Prolonged maturation of cochlear function in the barn owl after hatching.

Cochlear microphonics (CMs), which represent the electrical activity of hair cells, and compound action potentials (CAPs), which represent the activity of the auditory nerve, were recorded from the round window of the inner ear, in owlets aged between 5 and 97 days posthatching, i.e., from soon after hatching to beyond fledgling. At the earliest ages examined, animals showed very insensitive CM and virtually no CAP responses. Thus, hearing in barn owls develops entirely posthatching and the birds appear to be profoundly deaf well into the second week. Thresholds improved gradually after that and CMs reached their adult sensitivity at 5 weeks posthatching at all frequencies. Compound action potential responses appeared progressively later with increasing frequency. Adult neural sensitivity was achieved about 1 week later than for the CM responses at most frequencies, but took until 9-10 weeks posthatching at the highest frequencies (8-10 kHz). This indicates an apex-to-base maturation sequence of neural sensitivity within the cochlea, with a disproportionately long period to maturity for the most basal regions. Compound action potential amplitudes matured even later, at about 3 months posthatching, at all frequencies. This suggests a prolonged immaturity in the temporal synchrony of spiking in the auditory nerve.