Two Modes of Cerebellar Input to the Parietal Cortex in the Cat

The characteristics of cerebellar input to the parietal cortex through the ventroanterior-ventrolateral (VA-VL) complex of the thalamus were investigated in the adult cat by using combined electrophysiological and anatomical methods. Two distinct parietal regions were activated by stimulation of the cerebellar nuclei (CN). In the first region located in the depth of the bank of the ansate sulcus, stimulation of the CN induced early surface positive-deep negative potentials and late surface negative-deep positive potentials. In this cortical area, potentials of similar shape and time course were evoked at a shorter latency by stimulation of the ventrolateral part of the VA-VL complex where large negative field potentials were evoked by stimulation of the CN. After injection of the anterograde tracer Phaseolus vulgaris leucoagglutinin (PHA-L) in this part of the VA-VL complex, axon terminals of thalamocortical (TC) fibers were found in layers I, III and IV in the depth of the bank of the ansate sulcus and layers I and III in the motor cortex. In the second region located in the suprasylvian gyrus, late surface negative-deep positive potentials were evoked by stimulation of the CN and similar potentials were evoked at a shorter latency from the dorsomedial part of the VA-VL complex where large cerebellar-evoked potentials could be recorded. PHA-L injection in this thalamic region stained TC fibers and their terminals in layer I of the suprasylvian gyrus, and in layers I and III of the motor cortex. The laminar distribution of TC axon terminals in two different regions of the parietal cortex could account for the depth profiles of the cerebellar- and the thalamic-evoked potentials in each region. These results show that cerebellar information is conveyed to two separate areas in the parietal cortex by two different TC pathways.