The posterior parietal cortex is thought to influence the motor cortex both directly, via cortical connections, and, indirectly, via subcortical pathways through the cerebellar loop.

In this study, carried out on seven macaque monkeys, we aimed to define the contribution of thalamic nuclei, target of cerebellar afferents, to projections towards the posterior parietal cortex. Experimental protocols complied with Italian law on the care and use of laboratory animals. Tracer injections were made under general anaesthesia (ketamine I.M. 15 mg kg^-1). Following survival time, animals were killed with an overdose of pentobarbital I.V. In two animals, injections of WGA-HRP in cerebellar nuclei labelled, in addition to motor thalamic nuclei, nuclei Cl and Pcn and also some nuclei of the posterior thalamus (VLps, LP and Pul.m). In the posterior thalamus cerebellar afferents showed two patterns of termination: in VLps the labelling was strong and evenly distributed, whereas it showed a spot-like organization in LP and in rostro-dorsal part of Pul.m. In six monkeys tracers (WGA-HRP, fluorescent dyes) were injected into rostral and caudal areas of the superior parietal lobule (SPL; 3 injections) and in rostral, intermediate and caudal sectors of the inferior parietal lobule (IPL; 3 injections). Following all cortical injections the labelling in the thalamus formed a slab of marked neurons mostly involving posterior thalamic nuclei, but extending also in the caudal part of the motor thalamus. SPL areas receive a predominant projection from LP and additional projections from Pul.o, VLps, VLc, Pcn and Cl. However, rostral SPL injections labelled the ventral part of LP and VLc while caudal SPL injection produced a slab of labelling more dorsally located, involving dorsal LP and VLps. Following injections in IPL the most labelled nuclei were Pul.o and the ventral part of Pul.m. Additional projections originate from the intralaminar nuclei (Pcn and Cl) and from VLps, LP and VLc. Within these last nuclei, similarly to projections to caudal SPL, caudal IPL injection produced labelling more dorsally located than that following the rostral injection.

These findings suggest that the cerebellar input may reach both IPL and SPL through three different routes: motor thalamus, intralaminar nuclei, and posterior thalamus. However, none of these nuclei project exclusively to the parietal cortex, rather they project, although with different strength, to both frontal and parietal areas. Furthermore the observation that the cerebellar thalamic territory projecting to SPL appears to be larger than that projecting to IPL suggests a relevant link between SPL and cerebellum, and fits well with the recent proposal that, in respect to IPL, SPL is more involved in ‘on-line’ control of actions.

This work was supported by grants from MIUR.

All procedures accord with current National guidelines.Sensory PhysiologySensory PhysiologySensory PhysiologySensory PhysiologySensory PhysiologySensory Physiologyits well with the recent proposal that, in respect to IPL, SPL is more involved in ‘on-line’ control of actions.

This work was supported by grants from MIUR.All procedures accord with current National guidelines.