Brain Struct Funct (2008) 212:443–463 lenticularis, and a part of the dorsal extension of the fct near its entry into the thalamus. Nevertheless, the deﬁnition of the H ﬁeld as part of the pallidothalamic pathways is still used in the literature and often misleading. A continuity of the ﬁbres from H2, to H2 + H1, and H1 was ﬁrst described by Ce´cile Vogt and Oskar Vogt (1920) and corresponds to the ﬂ, ﬂ/al mergence and the ft in our study. Other ﬁbre degeneration studies in monkeys adopted this nomencla- ture (Vogt 1909; Ranson 1939; Nauta and Mehler 1966), changing also the names of the components of the ansa lenticularis of von Monakow (Von Monakow 1895; Nauta and Mehler 1966) to take into account their different pro- jections, such as the STh for the middle component. For the cerebellothalamic ﬁbres, the choice of the term fasciculus cerebellothalamicus participates to the same effort to homogenize the nomenclature for subthalamic ﬁbre tracts. Pallidothalamic tract: origin of the al and ﬂ The origin of the pallidothalamic tract through al and ﬂ from the internal segment of the globus pallidus has been conﬁrmed by a number of studies in primates (Nauta and Mehler 1966; Kuo and Carpenter 1973; Inase and Tanji 1994; Parent and Hazrati 1995a; Sakai et al. 1996; Sidibe et al. 1997, 2002; Baron et al. 2001; Parent et al. 2001; Haber 2003; Parent and Parent 2004) but how the cells of origin of these two ﬁbre systems distribute within the GPi is still controversial. On one hand, Parent and col. (Parent et al. 2001; Parent and Parent 2004), using single-axon tracing method propose a wide distribution of pallidofugal ﬁbres through the al or ﬂ, without particular anteroposterior or lateromedial segregation of their parent cells in GPi. Others using tracer injections or RFL lesions in GPi (Baron et al. 2001, 2006) put forward a different innervation by the al and ﬂ, with the ﬂ originating principally from posterior (motor-related) and the al from anterior (non-motor rela- ted) GPi, in line with earlier model (Kuo and Carpenter 1973). The origin of the al and ﬂ within the human GPi cannot be assessed in the present study, but we infer from the anteroposterior extent of the medial and ventral com- ponents of the ﬁbres emerging from GPi that al comprises ﬁbres originating from more anterior portions of the nucleus than the ﬂ, although the al can also be followed relatively far posteriorly at the ventral edge of the GPi. Course of al and ﬂ in the subthalamic region The merging together of the al and ﬂ (H2) to form the ft (H1) in subthalamic area was ﬁrst described by Vogt (1909). The exact location where the ﬂ and al meet 459 together, becoming ft, could not be determined with our methods, but the data suggest that the two tracts merge in the dorsal part of the H1 + H2 ﬁeld of Forel, about 2.5 mm ventral to the DV0 plane and about 2 mm posterior to mcl level. The thalamic projections of the ft, based on studies in monkeys, distribute principally to VApc and VLa, but also VLp, VM and the centre me´dian-parafascicular (CM-Pf) complex (Nauta and Mehler 1966; Kuo and Carpenter 1973; Parent and De Bellefeuille 1983; Percheron et al. 1993; Inase and Tanji 1994, 1995; Rouiller et al. 1994; Parent and Hazrati 1995a; Sakai et al. 1996, 1999, 2000; Sidibe et al. 1997; Haber and McFarland 2001; Parent and Parent 2004). They overlap to some extent with terminals of the fct in VLp and VLa (Ilinsky and Kultas-Ilinsky 1987; Rouiller et al. 1994; Sakai et al. 1996), though direct convergence of the pallido- and cerebellothalamic path- ways onto thalamocortical neurons appears very rare, as also demonstrated physiologically (Yamamoto et al. 1983; Anderson and Turner 1991). The VLa, Vapc, and VM are likely to be also primary targets of the ft in the human brain, as indicated by the course of the tract, particularly on sagittal sections (e.g. Fig. 5). Furthermore, in spite of the fact that the ft is negatively stained for CB (see Fig. 11b), the VLa and VApc show enhanced immunostaining (Figs. 4b, 11b; Morel 2007) and pallidothalamic terminals have been reported to be conﬁned to the CB-positive ter- ritory of the motor thalamus in monkeys (see Percheron et al. 1996; Parent et al. 2001; also suggested by Calzavara et al. 2005). In terms of functional aspects related to the ﬁbres coursing along al and ﬂ in the human subthalamic area, our surgical experience revealed that in a PD patient, a partial PTT due to lateral shift of the electrode that left the medial part of the ft (most probably containing mainly ﬁbres of the al) intact, resulted in only partial relief of the motor symptoms. A second, complementary surgical intervention with a medially placed RFL resulted in com- plete relief (unpublished observation). This clinical observation would agree with the scheme proposed by Parent and colleagues (Parent and Parent 2004), where motor ﬁbres course through both ﬂ and al, more than with that of Baron et al. (Baron et al. 2006), where those ﬁbres are mainly restricted to the ﬂ. Other subthalamic ﬁbre tracts related to basal ganglia efferent pathways Two ﬁbre systems apart from the pallidothalamic tracts could be recognized on the basis of immunohistochemical stainings. The ﬁrst is related to CB positive ﬁbres running through the pallidal segments, the internal capsule and penetrating the cerebral peduncle on the ventral aspect of 123