Investigating the effect of rate on frequency modulation and frequency change detection of harmonic complex tones with resolved or unresolved harmonics

The dual-code theory posits two neural mechanisms for frequency-modulation (FM) detection with pure tones or complex tones with spectrally resolved harmonics: At slow rates, precise neural phase-locking to the tones’ instantaneous frequencies conveys FM; at faster rates, detection is aided by amplitude-modulation (AM) cues, produced via cochlear filtering and FM-to-AM conversion. With unresolved harmonics, FM detection seems based on extraction of the instantaneous fundamental frequency (F0), via phase-locking to the temporal envelope, with no alternative cues available at faster FM rates. To test this theory, FM sensitivity was measured for complex tones with resolved or unresolved harmonics (F0 = 100 Hz) at four rates (2, 5, 10, or 20 Hz) and compared with performance on equivalent F0 change-detection tasks. The theory predicts more degradation with rate for FM and frequency-change detection with unresolved harmonics, and for frequency-change detection with resolved harmonics, than for FM detection with resolved harmonics, where FM-to-AM conversion may aid performance at faster rates. Preliminary results do not fully match these predictions, suggesting instead similar patterns as a function of rate for resolved and unresolved harmonics in each task, but some differences in patterns with rate between the FM-detection and frequency-change-detection tasks. [Work supported by NIH, Grants R21DC019409 and R01DC005216.]