Language-trained Chimpanzees Research Articles

A chimpanzee recognizes varied acoustical versions of sine-wave and noise-vocoded speech.

Previous research demonstrated that a language-trained chimpanzee recognized familiar English words in sine-wave and noise-vocoded forms (Heimbauer et al. Curr Biol 21:1210-1214, 2011). However, those results did not provide information regarding processing strategies of the specific acoustic cues to which the chimpanzee may have attended. The current experiments tested this chimpanzee and adult humans using sine-wave and noise-vocoded speech manipulated using specific sine-waves and a different number of noise bands, respectively. Similar to humans tested with the same stimuli, the chimpanzee was more successful identifying sine-wave speech when both SW1 and SW2 were present - the components that are modeled on formants F1 and F2 in the natural speech signal. Results with noise-vocoded speech revealed that the chimpanzee and humans performed best with stimuli that included four or five noise bands, as compared to those with three and two. Overall, amplitude and frequency modulation over time were important for identification of sine-wave and noise-vocoded speech, with further evidence that a nonhuman primate is capable of using top-down processes for speech perception when the signal is altered and incomplete.

Post-event misinformation effects in a language-trained chimpanzee (Pan troglodytes).

Errors of source monitoring are widespread human memory challenges, and our memories are subject to distortion upon the presentation of subsequent misinformation. Less is known about if and when misinformation effects occur in nonhuman species' memory. Here we tested a symbol-trained chimpanzee's recall memoryof a hidden food item's identity after a 10-min delay. During this delay, the subject was sometimes (depending onthecondition) shown consistent or inconsistent video information about the identity of the food, before being asked to name the item to a second experimenter blind to the reward and condition. Across all conditions, our subject, Sherman, correctly named the food item at above chance levels. In the Inconsistent condition, in which Sherman was shown a video with misleading information, his performance was the worst of all conditions (although accuracy was still high). Interestingly, however, during three of the four trials in this condition in which Sherman made a mistake, he incorrectly named the food item shown during the misleading video information. These results provide evidence that chimpanzees, like humans, may be vulnerable to misinformation effects, even when that misleading information is presented in a different modality (video) than the original live event memory, demonstrating further commonality between human and ape memory systems.

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.

Chimpanzees modify intentional gestures to coordinate a search for hidden food

Humans routinely communicate to coordinate their activities, persisting and elaborating signals to pursue goals that cannot be accomplished individually. Communicative persistence is associated with complex cognitive skills such as intentionality, because interactants modify their communication in response to another's understanding of their meaning. Here we show that two language-trained chimpanzees effectively use intentional gestures to coordinate with an experimentally-naïve human to retrieve hidden food, providing some of the most compelling evidence to date for the role of communicative flexibility in successful coordination in nonhumans. Both chimpanzees (named Panzee and Sherman) increase the rate of non-indicative gestures when the experimenter approaches the location of the hidden food. Panzee also elaborates her gestures in relation to the experimenter's pointing, which enables her to find food more effectively than Sherman. Communicative persistence facilitates effective communication during behavioural coordination and is likely to have been important in shaping language evolution.

Evidence for cognitive restoration of time-reversed speech by a language-trained chimpanzee (Pan troglodytes)

Previously, we reported on the ability of Panzee, a language-trained chimpanzee, to identify sine-wave and noise-vocoded speech by attending to the amplitude and frequency modulations in the altered signals. Here, we report on her ability to perceive phoneme-length information in words reproduced in time-reversed form. While this manipulation preserves the amplitude of frequency components, it reverses the pattern of energy changes within each reversal window. Listeners easily recognize speech at reversal windows up to 100-ms length, but at longer reversal lengths unintelligibility begins to occur (Saberi and Perrott, 1999). The theoretical interpretation is that individual phonetic segments range from 50 to 100 ms (Crystal and House, 1988), and reversal-windows less than 100 ms provide for restoration of phoneme perception. Hypothesizing that Panzee also perceives speech based on phonemic segments, we tested her and humans with words in eight reversal forms ranging from 25 to 200 ms. Results revealed time-reversal window length significantly predicted percentage-correct word identification for Panzee and the humans. Additionally, window lengths exceeding 100 ms produced partial word intelligibility, with 50% intelligibility occurring at approximately 130 ms for both species. We therefore conclude that Panzee attends to phoneme-related cues in time-reversed speech, and hence in natural speech.

Language-Trained Chimpanzees (Pan troglodytes) Name What They Have Seen but Look First at What They Have Not Seen

Metacognition can be defined as knowing what one knows, and the question of whether nonhuman animals are metacognitive has driven an intense debate. We tested 3 language-trained chimpanzees in an information-seeking task in which the identity of a food item was the critical piece of information needed to obtain the food. The chimpanzees could either report the identity of the food immediately or first check a container in which the food had been hidden. In two experiments, the chimpanzees were significantly more likely to visit the container first on trials in which they could not know its contents but were more likely to just name the food item without looking into the container on trials in which they had seen its contents. Thus, chimpanzees showed efficient information-seeking behavior that suggested they knew what they had or had not already seen when it was time to name a hidden item.

