Differences in American English, Spanish, and monkey perception of the say-stay trading relation.

An interesting phenomenon in human speech perception is the trading relation, in which two different acoustic cues both signal the same phonetic percept. The present study compared American English, Spanish, and monkey listeners in their perception of the trading relation between gap duration and F1 transition onset frequency in a synthetic say-stay continuum. For all the subjects, increased gap duration caused perception to change from say to stay; however, subjects differed in the extent to which the F1 cue traded with gap duration. For American English listeners, a change from a low to a high F1 onset caused a phoneme boundary shift of 26 msec toward shorter gap durations, indicating a strong trading relation. For Spanish listeners, the shift was significantly smaller at 13.7 msec, indicating a weaker trading relation. For monkeys, there was no shift at all, indicating no trading relation. These results provide evidence that the say-stay trading relation is dependent on perceptual learning from linguistic exposure.

Can macaques perceive place of articulation from formant transition information?

An important problem in speech perception is to determine how humans extract the perceptually invariant place of articulation information in the speech wave across variable acoustic contexts. Although analyses have been developed that attempted to classify the voiced stops /b/ versus /d/ from stimulus onset information, most of the human perceptual research to date suggests that formant transition information is more important than onset information. The purpose of the present study was to determine if animal subjects, specifically Japanese macaque monkeys, are capable of categorizing /b/ versus /d/ in synthesized consonant-vowel (CV) syllables using only formant transition information. Three monkeys were trained to differentiate CV syllables with a "go-left" versus a "go-right" label. All monkeys first learned to differentiate a /za/ versus /da/ manner contrast and easily transferred to three new vowel contexts /[symbol: see text], epsilon, I/. Next, two of the three monkeys learned to differentiate a /ba/ versus /da/ stop place contrast, but were unable to transfer it to the different vowel contexts. These results suggest that animals may not use the same mechanisms as humans do for classifying place contrasts, and call for further investigation of animal perception of formant transition information versus stimulus onset information in place contrasts.

Effects of syllable duration on stop-glide identification in syllable-initial and syllable-final position by humans and monkeys.

Humans and monkeys were compared in their identification of phoneme boundaries along synthetic stop-glide continua in syllable-initial /bo/-/wa/ or syllable-final /bab/-/baw/ contrasts differing in overall syllable duration. For both contrasts, humans were first tested with a conventional written identification procedure. Here, similar phoneme boundaries emerged and shifted with increases in syllable duration toward longer transitions, as has previously been reported in the literature for syllable-initial data (Miller & Liberman, 1979). Humans and monkeys were then tested on these contrasts, using a go/no-go identification procedure specifically designed for monkeys. Here also, stop-glide boundaries emerged and shifted with increased syllable duration for both species, although monkey "boundaries" were at longer durations than humans' in syllable-final position. The results indicate that there are both gross similarities and subtle differences between humans and monkeys with regard to the stop-glide context effect. The results are discussed in relation to the hypothesis that general mammalian auditory mechanisms are responsible for this effect.

A multidimensional scaling analysis of vowel discrimination in humans and monkeys.

Multidimensional scaling (MDS) was used to compare perceptual maps for 10 synthetic English vowels in humans and Old World monkeys (Macaca fuscata and Cercopithecus albogularis). Subjects discriminated among the vowels using a repeating background procedure, and reaction times were submitted to an MDS analysis to derive measures of perceive similarity. The dimensions that emerged related to the frequencies of the first (F1), second (F2), and third (F3) formants. Human data indicated a good match to previous MDS studies using rating procedures or confusion matrices: The dominant dimension mapped onto vowel F2, the phonetically most important formant, and the second and third dimensions mapped onto F1 and F3, respectively. For monkeys, equal weightings occurred for F1 and F2, and F3 was not clearly represented. Monkey sensitivity to the formants appeared to relate to formant amplitudes. If monkeys are giving an accurate representation of the psychoacoustic relations among the formants, then our human results suggest that species-specific mechanisms, reflecting the salience of the phonetic feature of advancement, may contribute to vowel coding in humans.

Behavioral measures of vowel sensitivity in Mongolian gerbils (Meriones unguiculatus): effects of age and genetic origin.

