458 Fig. 14 Targeting in functional neurosurgery. a, b Postoperative T1-weighted axial MRI and c atlas reconstruction of pallidothalamic (PTT) and cerebellothalamic (CTT) tractotomies in two patients with PD and ET, respectively (see text for more detail). The horizontal atlas map is 1.8 mm ventral (V1.8) to the intercommissural plane. Scale bars: 5 mm in a and b, 2 mm in c a b ac mcl pc ac mcl pc PTT CTT c ac mcl AP 0 PAG Acb ac fx mtt RN al fl ft PTT STh CTT fct VLpv sPf VPMpc VPM VPLp VPI SG Po ml bic PuM PuL R Brain Struct Funct (2008) 212:443–463 ic GPi GPe PuT V 1.8 (Schaltenbrand and Bailey 1959; Schaltenbrand and Wah- ren 1977; Mai et al. 2004) provide a basis for coordinate determination of the tracts, though with low-spatial reso- lution and insufﬁcient 3D stereotactic precision. In spite of these limitations, the reproducibility of our results in many cases and comparison with the existing data in human allow us to identify the main bulks of the ﬁbre tracts with a certain conﬁdence. Furthermore, the use of immunohisto- chemical staining procedures in addition to myelin and Nissl stainings, provide further criteria to ascertain the delimitations and discriminate between the different tracts. This was particularly the case for the passage of ﬁbres through the internal capsule and cerebral peduncle, where identiﬁcation of tracts can only be inferred, but not ﬁrmly assessed, on myelin stained sections. One critical problem with myelin stain, i.e. deformation and shrinkage due to alcohol pre-treatment in the myelin staining procedure could be partly corrected in a most recent case (Table 1, Hb5) by applying a protocol on mounted, instead of free- ﬂoating sections (Burgel et al. 1997). The delineations of the ﬁbre bulks were also superposed on Nissl sections (where they appear negatively stained), which undergo minimal deformations and served as primary basis for map drawings in the Morel atlas. Moreover, high-resolution post-mortem MRI scans prior to guillotine section and histological processing, were very helpful not only to visualize the subthalamic ﬁbre tracts and thalamic struc- tures with no additional deformation (except that caused by removing the brain from the skull), but also to better assess 123 the positions of the reference stereotactic intercommissural plane and estimate the mediolateral extent of thalamic and subthalamic areas. Nomenclature The nomenclature of the subthalamic ﬁbres was revised and adapted in an effort to combine old terminology with more recent studies in primates. Since Forel’s seminal morphological studies of the pallidothalamic ﬁbres (with- out knowing their origin) that were described as ﬁelds (or ‘‘Campi’’) (Forel 1877), the terms H, H1, and H2 ﬁelds of Forel have been consecutively used by different neuro- anatomists. Initially, ﬁeld H1 designated the dorsal extension of the ﬁbres from H en route to the thalamus, and H2 the ventral continuity of H to the substantia innominata and the Pes Pedunculi. The origin of H ﬁeld was supposed to be the red nucleus. Forel described those ﬁbres with the idea of the logical sequence of H, H1, and H2, but not knowing the relationship between the ﬁbres coursing in his ﬁelds and the thalamus or the pallidum. He was not able to differentiate between cerebellothalamic ﬁbres running in his H ﬁeld (the so-called prerubral ﬁeld of Forel) and the pallidothalamic (ft) ﬁbres more anteriorly. By close examination of the illustrations in Forel’s original paper (Forel 1877) and according to our data, we consider ﬁeld H as comprising the posterior extension of the ft containing ﬁbres from both the ansa lenticularis and the fasciculus