Corticothalamic connections of the posterior parietal cortex in the rhesus monkey.

Corticothalamic connections of posterior parietal regions were studied in the rhesus monkey by using the autoradiographic technique. Our observations indicate that the rostral superior parietal lobule (SPL) is connected with the ventroposterolateral (VPL) thalamic nucleus. In addition, whereas the rostral SPL is connected with the ventrolateral (VL) and lateral posterior (LP) thalamic nuclei, the rostral IPL has connections with the ventroposteroinferior (VPI), ventroposteromedial parvicellular (VPMpc), and suprageniculate (SG) nuclei as well as the VL nucleus. The caudal SPL and the midportion of IPL show projections mainly to the lateral posterior (LP) and oral pulvinar (PO) nuclei, respectively. These areas also have minor projections to the medial pulvinar (PM) nucleus. Finally, the medial SPL and the caudal IPL project heavily to the PM nucleus, dorsally and ventrally, respectively. In addition, the medial SPL has some connections with the LP nucleus, whereas the caudal IPL has projections to the lateral dorsal (LD) nucleus. Furthermore, the caudal and medial SPL and the caudal IPL regions have additional projections to the reticular and intralaminar nuclei-the caudal SPL predominantly to the reticular, and the caudal IPL mainly to the intralaminar nuclei. These results indicate that the rostral-to-caudal flow of cortical connectivity within the superior and inferior parietal lobules is paralleled by a rostral-to-caudal progression of thalamic connectivity. That is, rostral parietal association cortices project primarily to modality-specific thalamic nuclei, whereas more caudal regions project most strongly to associative thalamic nuclei.