Abstract:

In budgerigars, as in humans, the detection of amplitude modulation (AM) remains relatively constant as modulation frequency increases while detection of frequency modulation (FM) improves. The point at which FM and AM are equal defines the critical modulation frequency (CMF). The CMF is approximately half the size of the critical band in humans because phase information is lost outside the critical band. At small modulation indices, the power spectrum of FM is almost identical to the power spectrum of AM with the difference being the relative phase of the components. The power spectrum of quasifreqeuncy modulation (QFM) is exactly the same as AM even at high-modulation indices. In this experiment, two budgerigars were trained by operant conditioning to detect AM, FM, and QFM at several modulation rates at three carrier frequencies. Budgerigars show nearly identical thresholds for detecting modulation in FM and QFM tones at low-modulation rates and similar thresholds for detecting modulation in FM, AM, and QFM tones at higher modulation rates. These results argue for an insensitivity to phase differences in budgerigars when they fall outside the frequency bandwidths of the auditory system. [Work supported by NIH Grant Nos. DC-00198 and MH-00982 to RJD.]