Diagram of connections involved in vibrissal information processing in ZI and Po. These nuclei receive vibrissal input from the axonal branches of large-sized, multiwhisker-responsive cells located in the PrV and the SpVi (Jacquin et al., 1989; Williams et al., 1994; Veinante and Deschênes, 1999; Veinante et al., 2000a). Both groups of axons terminate profusely in ZI, but their projections in Po differ; that provided by the PrV is sparse and principally concentrated in the caudal part of the nucleus, whereas that issued from the SpVi is dense and forms a shell-like terminal field at the margin of the VPM (Williams et al., 1994; Veinante et al., 2000a). Like most higher-order thalamic nuclei, Po receives a dual corticothalamic input: one that arises from lamina 6 cells in cortical areas innervated by Po axons (omitted for simplicity), and another one from layer 5 cells that are exclusively located in the granular and dysgranular zones of the barrel field (Hoogland et al., 1987; Bourassa et al., 1995; Veinante et al., 2000b). Latter projection consists of collaterals of corticofugal cells whose main axons also project to the striatum, tectum, ZI, pontine nuclei, and trigeminal nuclei. These collaterals do not supply branches in the reticular thalamic nucleus (Veinante et al., 2000b) but make large synaptic contacts on the proximal dendrites of Po neurons (Hoogland et al., 1991). Inhibitory inputs from the reticular thalamic nucleus and APT are not shown. The dashed line connecting PrV to Po indicates a sparse contribution to the dorsal part of Po, which contains the whisker representation.