Visual agnosia is an impairment in recognition of visually presented objects. It is not due to a deficit in vision (acuity, visual field, and scanning), language, memory, or intellect. While cortical blindness results from lesions to primary visual cortex, visual agnosia is often due to damage to more anterior cortex such as the posterior occipital and/or temporal lobe(s) in the brain. [2] There are two types of visual agnosia: apperceptive agnosia and associative agnosia. Recognition of visual objects occurs at two primary levels. At an apperceptive level, the features of the visual information from the retina are put together to form a perceptual representation of an object. At an associative level, the meaning of an object is attached to the perceptual representation and the object is identified. If a person is unable to recognize objects because they cannot perceive correct forms of the objects, although their knowledge of the objects is intact (i.e. they do not have anomia), they have apperceptive agnosia. If a person correctly perceives the forms and has knowledge of the objects, but cannot identify the objects, they have associative agnosia. Symptoms and signs
While most cases of visual agnosia are seen in older adults who have experienced extensive brain damage, there are also cases of young children with less brain damage during developmental years acquiring the symptoms.
Cavina-Pratesi C, Kentridge RW, Heywood CA, Milner AD (February 2010). "Separate processing of texture and form in the ventral stream: evidence from FMRI and visual agnosia". Cereb. Cortex. 20 (2): 433–46. doi:10.1093/cercor/bhp111. PMID 19478035. Goodale MA; Milner AD (2004). Sight Unseen: An Exploration of Conscious and Unconscious Vision. Oxford UK: Oxford University Press. pp. 139. ISBN 978-0-19-856807-0. OCLC 54408420. Farah M (2004). Visual Agnosia. 2nd Edition. Cambridge MA: MIT Press: Bradford Books. p. 192. ISBN 978-0-262-56203-4. OCLC 57182718.