Conditioned enhancement and suppression in the developing auditory midbrain

Neural responses in the adult central auditory system to binaural stimuli c an be altered by preceding acoustic events, including auditory motion. To d etermine whether the juvenile auditory system also exhibits this feature, w e have examined interaural level difference (ILD) processing in the develop ing gerbil. A long binaural stimulus was followed without interruption by m odulation of the level difference (virtual acoustic motion), which in turn was followed smoothly by a new steady state ILD. Auditory responses of sing le neurons in the inferior colliculus (IC) were assessed for sensitivity to the final steady state ILD. The response of El neurons (excited by contral ateral stimulation and inhibited ipsilaterally) was examined at postnatal ( P) days 17-18, P24-25, and in adult animals. In adult animals, a sudden red uction of the inhibitory stimulus level resulted in a long-lasting (median = 4.3 s) enhanced discharge rate (conditioned enhancement). In P17-18 anima ls, conditioned enhancement only lasted for 1.2 s. When the inhibitory stim ulus level was suddenly increased, adult neurons often displayed a conditio ned suppression of discharge rate (median = 4.5 s), whereas P17-18 neurons remained suppressed for a much briefer period (median = 1.2 s). Moreover, t he difference between conditioned responses and control discharge rates was three-four times greater in adult neurons compared to those recorded in P1 7-25 animals. Because conditioned responses are sensitive to the relative balance of cont ralateral excitation and ipsilateral inhibition, we examined the relationsh ip between excitatory and inhibitory thresholds. In adult animals, excitato ry thresholds were an average of 12 dB lower than inhibitory thresholds, wh ile at P17-25 excitatory and inhibitory thresholds were roughly the same, T hese results indicate that computational properties of juvenile and adult I C neurons differ quantitatively, and this may reflect an imbalance between excitation and inhibition. The developmental differences described herein m ay limit the ability of young animals to locate a sound source with the lat ency and accuracy of an adult.