Functional properties of auditory-nerve fibers during postnatal development in the kitten

The discharges of the auditory-nerve fibers were studied in kittens between 2-40 days of age. Up to the 10th postnatal day, fibers could be divided into two main categories: fibers with spontaneous activity (SA) that respond to sound and fibers without SA but with evoked responses. A third, smaller, category, fibers having neither SA nor evoked activity, was also present. The development of SA comprises two phases. The first, lasting from birth up to the third postnatal week, shows a relatively fast increase and the second, lasting up to adulthood, a slower increase. Typical tone burst responses can be recorded at the end of the first postnatal week. Thereafter reactivity steadily increases especially after the 10th postnatal day. In young animals, rate level function is characterized by a steep segment with a low dynamic range followed by a decrease in activity that lasts until the end of the second week. At this point adult-like functions may be observed, although maximal firing still increases for some weeks. Tuning curves and threshold sensitivity tend to develop inversely at corresponding frequencies. Fibers with low characteristic frequencies reach adult threshold before that of high frequency fibers and high frequency fibers reach adult tuning before low frequency fibers. A comparison of auditory-nerve fiber activity in kittens show that maturation of most functional characteristics lasts several weeks after birth and in some cases continues after the first postnatal month.