Representational Geometries of Telencephalic Auditory Maps in Birds and Mammals

The evolution of the mammalian as well as of the bird brain is highlighted by the parallel acquisition of a voluminous telencephalon. Yet the routes of morphological forebrain differentiation taken by the two forms are strikingly divergent. Birds lack a true cortex with its specific cell types and a segregation of gray and white matter areas is not conspicuous. Instead the telencephalic mass is entirely composed of basal forebrain nuclei and multiple and thick dorsal layers of stellate cells, the latter being separated by several thin fibrous laminae and crossed by diffuse fiber tracts. In the layers of this dorsal roof, traditionally misnamed “striatum”, local variations of cytoarchitecture are present but rarely with sharp boundaries. Only with modern connectivity studies it has been recognized over the past three decades that in spite of this low degree of overt organization the bird telencephalon follows a plan very similar to that of mammalian forms. This covers functional subsystems (including neocortical equivalents) and their intratelencephalic connections as well as connections with subtelencephalic structures (Karten, 1969). It is the aim of this contribution to characterize the functional organization of the auditory cortex analogue, Field L, of birds and to compare it to auditory cortex in the mongolian gerbil (Meriones unguiculatus). Field L of birds from various families has been studied in greater detail over the years in this laboratory and the gerbil has been chosen recently as a mammalian model which allows specific comparisons with bird auditory systems. These species share low frequency hearing with most space in the telencephalic auditory maps devoted to the analysis of frequencies below 10 kHz. In chicks and gerbil there seems to be no specialization for communication sounds greatly distorting the spatial organization of their auditory maps.