Connections of the parietal lobe

Abstract

The efferent connections of the posterior parietal cortex were studied in rhesus monkeys subjected to selective lesions of the superior and inferior parietal lobules, which correspond approximately to Brodmann's areas 5 and 7, respectively. Following ablations of either the superior or inferior parietal lobule, axon degeneration, stained with the Nauta and Fink-Heimer methods, was traced into the extreme, external, and internal capsules, and into the cerebral peduncle. This degeneration extended into the ipsilateral insular cortex, cingulate gyrus, prefrontal and premotor cortices, and the precentral and postcentral gyri. In addition to these connections, the superior lobule sends fibers to the ipsilateral inferior parietal lobule and superior temporal gyrus, and via the corpus callosum to the contralateral superior and inferior parietal lobules, whereas the inferior parietal lobule sends fibers to the ipsilateral superior parietal lobule and to the contralateral superior and inferior parietal lobules. A prominent fiber system to the ipsilateral temporal lobe degenerates following lesions in the inferior parietal lobule (area 7); in such cases fiber degeneration appears in the superior, middle and inferior temporal convolutions, and in the fusiform and parahippocampal gyri. Both lobules evidently project to the claustrum and body of the caudate nucleus. Both, moreover, have massive efferent connections with the dorsal two-thirds of the putamen. By contrast, no evidence of projections from the parietal cortex to the globus pallidus was found in any of the cases studied. A further subcortical projection from the posterior parietal cortex involves the nucleus reticularis thalami and the nucleus lateralis posterior thalami. The inferior lobule projects directly to the nucleus lateralis dorsalis and to the mediodorsal region of the nucleus lateralis posterior that closely adjoins two thalamic cell groups: the n. lateralis dorsalis and the intralaminar nucleus centralis lateralis. The superior parietal lobule, by contrast, projects massively to a ventrolateral district of the nucleus lateralis posterior. Parietosubthalamic connections could be traced from areas 5 and 7 to the zona incerta and fields H 2 and H of Forel, but evidence for terminal connections with the n. subthalamicus (Luys) could not be foud. Both areas 5 and 7 project massively to the pretectal area and the deeper layers of the superior colliculus. This parieto-mesencephalic connection is amplified by a fiber connection from the inferior parietal lobule (area 7) to the lateral, densocellular region of the circumaqueductal gray matter. No evidence of parietal corticonigral fibers connections was found. Finally, both parietal lobules were found to project to the pontine nuclei. Speculations regarding the associative functions of the parietal lobules at the cortical and subcortical levels are presented, with particular emphasis upon the possible significance of the projections from the inferior parietal lobule to insular, cingulate and temporal regions of the cortex.