Neuro-Ophthalmology & Neuro-Otology
Isolated sixth nerve palsy
Dec. 07, 2022
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The term “visual agnosia” encompasses a group of neurologic disorders. Patients with one of these disorders cannot recognize some or all objects by sight but can do so using other sensory modalities, such as sound or touch. Their problem also cannot be attributed to broader problems with language, memory, or basic visual functions. Agnosia can be general, affecting all types of objects, or specific to certain types of objects. In this article, the authors review the different forms of visual agnosia with respect to clinical characteristics, pathogenesis, structural localization, differential diagnosis, diagnostic evaluation, and rehabilitation.
• Visual agnosia is a rare problem characterized by an inability to recognize objects by sight, despite preserved ability to do so by sound or touch.
• The two main streams of visual processing beyond the primary visual cortex are the dorsal (“where”) stream and the ventral (“what”) stream. Visual agnosia is a problem of the latter.
• Visual agnosia is classified into general and selective visual agnosias.
• Subtypes of selective visual agnosia are named according to the class of visual objects affected.
• Rehabilitation programs for agnosia remain largely experimental.
Visual agnosia is the inability to recognize objects by sight. Descriptions of visual agnosia have been around for over a century. One of the earliest experimental observations was that a dog with partial ablations of both occipital lobes lost its usual responses to familiar objects but could still navigate around them, suggesting that it could see but could not recognize (100). In 1890, Lissauer described this “mind-blindness” in humans (87). He also distinguished between two forms: an apperceptive one, in which subjects did not perceive objects well, and an associative one, in which subjects could not link what they saw with their stored knowledge about objects (110). In 1891, Freud introduced the term "agnosia," which was later defined more specifically as a failure to recognize objects that was not attributable to poor visual acuity, cognitive impairment, or aphasia.
General visual agnosia. General visual agnosia is the loss of visual recognition for all types of objects. Patients make gross misidentifications, such as the man who mistook his wife for a hat (109). Beyond the broad distinction between apperceptive and associative forms, recent years have seen the description of further subtypes of each and the emergence of a more refined taxonomy (73; 52).
(Contributed by Dr. Jason J S Barton.)
Form agnosia. Form agnosia is a type of apperceptive agnosia. Patients with this type of agnosia have difficulty seeing even basic shapes, which are the building blocks of object structure (21); however, they can see elementary visual properties, such as brightness, contrast, and motion.
Integrative agnosia. Integrative agnosia is another type of apperceptive agnosia. Patients with this type of agnosia can see basic shapes but cannot integrate these shapes into the more complex structures of real objects (73). Hence, they may see two round wheels and two triangles but not realize they form a bicycle. These patients have trouble parsing a complex scene with multiple objects and figuring out which elements belong to which object. This deficit can be shown in their struggles with overlapping figures (73; 65).
Transformation agnosia. Transformation agnosia is a rare condition in which subjects cannot recognize objects shown from unusual (“non-canonical”) viewpoints. One interpretation is that this deficit reflects a problem with deriving a viewpoint-independent representation of an object’s three-dimensional structure (130). These subjects have no trouble with more typical views of objects and, thus, may not struggle with recognizing objects in daily life.
These perceptual functions are relatively intact in patients with associative forms of general visual agnosia. Patients with associative agnosias have trouble linking what they see with what they know, as in semantic access agnosia, or may have lost that visual knowledge, as in semantic agnosia (73). This knowledge may concern either what objects look like (ie, their “structural representations”) or general “semantic” facts about objects (eg, their function and where they are found). Some of these general agnosias exhibit a degree of category-specificity; knowledge about living things like plants and animals is more deficient than knowledge about man-made objects like tools, furniture, or vehicles (51; 84; 25). This deficit may be due to the greater visual complexity and similarity of natural objects (74).
Beyond the general visual agnosias, there are also selective visual agnosias. Patients with these deficits do not confuse people for apparel but struggle with finer distinctions. Sometimes their difficulties are referred to as problems with making “within-category” judgments (50) about objects (eg, “what type of car is this?”), in contrast to the “between-category” difficulties (eg, “is this a car or a boat?”) characteristic of patients with general visual agnosias. These deficits affect certain types of objects more than others for the different selective agnosias.
Prosopagnosia. Prosopagnosia is the inability to recognize the identity of previously learned faces as being familiar (08; 39). This deficit may or may not spare the perception of other types of face information, such as expression, age, and gender. Whether this problem with object identity is truly specific to faces alone is a matter of much debate. More rigorous testing suggests that most but not all patients with prosopagnosia have some difficulty identifying other types of objects, such as cars, bicycles, and animals (60; 09).
There are apperceptive, associative, and amnestic variants of prosopagnosia (37). Those with the apperceptive variant cannot see differences in the facial structure of different people. Those with the amnestic variant do better but cannot recall what specific faces look like, which can be tested using a questionnaire about what they remember about famous people’s faces (10). Those with the associative form have good facial perception and preserved facial memories but cannot match the former to the latter. In addition to the acquired form, there is a developmental variant (118).
Pure alexia. Pure alexia is also known as “word blindness” or “alexia without agraphia.” Patients with this condition have an acquired inefficiency in the reading of words. At the severe end are patients with global alexia, who cannot read words at all (22); this deficit may extend to letters and numbers and other forms of visual symbolic communication, such as musical notation or map symbols (72; 19). At the milder end are “letter-by-letter” readers, who can read but do so slowly, exhibiting a characteristically increased “word-length effect,” which is the correlation between the number of letters in the word and the time it takes to read it (14).
Topographic disorientation. Topographic disorientation is manifest as getting lost in familiar surroundings. This navigation failure has a number of possible causes (28). Landmark agnosia is the inability to recognize landmarks, such as buildings and scenes (119). Impaired cognitive map formation is the inability to place these landmarks in a mental map of the environment, which is the most flexible means of navigation. Heading disorientation is the inability to orient oneself correctly within this environment. Topographic disorientation also has a developmental variant (76; 75).
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