Frequency modulation versus amplitude modulation discrimination: Evidence for a second frequency modulation encoding mechanism

TLDR: The encoding mechanisms for amplitude modulation (AM) and frequency modulation (FM) were investigated using AM-FM discrimination tasks and the fact that minima in the discrimination psychometric functions increase from d' = 0 as beta increases indicates that the information encoded by the second mechanism becomes more detectable with increasing beta.

Abstract: The encoding mechanisms for amplitude modulation (AM) and frequency modulation (FM) were investigated using AM–FM discrimination tasks In the first experiment, AM and FM were set at equally detectable levels within a trial, and discrimination thresholds were obtained adaptively in a 3IFC task Here, AM–FM discrimination thresholds were considerably larger than both AM and FM detection thresholds This is consistent with an encoding system whereby AM and FM are partially encoded by the same mechanism In the second experiment, performance on AM–FM discrimination is measured with a fixed‐level procedure Psychometric functions obtained for a constant modulation depth of AM were nonmonotonic with FMs modulation index β and each displayed a single minimum The nonmonotonic nature of the functions is consistent with a model in which FM is encoded primarily with the same mechanism that encodes AM but also with a second mechanism, probably related to changes in instantaneous frequency, that is independent of th