Visual object agnosia This rare condition, first described by Lissauer in 1890, consists of a failure to name and indicate the use of a seen object by spoken or written word or by gesture. The patient cannot even determine the generic class of the object presented. Visual acuity is intact, the mind is clear, and the patient is not aphasic—conditions requisite for the diagnosis of agnosia. If the object is palpated, it is recognized at once, and it can also be identified by smell or sound if it has an odor or makes a noise. Moving the object or placing it in its customary surroundings facilitates recognition. In most reported instances of object agnosia, the patient retains normal visual acuity but cannot identify, match, or name objects presented in any part of the visual fields; if misnamed, the object is used in a fashion that reflects the incorrect perception.
Lissauer conceived of visual object recognition as consisting of two distinct processes, the construction of a perceptual representation from vision (perception) and the mapping of this perceptual representation onto stored percepts or engrams of the object’s functions and associations (apperception), and he proposed that impairment of either of these processes could give rise to a defect in visual object recognition.
One rarely encounters patients who have lost the capacity to recognize only one class of objects, for example, animals or colors, a problem that may be termed a category anomia. We have encountered several patients who, remarkably, when presented with an orange (the fruit), can name it but not its color (orange), or conversely, can name its color but not the object itself. There is a dissociation of ability to retrieve the name of an object (a noun) and its attribute (an adjective) even though, in the case of an orange, they are the same word.
As indicated in Chap. 12, visual object agnosia is often associated with visual verbal agnosia (alexia) and homonymous hemianopia. Prosopagnosia (the inability to identify faces; see further on) is also present in most cases. The underlying lesions are usually bilateral, although McCarthy and Warrington have related a case with a restricted lesion of the left occipitotemporal region (by MRI). Two of our patients with visual object agnosia had an incomplete amnesic syndrome from a left-sided inferior occipital and mediotemporal infarction, reflecting a proximal occlusion of the posterior cerebral artery.
Prosopagnosia This term (from the Greek prosopon, “face,” and gnosis, “knowledge”) was introduced by Bodamer for a type of visual defect in which the patient cannot identify a familiar face by looking at either the person or a picture, even though he knows that a face is a face and can point out its features. Such patients also cannot learn to recognize new faces. They may also be unable to interpret the meaning of facial expressions or to judge the ages or distinguish the genders of faces. In identifying persons, the patient depends on other data, such as the presence and type of glasses or moustache, the type of gait, or sound of the voice. Similarly, species of animals and birds and specific models or types of cars cannot be distinguished from one another, but the patient can still recognize an animal, bird, or car as such. Other agnosias may be present in such cases (color agnosia, simultanagnosia) and there may be topographic disorientation, disturbances of body schema, and constructional or dressing apraxia. Visual field defects are nearly always present. Some neurologists have interpreted this condition as a simultanagnosia involving facial features. Another view is that the face, though satisfactorily perceived, cannot be matched to a memory store of faces. Levine has found a deficit in perception, characterized by insufficient feature analysis of all visual stimuli.
The small number of cases that have been studied anatomically and by CT and MRI indicate that prosopagnosia is most often associated with bilateral lesions of the ventromedial occipitotemporal regions (Damasio et al) including the inferior occipital or midfusiform gyri, but there are exceptions that are attributable to unilateral damage, almost always on the right side. The notion that there is a “face area” in the fusiform gyrus is expressed uncritically in the literature and seems to be an oversimplification.