Function of the GABA-Ergic Inhibitory System in the Chicken Auditory Forebrain

Abstract

The auditory structures in the caudal auditory telencephalon (CAT) of birds consist of the trilaminated field L and the auditory part of the hyperstriatum ventrale (HV). These substructures share a common tonotopic organization, whereas iso-frequency planes cut perpendicular to all laminae of field L and the HV (Scheich et al., 1979b; Müller and Leppelsack, 1985). The input to the CAT arises from the diencephalic n. ovoidalis and terminates in the central lamina (L2) of field L (Bonke et al., 1979a). From there information is further relayed to the adjacent laminae L1 and L3, as well as to the HV. This hierarchical pattern of interconnectivity is paralleled by an increase in neuronal response selectivity. While cells in the thalamorecipient lamina usually respond to pure tones, about 15 to 20% of the neurons in the postsynaptic structures respond only to complex auditory stimuli, e.g. noise bands or natural sounds (Scheich et al., 1979a; Müller and Leppelsack, 1985). In addition, tone responsive units in the higher order laminae often reject broad band sounds although they contain frequencies that are responded to when presented as pure tones. The increase of response selectivity from the input layer L2 to the postsynaptic structures has been attributed especially to inhibitory interactions in the CAT (Langner et al., 1981; Müller and Leppelsack, 1985).