Clinical observations indicate that patients with right parietal lesions show variable but lesser elements of ipsilateral neglect in addition to the striking degree of contralateral neglect, suggesting that, in respect to spatial attention, the right parietal lobe is truly dominant (Weintraub and Mesulam). Damage of the superior parietal lobule, in addition to producing agnosias and apraxias, may interfere with voluntary movement of the opposite limbs, particularly the arm, as pointed out by Holmes. In reaching for a visually presented target in the contralateral visual field, and to a lesser extent in the ipsilateral field, the movement is misdirected and dysmetric (the distance to the target is misjudged).
Another subtle aspect of parietal lobe physiology revealed by human disease is the loss of exploratory and orienting behavior with the contralateral arm and even a tendency to avoid tactile stimuli. Mori and Yamadori call this rejection behavior. Denny-Brown and Chambers attributed the released grasping and exploring that follow frontal lobe lesions to a disinhibition of inherent parietal lobe automatisms but there is no way of confirming this. It is of interest that demented patients with prominent grasp reflexes tend not to grasp parts of their own bodies, but if there has been an additional parietal lesion, there is “self-grasping” of the forearm opposite the lesion (Ropper).
Conventional treatments for hemispatial neglect use prismatic glasses and training in visual exploration of the left side. Another approach demonstrates improvement by the application of vibratory stimulation to the right side of the neck, as reported by Karnath and colleagues, or of the ipsilateral labyrinth by caloric or electrical means (a similar treatment has been successful in some cases of dystonic torticollis, see Chap. 4). Based on the work of Ramachandran and colleagues, mirrors have been used to assist recovery of the side with agnosia. With a mirror in the right parasagittal plane, the patient observes the mirror image of their neglected hand and space and is induced to use that side more naturally. The larger problem is that these patients may not respond to rehabilitation if they lack an innate body schema.
Ideomotor and Ideational Apraxia (See Also Chap. 3)
As discussed extensively in Chap. 3, patients with parietal lesions of the dominant hemisphere who exhibit no defects in motor or sensory function, lose the ability to perform learned motor skills on command or by imitation. They can no longer use common implements and tools, either in relation to their bodies (e.g., brushing teeth, combing hair) or in relation to objects in the environment (e.g., a doorknob or hammer). The patient holds the implement awkwardly or seems at a loss to begin the act. It is as though the patient had forgotten the sequences of learned movements. The effects are bilateral. When defects of apraxia are intertwined with agnosic defects, the term apractognosia seems appropriate. A special type of visuospatial disorder, separable from neglect but also associated with lesions of the nondominant parietal lobe, is reflected in the patient’s inability to reproduce geometric figures (constructional apraxia). A number of tests have been designed to elicit these disturbances, such as indicating the time by placement of the hands on a clock, drawing a map, copying a complex figure, reproducing stick-pattern constructions and block designs, making three-dimensional constructions, and constructing puzzles.