With anterior parietal lobe lesions, there is sometimes an associated mild hemiparesis, as this portion of the parietal lobe contributes a considerable number of fibers to the corticospinal tract. Occasionally there is such a large degree of inability or disinclination to use the limb that it simulates a hemiplegia. More often, there is only a poverty of movement or a weak effort of the opposite side. The affected limbs, if involved with this apparent weakness, tend to remain hypotonic and the musculature may undergo slight atrophy of a degree possibly not explained entirely by inactivity alone. In some cases, as noted below, there is clumsiness in reaching for and grasping an object under visual guidance (optic ataxia), and exceptionally, at some phase in recovery from the hemisensory deficit, there is incoordination of movement and intention tremor of the contralateral arm and leg that closely simulates a cerebellar deficit (pseudocerebellar syndrome). While relatively rare, this type of ataxia is authenticated by our own case observations.
In instances of cortical sensory disturbance, the outstretched hand may display small random “searching” movements of the fingers that simulate playing a piano (pseudoathetosis); these are exaggerated when the eyes are closed. Fixed dystonic postures and asterixis have also been described after parietal lesions with sensory loss, but these are most often the result of thalamic damage.
A conceptual inability to recognize objects, persons, or sensory stimuli in the absence of a primary deficit in the sensory modality is termed agnosia, derived from the Greek for lack of knowledge. It was included as a form of loss of insight as part of the confusional state in Chap. 19.
The idea that visual and tactile sensory information is synthesized into a body schema or image (perception of one’s body and the relations of bodily parts to one another) was first formulated by Pick and elaborated by Brain. Long before their time, however, it was suggested that such information was the basis of our emerging awareness of ourselves, and philosophers had assumed that this comes about by the constant interplay between inherent percepts of ourselves and of the surrounding world.
The formation of the body schema is considered to be based on the constant influx and storage of sensations from our bodies as we move about; hence, motor activity is important in its development. A sense of extrapersonal space is central to this activity, and this also depends upon visual and labyrinthine stimulation. The mechanisms of these perceptions are best appreciated by studying their derangements in the course of neurologic disease of the parietal lobes.
Denny-Brown and Banker introduced the idea that the basic disturbance in all these defects is an inability to integrate a series of “spatial impressions”—tactile, kinesthetic, visual, vestibular, or auditory—a defect they referred to as amorphosynthesis. Examples of the loss of concept in their schema include finger agnosia, right-left confusion, acalculia, and all the apperceptive losses that attend damage of integrative sensory areas of the brain. The theoretical problem presented by agnosia is taken up in a later section.
Anosognosia and hemispatial neglect (Anton–Babinski syndrome) The observation that a patient with a dense hemiplegia, usually of the left side, may be indifferent to a paralysis, or is entirely unaware of it, was first made by Anton; later, Babinski named this disorder anosognosia. It expresses itself in several ways. For example, a lack of concern regarding paralysis was called anosodiaphoria by Babinski, an interesting term that is now little used. The term denial was introduced by Freud to explain the problem but is laden with psychic and psychoanalytical meaning and is less precise than “neglect.”