Vocal-tract resonances as indexical cues in rhesus monkeys

Abstract

Vocal-tract resonances (or formants) are acoustic signatures in the voice and are related to the shape and length of the vocal tract. Formants play an important role in human communication, helping us not only to distinguish several different speech sounds [1], but also to extract important information related to the physical characteristics of the speaker, so-called indexical cues. How did formants come to play such an important role in human vocal communication? One hypothesis suggests that the ancestral role of formant perception--a role that might be present in extant nonhuman primates--was to provide indexical cues [2-5]. Although formants are present in the acoustic structure of vowel-like calls of monkeys [3-8] and implicated in the discrimination of call types [8-10], it is not known whether they use this feature to extract indexical cues. Here, we investigate whether rhesus monkeys can use the formant structure in their "coo" calls to assess the age-related body size of conspecifics. Using a preferential-looking paradigm [11, 12] and synthetic coo calls in which formant structure simulated an adult/large- or juvenile/small-sounding individual, we demonstrate that untrained monkeys attend to formant cues and link large-sounding coos to large faces and small-sounding coos to small faces-in essence, they can, like humans [13], use formants as indicators of age-related body size.

Figure 1

Auditory and Visual Stimuli Used in the Current Experiments (A) Resynthesized coo vocalizations based on one of the two coo exemplars used in the preferential-looking paradigm. Diagram shows the spectrograms and waveforms of a coo vocalization resynthesized with two different vocal-tract lengths. The arrow in the spectrogram indicates the position of an individual formant, which increases in frequency as the apparent vocal-tract length decreases. (B) Power spectra (black line) and linear predictive coding spectra (gray lines) for the long vocal-tract length (10 cm, top panel) and short vocal-tract length (5.5 cm, bottom panel) used in the experiment and seen in (A). (C) Still frames extracted from the videos used in the preferential-looking experiments. The top row shows frames from the large monkey. Videos were synchronized and edited so that they appeared to be synchronously producing the coo vocalization shown in (A).

Figure 2

Monkeys Match the Acoustic Size Extracted from Formant Frequencies to the Matching Face (A) The mean percentage of total looking time spent looking at the matching video display; the dotted line indicates chance expectation (n = 24). Error bars represent the standard error of the mean. (B) A significant proportion of subjects looked longer at the match than the nonmatch screen.