Mechanisms of masking by Schroeder-phase harmonic tone complexes in the budgerigar (Melopsittacus undulatus)

Schroeder-phase harmonic tone complexes can have a flat temporal envelope and either rising or falling instantaneous-frequency sweeps within periods of the fundamental frequency (F0), depending on the phase-scaling parameter C. Human thresholds for tone detection in a concurrent Schroeder masker are 10-15 dB lower for positive C values (rising frequency sweeps) compared to negative (falling sweeps), potentially due to the impulse response of cochlear filtering, though this hypothesis remains controversial. Birds provide an interesting animal model for studies of Schroeder masking because prior reports suggest less behavioral threshold difference between maskers with opposite C values. However, most behavioral studies focused on relatively low masker F0s, and neurophysiological mechanisms in birds have not been explored. We performed behavioral Schroeder-masking experiments in budgerigars (Melopsittacus undulatus) using a wide range of masker F0 and C values. The signal frequency was 2800 Hz. Neural recordings at the midbrain processing level characterized encoding of behavioral stimuli in awake animals. Behavioral thresholds increased with increasing masker F0 and showed minimal difference between opposite C values, consistent with prior studies. Neural recordings showed prominent temporal and rate-based encoding of Schroeder F0, and in many neurons, marked response asymmetry between Schroeder stimuli with opposite C values. Neural thresholds for Schroeder-masked tone detection were (1) in most cases based on a response decrement compared to the masker alone, consistent with prominent modulation tuning in midbrain neurons, and (2) generally similar between opposite masker C values. These results highlight the likely importance of envelope cues in behavioral studies of Schroeder masking.