Diffuse hypoxic damage is the most common cause of visual object agnosias. Ennio De Renzi worked extensively with a variety of agnosias in 1963 and 1970. Pick's studies introduced autotopagnosia and other category specific agnosias, such as visual and tactile agnosia. Musical agnosias may be categorized based on the process which is impaired in the individual. Agnosias are sensory modality specific, usually classified as visual, auditory, or tactile.