460 the STh ‘‘en route’’ to the substantia nigra, pars reticulata. The latter is also enhanced in CB-ir and is known to receive important projections from the striatum (putamen and caudate) (Lynd-Balta and Haber 1994; Haber et al. 1995; Parent and Hazrati 1995a; Levesque and Parent 2005). These striatonigral ﬁbres were already described in terms of neurochemical characteristics in a previous report on the human basal ganglia (Morel et al. 2002). A second ﬁbre tract could be recognized at the anterior, mid-dorsal, and lateral aspect of the STh nucleus, both characterized by enhanced PV-ir. Although the course of the ﬁbres could not be followed back to the pallidum, their position between the ﬂ and the al strongly suggests that they belong to the fasciculus subthalamicus, or middle division of von Mo- nakow’s ansa lenticularis (Von Monakow 1895; Nauta and Mehler 1966) which connects the external globus pallidus (GPe) with the STh (Ranson 1939; Nauta and Mehler 1966; Parent and Hazrati 1995b; Shink et al. 1996). Indeed, dense degenerated ﬁbres were observed in this part of the tract and in the STh after lesion of GPe in human (Morel et al. 2001). Some ﬁbres coursing on the dorsal aspect of the STh and continuous with the ﬂ tract (e.g. Fig. 4b) may corre- spond to efferents from the GPi to the pedonculopontine nucleus (PPN) (Nauta and Mehler 1966; Shink et al. 1997; Parent et al. 2001; Parent and Parent 2004). These pallido- pedunculopontine connections are supposed to be affected by medial pallidotomies in PD patients and could explain part of the beneﬁcial results against akinesia. These would likely be also interrupted by the PTT and possibly, though only partially, by subthalamic DBS or RFL (targets too far anterior and lateral). Previous experiments in monkey have shown that SNr projects to the thalamus, particularly to VAmc, VM, MDpl, and Pf nuclei (Carpenter et al. 1976; Parent and De Bellefeuille 1983; Francois et al. 1984, 2002; Ilinsky et al. 1993; Percheron et al. 1996). Thalamic projections of the SNr share some territories with those from the GPi (e.g. VM), but others are different (e.g. VAmc) (Ilinsky et al. 1993). Nigral projections to the thalamus were not described in humans but by comparison with data gathered in monkeys, we can infer that they probably course along the posterior part of the ft. These would also be interrupted by the PTT, in contrast to pallidotomies or subthalamic DBS or RFL, the latter affecting only the ﬂ. Cerebellothalamic tract Auguste Forel already described ﬁbres originating in the dentate nucleus and passing through and lateral to the red nucleus (‘‘Bindearm’’) before entering the thalamus. The ﬁrst experimental anterograde study of the cerebellotha- lamic ﬁbres after destruction of the dentate nucleus in 123 Brain Struct Funct (2008) 212:443–463 monkey is due to Vogt (Vogt 1909). She described pro- jections to the thalamus, in particular to its ventral lateral part. The origin of the cerebellothalamic ﬁbres in the cer- ebellar nuclei (dentate, interposed, and fastigial nuclei) and its projections to the thalamus are now well established. Anterograde studies in monkeys have shown that after tracer injections in cerebellar nuclei, terminal labels are found primarily in VLp, in discrete zones of VLa and in the intralaminar CM/Pf and CL nuclei (Asanuma et al. 1983a, b; Jones 1990; Percheron et al. 1993; Rouiller et al. 1994; Sakai et al. 1996; Calzavara et al. 2005). Our observations that the bulk of the fct is mainly directed toward the VLp, which is enhanced in PV-ir (see also Fig. 3d), are consis- tent with reports in monkeys although with indirect demonstration (Calzavara et al. 2005). We cannot exclude however that the fct also innervates the VLa, as suggested in sagittal sections where some ﬁbres ascend toward the VLa through a more anterior part of the VM. Although some overlap exists between the cerebello- and pallido- thalamic terminals ﬁelds in the lateral thalamus, direct interaction between the two seems very limited, as dis- cussed above. For the medial thalamus, we noticed that some ﬁbres emerging from the RN and most probably associated with the fct, take a turn medially toward the CM and possibly CL nuclei. This may well correspond to the medial cerebellothalamic component directed to the intra- laminar nuclei described by others (Asanuma et al. 1983a, b; Rouiller et al. 1994; Sakai et al. 1996; Calzavara et al. 2005). Relations between cerebello- and pallidothalamic ﬁbres in the subthalamic region: consequences for neurosurgical targeting The present study demonstrates that the pallidothalamic and cerebellothalamic tracts do not to converge in the subthalamic region. We cannot exclude some ﬁbre exchanges between the two tracts, but we can demonstrate that the main bulks are clearly separated. Immunohisto- chemical stainings, especially PV immunoreactivity and AChE, provided additional evidence for a separation between the two tracts. At thalamic level, evidence for a convergence of pallidal and cerebellar ﬁbres onto the same nuclei, although with minor overlap, has been brought by experimental studies in monkeys (Rouiller et al. 1994; Sakai et al. 1996, 1999, 2000). The demonstration at subthalamic level of a separation between cerebello- and pallidothalamic ﬁbres is of major interest for functional neurosurgery. Being able to target a ﬁbre bundle while sparing the other is particularly relevant for the surgical treatments of motor disorders with different underlying pathophysiology, such as PD and ET. The