Adjustable frequency selectivity of auditory forebrain neurons recorded in a freely moving songbird via radiotelemetry.

One of the hearing system's basic properties that determines the detection of signals is its frequency selectivity. In the natural environment, a songbird may achieve an improved detection ability if the neuronal filters of its auditory system could be sharpened to adapt to the spectrum of the background noise. To address this issue, we studied 35 multi-unit clusters in the input layer of the primary auditory forebrain of nine European starlings (Sturnus vulgaris). Microelectrodes were chronically implanted in this songbird's cortex analogue and the neuronal activity was transmitted from unrestrained birds via a miniature FM transmitter. Frequency tuning curves (FTCs) and inhibitory sidebands were determined by presenting a matrix of frequency-level combinations of pure tones. From each FTC, the characteristic frequency (CF) and several parameters describing the neurons' filter characteristics were derived and compared to the same recording site's filter function while simultaneously stimulating with a continuous CF tone 20 dB above the response threshold. Our results show a significant improvement of frequency selectivity during two-tone stimulation, indicating that spectral filtering in the starling's auditory forebrain depends on the acoustic background in which a signal is presented. Moreover, frequency selectivity was found to be a function of the time over which the stimulus persisted, since FTCs were much sharper and inhibitory sidebands were largely expanded several milliseconds after response onset. Neuronal filter bandwidths during two-tone stimulation in the auditory forebrain are in good agreement with psychoacoustically measured critical bandwidths in the same species. Radiotelemetry proved to be a powerful tool in studying neuronal activity in freely behaving birds.