Monaural phase discrimination by macaque monkeys: use of multiple cues

Research examining the discrimination of monaural phase change has suggested that temporal envelope shape, which varies with phase, may be an important cue. Much of that research employed stimuli consisting of three components, a center frequency (Fc), which is varied in phase, and an upper and lower sideband separated from the carrier by some frequency (delta F). As the phase of the center component is varied, both temporal envelope and temporal fine structure change. The present research explored the salience of both envelope and fine structure as cues in a phase discrimination task. Monkeys were trained to report detection of a change from a three-tone complex with 90 degrees starting phase for the center component to one in which the starting phase was smaller. In general, for the values of Fc tested, thresholds for phase change decreased as delta F increased. When tested with comparison stimuli that had a temporal envelope closely matched to that of the standard, but 0 degree starting phase, subjects had difficulty discriminating these stimuli from the standard for smaller delta F, but readily discriminated them at larger delta F values. These findings suggest that temporal envelope is a critical cue in discrimination of three-tone complexes on the basis of the starting phase of the center component at small values of delta F, but that other cues are used at larger delta F values.