Brain Struct Funct (2008) 212:443–463 445 M1 PMc SMA-proper pre-SMA PMr PFC VLp V.im+D.im VM V.o.m VLa V.o.(p,a) VApc L.po Cortex Thalamus Pontine Nuclei Cerebellar cortex fct Cerebellar nuclei STh SNc ft fl al Fig. 1 Simpliﬁed diagram of the cortico-cerebello-thalamocortical and cortico-basal ganglia-thalamocortical circuitry based on data derived from tracing studies in monkeys. The cerebellar nuclei project to the motor thalamus via the fasciculus cerebellothalamicus (fct), with primary target in the VLp, but also in VLa and VM (projections to medial thalamic nuclei are not included in the schema). Efferent connections of the GPi to the thalamus course through the ansa lenticularis (al) and the fasciculus lenticularis (ﬂ), and the two ﬁbre bulks merge to form the fasciculus thalamicus (ft) which projects Tanji 1995; Parent and Hazrati 1995a; Matelli and Luppino 1996; Sakai et al. 1996, 2000; Shink et al. 1997; Sidibe et al. 1997; Middleton and Strick 2000; Baron et al. 2001, 2006; Francois et al. 2002; Haber 2003; Parent and Parent 2004; Calzavara et al. 2005). Thus, more is known about the origin and terminations than the exact pathway of those ﬁbres, particularly in the subthalamic region. In the human brain, the course of the pallidothalamic and cerebellotha- lamic ﬁbre tracts is still unclear. Indeed the description of the subthalamic ﬁbre pathways in the human brain dates from the late ninteenth and early twentieth centuries (Forel 1877; Von Monakow 1895; Vogt 1909; Vogt and Vogt 1920). The reference to Forel’s work (‘‘ﬁelds of Forel’’) is still used to describe the pallidothalamic ﬁbre system. Targeting the pallidothalamic and cerebellothalamic tracts for surgical treatment of PD and ET was already explored with some success in the 1950s–1970s (Spiegel and Wycis 1954; Spiegel et al. 1963; Mundinger 1965; Ve- lasco et al. 1972; Bertrand et al. 1973) but with a limited precision due to poor technical support at the time. This surgical approach was reappraised in the last 10 years based on pathophysiological evidence of dysfunctional GPi GPe Striatum Cerebellum Basal Ganglia primarily to the VLa, VApc, and VM nuclei, with only minor projections to the VLp (projections to the intralaminar nuclei are not shown). Equivalent Hassler’s nomenclature (Hassler 1982) for thalamic nuclei is also indicated. Connections to the thalamus and cortex are represented with different thicknesses according to their relative density and by different gradients in the thalamus. The two major motor pathways are also represented by different colors for clarity. See list of abbreviations thalamocortical system (thalamocortical dysrhythmia, TCD) following thalamic cells deactivation, in case of ET, or cell overinhibition, in case of PD (Magnin et al. 2001a, 2001b; Aufenberg et al. 2005). This pre-thalamic approach maxi- mizes the disinhibition of the thalamus with a lesion of the pallidothalamic ﬁbers, that is, at least three times smaller than the size of a pallidal lesion that would include the sen- sorimotor and the associative territories (Francois et al. 1994; Parent and Hazrati 1995a; Gross et al. 1999; Karachi et al. 2002; Morel et al. 2002). The relevance of the motor ﬁber tracts in the subthalamic area in surgical approach for motor disorders is also supported by recent reports on DBS [or in rare cases, of radiofrequency lesions (RFL)] of the subtha- lamic nucleus (STh) in PD, where the best clinical effect is obtained when the lesion or DBS electrode contact is dorsal to STh and involves at least part of the pallidothalamic ﬁbres (H2 or ﬂ) (Voges et al. 2002; Hamel et al. 2003; Zonenshayn et al. 2004; Alvarez et al. 2005; Breit et al. 2006; Godinho et al. 2006; Herzog et al. 2007a). Furthermore, direct tar- geting of the posterior subthalamic area has also recently been reported to be clearly more efﬁcient than thalamic DBS in ET (Hamel et al. 2007; Herzog et al. 2007b). 123