“…Auditory-evoked potentials and multi-unit activity have been recorded in the core and shell regions of auditory nuclei, suggesting that both regions process auditory information [reptiles: Khachunts, 1982;Khachunts and Belekhova, 1986;birds: Durand et al, 1992;Cheng and Havens, 1993;mammals: Syka et al, 2000]. There are, however, substantial differences between core and shell regions, specifically in electrophysiological properties [reptiles: Khachunts, 1982;Khachunts and Belekhova, 1986;birds: Wild et al, 1990;Durand et al, 1992;mammals: Aitkin and Webster, 1972;Tawil et al, 1983;Morest and Oliver, 1984;Huffman and Henson, 1990], cytoarchitecture [reptiles: Balaban and Ulinski, 1981;birds: Durand et al, 1992;mammals: Morest and Oliver, 1984;Huffman and Henson, 1990], and neural connections or neurochemistry [reptiles: Pritz, 1974;Balaban and Ulinski, 1981;Brauth and Reiner, 1991;Pritz and Stritzel, 1992;Belekhova et al, 2002;birds: Brauth and Reiner, 1991;Durand et al, 1992;Zeng et al, 2004;mammals: Tawil et al, 1983;Morest and Oliver, 1984;Ledoux et al, 1985;Huffman and Henson, 1990]. These data lead to two interesting questions: (1) What is the evolutionary relationship between the core and shell regions of the auditory nuclei?, and (2) How does the core/ shell organization evolve?…”