454 Brain Struct Funct (2008) 212:443–463 Fig. 10 Multiarchitectonic characteristics of the pallidal and subpal- lidal areas on photomicrographs of frontal sections stained for a myelin, b AChE or c immunoreacted for CB and d CR. Note the clear contrast between al (only moderately stained) and the GPi (enhanced in CB, c), and the area underneath the pallidum (or sublenticular area, enhanced for CR, d). A continuity between the sublenticular area and the ansa peduncularis (ap) is seen with CR immunostaining. Case Hb4. Scale bar: 3 mm lateral, and anterior thalamic nuclei (AV, LD, MD, LP, and the intralaminar CM and CL nuclei). The reticular nucleus (R) is also distinguishable between the internal capsule and the lateral nuclei (Fig. 7, A5–A7.5). The fct tract is illus- trated in Fig. 7 at three anteroposterior levels (A7.5, A5.0, and A2.5), highlighting its entrance into the thalamus. The course of the pallidothalamic ﬁbres is shown in Fig. 8, from their emergence from the internal part of the pallidum (A20.0 and A17.5 for the al and A17.5 and A15.0 for the ﬂ) up to their entry in the thalamus (A12.25). At levels A17.5 and A15.0, the course of the tract in the subthalamic area and the most medial extension of the al just before merging with the ﬂ to form the ft are also clearly recognized. Other ﬁbre tracts such as the fornix (fx), the mtt and the optic tract (ot) are strongly contrasted. Stereotactic localization and interindividual variability The stereotactic position of the cerebello- and pallidotha- lamic tracts was determined in six hemispheres (three brains; cases Hb1, Hb4, and Hb5; Table 1). As seen on 123 sagittal and horizontal planes (Figs. 5, 6), the fct enters the thalamus more posterior and lateral than the ft, but the two come into close proximity at several mediolateral levels (e.g. Fig. 5, L7.2–L9) and in horizontal planes, ventral to the intercommissural level (e.g. Fig. 6, level V0.9). Nev- ertheless, there is a gap between the two tracts, as best seen in sagittal sections stained for myelin or immunoreacted for PV (arrows in Fig. 3a, d) and the bulks of the fct and ft are clearly separated. The position of the two tracts in different brains is illustrated on sagittal and horizontal atlas sections in Fig. 12. The fct, ft, and ﬂ from two different brains (Hb1 in red and Hb4 in gray) are compared on sagittal sections (L7 and L8) in upper panels. In these representations, the tracts show similar dorsoventral and anteroposterior posi- tions. More variations are seen in the mediolateral axis, as best illustrated on horizontal section (lower panel) where the ﬁber tracts from four different brains (Hb1, Hb2, Hb3, and Hb5) are superposed. The area of maximal overlap between three of the four cases (represented in yellow) is located in the medial more than the lateral part of the tracts. This is related to overall differences in mediolateral width of the thalamus and subthalamic area in the different brains