Absolute thresholds for complex vowel stimuli were compared in Mongolian gerbils (Meriones unguiculatus) as a function of age and genetic origin. For a group of 12-month-old 'domestic' gerbils obtained from Tumblebrook Farms, lowest thresholds averaging 14 dB SPL occurred for the vowel /alpha/, which had its most intense formant (F1) at 730 Hz. Thresholds increased to 22 dB SPL for /i/, which had its two most intense formants (F1 and F3) at 270 and 3000 Hz, respectively. Highest thresholds of 30 dB SPL occurred for /u/, which had its most intense formant (F1) at 300 Hz. Thresholds increased by about 10 dB per year through the ages of 12-36 months, with most of the loss occurring for /alpha/ and /u/. The domestic gerbils' /alpha/ thresholds corresponded well to those measured in aging gerbils in electrophysiological studies. Vowel thresholds were also measured in a group of first-generation offspring of 'wild' gerbils imported from Asia, first tested at the ages of 18-24 months. Thresholds were similar to those of the 12-month-old domestic gerbils, and showed no hearing loss with age up to 36 months. The wild gerbils were also free of ear impactions, which commonly occurred in the domestic gerbils. The hearing loss with age in the domestic gerbils may have a genetic basis, and might be due to inbreeding in the domestic strain, in contrast to the hybrid vigor of the wild gerbils.

Perception of the American English liquid /ra-la/ contrast by humans and monkeys.

Human and monkey perception of the American English liquid /ra-la/ contrast was compared using various synthetic continua in which the normal spectral and temporal cues were higher either complete, partial, or altered in various ways. Two experiments compared human and monkey discrimination of the various continua using a low-uncertainty repeating-standard procedure. Results showed that, while human sensitivity was best at the human phoneme boundary, monkey sensitivity was best inside the /ra/ category. Also, while humans were more sensitive than monkeys to temporal variation in the stimuli, monkeys were more sensitive than humans to spectral variation, particularly for stimuli inside the /ra/ category. Two additional experiments compared human and monkey identification of the /ra-la/ continua using a higher-uncertainty go/nogo identification procedure. Monkeys performed as accurately as humans (including one native Spanish and one native Hindi listener) in identifying the stimuli. However, human and monkey "phoneme boundaries" were in different places, with monkey boundaries shifted more toward /ra/ than human boundaries. These results suggest that human boundaries may be based on some sort of specific linguistic knowledge. Despite these boundary differences, monkeys showed a trading relation comparable to that of humans, indicating that the /ra-la/ trading relation has a psychoacoustic basis and that linguistic knowledge is not a necessary prerequisite for it to occur.

Perception of chirps by Sykes's monkeys (Cercopithecus albogularis) and humans (Homo sapiens).

Four Sykes's monkeys (Cercopithecus albogularis) and 4 humans (Homo sapiens) discriminated among 12 chirps presented in a repeating background paradigm. The test stimuli consisted of sets of 4 chirps recorded from Sykes's monkeys, red-tailed monkeys (C. ascanius), and small East African birds. Reaction times were submitted to a multidimensional scaling analysis. All monkey listeners perceived the bird chirps as similar to each other and distinct from the monkey calls, whereas 3 of the 4 human listeners had difficulty distinguishing the bird chirps from the monkey calls. Both human and monkey subjects tended to perceive Sykes's and red-tailed monkey calls as very similar to one another, but the degree of perceived similarity was greatest for the monkey listeners. The data suggest that the perceptual map of these calls is influenced by their biological significance in nature.

Effects of varying signal and noise levels on pure-tone frequency discrimination in humans and monkeys.

This study examined the effects of presenting various signal and noise levels on pure-tone frequency difference limens (FDLs) in humans and monkey. Signal levels for two frequencies of 500 Hz and 4 kHz were varied both in quiet and in noise. For the lower frequency of 500 Hz, the two species exhibited qualitatively divergent FDLs as level increased in quiet: FDLs decreased at high levels for humans, but increased for monkeys. In noise, differences between human and monkey FDLs were less pronounced. At 500 Hz, monkeys exhibited slightly lower FDLs in noise than in quiet. At the higher frequency of 4 kHz, under all conditions, FDLs for both species were qualitatively similar. Results are discussed in relation to (1) different use of "temporal" coding mechanisms by humans and monkeys at 500 Hz, and (2) similar use of "rate-place" coding mechanisms at 4 kHz.

