Masking patterns and filter characteristics of auditory nerve fibers in the green treefrog (Hyla cinerea)

Abstract

1. Absolute thresholds to tone and noise bursts were measured for 177 single auditory fibers in the eighth nerves of 8 adult green treefrogs (Hyla cinerea). The lowest tonal thresholds were found between 300 and 600 Hz (at about 30 dB SPL) and lowest noise thresholds between 500 and 1,200 Hz (at about 10 dB spectrum level, noise bandwidth 20–10,000 Hz). 2. Masking effects of continuous broad-band noise on the response to tone bursts were studied using rate-intensity and latency-intensity curves and PST-histograms. The dynamics of masking can be described by two noise levels, one at which a fiber starts responding to background noise (in the presence of tone bursts), and the other at which the response of a fiber is totally dominated by noise (the response to the tone bursts is masked). This ‘dynamic range of masking’ varies around 20 dB measured in rateintensity curves and around 10 dB in latency-intensity curves. 3. The fibers are divided into four response types according to their response rate change in the masking paradigm. The differences can be explained by different influences of summation, adaptation and suppression in the four groups of fibers. 4. Masked thresholds of the fibers were used for calculation of the masking susceptibility, which is equal to the bandwidth of the critical ratio (CR-band) filter of a fiber. Fibers with best excitatory frequencies (BEFs) below 700 Hz have CR-bandwidths mostly between 10 and 100 Hz, those with higher BEFs have CR-bands mostly between 50 and 1,000 Hz. CR-bands were found in nerve fibers originating in both hearing organs (amphibian and basilar papilla) in treefrogs. 5. The neural CR-bandwidths are significantly related to the sharpness of tuning (Q10-values) and they agree well with the 3 dB bandwidths of the tuning curves of the respective fibers. 6. The mean neural CR-bandwidths near 900 and 3,000 Hz mid-frequency are very similar to those behaviorally measured at these frequencies. This similarity suggests that the mechanism of the CR-band (critical band) filter is of peripheral origin and may be identical with the filtering process leading to the frequency tuning curve of a single fiber in the auditory nerve.