Differential effects of age on click-rate and amplitude modulation-frequency coding in primary auditory cortex of the cat.

Recordings were made from 185 neurons in the primary auditory cortex of cats in the age range of 15 to 297 days. A comparison was made between the tuning for click repetition-rate and for amplitude modulation-frequency of a noise burst on the basis of temporal Modulation Transfer Functions (tMTF). 90 of the 185 units had a clear band-pass type tMTF for both repetition rate and modulation frequency, there was, however, no correlation between the respective Best Modulation Frequencies (BMF). Amplitude modulated noise (AMnoise) was the more effective stimulus in young kittens while click-train stimulation was more effective in adult cats. For all neurons with significant synchronization, BMFs for both click-train and AMnoise increased with age from about 4 Hz in kittens younger than 30 days to about 10 Hz in adult cats. In the approximately 50% of the neurons that were tuned both to click rate and modulation frequency, however, the BMF to AMnoise was consistently and significantly higher than that for clicks. In this group the mean BMF for kittens younger than 30 days were 7.94 Hz for AMnoise and 3.29 Hz for clicks and in the adults 10.91 Hz for AMnoise and 7.71 Hz for clicks.