Despite Critchley’s pessimistic prediction that establishing a formula of normal parietal function would prove to be a “vain and meaningless pursuit,” our concepts of the activities of this part of the brain have assumed some degree of order, in part from his own work. There is little reason to doubt that the anterior parietal cortex contains the mechanisms for tactile percepts. Discriminative tactile functions, listed below, are organized in the more posterior, secondary sensory areas. But the greater part of the parietal lobe functions as a center for integrating somatosensory with visual and auditory information in order to construct an awareness of one’s own body (body schema) and its relation to extrapersonal space. Connections with the frontal and occipital lobes provide the necessary proprioceptive and visual information for movement of the body and manipulation of objects and for certain constructional activities (constructional apraxia). Impairment of these functions implicates the parietal lobes, more clearly the nondominant one (on the right).
The conceptual patterns on which complex voluntary motor acts are executed also depend on the integrity of the parietal lobes, particularly the dominant one. Defects in this region give rise to ideomotor apraxia, as discussed in Chap. 3 and further on. The understanding of spoken and written words is partly a function of the supramarginal and angular gyri of the dominant parietal lobe as elaborated in Chap. 22. The recognition and utilization of numbers, arithmetic principles, and calculation, which have important spatial attributes, are other functions integrated principally through these structures.
Clinical Effects of Parietal Lobe Lesions
Within the brain, perhaps no other territory surpasses the parietal lobes in the rich variety of clinical phenomena exposed under conditions of disease. Our current understanding of the effects of parietal lobe disease contrasts sharply with that of the late nineteenth century, when these lobes, in the textbooks of Oppenheim and Gowers, were considered to be “silent areas.” However, some of the clinical manifestations of parietal lobe disease may be subtle, requiring special techniques for their elicitation.
Close to the core of the complex behavioral features that arise from lesions of the parietal lobes is the problem of agnosia. Allusion has already been made to agnosia in the discussion of lesions of the temporal lobes that affect language, and similar findings occur with lesions of the occipital lobe as discussed further on. In those contexts, agnosia refers to a loss of recognition of an entity that cannot be attributed to a defect in the primary sensory modality. The term agnosia extends to a loss of more complex integrated functions and mental symbolism as described below, a number of intriguing deficits arise. These syndromes expose properties of the parietal lobe that have implications regarding a map of the body schema and of external topographic space, of the ability to calculate, to differentiate left from right, to write words, and other problems discussed below. The fact that apraxia, an inability to carry out a commanded task despite the retention of motor and sensory function, may also arise from parietal lobe damage, and the relationship of the apraxias to language and to agnosias, exposes some of the most complicated issues in behavioral neurology. Some of the theoretical aspects of agnosia, particularly those related to the disturbances of visual processing, are discussed later in the chapter.
The effects of a parietal lobe lesion on somatic sensation were first described by Verger and then more completely by Dejerine, in his monograph L’agnosie corticale, and by Head and Holmes. The latter, in their important paper of 1911, noted the close interrelationships between the thalamus and the sensory cortex. Although difficult to study, it is apparent that a large lesion of the primary sensory cortex, or beneath it, results in a circumscribed loss or reduction in sensation on the opposite side of the body. When primary sensory perception is altered, analysis of more complex and integrative sensory function is rendered less accurate.