Investigating Speech Perception in Evolutionary Perspective: Comparisons of Chimpanzee (Pan troglodytes) and Human Capabilities

There has been much discussion regarding whether the capability to perceive speech is uniquely human. The “Speech is Special” (SiS) view proposes that humans possess a specialized cognitive module for speech perception (Mann & Liberman, 1983). In contrast, the “Auditory Hypothesis” (Kuhl, 1988) suggests spoken-language evolution took advantage of existing auditory-system capabilities. In support of the Auditory Hypothesis, there is evidence that Panzee, a language-trained chimpanzee (Pan troglodytes), perceives speech in synthetic “sine-wave” and “noise-vocoded” forms (Heimbauer, Beran, & Owren, 2011). Human comprehension of these altered forms of speech has been cited as evidence for specialized cognitive capabilities (Davis, Johnsrude, Hervais-Adelman, Taylor, & McGettigan, 2005). In light of Panzee’s demonstrated abilities, three experiments extended these investigations of the cognitive processes underlying her speech perception. The first experiment investigated the acoustic cues that Panzee and humans use when identifying sine-wave and noise-vocoded speech. The second experiment examined Panzee’s ability to perceive “time-reversed” speech, in which individual segments of the waveform are reversed in time. Humans are able to perceive such speech if these segments do not much exceed average phoneme length. Finally, the third experiment tested Panzee’s ability to generalize across both familiar and novel talkers, a perceptually challenging task that humans seem to perform effortlessly. Panzee’s performance was similar to that of humans in all experiments. In Experiment 1, results demonstrated that Panzee likely attends to the same “spectro-temporal” cues in sine-wave and noise-vocoded speech that humans are sensitive to. In Experiment 2, Panzee showed a similar intelligibility pattern as a function of reversal-window length as found in human listeners. In Experiment 3, Panzee readily recognized words not only from a variety of familiar adult males and females, but also from unfamiliar adults and children of both sexes. Overall, results suggest that a combination of general auditory processing and sufficient exposure to meaningful spoken language is sufficient to account for speech-perception evidence previously proposed to require specialized, uniquely human mechanisms. These findings in turn suggest that speech-perception capabilities were already present in latent form in the common evolutionary ancestors of modern chimpanzees and humans

Perception of Synthetic Speech by a Language-Trained Chimpanzee (Pan troglodytes)

Ability of human listeners to understand altered speech is argued as evidence of uniquely human processing abilities, but early auditory experience also may contribute to this capability. I tested the ability of Panzee, a language-trained chimpanzee (Pan troglodytes), reared and spoken to from infancy by humans, to recognize synthesized words. Training and testing was conducted with different sets of English words in natural, “harmonics-only” (resynthesized using only voiced components), or “noise-vocoded” (based on amplitude-modulated noise bands) forms, with Panzee choosing from “lexigram” symbols that represented words. In Experiment 1 performance was equivalent with words in natural and harmonics-only form. In Experiment 2 performance with noise-vocoded words was significantly higher than chance but lower than with natural words. Results suggest specialized processing mechanisms are not necessary to speech perception in the absence of traditional acoustic cues, with the more important factor for speech-processing abilities being early immersion in a speech-rich environment

Correlational matrices for Sherman and Lana comparing performance (Known or Not Known) between all pairs of years for all words that were tested.

<p>Note. All correlations were statistically significant, <i>p</i> < .01. The df for correlations that included years 1999–2003 was 184. The df for correlations that only included years 2004–2008 was 198.</p><p>Correlational matrices for Sherman and Lana comparing performance (Known or Not Known) between all pairs of years for all words that were tested.</p

A Chimpanzee’s (Pan troglodytes) Perception of Variations in Speech: Identification of Familiar Words when Whispered and When Spoken by a Variety of Talkers

When humans perceive speech they process the acoustic properties of the sounds. The acoustics of a specific word can be different depending on who produces it and how they produce it. For example, a whispered word has different acoustic properties than a word spoken in a more natural manner; basically, the acoustics are “noisier.” A word will also sound differently depending on who speaks it, due to the different physical and physiological characteristics of the talker. In this instance, humans routinely normalize speech to retrieve the lexical meaning by solving what is termed the “lack of invariance” problem. We investigated these speech perception phenomena in a language-trained chimpanzee (Pan troglodytes) named Panzee to ascertain if more generalized auditory capabilities, as opposed to specialized human cognitive processes, were adequate to accomplish these perceptual tasks. In Experiment 1 we compared the chimpanzee’s performance when identifying words she was familiar with in natural versus whispered form. In Experiment 2 we investigated Panzee’s ability to solve the “lack of invariance” problem when familiar words were spoken by a variety of talkers (familiar and unfamiliar male and female adults, and children). The results of Experiment 1 demonstrated that there was no difference in her recognition for the two word types. The results of Experiment 2 revealed no significant difference in Panzee’s performance across all talker types. Her overall performance suggests that more generalized capabilities are sufficient for solving for uncertainty when processing the acoustics of speech, and instead favor a strong role of early experience

The list of words used for testing the vocabularies of the chimpanzees separated into categories.

<p>Note. Words with asterisks (*) next to them were not part of the tests given in 1999–2003. They were included in 2004–2008 testing.</p><p>The list of words used for testing the vocabularies of the chimpanzees separated into categories.</p

The list of Known words in at least 8 of 10 testing years for each chimpanzee.

<p>The list of Known words in at least 8 of 10 testing years for each chimpanzee.</p

An example trial.

<p>The sample is at center (TV) and the four match choices are presented around the perimeter of the screen. The chimpanzee must move the cursor into contact with one of the match choices to make a selection. For Panzee, auditory samples were played as .wav files, and then the four match choices appeared onscreen, with nothing in the center of the screen.</p

Number of Known words in each testing year for each chimpanzee.

<p>For Panzee, this is shown for visual samples and spoken English samples whereas for Lana and Sherman it is shown only for visual samples.</p

Correlational matrices for Panzee for visual and spoken English conditions comparing performance (Known or Not Known) between all pairs of years for all words that were tested.

<p>Note. All correlations were statistically significant, <i>p</i> < .01. The df for correlations that included years 1999–2003 was 184. The df for correlations that only included years 2004–2008 was 198.</p><p>Correlational matrices for Panzee for visual and spoken English conditions comparing performance (Known or Not Known) between all pairs of years for all words that were tested.</p