Response patterns and their relationship to frequency analysis in auditory forebrain centers of a songbird

In the field L complex, the auditory part of the caudal telencephalon, multi-unit recordings were performed in seven awake, adult male starlings (Sturnus vulgaris, L). Pure tones in a frequency range between 0.5 and 6.0 kHz were used as stimuli. The field L complex of starlings consists of at least 11 functionally separated, tonotopically organized subcenters. The auditory processing of frequency information was investigated in eight of these areas. Two kinds of response patterns could be distinguished. The centers NA-L, NA2a, NA3 and NA4 showed phasic and sustained excitation, the other areas responded with phasic excitation only. All these auditory areas show strong tonotopic gradients, each of them representing the complete hearing range. The sharpest picture of the stimulus frequency is represented in the functional area NA-L. In relation to its total size, NA-L shows the smallest active area if stimulated by pure tones. In addition, in NA-L, only the excited neurons are surrounded by inhibited neurons during the response to a pure tone. This leads to an additional sharpening of frequency representation. In comparison with the other auditory areas, NA-L shows the greatest spatial extension of the tonotopic gradient. This, in combination with the smallest active size in NA-L, leads to the conclusion that in the primary projection field NA-L, the most neural space is available for the processing of a given frequency range. (C) 1999 Elsevier Science B.V. All rights reserved.