Effects of varying signal duration on pure-tone frequency discrimination in humans and monkeys.

This study examined the effects of varying signal duration and pulse repetition rate on pure tone frequency difference limens (FDLs) at 500 Hz and 4 kHz in humans and monkeys. Two experiments examined FDLs as a function of tone duration from 12-400 ms, using stimuli presented either with a constant pulse rate, or with a constant interstimulus interval. In both cases, at 500 Hz, human FDLs increased more than monkey FDLs as duration decreased from 400 to 12 ms. This effect resulted in monkey FDLs that were about 14 times larger than human FDLs at 400 ms, but only about 4 times larger at 12 ms. At 4 kHz, human and monkey FDLs showed more similar rates of increase as duration decreased. A third experiment examined FDLs at 500 Hz for a 100-ms tone as a function of pulse rate (1 per 250-2000 ms). Here, FDLs for humans and monkeys varied in a similar fashion, indicating that both species' sensitivity was similarly affected by manipulating temporal variables relating to memory load, rather than sensory coding. These data provide evidence that, at 500 Hz, humans have a qualitatively different pure tone frequency analyzing mechanism from monkeys, possibly based on better access to phase-locking mechanisms.

Silhouettes elicit alarm calls from captive vervet monkeys (Cercopithecus aethiops).

In Part 1, we analyzed alarm calls produced by captive vervets (Cercopithecus aethiops) in response to naturally occurring stimuli. Females and juveniles regularly alarm called to airplanes, birds, and barking dogs. Juvenile females accounted for 60% of these alarm calls. In Part 2, we isolated several monkeys from the colony and presented them with life-sized silhouettes of a leopard, snake, eagle, baboon, vervet, and goose. Adult monkeys alarm called more than did juveniles. Alarm calls were elicited by leopard, snake, baboon, and vervet silhouettes, but none were elicited by eagle or goose silhouettes. Some leopard and snake alarm calls matched those recorded in the wild in the context of the vervets' natural predators. Results indicate that silhouette stimuli are a useful technique for eliciting monkey vocalizations in the laboratory.

Differential sensitivity of Japanese macaques (Macaca fuscata) and humans (Homo sapiens) to peak position along a synthetic coo call continuum.

Difference limens (DLs) for changes in the temporal position of a pitch peak along a synthetic early-high to late-high coo continuum were measured in 2 Japanese macaques (Macaca fuscata) and 2 humans (Homo sapiens) in a low-uncertainty, repeating standard discrimination procedure. Lowest DLs (19-32 ms for monkeys; less than 10 ms for humans) occurred near the endpoints of the continuum. Highest DLs (59-73 ms for monkeys; 25-27 ms for humans) occurred near the center of the continuum. DLs for both monkeys and humans corresponded to previously reported measures of temporal resolution. Neither monkeys nor humans exhibited categorical perception of the coo continuum, with a central area of enhanced sensitivity, a result previously reported by May, Moody, and Stebbins (1989) for similar stimuli. We conclude that our subjects discriminated variation in coo peak position by using general psychoacoustic mechanisms related to temporal discrimination.

Frequency and intensity discrimination in Mongolian gerbils, African monkeys and humans.

Frequency (delta F) and intensity (delta I) difference limens were directly compared in Mongolian gerbils (Meriones unguiculatus), Old World African Monkeys (Cercopithecus mitis, Cercocebus albigena), and humans. Methods employed a repeating background AX discrimination procedure, and positive (food) reinforcement for animals. For delta I, there were small quantitative differences between the species. At 1 kHz, 70 dB SPL, DLs averaged 2.82 dB for gerbils, 2.29 dB for monkeys, and 0.75 dB for humans. For delta F, there were larger differences between the species. At 1 kHz, 60 dB SPL, frequency DLs were highest for gerbils, averaging 108 Hz. DLs were lower for monkeys, averaging 32.6 Hz, similar to recently reported DLs for other Old World monkeys (Prosen et al., 1990). Human DLs, averaging 2.27 Hz, were markedly lower than those of either monkeys or gerbils. These results suggest that animals provide better models of human delta I than delta F.

