Discrimination of modulation type (amplitude modulation or frequency modulation) with and without background noise.

These experiments compare the ability to detect amplitude modulation (AM) and frequency modulation (FM) with the ability to discriminate AM from FM, using 10-Hz sinusoidal modulation of a 1000-Hz carrier. Initially, psychometric functions were measured for the detection of AM and FM alone, using a two-alternative forced-choice (2AFC) task. In experiment 1, pairs of values of AM and FM were selected that would be equally detectable, and psychometric functions were measured for the discrimination of AM from FM, again in a 2AFC task. Values of d' for discriminating AM from FM were always lower than the values of d' for detection of the AM or FM. When the detectability of the AM and FM was low (d' = 0.66), two subjects were essentially unable to discriminate AM from FM. This was true both for stimuli presented in quiet and for stimuli presented with continuous noise chosen to mask either the lower or the upper side of the excitation pattern. In experiment 2, subjects were again required to discriminate AM from FM, but the AM depth was fixed within a block of trials, while the FM depth was varied across trials. The discriminability of AM from FM did not show distinct minima at specific FM depths. Again, this was true both for stimuli presented in quiet and for stimuli presented with continuous noise chosen to mask either the lower or the upper side of the excitation pattern. This result suggests that the discrimination of AM from FM was not based on monitoring just one side of the excitation pattern of the carrier.(ABSTRACT TRUNCATED AT 250 WORDS)