study revealed continuous rather than discrete differences in neuropsychological profile between three previously-claimed variant PCA syndromes held to reflect predominant dorsal, ventral and caudal tissue loss. The object and space perception problems commonly reported in PCA are underpinned by distinct and identifiable patterns of disruption to more basic visual processes associated with occipital lobe function. These behavioural data also complement neuroimaging findings from the same participants showing largely overlapping patterns of atrophy and cortical thinning, and indicate that different PCA phenotypes represent points within the space of continuous variation within the wider PCA syndrome.