Fig. 3.

Frontal aspect of 3-D reconstructions with the MNTB marked in red, the MSO in black, and the LSO in blue. The rostro-caudal-dorso-ventral plane labels the idealized median line of the auditory brainstem. Examples were given for three species. Both the MNTB and, to a smaller extent, the LSO decreased from macaque to bonobo despite the increasing brain sizes. The MNTB separated from the MSO and the former compact MNTB decayed during phylogeny. The orangutan is the link between the condensed SOC of the macaca and the split one of the bonobo. The pearl-like pattern of bonobo MNTB pointed to that development. MNTB, medial nucleus of the trapezoid body; MSO/LSO, medial/lateral superior olive; SOC, superior olivary complex