Brain Struct Funct (2008) 212:443–463 453 Fig. 9 a Illustration of the pallidal emergence of al in horizontal and b frontal sections stained for myelin. The levels are 4.5 mm ventral to the intercommissural plane (in a) and 22 mm anterior to pc (in b). Note the relatively large anteroposterior extent of the al at the ventral limit of the GPi in a. Cases Hb5 (left panel) and Hb4 (right panel). Scale bars: 3 mm clearly in contrast with the very intense CR-ir in the sub- lenticular area, associated with the extended amygdala deﬁned by others (Heimer et al. 1997; Alheid 2003), as well as with enhanced CB-ir in the GPi (see Fig. 10c, d). The CR-ir ﬁbre tract ascending into the thalamus at the same level and which is continuous with the sublenticular CR-ir ﬁbres (Fig. 10d) corresponds to the ansa peduncu- laris (ap) known to connect the amygdala to the mediodorsal (MD) and reticular (R) thalamic nuclei. Striatonigral and pallidosubthalamic ﬁbres Two other ﬁbre systems related to the basal ganglia are the pallidosubthalamic and striatonigral ﬁbres, which are part of the so-called ‘‘comb system’’ (Nauta and Mehler 1966). Pallidosubthalamic ﬁbres form the fasciculus subthalami- cus, which was associated with the middle division of the ansa lenticularis of von Monakow (1895) and connects the external pallidum (GPe) to the STh. The tract cannot be distinguished from the ﬂ at its emergence from the GPi, but further posteriorly and medially, can be seen as thick myelin bundles coming into the STh, ventral to the ﬂ and dorsal to al (Figs. 3a, 11a). In contrast to ﬂ and al, which remain largely unstained, some of pallidosubthalamic ﬁbres are characterized by enhanced PV-ir and can be traced through the internal capsule up to the level of the subthalamic nucleus, which itself contains very high level of PV (Figs. 3d, 11d). Interestingly, these PV-ir ﬁbres appear in negative contrast to the thick myelinated ﬁbres seen at the same level (Figs. 3a, 11a). The tract corre- sponding to this position contained very dense degenerated ﬁbres after lesion of GPe (Morel et al. 2001). The large ﬁbre bundles characterized by high CB-ir and AChE staining reaching the substantia nigra (SN) corre- spond to the striatonigral ﬁbres described previously (Morel et al. 2002). In the subthalamic area, these ﬁbres run ventral to the STh and to the fasciculus subthalamicus to continue posteriorly toward the SN, as shown in Fig. 11b. Through not illustrated in the present report, these ﬁbres can be followed from their origin in the stria- tum (mostly from the posterior two-third of the putamen), through the two pallidal segments and the internal capsule, to their entry in the pars reticulata (SNr). The striatonigral ﬁbres are also enhanced in SMI-32-ir, while only moder- ately for PV- and CR-ir (Figs. 4c, e; 11d). They appear largely unstained in myelin (Fig. 11a), which is presum- ably related to their smaller size. MRI correlations High-resolution proton density MRI was obtained from one brain (Hb5, Table 1; see also MRI acquisition protocole in ‘‘Materials and methods’’) prior to guillotine sectioning and histological processing. After MRI acquisition, one hemisphere was cut in the frontal plane and the other in the horizontal plane. The MRI slices (1.3-mm thick) were correlated with the closest myelin stained sections and drawings of the same brain. Examples of these correlations are shown on frontal sections in Figs. 7 and 8, at different anteroposterior levels of the thalamus. On these high- resolution MRI, several thalamic and basal ganglia subdi- visions can be well recognized: the striatum (PuT, Cd), both segments of the pallidum (GPe/GPi) separated by the internal medullary lamina (Fig. 8, A20–A15), medial, 123