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.
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).
Categories and subtypes of visual agnosia
The two main categories of visual agnosia are:
- Apperceptive visual agnosia, impaired object recognition. Individuals with apperceptive visual agnosia cannot form a whole percept of visual information.
- Associative visual agnosia, impaired object identification. Individuals with associative agnosia cannot give a meaning to a formed percept. The percept is created, but it would have no meaning for individuals who have an associative agnosia.
- Subtypes of Associative Visual Agnosia
- Achromatopsia, an inability to distinguish different colors.
- Prosopagnosia, an inability to recognize human faces. Individuals with prosopagnosia know that they are looking at faces, but cannot recognize people by the sight of their face, even people whom they know well.
- Orientation Agnosia: an inability to judge or determine orientation of objects.
- Pantomime Agnosia: an inability to understand pantomimes (gestures). It appears that the inferior cortical visual cortex is critical in recognizing pantomimes.
While most cases of visual agnosia are seen in older adults who have had extensive brain damage, there are also cases of young children with less brain damage during developmental years acquiring the symptoms. Commonly, visual agnosia presents as an inability to recognize an object by its shape, in the absence of other explanations, such as vision problems. This is most easily seen in prosopagnosia, where the individual can clearly see the faces of those people nearby, but is unable to draw the mental connections and recognize the person. Other common manifestations of visual agnosia include difficulty identifying objects that look similar in shape, difficulty with line drawings that lack much detail, and recognizing objects that are shown from less common views.
Visual agnosia occurs after damage to the visual association cortex. This occurs even when no damage has been done to the eyes or optic tract that leads visual information into the brain. Visual agnosia is caused by damage to parts of the ventral stream of vision. The ventral stream is also known as the "what pathway of vision" because this pathway allows the individual to identify the objects they see. The other stream is the dorsal stream or "where/how" pathway; and this pathway remains undisturbed, allowing individuals with visual agnosia to show relatively normal visually guided behavior. Damage to specific areas of the ventral stream impair the ability to recognize certain categories of visual information.
Visual agnosia occurs due to damage in the association areas of the visual cortex. The lateral occipital complex appears to respond to many different types of objects. Prosopagnosia (inability to recognize faces) is due to damage of the fusiform face area (FFA). Studies with functional imaging show that there is a certain area specialized for facial recognition, known as the fusiform face area, in the fusiform gyrus of the temporal lobe. However, this area is not exclusive to faces; recognition of other objects of expertise are also processed in this area. The extrastriate body cortex (EBA) was found to be activated by photographs, silhouettes, or stick drawings of human bodies. The parahippocampal place area (PPA) of the limbic cortex has been found to be activated by the sight of scenes and backgrounds. Cerebral achromatopsia (the inability to discriminate between different hues) is caused by damage to the V8 area of the visual association cortex. The left hemisphere seems to play a critical role in recognizing the meaning of common objects.
In popular culture
- A famous report on this condition is the title essay of Oliver Sacks' book, The Man Who Mistook His Wife for a Hat.
- The murder suspect in the Picket Fences episode "Strangers" supposedly suffered from agnosia.
- The patient in the House episode "Adverse Events" suffered from agnosia.
- Val Kilmer's character suffers from visual agnosia in the film At First Sight.
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