444 ﬂ frf ft fx GPi GPe Hip Hyp ic IC iml LD LGN Li LP M1 MB MD (pc, mc, pl) MGN ml mtt MV ot PAG pc Pf PF PMc PMr Po pre-SMA PTT PuA PuM PuT PV R RN SC SG sm SMA-proper SNc SNr sPf STh stt VA (pc, mc) VLa 123 Fasciculus lenticularis Fasciculus retroﬂexus Fasciculus thalamicus Fornix Globus pallidus, internal segment Globus pallidus, external segment Hippocampus Hypothalamus Internal capsule Inferior colliculus Internal medullary lamina Lateral dorsal nucleus Lateral geniculate nucleus Limitans nucleus Lateral posterior nucleus Primary motor cortex Mammillary body Mediodorsal nucleus (parvocellular, magnocellular, and paralamellar divisions) Medial geniculate nucleus Medial lemniscus Mammillothalamic tract Medioventral nucleus Optic tract Peri-aqueductal gray Posterior commissure Parafascicular nucleus Prefrontal cortex Premotor cortex, caudal part Premotor cortex, rostral part Posterior nucleus Pre-supplementary motor area Pallidothalamic tractotomy Anterior pulvinar Medial pulvinar Putamen Parvalbumin Reticular thalamic nucleus Red nucleus Superior colliculus Suprageniculate nucleus Stria medullaris Supplementary motor area Substantia nigra, pars compacta Substantia nigra, pars reticulata Subparafascicular nucleus Subthalamic nucleus Spinothalamic tract Ventral anterior nucleus (parvocellular and magnocellular divisions) Ventral lateral anterior nucleus VLp (d, v, pl) VM VPI VPL (p) VPM VPMpc ZI Introduction Brain Struct Funct (2008) 212:443–463 Ventral lateral posterior nucleus (dorsal, ventral, and paralamellar divisions) Ventral medial nucleus Ventral posterior inferior nucleus Ventral posterior lateral nucleus (posterior division) Ventral posterior medial nucleus Ventral posterior medial nucleus, parvocellular division Zona incerta The surgical treatment of motor disorders such as Parkin- son’s disease (PD) and essential tremor (ES) has evolved over the years, targeting alternatively the motor thalamus, the basal ganglia, and more rarely, subthalamic area. The different approaches are based on extensive knowledge of the motor circuitry and of the pathophysiology known from animal models, such as the MPTP monkey (DeLong 1990; Obeso et al. 2000; Wichmann and DeLong 2003), and peroperative recordings in patients undergoing surgery (Lenz et al. 1994; Jeanmonod et al. 1996; Lozano et al. 1997; Hutchison et al. 1998; Vitek et al. 1998; Magnin et al. 2000, 2001a; Zonenshayn et al. 2000; Garonzik et al. 2002; Kobayashi et al. 2003). The two major motor path- ways involved in PD and ET are illustrated schematically in Fig. 1. Their principal access to the thalamus and thal- amocortical network is through the pallidothalamic and cerebellothalamic tracts, respectively. The pallidothalamic tract is composed of the ansa lenticularis (al) and of the fasciculus lenticularis (ﬂ, coursing through ﬁeld H2 of Forel), both take their origin in the internal part of the pallidum (GPi). The two ﬁbre tracts merge into the fas- ciculus thalamicus (ft) (or ﬁeld H1 of Forel) before entering the thalamus. The cerebellothalamic tract connects the deep cerebellar nuclei (dentate, interposed, and fastigial nuclei) with the thalamus and courses through the superior cerebellar peduncle, decussates, and passes through and anterior to the red nucleus before reaching the thalamus. The current knowledge on the pallidothalamic and cerebellothalamic connections is dominantly derived from data in monkeys. Degeneration studies and the use of modern tracing techniques with multiple anterograde and retrograde tracers provided much information about the origin of these ﬁbre tracts and their projections to the thalamus and thalamocortical circuits (Nauta and Mehler 1966; Carpenter et al. 1976; Asanuma et al. 1983a, b; Parent and De Bellefeuille 1983; Ilinsky et al. 1985; Per- cheron et al. 1993, 1996; Rouiller et al. 1994; Inase and