Differential sensitivity to vowel continua in Old World monkeys (Macaca) and humans.

Previous studies indicate that monkey pure tone frequency discrimination is quantitatively and qualitatively very different from that of humans: Monkey DLs at 1.0 and 2.0 kHz are up to 20 times larger than human DLs, and monkeys DLs increase as sensation level increases, in contrast to human DLs [Sinnott et al., J. Acoust. Soc. Am. 78, 1977-1985 (1985); Sinnott et al., J. Comp. Psychol. 101, 126-131 (1987)]. These results led to an hypothesis that monkey frequency discrimination is more dependent upon "rate" coding than is that of humans. The present study compared monkey and human DLs for formant frequency changes along three synthetic vowel continua /I-i/, /ae-epsilon/, and /a-v/. Here, monkey DLs for formants near 1.0 and 2.0 kHz (32-48 Hz) were only about two to three times larger than human DLs (11-21 Hz), and both monkeys and humans exhibited relatively similar, flat sensation level functions. Taken together, these data indicate that monkey and human frequency discrimination is more similar in the case of a complex vowel stimulus than in the case of a simple pure tone stimulus. Results are discussed in relation to "rate" versus "temporal" coding of tones and vowels in the auditory system.

Detection and discrimination of synthetic English vowels by Old World monkeys (Cercopithecus, Macaca) and humans.

Abilities to detect and discriminate ten synthetic steady-state English vowels were compared in Old World monkeys (Cercopithecus, Macaca) and humans using standard animal psychophysical procedures and positive-reinforcement operant conditioning techniques. Monkeys' detection thresholds were close to humans' for the front vowels /i-I-E-ae-E), but 10-20 dB higher for the back vowels /V-D-C-U-u/. Subjects were subsequently presented with groups of vowels to discriminate. All monkeys experienced difficulty with spectrally similar pairs such as /V-D/, /E-ae/, and /U-u/, but macaques were superior to Cercopithecus monkeys. Humans discriminated all vowels at 100% correct levels, but their increased response latencies reflected spectral similarity and correlated with higher error rates by monkeys. Varying the intensity level of the vowel stimuli had little effect on either monkey or human discrimination, except at the lowest levels tested. These qualitative similarities in monkey and human vowel discrimination suggest that some monkey species may provide useful models of human vowel processing at the sensory level.

Absolute auditory thresholds in three Old World monkey species (Cercopithecus aethiops, C. neglectus, Macaca fuscata) and humans (Homo sapiens).

We investigated the absolute auditory sensitivities of three monkey species (Cercopithecus aethiops, C. neglectus, and Macaca fuscata) and humans (Homo sapiens). Results indicated that species-typical variation exists in these primates. Vervets, which have the smallest interaural distance of the species that we tested, exhibited the greatest high-frequency sensitivity. This result is consistent with Masterton, Heffner, and Ravizza's (1969) observations that head size and high-frequency acuity are inversely correlated in mammals. Vervets were also the most sensitive in the middle frequency range. Furthermore, we found that de Brazza's monkeys, though they produce a specialized, low-pitched boom call, did not show the enhanced low-frequency sensitivity that Brown and Waser (1984) showed for blue monkeys (C. mitis), a species with a similar sound. This discrepancy may be related to differences in the acoustics of the respective habitats of these animals or in the way their boom calls are used. The acuity of Japanese monkeys was found to closely resemble that of rhesus macaques (M. mulatta) that were tested in previous studies. Finally, humans tested in the same apparatus exhibited normative sensitivities. These subjects responded more readily to low frequencies than did the monkeys but rapidly became less sensitive in the high ranges.

Modes of perceiving and processing information in birdsong (Agelaius phoeniceus, Molothrus ater, and Homo sapiens).

