A variant of this disorder is characterized by specific difficulty with facial matching or discrimination from partial cues, such as portions of the face or a profile. The distinction between this deficit and the usual type of prosopagnosia rests on the use of tests that do not require memory of a specific face. This difficulty with facial matching and discrimination is more likely to be seen with lesions of the right than of the left posterior hemisphere.
Closely allied and often associated with prosopagnosia is a subtle syndrome of loss of environmental familiarity, in which the patient is unable to recognize familiar places. The patient may be able to describe a familiar environment from memory and locate it on a map, but he experiences no sense of familiarity and gets lost when faced with the actual landscape. In essence, this is an environmental agnosia. This syndrome is associated with right-sided, medial temporooccipital lesions, although in some patients, as in those with prosopagnosia, the lesions are bilateral (Landis et al).
Environmental agnosia can be distinguished from the visual disorientation and disorder of spatial (topographic) localization discussed earlier. Patients with the latter disorder are unable to orient themselves in an abstract spatial setting (topographagnosia, or loss of topographic memory). They cannot draw the floor plan of their house or a map of their town or the United States and cannot describe a familiar route, as from their home to their place of work, or find their way in familiar surroundings.
Visual agnosia for words (alexia without agraphia) See Chap. 22 and further on in this chapter in the discussion of alexia without agraphia, under the “Disconnection Syndromes.”
Color agnosia Here one must distinguish several different aspects of identification of colors, such as the correct perception of color (the loss of which is called color blindness) or the naming of a color. The common form of retinal color blindness is congenital and is readily tested by the use of Ishihara plates. Acquired color blindness caused by a cerebral lesion, with retention of form vision, is referred to as central achromatopsia. Here the disturbance is one of hue discrimination; the patient cannot sort a series of colored wools according to hue (Holmgren test) and may complain that colors have lost their brightness or that everything looks gray. Achromatopsia is frequently associated with visual field defects and with prosopagnosia. Most often, the field defects are bilateral and tend to affect the upper quadrants. However, full-field achromatopsia may exist with retention of visual acuity and form vision. There may also be a hemior quadrant-achromatopsia without other abnormalities, although special testing is required to reveal this defect. These features, together with the usually associated prosopagnosia, point to involvement of the inferomedial, occipital, and temporal lobe(s) and the lower part of the striate cortex or optic radiation (Meadows et al, 1974a). The existence of a central achromatopsia is not surprising in view of the animal studies of Hubel, which identified sets of cells in areas 17 and 18 that are activated only by color stimuli.