Connections of the parietal lobe.

THE PARIETAL lobes of the brains of primates exhibit an increase in fissuration and gyral complexity when comparing prosimians with New World and Old World monkeys, and some species of ceboids and cercopithecids with the apes and man. I-4 Increases in fissuration and gyration can be seen among the New World species themselves.4 Ample attention has been given to the great increase in size of the frontal lobes among primates, but a similar importance clearly should be attached to the parietal lobes of primates, and most probably to the temporal lobes as well. Our findings with respect to the anatomy of the parietal lobe suggest that the posterior part of the parietal lobe, i.e. the expanse of parietal cortex adjoining the somatic sensory region on the caudal side, undergoes an equally rapid development in several primate lines, such as ceboids and anthropoids, and that this development may indeed proceed somewhat in concert with the growth of the frontal lobes. The present account will serve to report and discuss evidence that the parietal lobe is connected with (i) the granular frontal cortex, directly as well as via a transcortical and transthalamic circuit with the temporal cortex, and (ii) the limbic system, more especially the cingulate gyrus, by direct associations and also more indirectly by way of a transthalamic pathway involving the nucleus lateralis dorsalis thalami. Severe agnosias, apraxias and aphasias may beset man following infarction of the parietal lobes.2,5,6 Cerebrovascular accidents and cerebral tumors commonly cause tissue destruction extensive enough to involve both parietal lobules as well as adjoining regions of the occipital or temporal isocortex. Clinico-pathological studies have led to the identification of polyesthesias, hallucinations, graphesthesia, ahylognosia, amorphognosia and constructional apraxia as symptoms attributable to parietal lobe involvement. Dyscalculia, dysgraphia, finger-agnosia, right and left-sided disorientation (Gerstmann’s syndrome) have been found symptomatic of lesions involving the supramarginal and angular gyri of the inferior parietal lobule together with adjacent occipital gyri, with or without additional involvement of the adjoining superior temporal convolution. In such cases, visual agnosias and disorientation are also commonly present. Disorders of body-image such as lack of awareness or complete denial of one-half of the body may be caused by lesions of the posterior parietal cortex and adjacent cortical territories. In experimental studies in the rhesus monkey and chimpanzee738 tactile agnosias such as barognosis, amorphogenesis and ahylognosia have been found to follow ablations of the posterior parietal cortex. It thus