Figure 2.

Schematic summary comparing the neuronal circuitry of auditory pathways in the dorsal ventricular ridge of the avian telencephalon and the equivalent neocortical circuit of the mammalian auditory cortex. In the avian forebrain, the populations of neurons corresponding to the individual laminae of cortex are organized as clusters, rather than as laminae. This circuit in the mammal is represented as a simplified three-layered cortex, consisting of a layer of thalamic recipient neurons (blue) forming layer IV, a group of interneurons (yellow) forming the more superficial layers, and a group of descending telencephalic efferents (DTEs) (red), the output neurons of this cortical region, forming layers V and VI. The morphology of individual neurons, their transmitters, and physiological properties at each parallel step of the circuit are virtually identical in bird dorsal ventricular ridge and mammalian cortex (6).