Developmental plasticity in the central auditory system of the Mongolian gerbil

Interaural intensity differences (IIDs) are salient localization cues for high frequency sounds because the head acts as an acoustic barrier to wavelengths shorter than the head's diameter. Because head diameter changes with maturation, IIDs generated by the head also change. The goal of our experiments was to determine if the IID sensitivity of central auditory neurons differed between adult and juvenile animals. In addition, to determine how rapidly IID sensitivity might change, we used a monaural earplug on juvenile animals. We compared 20--25 day-old juvenile with adult gerbils, and we compared normal juveniles (20--25 days old) with juveniles (20--25 days old) that were raised with an earplug.;Average head diameter for juveniles was 18 mm vs. 25 mm for adults. Free-field measures showed that IIDs generated at the head differed for adults and juveniles with substantial IIDs generated above 12 kHz by adult heads and above 18 kHz by juvenile heads. In the central auditory system, many cells are sensitive to IIDs in that they are excited by stimulation of one ear and inhibited by stimulation of the other. Hence, response magnitude depends on the relative strengths of the two inputs, which in turn depends on the sound intensities at the ears. We assessed IID sensitivity of neurons in the inferior colliculus using extracellular recordings. Comparison of IID functions for adult and juvenile cells showed differences that were consistent with the head sizes of the different age groups, suggesting that the cells adjust their sensitivity with maturation.;If IID sensitivity is mediated by experience, then IID sensitive cells in the monaurally occluded animals should acclimate to the altered cues. The earplugs were in place at 10 days, until the day of recording. The results show there were differences in both monaural and binaural responses between the normal juveniles and the plugged juveniles. In general, the results indicate that changes in sensitivity were relatively slower than we expected. Furthermore, both populations included many cells sensitive to IIDs outside of the biologically relevant range of IIDs generated in free field.