Two-tone rate suppression boundaries of cochlear ganglion neurons in normal chickens

The purpose of the present study was to provide a quantitative description of two-tone rate suppression boundaries in normal chickens. The boundaries were measured in 249 cochlear ganglion neurons using a tone 20 dB above threshold at the characteristic frequency (CF). The boundaries were present in 90.4% of neurons either on both sides or only one side of CF but more frequently above CF than below CF. The best suppression thresholds were positively correlated with and, on the average, 19-25 dB higher than CF thresholds. The boundary was farther from CF and shallower below CF than above CF. The boundary slope varied slightly with CF threshold and the tuning curve slope. These results are generally consistent with previous reports from mammals except that: (1) the boundary below CF did not follow and lie above the tuning curve flank; (2) the average best suppression threshold was slightly lower below CF than above CF; (3) the boundaries below and above CF were not particularly asymmetrical.