Prospective memory in a language-trained chimpanzee (Pan troglodytes)

Prospective memory involves the encoding, retention, and implementation of an intended future action. Although humans show many forms of prospective memory, less is known about the future oriented processes of nonhuman animals, or their ability to use prospective memory. In this experiment, a chimpanzee named Panzee, who had learned to associate geometric forms called lexigrams with real-world referents, was given a prospective memory test. Panzee selected between two foods the one she wanted to receive more immediately. That food was scattered in an outdoor yard where she could forage for it. Also outdoors were lexigram tokens, one of which represented the food item that remained indoors throughout a 30min period, and that could be obtained if Panzee brought in the token that matched that food item. After foraging for the selected food item, Panzee consistently remembered to retrieve and return the correct token when food was available indoors, whereas on control trials involving no indoor food she rarely returned a token. This indicated that Panzee encoded information relevant to the future action of token retrieval after extended delays for one type of food, even when a more immediately preferred food was available.

Language‐Trained Chimpanzees (Pan troglodytes) Delay Gratification by Choosing Token Exchange Over Immediate Reward Consumption

Token exchange inherently introduces an element of delay between behavior and reward and so token studies may help us better understand delay of gratification and self-control. To examine this possibility, we presented three language-trained chimpanzees with repeated choices involving different foods that could be eaten immediately or lexigram (graphic symbol) tokens that represented (and could be traded for) foods later. When both options were foods, chimpanzees always chose more preferred foods over less preferred foods. When both options were lexigram tokens representing those same foods, performance remained the same as chimpanzees selected the higher value token and then traded it for food. Then, when faced with choosing a token that could be traded later or choosing a food item that could be eaten immediately, most chimpanzees learned to make whatever response led to the more preferred food. They did this even when that meant selecting a high value lexigram token that could be traded only 2 to 3 min later instead of a medium value, but immediately available, food item. Thus, chimpanzees flexibly selected tokens even though such selections necessarily delayed gratification and required forgoing immediately available food. This finding illustrates the utility of symbolic token exchange for assessing self-control in nonhuman animals.

A chimpanzee responds best to sine-wave speech with formant tones 1 and 2 present

A seminal study by Remez and colleagues [R. E. Remez, et al., Science, 212, 947–949 (1981)] demonstrated that listeners were more successful in identifying sine-wave speech when the first two (T12) or all three formant tones (T123) were present than when either were absent (forms T13 and T23). To determine whether a language-trained chimpanzee (Panzee) with the ability to recognize English words in sine-wave form [L. A. Heimbauer, et al., Curr. Biol., doi:10.1016/j.cub.2011.06.007 (2011)] would respond similarly, she and 13 humans were tested with words synthesized in the four forms used by Remez et al. Indeed, for each species, perception of speech was significantly better when the first and second tones were both present. Panzee's performance suggests that she is attending to the same spectro-temporal features of sine-wave speech that are critical to humans. The outcomes further indicate that basic capabilities involved in speech perception could have been present in latent form in the common ancestor o...

Speech Perception: A Language-Trained Chimpanzee Weighs In

A language-trained chimpanzee is able to interpret synthetic 'auditory caricatures' as speech. Important components of human speech perception thus rely upon general auditory mechanisms that predated the evolution of spoken language.

Delay of gratification in chimpanzees(Pan troglodytes)

Delay of gratification in 3 chimpanzees (Pan troglodytes) was examined by using a paradigm based on research with children. The chimpanzees either pressed a door-bell button during a trial and received one reward (the immediate reward) or did not press the doorbell and received another reward (the delayed reward). Two chimpanzees were language-trained, and a 3rd was non-language-trained. Foods (one more-preferred and one less-preferred), photographs of those foods, or lexigrams representing those foods were presented to the chimpanzees. All 3 chimpanzees delayed gratification when foods were physically present. One language-trained chimpanzee also delayed gratification with lexigrams present, and the 2nd language-trained chimpanzee delayed gratification in all three conditions. Language competence and early rearing are proposed as explanations for the different performances of these chimpanzees.

Indexical pointing in language-trained chimpanzees: Methodological issues and a comparison of rearing histories.

Indexical pointing in language-trained chimpanzees: Methodological issues and a comparison of rearing histories.

Language-naive chimpanzees (Pan troglodytes) judge relations between relations in a conceptual matching-to-sample task.

Three chimpanzees with a history of conditional and numeric token training spontaneously matched relations between relations under conditions of nondifferential reinforcement. Heretofore, this conceptual ability was demonstrated only in language-trained chimpanzees. The performance levels of the language-naive animals in this study, however, were equivalent to those of a 4th animal--Sarah--whose history included language training and analogical problem solving. There was no evidence that associative factors mediated successful performance in any of the animals. Prior claims of a profound disparity between language-trained and language-naive chimpanzees apparently can be attributed to prior experience with arbitrary tokens consistently associated with abstract relations and not language per se.

