Visual agnosia is a disorder of visual object recognition. The term derives from Freud (1891), but it was Lissauer (1890), speaking of see-lenblindheit (psychic blindness), who suggested the categorization into two types which continues to be used:
- Apperceptive visual agnosia:
A defect of higher order visual perception leading to impaired shape recognition, manifested as difficulty copying shapes or matching shapes, despite preserved primary visual capacities, including visual acuity and fields (adequate to achieve recognition), brightness discrimination, color vision and motion perception (indeed motion may facilitate shape perception; see Riddoch’s phenomenon). Reading is performed with great difficulty, with a "slavish" tracing of letters which is easily derailed by any irrelevant lines; such patients may appear blind.
- Associative visual agnosia:
An impairment of visual object recognition thought not to be due to a perceptual deficit, since copying shapes of unrecognized objects is good. The scope of this impairment may vary, some patients being limited to a failure to recognize faces (prosopagnosia) or visually presented words (pure alexia, pure word blindness).
Visually agnosic patients can recognize objects presented to other sensory modalities. Clinically, apperceptive visual agnosia lies between cortical blindness and associative visual agnosia.
Apperceptive visual agnosia results from diffuse posterior brain damage; associative visual agnosia has been reported with lesions in a variety of locations, usually ventral temporal and occipital regions, usually bilateral but occasionally unilateral. Pathological causes include cerebrovascular disease, tumor, degenerative dementia (visual agnosia may on occasion be the presenting feature of Alzheimer’s disease, the so-called visual variant, or posterior cortical atrophy), and carbon monoxide poisoning.
A related syndrome which has on occasion been labeled as apperceptive visual agnosia is simultanagnosia (q.v.), particularly the dorsal variant in which there is inability to recognize more than one object at a time. Associative visual agnosia has sometimes been confused with optic aphasia (q.v.).
Farah MJ. Visual agnosia: disorders of object recognition and what theytell us about normal vision. Cambridge: MIT Press, 1995
Riddoch MJ, Humphreys GW. Visual agnosia. Neurologic Clinics
2003; 21: 501-520