We recorded responses of single AN fibers in anesthetized cats to periodic complex tones with a missing fundamental at moderate sound levels. The harmonics were all of equal amplitude (10-70 dB SPL) and spanned a spectral region of 2.2 octaves around a fiber's CF. We used three phase conditions: cosine-phase, Schroeder-phase and alternated sine-cosine phases.

The average discharge rate of a fiber was greater when the CF was a small integer multiple of F0, than when the CF fell between two harmonics. In general, harmonics up to the 5^th were thus resolved in rate responses, although this number decreased at higher stimulus levels. Using data from 12 to 47 single units, we could estimate F0 from rate-vs-CF profiles with errors smaller than 2-3%. However, few reliable estimates were obtained below 400 Hz, due to the broad cochlear tuning at low frequencies in the cat.

We generated "pooled" interspike interval distributions by summing all-order interval histograms from all the sampled fibers. By finding the best fitting periodic template to the pooled distributions, we were able to estimate F0 with error smaller than 1%, up to 1300 Hz. Phase had no obvious effect on either rate-CF profiles or pooled interval distributions.

In summary, both rate-place and interspike-interval codes are viable over a wide range of F0 in the cat, but the intervals code is more useful at lower F0s, and the rate-place code at higher F0s.

Supported by NIH Grant DC02258.