Cerebral Forms of Blindness and Visual Agnosia (See Also Chap. 21)
The ability to recognize visually presented objects and words depends on the integrity not only of the visual pathways and primary visual area of the cerebral cortex (area 17 of Brodmann) but also of those cortical areas that lie just anterior to area 17 (areas 18 and 19 of the occipital lobe and area 39—the angular gyrus of the dominant hemisphere). Blindness that is the result of destruction of both visual and adjacent regions of the occipital lobes is termed cortical or cerebral blindness. Another remarkable condition exists in which the patient denies or is oblivious to blindness despite overt manifestations of the defect (Anton syndrome).
In distinction to these forms of blindness, there is a less-common category of visual impairment in which the patient cannot understand the meaning of what he sees, that is, visual agnosia. Primary visual perception is more or less intact, and the patient may accurately describe the shape, color, and size of objects and draw copies of them. Despite this, he cannot identify the objects unless he hears, smells, tastes, or palpates them. The failure of visual recognition of words alone is called pure word blindness, or alexia. Visual-object agnosia rarely occurs as an isolated finding: as a rule, it is combined with alexia, homonymous hemianopia, or both. These abnormalities arise from lesions of the dominant occipital cortex and adjacent temporal and parietal cortex (angular gyrus) or from a lesion of the left calcarine cortex combined with one that interrupts the fibers crossing from the right occipital lobe (see Fig. 21-6). In the latter case, fibers responsible for writing are spared, and the patient remains with a syndrome of alexia without agraphia.
Failure to understand the meaning of an entire picture even though some of its parts are recognized is referred to as simultanagnosia, and is found in bilateral lesions of the occipital–parietal junction. When combined with deficits in visual control of hand and eye movements (optic ataxia and ocular apraxia), the resulting condition is referred to as Balint syndrome. A failure to recognize familiar faces is called prosopagnosia and typically results from occipital–temporal lesions. These and other variants of visual agnosia (including visual neglect) and their pathologic bases are dealt with more fully in Chap. 21.
Other cerebral disturbances of vision include various types of distortion in which images seem to recede into the distance (teleopsia), appear too small (micropsia), or, less frequently, seem too large (macropsia). If such distortions are perceived with only one eye, a retinal lesion should be suspected. If perceived with both eyes, they usually signify disease of the temporal lobes, in which case the visual disturbances tend to occur in attacks and are accompanied by other manifestations of temporal lobe seizures (see Chap. 15). Palinopsia, a persistence of repetitive afterimages, similar to the appearance of a celluloid movie strip, occurs with right parietooccipital lesions; it has been a consequence of seizures in the cases we have encountered, but instances associated with static disorders (tumor, infarction) have been described as well. Patients describe the images as “trailing” or “echoing.” With parietal lobe lesions, objects may appear to be askew or even turned upside down. More often, lesions of the vestibular nucleus or its immediate connections produce the illusion that objects are tilted or turned upside down (tortopsia), or that straight lines are curved. Presumably this is the result of a mismatch between the visual image and the otolithic, vestibular input to the visual system.
Abnormalities of Color Vision