In cats, territories of basal ganglia were found mainly anteriorly and laterally, whereas cerebellar territories were situated more posteriorly and medially. Pallidal and cerebellar territories interdigitated with minor overlaps (Ilinsky and Kultas-Ilinsky
1984). However, data from intraindividual tracing studies that pursue a multi-tracer approach are missing. In dogs, such techniques were employed to investigate the relation of cerebellar and nigral territories in the thalamus (Sakai and Patton
1993). The findings support the notion of interdigitating territories of cerebellar and basal ganglia projections with minor overlap in VA–VL of cats and dogs. Evidence for overlapping territories in the thalamus was provided by a retrograde tracing study in cats: Anderson and DeVito (
1987) placed multiple very small tracer injections in different regions of VA–VL, VM, IL, and MDpl. They found the following patterns of retrogradely labeled neurons: (1) injections in dorsal parts of VL (VLd,
n = 3) labeled numerous neurons in DCN (338–1089 neurons), few-to-moderate amounts in GPi (2-318 neurons), and no-to-moderate amounts in SNr (0-102 neurons); (2) injections in caudal and caudolateral VL (VLc,
n = 6) labeled few to moderate amounts of neurons in DCN (21–506 neurons), no-to-moderate amounts in GPi (0–94 neurons), and none up to few in SNr (0–2 neurons); (3) injections in CL and MDpl (
n = 1) labeled few neurons in DCN (14 neurons), moderate amounts in GPi (116 neurons), and few in SNr (4 neurons); (4) injections in VM (
n = 2) labeled few-to-moderate amounts of neurons in DCN (10–72 neurons), few-to-moderate amounts in GPi (9–86 neurons), and moderate amounts in SNr (82–107 neurons). We considered the relative frequencies of labeled neurons between these structures in selected experiments of this study to reveal the complementary relation of these projections in the different nuclei: (1) VLd: BG 0.28, DCN 0.72; (2) VLc: BG 0.21, DCN 0.79; (3) IL: BG 0.9, DCN 0.1; and (4) VM: BG 0.73, DCN 0.27. These observations might be interpreted as evidence for overlapping territories with complementary projection densities. However, tracer placement in regions of interdigitating territories would label the projecting structures, even if no overlap was present. This especially concerns the injections into VLd and VLc. (Ilinsky and Kultas-Ilinsky
1984) describe the relationship of pallidal and cerebellar territories in an interindividual anterograde tracing study in cats as being complementary in the whole thalamus. However, they admit that a small zone of overlap in border regions of the territories cannot be ruled out by this evidence. Similar evidence was provided by another interindividual anterograde tracing study in cats with injections in DCN, GPi, and SNr (Hendry et al.
1979). It is worth mentioning that in this study, no projections of the DCN to VM were found, contradicting evidence from other studies in cats (Kultas-Ilinsky et al.
1978; Sugimoto et al.
1981; Anderson and DeVito
1987; Ilinsky and Kultas-Ilinsky
1984). In dogs, the VM was found to be a site of clearly overlapping cerebellar and nigral territories [see Fig. 4 in Sakai and Patton (
1993)]. Furthermore, it was identified as being the main territory of fastigial projections in dogs (Person et al.
1986; Sugimoto et al.
1981; Jimenez-Castellanos and Reinoso-Su rez
1985). One could hypothesize that these inconsistencies might be due to an incomplete coverage of the DCN, differences in nomenclature or interspecies differences in dogs and cats. Indeed, the fastigial nucleus has been neglected in some studies without addressing this issue sufficiently (Ilinsky and Kultas-Ilinsky
1984). Other studies did not show the injection sites (Hendry et al.
1979). Thus, it is not clear to what extent the discrepancy between observations can be attributed to an omission of the fastigial nucleus. For a detailed discussion, see the section, “
Interspecies differences—VM”.