Whiskers, barrels, and cortical efferent pathways in gap crossing by rats

Rats can readily be trained to jump a gap of around 16 cm in the dark and a considerably larger gap in the light for a food reward. In the light, they use vision to estimate the distance to be jumped. In the dark, they use their vibrissae at the farthest distances. Bilateral whisker shaving or barrel field lesions reduce the gap crossed in the dark by about 2 cm. Information from the barrel fields reaches motor areas via cortico-cortical, basal ganglia, or cerebellar pathways. The cells of origin of the ponto-cerebellar pathway are segregated in layer Vb of the barrel field. Efferent axons of Vb cells occupy a central position within the basis pedunculi and terminate on cells in the pontine nuclei. Pontine cells, in turn, project to the cerebellar cortex as mossy fibers. We trained normal rats to cross a gap in the light and in a dark alley that was illuminated with an infra-red source. When the performance was stable, we made unilateral lesions in the central region of the basis pedunculi, which interrupted connections from the barrel field to the pons while leaving cortico-cortical and basal ganglia pathways intact. Whisking was not affected on either side by the lesion, and the rats with unilateral peduncle lesions crossed gaps of the same distance as they did pre-operatively. Shaving the whiskers on the side of the face that retains its input to the pontine nuclei reduced the maximal gap jumped in the dark by the same amount as bilateral whisker shaving. Performance in the light was not affected. Regrowth of the shaved whiskers was associated with the recovery of the maximum distance crossed in the dark. In control cases, shaving the whiskers on the other side of the face did not reduce the distance jumped in the dark or in the light. These results suggest that the cerebellum must receive whisker information from the barrel fields for whisker-guided jumps.