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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 low intellect. 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, it is apperceptive agnosia. If a person correctly perceives the forms and has knowledge of the objects, but cannot identify the objects, it is associative agnosia. Visual agnosia is often due to bilateral damage in the posterior occipital and/or temporal lobe(s) in the brain. == Classification == The two major types of visual agnosia are apperceptive and associative visual agnosia. Apperceptive agnosia is failure of object recognition even when the basic visual functions (acuity, color, motion) are normal.〔Kolb, B. & Whishaw, I. Q. (2009). "Fundamentals of Human Neuropsychology 6th ed.'' New York, NY., Worth Publishers. ISBN 978-0-7167-9586-5.〕 The brain must correctly integrate such features as lines, brightness, and color of visual information to form a whole percept of an object. If a failure occurs during this process, a percept of an object is not fully formed and thus it cannot be recognized.〔Heilman, K. M. (2002). "Matter of Mind.'' New York, NY., Oxford University Press. ISBN 978-0-19-514490-1.〕 Tasks requiring copying or matching simple figures can distinguish the individuals with apperceptive agnosia because they have impaired abilities and cannot perform such tasks. Associative agnosia is an inability to identify objects even with apparent perception and knowledge of them. It involves a higher level of processing than apperceptive agnosia.〔 Individuals with associative agnosia can copy or match simple figures, indicating that they can perceive objects correctly. They also display the knowledge of objects when tested with tactile or verbal information. However, when tested visually, they cannot name or describe common objects.〔 This means that there is an impairment in associating the perception of objects with the stored knowledge of them. Known clinical variants of visual agnosia include prosopagnosia (inability to recognize faces), pure word blindness (inability to recognize words), agnosias for colors (inability to differentiate colors), agnosias for the environment (inability to recognize landmarks etc.) and simultanagosia (inability to sort out multiple objects in a visual scene).〔Biran I. and Coslett H.B. (2003). Visual Agnosia.Current neurology and neuroscience reports, 3(6):508 - 512. ISSN 1528-4042. 〕 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「visual agnosia」の詳細全文を読む スポンサード リンク
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