In a previous study I examined the abilities of red-winged blackbirds and brown-headed cowbirds trained with a go-right/go-left procedure to identify conspecific and alien song themes (Sinnott, 1980). Results showed that each bird species exhibited superior identification of conspecific final "trill" or "whistle" elements, relative to the alien species. The present study extends these results by examining human perception of these same song stimuli, by examining the effects of tutoring birds with alien final song elements, and by using latency analyses to investigate processing modes that are not apparent from analyses of percent-correct scores. Results suggest three different processing modes: First, humans attend primarily to the final song elements. Second, birds identifying alien songs attend primarily to the introductory elements and disregard information in the final elements. Third, birds identifying conspecific songs process both the initial and final elements, but their response latencies indicate that they direct their attention primarily to the initial elements and process the final elements without investing more time than do the alien birds that fixate on the initial elements. Conspecific special processing is discussed in relation to various psychophysical, ethological, and psycholinguistic frameworks. Human perception of birdsong is discussed in relation to backward recognition masking.

Differences in human and monkey sensitivity to acoustic cues underlying voicing contrasts.

Humans and monkeys were compared in their differential sensitivity to various acoustic cues underlying voicing contrasts specified by voice-onset time (VOT) in utterance-initial stop consonants. A low-uncertainty repeating standard AX procedure and positive-reinforcement operant conditioning techniques were used to measure difference limens (DLs) along a VOT continuum from--70 ms (prevoiced/ba/) to 0 ms (/ba/) to + 70 ms (/pa/). For all contrasts tested, human sensitivity was more acute than that of monkeys. For voicing lag, which spans a phonemic contrast in English, human DLs for a/ba/(standard)-to-/pa/ (target) continuum averaged 8.3 ms compared to 17 ms for monkeys. Human DLs for a/pa/-to-/ba/ continuum averaged 11 ms compared to 25 ms for monkeys. Larger species differences occurred for voicing lead, which is phonemically nondistinctive in English. Human DLs for a /ba/-to-prevoiced/ba/ continuum averaged 8.2 ms and were four times lower than monkeys (35 ms). Monkeys did not reliably discriminate prevoiced /ba/-to-/ba/, whereas humans DLs averaged 18 ms. The effects of eliminating cues in the English VOT contrasts were also examined. Removal of the aspiration noise in /pa/ greatly increased the DLs and reaction times for both humans and monkeys, but straightening out the F1 transition in /ba/ had only minor effects. Results suggest that quantitative differences in sensitivity should be considered when using monkeys to model the psychoacoustic level of human speech perception.

Auditory duration discrimination in Old World monkeys (Macaca, Cercopithecus) and humans.

Auditory duration DLs at 2.0 kHz were measured in Old World monkeys (Macaca, Cercopithecus) and humans using a go, no-go repeating standard AX procedure and positive reinforcement operant conditioning techniques. For a 200-ms standard, monkey DLs were 45-125 ms, compared to 15-27 ms for humans. Weber fractions (delta T/T) for all species were smallest at standard durations of 200-400 ms and increased as standard duration decreased to 25 ms. Varying intensity from 30-70 dB SPL had only minor effects on DLs, except at the lowest levels tested, where DLs were elevated slightly. Monkeys had difficulty discriminating duration decrements, in contrast to humans. Results are discussed in relation to other comparative psychoacoustic data and primate vocal communication, including human speech.

Frequency and intensity discrimination in humans and monkeys.

Frequency and intensity DLs were compared in humans and monkeys using a repeating standard "yes-no" procedure in which subjects reported frequency increments, frequency decrements, intensity increments, or intensity decrements in an ongoing train of 1.0-kHz tone bursts. There was only one experimental condition (intensity increments) in which monkey DLs (1.5-2.0 dB) overlapped those of humans (1.0-1.8 dB). For discrimination of both increments and decrements in frequency, monkey DLs (16-33 Hz) were approximately seven times larger than those of humans (2.4-4.8 Hz), and for discrimination of intensity decrements, monkey DLs (4.4-7.0 dB) were very unstable and larger than those of humans (1.0-1.8 dB). For intensity increment discrimination, humans and monkeys also exhibited similar DLs as SL was varied. However, for frequency increment discrimination, best DLs for humans occurred at a high (50 dB) SL, whereas best DLs for monkeys occurred at a moderate (30 dB) SL. Results are discussed in terms of various neural mechanisms that might be differentially engaged by humans and monkeys in performing these tasks; for example, different amounts of temporal versus rate coding in frequency discrimination, and different mechanisms for monitoring rate decreases in intensity discrimination. The implications of these data for using monkeys as models of human speech sound discrimination are also discussed.