Influence of urethane anesthesia on neural processing in the auditory cortex analogue of a songbird.

Functional maps of auditory response areas were derived from multi-unit recordings in the caudal telencephalon of the starling (Sturnus vulgaris L.). A regular grid of recording sites with distances of 200 microns horizontally and 100 microns vertically was placed over the auditory cortex analogue. Within one plane, mapping of auditory responses was first performed in the awake bird and then repeated under urethane anesthesia. The data from both experimental approaches differ considerably. Urethane reduces the spontaneous discharge rate significantly. Under anesthesia, inhibition decreases in all auditory subunits. Excitation is less affected. Eight auditory subcenters were divided into three groups according to the changes in their excitatory responses. In the first group 'on' and sustained excitation changed only weakly. These areas are thought to receive direct inputs from the diencephalon. In the second group, 'on' and sustained excitation are substantially reduced. These subcenters seem to receive projections from other forebrain areas. In subunits of the third group, an increase in sustained excitation is correlated to a decrease in inhibition. Within some specific centers, distinct natural calls, for example the bird's own song, elicit stronger responses under anesthesia than other stimuli.