Assessment of hand preference in two language-trained chimpanzees (Pan troglodytes): a multimethod analysis.

The nonhuman primate literature concludes that monkeys and apes do not exhibit handedness preferences at the population level. This discrepancy between human and nonhuman primate findings may be due to differences in the methods of assessment of handedness, lack of reliability between various measures of handedness, and a paucity of studies involving great apes. This paper presents the results of extensive hand preference studies with two language-trained chimpanzees (Pan troglodytes). Both naturalistic and experimental paradigms are described. In particular, various methodologic factors involved in handedness assessment, including the use of a hand preference measure previously used with human subjects were examined. Both chimpanzees exhibited a right-hand preference for fine motor tasks requiring manual dexterity.

Hemispheric priming by meaningful and nonmeaningful symbols in language-trained chimpanzees (Pan troglodytes): further evidence of a left hemisphere advantage.

Hemispheric priming was examined in 3 language-trained chimpanzees (Pan troglodytes) using a simple reaction time paradigm. Subjects were required to hold down a response button until the occurrence of a response cue. A warning stimulus was presented to either the left visual field (LVF) or the right visual field (RVF) before the response cue occurred. No warning stimulus was presented on control trials. The warning stimuli were geometric communicative symbols from two semantic categories: foods and tools. A third set of warning stimuli were familiar geometric symbols. Dependent measures included reaction time and the number of false-positive responses. Reaction-time data indicated an RVF advantage in priming when the warning stimuli were food or tool symbols. No significant visual half-field differences were found for familiar symbols, but a trend toward an RVF advantage was observed. These effects were enhanced when subjects responded with their left hand. False-positive data also indicated an RVF advantage for the food and tool warning stimuli. The data indicate that hemispheric asymmetries for processing communicative symbols are present in language-trained chimpanzees.

Hemispheric priming by meaningful and nonmeaningful symbols in language-trained chimpanzees (Pan troglodytes): Further evidence of a left hemisphere advantage.

Hemispheric priming by meaningful and nonmeaningful symbols in language-trained chimpanzees (Pan troglodytes): Further evidence of a left hemisphere advantage.

Evidence for asymmetrical hemispheric priming using known and unknown warning stimuli in two language-trained chimpanzees (Pan troglodytes).

In this article results are reported from 3 warning stimulus-priming experiments that assessed hemisphere-specific activation and lateralization in 2 language-trained chimpanzees (Pan troglodytes). Results from Experiment 1 indicated a right visual field advantage in priming for meaningful warning stimuli presented in blocks of 36 trials. In Experiments 2 and 3, randomized presentations of meaningful, familiar, and meaningless stimuli resulted in right visual field advantages for meaningful warning stimuli. No visual half-field differences were found for familiar or meaningless warning stimuli. The findings are similar to those found in human subjects using known-unknown symbol paradigms; they suggest that basic phylogenetic neuropsychological systems related to activation and priming processes may link nonhuman primate and human studies of lateralization.

Evidence for asymmetrical hemispheric priming using known and unknown warning stimuli in two language-trained chimpanzees (Pan troglodytes).

Evidence for asymmetrical hemispheric priming using known and unknown warning stimuli in two language-trained chimpanzees (Pan troglodytes).

Laterality for visual-spatial processing in two language-trained chimpanzees (Pan troglodytes).

Most reviews of laterality in nonhuman primates indicate that hemispheric asymmetries, similar to those found in humans, are not evident. With the growing evidence for cognitive processes germane to language in apes, in addition to their phylogenetic similarity to humans, they appear to be useful candidates for studies of laterality. Laterality for visual-spatial processing in two language-trained chimpanzees (Pan troglodytes) was investigated with a visual half-field paradigm. Initially, the chimpanzees were taught to manipulate a joystick that controlled the movement of a cursor on a computer monitor to a central fixation point. Subjects were then taught a visual discrimination based on the location of a short line contained within a geometric form. Testing consisted of systematic presentation rates of 15, 122, and 226 ms to the left and right visual fields. For half of the trials, subjects used their left hand to respond and, alternately, used the right hand for the remaining trials. Accuracy and reaction time were the dependent measures. One subject demonstrated significantly faster reaction times to stimuli presented to the left visual field (LVF) regardless of which hand was used to respond. The other subject demonstrated a significant LVF bias when using the left hand to respond. The results are discussed in the context of current theories on the evolution of laterality and its relation to linguistic functioning.

Laterality for visual-spatial processing in two language-trained chimpanzees (Pan troglodytes).

Laterality for visual-spatial processing in two language-trained chimpanzees (Pan troglodytes).