New Model of Human Red Nucleus Figure 8. WGA-HRP injected sites in macaque’s cerebral cortex (case 5). Upper drawing shows the expansion of the injected WGA-HRP solution indicated by oblique lines in a lateral view of the macaque’s cerebral cortex. Lower drawing shows multiple injection sites (small dots) in the frontal section. A, B and C indicates a-a’, b-b’ and c-c’ in upper drawing, respectively. doi:10.1371/journal.pone.0006623.g008 proper (Gerfen et al., 1982 [26]; Deniau and Chevalier, 1992 [27]) in contrast to the cat’s ND which projects to the rostral MAO (Onodera, 1984 [3]) and receives terminals from the SNr (Onodera and Hicks, 1998 [28]). Therefore, there is no neuroanatomical homology between the rat’s ND (proper) and the feline ND. As the rat’s ventrolateral central gray, ventral to the ND proper, receives terminals from the SNr [27,26] and projects to the inferior olive [23,25], we consider that this region of the ventral central gray corresponds not to the cat’s NB but the cat’s ND. In an autoradiographical study, Saint-Cyr and Courville (1982) [19] and Holstege and Tan (1988) [29] were not able to define a detailed topographic organization within the mesodiencephalo- olivary projections in the cat. By contrast, our previous study in the cat presented findings that showed a precise topographical organization within these olivary projections and a well-developed MTT belonging to the well-developed ND and NB and a poorly- developed CTT belonging to the poorly-developed dm- and vl- pNr (Onodera, 1984 [3], see Fig. 13A). We then confirmed that in cats, the ND projects by way of the MTT, while the Nint and the rostral interstitial nucleus of MLF project only to the MLF (Onodera and Hicks, 1998 [28]). Our data from this study in the cat show that the border among ND, NB and dmpNr has become clear and that this well-restricted NB area strongly projects to the lateral bend of the PO more than to the vl of PO, judging from a comparison of case 1 with case 2. A question of interest at this point was whether quadrupeds other than the cat also have a similar topographically organized olivary projection from their mesodiencephalic regions. Previous studies in the rat (Swenson and Castro, 1983 [30]), macaque (Strominger et al., 1979 [31]) and chimpanzee (Strominger et al., 1985 [32]) suggested that they do. As the so-called ‘‘rat’s rostral ND’’ projects only to the rostral MAO in the rat, as supported by use of the DiI method (Onodera et al., 2004 [24]), we propose that the rat also has a similar topographically precise projection of the mesodiencephalo-olivary system. The macaque, a species that also practises quadrupedal locomotion but has a manipulative hand, possesses a well-developed pNr and a well-developed mNr. Using autoradiographic tracing methods, Strominger et al. (1979) have demonstrated that the lateral and medial parts of the macaque’s pNr projects to the dorsal lamella (dl-) and ventral lamella of (vl-) PO, respectively, and the intermediate part projects to the bend region of PO [31]. In that study medial and intermediate parts of the pNr contain the NB. Our present data from the macaque show that 1) the dmpNr projects to the rostral part of dl of PO in case 3; 2) the vlpNr projects to the caudal part of dl of PO in case 4; and 3) the rostral ND projects to the caudal part of the rostral half of MAO in case 4. Our previous study in the cat (Onodera, 1984 [3]) showed PLoS ONE | www.plosone.org 10 August 2009 | Volume 4 | Issue 8 | e6623