TITLE: 
Frequency modulation versus amplitude modulation discrimination:
evidence for a second frequency modulation encoding mechanism.
AUTHORS: 
Edwards BW; Viemeister NF
AUTHOR AFFILIATION: 
Department of Psychology, University of Minnesota, Minneapolis 55455.
SOURCE: 
J Acoust Soc Am 1994 Aug;96(2 Pt 1):733-40
CITATION IDS: 
PMID: 7930074 UI: 95015292
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 beta 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 the mechanism that
extracts AM. 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.
MAIN MESH HEADINGS: 
*Auditory Perception