Adjustable frequency selectivity of auditory forebrain neurons recorded ina 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 song bird may achieve an improved detection ability if the neuronal filters of i ts auditory system could be sharpened to adapt to the spectrum of the backg round noise. To address this issue, we studied 35 multi-unit clusters in th e input layer of the primary auditory forebrain of nine European starlings (Sturnus vulgaris). Microelectrodes were chronically implanted in this song bird's cortex analogue and the neuronal activity was transmitted from unres trained birds via a miniature FM transmitter. Frequency tuning curves (FTCs ) and inhibitory sidebands were determined by presenting a matrix of freque ncy-level combinations of pure tones. From each FTC, the characteristic fre quency (CF) and several parameters describing the neurons' filter character istics were derived and compared to the same recording site's filter functi on while simultaneously stimulating with a continuous CF tone 20 dB above t he response threshold. Our results show a significant improvement of freque ncy selectivity during two-tone stimulation, indicating that spectral filte ring in the starling's auditory forebrain depends on the acoustic backgroun d 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 m illiseconds after response onset. Neuronal filter bandwidths during two-ton e stimulation in the auditory forebrain are in good agreement with psychoac oustically measured critical bandwidths in the same species. Radiotelemetry proved to be a powerful tool in studying neuronal activity in freely behav ing birds. (C) 1999 Published by Elsevier Science B.V. All rights reserved.