A lesion deep to the inferior part of the parietal lobe, at its junction with the temporal lobe, involves the geniculocalcarine radiations and results in an incongruous homonymous hemianopia or an inferior quadrantanopia on the opposite side; but just as often, in practice, the defect is complete or almost complete and congruous. If the lesion is small and predominantly cortical, optokinetic nystagmus is usually retained; with deep lesions, it is abolished, with the target moving ipsilaterally (see Chap. 13).
Visual neglect is a typical feature of posterior parietal lesions on either side, more prominent with right-sided lesions. The problem that often arises is of distinguishing visual hemineglect (particularly of the left side) from a hemianopia. In its more severe forms the neglect is evident from casual observation of the patient’s behavior or in drawings made by the patient that omit features on the left side; but here a more pervasive syndrome of hemispatial neglect, discussed earlier, may underlie the visual behavior. Occasionally, severe left-sided visual neglect results from a lesion in the right angular gyrus (see Mort et al). Visual neglect can also occur after focal lesions in the posterior medial temporal lobe (supplied by a branch of the posterior cerebral artery, in contrast to the middle cerebral artery supply of the angular gyrus of the inferior parietal lobule).
With posterior parietal lesions, as noted by Holmes and Horrax, there are deficits in localization of visual stimuli, inability to compare the sizes of objects, failure to avoid objects when walking, inability to count objects, disturbances in smooth-pursuit eye movements, and loss of stereoscopic vision. Cogan observed that the eyes may deviate away from the lesion upon forced lid closure, a “spasticity of conjugate gaze.”
A common disorder of motor behavior of the eyelids is seen in many patients with large acute lesions of the right parietal lobe. Its mildest form is a disinclination to open the lids when the patient is spoken to. This gives the erroneous impression that the patient is drowsy or stuporous, but it will be found that a quick reply is given to whispered questions. In more severe cases, the lids are held shut and opening them is strongly resisted, to the point of making an examination of the pupils and fundi impossible.
Visual disorientation and disorders of extrapersonal space (topographic localization) Spatial orientation depends on the integration of visual, tactile, and kinesthetic perceptions, but there are instances in which the defect in visual perception predominates. Patients with this disorder are unable to orient themselves in an abstract spatial setting (topographagnosia). Such patients cannot draw the floor plan of their house, a map of their town, or of the United States and cannot describe a familiar route, as from home to work, for example, or find their way in familiar surroundings. In brief, such patients have lost topographic memory. This disorder is almost invariably caused by lesions in the white matter deep to the inferior and superior parietal lobules and it is separable from anosognosia as summarized by Levine and colleagues.
A clever mental experiment posed to patients by Bisiach and Luzzatti has suggested that the loss of attention to one side of the environment extends to, or perhaps is derived from, the mental representation of space. Their patient with a right parietal lesion was asked to describe from memory the buildings lining the Piazza del Duomo, first as if seen from one corner of the piazza and then from the opposite corner. In each instance, the description omitted the left side of the piazza from the observer’s perspective.
An important and not infrequent disorder of visual agnosia, a disorder of visually directed reaching with the hand, difficulty directing gaze, and simultanagnosia, is given the name Balint syndrome. It is, strictly speaking, a bilateral disorder of the parietal lobes but we discuss it below for convenience in order to append it to the clinically similar entity of cortical blindness.