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General description technique/protocol: In each experiment, restricted injections of BDA and FR were placed into separate regions of the SI barrel cortex (Fig. 1). Ten of the 13 experiments were selected by criterion of labeling in the neostriatum and thalamus and were used previously in a study of corticostriatal organization (Alloway et al., 1999). All cases contained anterograde BDA and FR labeling in the pontine nuclei. All injections were restricted to the gray matter. The outlines and position of the injection sites were estimated with respect to the whisker barrels by comparison of adjacent BDA- and CO-processed tangential sections through cortical layers IV and V. The position of injections in the whisker barrel field corresponded to the electrophysiological recordings made before tracer injection. The diameter of the injection sites (Table 1), defined by the maximum width of dense staining in layer V (the cortical layer containing cell bodies of corticopontine neurons), ranged from 200 to 800 µm. Each injection involved one to two barrels (Table 1). In five cases, both tracers were confined to the same row of barrels ("within rows"), and in eight cases the tracers were injected into separate rows of barrels ("across rows"). The edge-to-edge separation between the two injection sites was measured in layer V (Fig. 2). Injection sites never overlapped. Photomicrographs of representative injection sites are shown in Figure 2.
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