Vocal-Tract Resonances as Indexical Cues in Rhesus Monkeys

Abstract

SummaryVocal-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.

Zebra finches exhibit speaker-independent phonetic perception of human speech

“…Rhesus monkeys use formants in species-specific vocalizations as indexical cues as well (Ghazanfar et al 2007) and although not many studies have investigated similar phenomena in bird vocalizations it has been shown that whooping cranes, for example, can perceive changes in formant frequencies in their own species calls and exhibit a different response pattern to calls with higher formants compared with lower formants (Fitch & Kelley 2000). These results led to the speculation that formant perception originally emerged in a wide range of species to assess information about the physical characteristics of conspecifics, and that human speech has exploited the already existing sensitivity for formant perception (Fitch 2000;Ghazanfar et al 2007). It can, of course, not be ruled out completely that unique perceptual abilities to facilitate speech perception did evolve in humans, or that the observed abilities evolved separately in birds and humans.…”

The effect of varying talker identity and listening conditions on gaze behavior during audiovisual speech perception

“…Identity information is present in both the face and the voice and recently there has been considerable interest in the ability of both humans and animals to use this information (e.g., Ghazanfar et al, 2007). Individuals can estimate body size, age and gender from both faces and voices.…”

“…This would provide an account for the considerable fixations made to the eyes during audiovisual speech experiments (e.g. Vatikiotis-Bateson, Eigsti, Yano & Munhall, 1998;Paré, Richler, ten Hove & Munhall, 2003;Lansing & McConkie, 2003) even though the eyes are not a particularly informative facial feature for perceiving speech sounds.Identity information is present in both the face and the voice and recently there has been considerable interest in the ability of both humans and animals to use this information (e.g., Ghazanfar et al, 2007). Individuals can estimate body size, age and gender from both faces and voices.…”

Vocal Tract Morphology Determines Species-Specific Features in Vocal Signals of Lemurs (Eulemur)

“…Formants also correlate with vocal tract length in mammals (Canis familiaris: Riede and Fitch 1999;Cervus elaphus: Reby and McComb 2003) and may represent an indexical cue (Ghazanfar et al 2007) for receivers, thus providing information related to the physical characteristics of the emitter.…”