Detecting continuous and discrete frequency changes as a function of spectral resolvability and modulation rate

This study measured detection of frequency modulation (FM) and frequency differences in alternating tones with spectrally resolved and unresolved harmonics for modulation rates between 2 and 20 Hz. The hypotheses were that FM detection involves (i) conversion to amplitude modulation through cochlear filtering and (ii) sampling instantaneous frequency at discrete times. Contrary to the first hypothesis, thresholds worsened with increasing modulation rate similarly for both resolved and unresolved harmonics. Thresholds for alternating tones were higher than for FM, particularly at faster modulation rates. Outcomes can be reconciled with existing theories if performance is limited by central, not peripheral, processes.