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 and associative. Recognition of visual objects occurs at two 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 due to inability to perceive the correct forms of the objects, although knowledge of the objects is intact (i.e. the person does 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, the person has 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. Commonly, visual agnosia presents as an inability to recognize an object in the absence of other explanations, such as blindness or partial blindness, anomia, memory loss, etc. Other common manifestations of visual agnosia that are generally tested for include difficulty identifying objects that look similar in shape, difficulty with identifying line drawings of objects, and recognizing objects that are shown from less common views, such as a horse from a top-down view. Within any given patient, a variety of symptoms can occur, and the impairment of ability is not only binary but can range in severity. For example, Patient SM is a prosopagnosic with a unilateral lesion to left extrastriate cortex due to an accident in his twenties who displays behavior similar to congenital prosopagnosia. Although he can recognize facial features and emotions – indeed he sometimes uses a standout feature to recognize a face – face recognition is almost impossible purely from visual stimuli, even for faces of friends, family, and himself. The disorder also affects his memory of faces, both in storing new memories of faces and recalling stored memories. The symptoms reach to other domains. SM's object recognition is similarly impaired though not entirely; when given line drawings to identify, he was able to give names of objects with properties similar to the drawing, implying that he is able to see the features of the drawing. Similarly, copying a line drawing of a beach scene led to a simplified version of the drawing, though the main features were accounted for. For recognition of places, he is still impaired but familiar places are remembered and new places can be stored into memory.
Pathophysiology Visual agnosia occurs after damage to visual association cortex or to parts of the ventral stream of vision, known as the "what pathway" of vision for its role in object recognition. This occurs even when no damage has been done to the eyes or optic tract that leads visual information into the brain; in fact, visual agnosia occurs when symptoms cannot be explained by such damage. Damage to specific areas of the ventral stream impair the ability to recognize certain categories of visual information, such as the case of prosopagnosia. Patients with visual agnosia generally do not have damage to the dorsal stream of vision, known as the "where pathway" of vision because of its role determining object's position in space, allowing individuals with visual agnosia to show relatively normal visually guided behavior. For example, patient DF had lesions to the ventral surface that gave her apperceptive agnosia. One of the tasks she was tested on required her to place a card through a thin slot that could be rotated into all orientations. As an apperceptive agnosic, it would be expected that since she cannot recognize the slot, she should not be able to correctly place the card into the slot. Indeed, when she was asked to give the direction of the slot, her responses were no better than chance. Yet, when she was asked to place the card into the slot, her success was almost to the level of the controls. This implies that in the event of a ventral stream deficit, the dorsal stream can help with processing of special information to aid movement regardless of object recognition. More specifically, 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). An area in the fusiform gyrus of the temporal lobe that has been strongly associated with a role in facial recognition. 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.
Diagnosis
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