CONNECTIONS OF THE AUDITORY FOREBRAIN IN THE PIGEON (COLUMBA-LIVIA)

Ascending auditory efferents in birds terminate mainly within Field L2 , a cytoarchitectonically distinct region of the caudomedial telenceph alon. The organization of Field L2, and that of its flanking regions, Ll and L3, was investigated with 14C-2-deoxyglucose (14C-2-DG), cytoch rome oxidase, and both retrograde and anterograde tracing techniques. Field L2 was found to contain a high concentration of cytochrome oxida se. Following auditory stimulation, 14C-2-DG autoradiography revealed that Field L2 consists of two adjacent but seemingly discontinuous zon es, designated Field L2a, which lies ventromedially, and Field L2b, wh ich lies dorsolaterally. Termination of thalamic efferents: The thalam ic auditory nuclei ovoidalis (Ov) and semilunaris parovoidalis (SPO) p roject predominantly upon Field L2, and possibly sparsely upon L1, L3 and the overlying hyperstriatum ventrale (HV). Ov subnuclei project up on L2a and SPO projects predominantly upon L2b. The topography of the projections is inverted along the ventromedial-to-dorsolateral axis of L2, and is in accord with an inverted tonotopic representation of fre quencies; high frequencies (< 3.5 kHz) being found in the more ventrom edial parts of L2a, and low frequencies and broad band responses in L2 b. Intra- and extratelencephalic connections: Field L2a also receives a substantial projection from HV, but the efferent projections of L2a appear confined to adjacent ''neostriatal'' regions. The subsequent pr ojections of L2b were not identified in this study. L1 and L3 project predominantly to the dorsal neostriatum (Nd) caudolateral to Field L, and have fewer projections to the caudomedial paleostriatum and anteri or hyperstriatum accessorium. Nd projects massively upon the ventromed ial nucleus of the intermediate archistriatum (Aivm), which has bilate ral projections upon the caudomedial telencephalon and is the origin o f a major descending pathway having dense terminations surrounding the ovoidalis complex (Ov and SPO), MLd, the lateral lemniscal nuclei, an d sparse terminations within SPO itself. It is suggested that within t he telencephalon the major components of the auditory pathway consist of cell groups which collectively correspond to the populations of neu rons found within the auditory cortex of mammals. (C) 1993 Wiley-Liss, Inc.