| Popis: |
Schroeder-phase harmonic tone complexes have a flattened temporal envelope and either rising or falling instantaneous frequency glides within F0periods, depending on the polarity of the phase relationship between harmonics. Human detection thresholds for a pure-tone target added to a Schroeder masker can be 10–15 dB lower for the negative Schroeder (upward gliding) compared to positive (downward gliding), possibly due to human cochlear mechanics though this hypothesis remains controversial. In contrast, several avian species show minimal behavioral masking differences between Schroeder polarities for F0s of 50 and 100 Hz. To gain further insight into the mechanisms of Schroeder masking, we performed operant-conditioning experiments in budgerigars, a parakeet species, including F0s up to 400 Hz that are potentially better matched to avian cochlear mechanics. Awake neurophysiological recordings at the midbrain processing level characterized neural encoding of behavioral test stimuli. Behavioral Schroeder masking thresholds decreased with increasing F0 and showed minimal difference between Schroeder polarities even at high F0s. Neural recordings showed prominent temporal and rate-based encoding of F0 periodicity and, in some neurons, marked response asymmetries between Schroeder polarities. These results suggest that neural response asymmetries between Schroeder polarities need not produce large behavioral masking differences for pure-tone targets. |
