In most recorded autopsy studies, the lesions have been bilateral, in the middle third of the superior temporal gyri, in a position to interrupt the connections between the primary auditory cortex in the transverse gyri of Heschl and the associated areas of the superoposterior cortex of the temporal lobe. In a few cases, unilateral lesions have been localized in this part of the dominant temporal lobe (see Chap. 21). Requirements of small size and superficiality of the lesion in the cortex and subcortical white matter are best fulfilled by an embolic occlusion of a small branch of the lower division of the middle cerebral artery.
Pure Word Blindness (Alexia Without Agraphia, Visual Verbal Agnosia)
The most striking feature of this syndrome is the retained capacity to write fluently, after which the patient cannot read what has been written (alexia without agraphia). In fact, reading of any material is greatly impaired. When the patient with alexia also has difficulty in auditory comprehension and in repeating spoken words, the syndrome corresponds more closely to Wernicke aphasia. In such cases, the individual loses the ability to understand written script, and, often, to name colors, that is, to match a seen color to its spoken name, visual verbal color anomia. Such a person can no longer name or point on command to words, although he is sometimes able to read letters or numbers. However, understanding spoken language, repetition of what is heard, writing spontaneously and to dictation, and conversation are all intact. The ability to copy words is impaired but is better preserved than reading, and the patient may even be able to spell a word or to identify a word by having it spelled to him or by reading one letter at a time (letter-by-letter reading). In some cases, the patient manages to read single letters but not to join them together (asyllabia).
Autopsies of such cases have usually demonstrated a lesion that destroys the left visual cortex and underlying white matter, particularly the geniculocalcarine tract, as well as the callosal connections of the right visual cortex with the intact language areas of the dominant hemisphere (see “Disconnection Syndromes” in Chap. 23). In the case originally described by Dejerine in 1892, the disconnection occurred in the posterior part (splenium) of the corpus callosum, wherein lie the connections between the visual association areas of the two hemispheres (see
Fig. 23-6). More often, the callosal pathways are interrupted in the forceps major or in the periventricular region (Damasio and Damasio, 1983). In either event, the patient is blind in the right half of each visual field by virtue of the left occipital lesion, and visual information reaches only the right occipital lobe; however, this information cannot be transferred, via the callosal pathways to the language area of the left hemisphere.
A rare variant of this syndrome takes the form of alexia without agraphia and without hemianopia. A lesion deep in the white matter of the left occipital lobe, at its junction with the parietal lobe, interrupts the projections from the intact (right) visual cortex to the language areas, but spares the geniculocalcarine pathway (Greenblatt). This lesion, coupled with one in the splenium, prevents all visual information from reaching the language areas, including the angular gyrus, and Wernicke area.
In yet other cases, the lesion is confined to the angular gyrus or the subjacent white matter. In such cases also, a right homonymous hemianopia will be absent, but the alexia may be combined with agraphia and other elements of the Gerstmann syndrome, that is, right-left confusion, acalculia, and finger agnosia (see “Gerstmann Syndrome,” Chap. 21). This entire constellation of symptoms is sometimes referred to as the syndrome of the angular gyrus. Anomic aphasia may be added (see later).