Visual Agnosia - Classification

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. 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. 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).