Formant frequency discriminations by Japanese monkeys.

Abstract

The studies reported in this dissertation investigated the general characteristics and significant features of formant frequency and intensity discriminations by the Japanese macaque (Macaca fuscata). The first study compared thresholds for discriminating frequency and intensity changes in pure-tones and single-formants. Difference limens (DL's) for detecting increases in the center frequency of the first (F1) or second (F2) formant were significantly less than corresponding pure-tone frequency thresholds. In contrast, DL's for intensity shifts in pure-tones and single-formants were almost identical. In the second study, additional formants and frequency transitions were added to the single-formant stimuli. The multi-formant signals were first synthesized without frequency modulations and with F1 and F2 equal in intensity. After DL's were obtained for this condition, formant amplitudes were made appropriate for the vowel/ae/, as in "bat". Finally, to more closely approximate the spectral characteristics of naturally produced vowel tokens, formant frequency transitions were added to the fundamental and one of the first two formants. Formant frequency and intensity thresholds were unchanged relative to those for single F1 and F2 stimuli, following the addition of formants and frequency transitions. The third experiment investigated the importance of changes in the starting phase and relative harmonic level for mediating formant discriminations. Removing phase as a discriminative cue did not significantly alter formant frequency DL's. Next, the amplitude spectra of all stimuli were equated except within a restricted frequency bandwidth containing between 1 and 5 harmonics of the fundamental. The results of this study demonstrated that when discriminating formant frequency increases, nonhuman primates selectively attend to restricted frequency regions and rely on detecting relative amplitude changes in 1 or 2 harmonics within these regions. In the final experiment, thresholds were determined for detecting relative amplitude changes in single harmonics of multi-component stimuli modeled after the vowel-like complexes. For stimuli with both 20 and 3 harmonics, thresholds for discriminating increments in the relative level of individual components near formant center frequencies were generally between 1 and 2 dB. Initial results suggest that decrement thresholds may be somewhat elevated compared to those obtained for increments.