Abstract:

Phase-locking responses to long sinusoidally amplitude-modulated best-frequency tones were recorded in midbrain auditory units of the curarized grass frog. The reproduction of the periodic AM (with modulation depths in the range 10%--30% and modulation frequencies in the 20- to 100-Hz range) was studied in the presence of low-frequency (0--50 or 0--150 Hz) noise added to the envelope waveform. The remarkable steadiness of periodicity extraction was observed in the presence of a low-frequency masker. Moreover, in some units, both the average firing rate and the phase-locking with a periodical modulation increased in the presence of noise. The last effect was especially pronounced when the modulation frequency (70--100 Hz) was located outside of the masker frequency band (0--50 Hz). The responses of torus semicircularis units to the best frequency tone amplitude modulated by a low-frequency two-tone complex were also studied. The typical observation was an increase in the synchronization index for the modulation frequency in the 20- to 40-Hz range with a low-frequency modulation (in 2- to 5-Hz range) being added to the signal. For both effects, the enhancement was even more pronounced if the Rayleigh statistic which took into account average firing rate was computed.