Brain Struct Funct (2008) 212:443–463 455 Fig. 11 Photomicrographs of adjacent frontal sections 15 mm ante- rior to the posterior commissure stained for a myelin, c AchE, and b immunostained for the calcium-binding proteins CB and d PV. Pallidothalamic ﬁbres (ﬂ, ft) are darkly stained for myelin, moderately for AChE and PV, and negatively for CB. The STh, thalamic VLp nucleus and the optic tract (ot) are strongly enhanced in PV-ir. The arrow in d points to presumed pallidosubthalamic ﬁbres (fasciculus subthalamicus) crossing the posterior limb of the internal capsule. The asterisks in a, b, and d indicate matching locations in the internal capsule corresponding to striatonigral ﬁbres which are strongly enhanced in CB-ir but unstained for myelin. Case Hb4. Scale bar (a): 2 mm (as also reported in Morel 2007). As seen on sagittal planes (upper panels of Fig. 12), the dorsoventral variation of the position of the two tracts is similar to that of the ventral part of the thalamus (e.g. CM, VPMpc, and ventral limit of VLpv and VM), but much smaller than the variation of dorsal thalamic nuclei (e.g. LD, dorsal extent of VLpd, MD, and CL). In order to evaluate more quantitatively the variability of the cerebello- and pallidothalamic tracts, the mediolateral and anteroposterior stereotactic coordinates of the bulk centers of the two tracts were determined on sagittal planes of two different brains (Hb1 and Hb4). In every sagittal map, the anteroposterior coordinate of the center of the ﬁbre bulks was measured at their intersecting point with the DV0, V1, and V2 horizontal planes, respectively. The mediolateral and anteroposterior coordi- nates of the ft, ﬂ, and fct are plotted in Fig. 13 for the three dorsoventral levels. The smallest anteroposterior distance between fct and ft centers is similar (2.4 mm) in both Hb1 and Hb4, when considering the three dorsoventral planes. At V2 level, the median anteroposterior value of ft is 11 mm in Hb1 and 12.7 mm in Hb4. In terms of mediolateral extension, there is a signiﬁcant difference between the two cases with an extension from L4.5 to L9 for Hb1, and from L2 to L5 for Hb4. The position of the ﬂ is more anterior to ft in both cases (median values of 15.6 and 15.2 mm anterior to pc for Hb1 and Hb4, respectively), and the mediolateral extension varies from 6.3 to 8.1 mm in case Hb1, and from 3 to 6 mm in case Hb4. For the fct, anteroposterior median value is 6.8 mm in Hb1 and 6.7 mm in Hb4. The medio- lateral extension is from L4.5 to L14 in Hb1, and from L2 to L10 in Hb4. As already shown in Fig. 12, the variations of the tracts in the different cases are not homogeneous in the three planes, with smallest variations in anteroposterior and dorsoventral axes, and largest variations in mediolat- eral axis. Implications for stereotactic surgery According to the orientations and stereotactic positions of the cerebello- and pallidothalamic tracts (Figs. 5, 6, 12), and because of the electrode trajectory restrained by a precoronal approach, our surgical targeting in PD and in ET are determined at 2 mm ventral to intercommissural plane (V2). The anteroposterior and mediolateral coordi- nates for the pallidothalamic tractotomy (PTT) are at midcommissural (mcl) level and 7 mm lateral to ventric- ular border, respectively. For the cerebellothalamic tractotomy (CTT), these are 5–6 mm posterior to mcl and 8 mm lateral to ventricular border, respectively. This strategy has been developed for (1) best inclusion of both ﬂ and ft in PD, with minimum involvement of STh and keeping a safe distance between the RFL and the internal capsule and mtt (see also Aufenberg et al. 2005), and (2) to take the cerebellothalamic tract between RN and the thal- amus, but avoid the risk of encroaching on the trigeminal thalamic relay VPM nucleus (more ‘‘eloquent’’ structure 123