Abstract
The extent to which human speech perception evolved by taking advantage of predispositions and pre-existing features of vertebrate auditory and cognitive systems remains a central question in the evolution of speech. This paper reviews asymmetries in vowel perception, speaker voice recognition, and speaker normalization in non-human animals – topics that have not been thoroughly discussed in relation to the abilities of non-human animals, but are nonetheless important aspects of vocal perception. Throughout this paper we demonstrate that addressing these issues in non-human animals is relevant and worthwhile because many non-human animals must deal with similar issues in their natural environment. That is, they must also discriminate between similar-sounding vocalizations, determine signaler identity from vocalizations, and resolve signaler-dependent variation in vocalizations from conspecifics. Overall, we find that, although plausible, the current evidence is insufficiently strong to conclude that directional asymmetries in vowel perception are specific to humans, or that non-human animals can use voice characteristics to recognize human individuals. However, we do find some indication that non-human animals can normalize speaker differences. Accordingly, we identify avenues for future research that would greatly improve and advance our understanding of these topics.
Highlights
The answer to how humans perceive speech has eluded researchers for over half a century (Jusczyk and Luce, 2002; Samuel, 2011)
Many researchers have adopted the general auditory approach outlined by Diehl et al (2004), which is a framework for the idea that human speech perception is achieved via general learning mechanisms and auditory principles common to humans and animals
From a general auditory approach, categorical perception occurs at natural psychophysical boundaries constrained by the functioning of the auditory system (Kuhl and Miller, 1975), compensation for coarticulation is possible by contrasting spectral patterns of high and low energy in particular frequency regions (Lotto et al, 1997b; Diehl et al, 2004), and the lack of invariance in speech can be solved in ways similar to concept formation for visual categories that cannot be defined by any single cue (Kluender et al, 1987)
Summary
The answer to how humans perceive speech has eluded researchers for over half a century (Jusczyk and Luce, 2002; Samuel, 2011). We encourage studies in human infants that test for perceptual asymmetries between low back contrasts that have already been examined in animals, such has the /U/ – /A/ contrast (where cats, red-winged blackbirds, and monkeys find the change from /U/ to /A/ easier than the change from /A/ to /U/), and the /u/ – /U/ and /A/ – /2/ contrasts (where monkeys’ performances contradict the predictions of the central-to-peripheral asymmetry hypothesis). Researchers may consider using a habituationdishabituation paradigm, where subjects are habituated to various speech sounds from one speaker and tested on whether they dishabituate to speech sounds of a different speaker (e.g., Johnson et al, 2011) Another fundamental component that these studies do not address is what auditory cues animals may be using to discriminate different human voices, and whether they use the same cues to identify conspecific calls. Some studies have found that adults use pitch and/or formants to identify speakers (e.g., Remez et al, 1987; Fellowes et al, 1997; Baumann and Belin, 2010), whereas others
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