Topographic distribution of pallidal neurons projecting to the thalamus in macaques.

The respective topographic distribution of the pallidal neurons projecting to the central complex (centre médian-parafascicular complex) and to the oral part of the lateral mass of the thalamus was studied by using a topographic technique based on ventricular landmarks. WGA-HRP was stereotactically injected into the central complex of 4 macaques and into the oral part of the lateral mass of the thalamus of 3 others. Neurons projecting to the central complex were located in the caudal, lateral and ventral region of the ipsilateral medial nucleus of the pallidum. Pallidal neurons projecting to the lateral mass of the thalamus were more numerous and occupied the entire volume of the medial pallidum apart from a small rostral and dorsomedial region. The location of the pallido-central complex neurons appeared to be included in that of the neurons projecting to the lateral mass. As very few neurons of this shared region were unlabeled, it is very likely that the same pallidal neurons project to both targets. The pallido-central complex neurons were located in the region crossed by axons coming from the putaminal sensorimotor territory of the striatum. These results provide further evidence that Nauta and Mehler's loop is a real closed loop probably involved in sensorimotor processing.