Factors influencing the salience of temporal cues in the discrimination of synthetic Japanese monkey (Macaca fuscata) coo calls

If temporal position of a frequency inflection is the most salient communication cue in Japanese macaque smooth early and smooth late high coos, then macaques should perceive coos differing only along the early-late dimension as belonging to different classes. The perceived similarity of synthetic coos and temporally reversed variants were evaluated, using multidimensional scaling of macaque discrimination latencies. Original calls and calls temporally reversed in the frequency domain could be discriminated if the peak was near a call endpoint but not if the frequency peak in the original call was near the coo midpoint. Perceived similarity of such calls was inversely related to the amount of frequency modulation. Temporal reversals of amplitude contours were also conducted. Although macaques are quite sensitive to amplitude increments, reversal of the relatively flat amplitude contours of these calls did not affect discrimination responses.     

Detection thresholds for intensity increments in a single harmonic of synthetic Japanese macaque (Macaca fuscata) monkey coo calls

There is evidence that Japanese macaques (Macaca fuscata) are extremely sensitive to dynamic changes in the relative amplitudes of coo call harmonics during discrimination tests. To verify this evidence using more controlled stimulus configurations, the authors examined threshold sensitivity of macaque monkeys to amplitude increments added to the standard level of coo call harmonics. Psychophysical threshold determination methods paralleled those used previously to determine macaque sensitivity to amplitude increments added to vowel-like stimuli. Variation was detectable although thresholds were elevated relative to those obtained with vowel-like stimuli in another investigation (C. G. Le Prell, A. J. Niemiec, & D. B. Moody, 2001). This elevation was probably a function of natural amplitude modulation in the standard stimuli.