Apperceptive agnosia
Each patient that suffers from apperceptive agnosia does not have brain damage in exactly the same area. However, brain damage in proximity to the occipital lobe is largely correlated with the patterns of deficit seen in apperceptive agnosics. For example, patient JB suffered extensive damage to the parietal-occipital areas to the left cerebral hemisphere leading to his deficit in the ability to name distinguish between structurally similar object.
Associative visual agnosia
Visual object agnosia (or semantic agnosia) is the most commonly encountered form of agnosia. The clinical "definition" of the disorder is when an affected person is able to copy/draw things that they cannot recognize. Individuals afflicted with the associative type often cannot identify, describe or mimic functions of items, though perception is intact, since images of objects can be copied or drawn. Individuals may retain semantic knowledge of the items, as exemplified during tasks where objects are presented through alternate modalities, through touch or verbal naming or description. Some associative visual object agnostics retain the ability to categorize items by context or general category, though unable to name or describe them. Diffuse hypoxic damage is the most common cause of visual object agnosias.