Neocortical-Like Organization of Avian Auditory ‘Cortex’

brous bands. In addition, some of these ‘layers’ in the avian Wulst and anterior DVR can be subdivided into narrower bands of cells that receive sensory-specific thalamic afferents . We emphasize this latter point because it was Karten’s pioneering work on the telencephalic projections of specific auditory and visual thalamic nuclei [Karten, 1968; Karten and Hodos, 1970] that underlies the data presented by Wang et al. [2010] on the layering and interlaminar connectivity of the auditory field L complex and associated structures in chicks. Some of this layering in the field L complex in chicks had already been noted and functionally explored electrophysiologically and with 2-deoxyglucose in the 1980s and 1990s [Scheich, 1983; Heil and Scheich, 1985, 1991, 1992], with the important finding being that frequency-specific bands of activity could be demonstrated perpendicular to and across all the layers of the auditory nidopallium and adjacent mesopallium. Anatomical tracing studies in guinea fowl and pigeon defined some of the interconnections between the thalamorecipient layer of the field L complex (L2) and the adjacent layers L1 and L3, with further connections to and from the caudomedial mesopallium (CM) and projections to the dorsal nidopallium [Bonke et al., 1979; Wild et al., 1993]. Further morphological and tracing studies extended these findings to define a complicated set of auditory interconnections in a vocalThe recent paper by Wang et al. [2010] is noteworthy in that it constitutes the first detailed anatomical evidence bearing on an influential proposal made by Karten over 40 years ago [Karten, 1969; Karten and Shimizu, 1989; Karten, 1991; Shimizu and Karten, 1991; Karten, 1997] regarding the organization and phylogeny of the avian telencephalon. In essence, that proposal suggested that large parts of the avian pallial telencephalon, namely the sagittal elevation (Wulst) and the dorsal ventricular ridge (DVR), are made up of collections of what are apparently non-laminated clusters of neurons that, nevertheless, correspond in their function and connections with identifiable populations of neocortical neurons in mammals; the defining characteristics of neocortex, of course, being the presence of six neuronal layers possessing a radial organization of functional columns, dependent on their specific interlaminar connectivity. Birds and reptiles do not possess a neocortex in this sense, i.e. nowhere in the sauropsid pallium is there evidence of a hexalaminated structure comparable with the neocortex of mammals, although the Wulst and some parts of the DVR are clearly layered in the sense that there are stacked bands of neurons (although considerably wider than neocortical layers) having characteristic size, morphology, packing density and gene expression, for example, and are separated from each other by narrower, fiPublished online: October 2, 2010