Elsevier

Hearing Research

Volume 162, Issues 1–2, December 2001, Pages 29-42
Hearing Research

Macaque thresholds for detecting increases in intensity: effects of formant structure

https://doi.org/10.1016/S0378-5955(01)00357-4Get rights and content

Abstract

Macaque monkeys, like humans, are more sensitive to differences in formant frequency than to differences in the frequency of pure tones (see Sinnott et al. (1987) J. Comp. Psychol. 94, 401–415; Pfingst (1993) J. Acoust. Soc. Am. 93, 2124–2129; Prosen et al. (1990) J. Acoust. Soc. Am. 88, 2152–2158; Sinnott et al. (1985) J. Acoust. Soc. Am. 78, 1977–1985; Sinnott and Kreiter (1991) J. Acoust. Soc. Am. 89, 2421–2429; for summary, see May et al. (1996) Aud. Neurosci. 3, 135–162). In the discrimination of formant frequency, it appears that the relevant cue for macaque monkeys is relative level differences of the component frequencies (Sommers et al. (1992) J. Acoust. Soc. Am. 91, 3499–3510). To further explore the result of Sommers et al., we trained macaque monkeys (Macaca fuscata) to report detection of a change in the spectral shape of multi-component harmonic complexes. Spectral shape changes were produced by the addition of intensity increments. When the amplitude spectrum of the comparison stimulus was modeled after the /ae/ vowel sound, thresholds for detecting a change from the comparison stimulus were lowest when intensity increments were added at spectral peaks. These results parallel previous data from human subjects, suggesting that both human and monkey subjects may process vowel spectra through simultaneous comparisons of component levels across the spectrum. When the subjects were asked to detect a change from a comparison stimulus with a flat amplitude spectrum, the subjects showed sensitivity that was relatively comparable to that of human subjects tested in other investigations (e.g. Zera et al. (1993) J. Acoust. Soc. Am. 93, 3431–3441). In additional experiments, neither increasing the dynamic range of the /ae/ spectrum nor dynamically varying the amplitude of the increment during the stimulus presentation reliably affected detection thresholds.

Introduction

The most striking difference between human and non-human primate sensitivity to specific acoustic features is seen in the detection of frequency differences in pure-tone stimuli (for review, see May et al., 1996). Monkey difference limens (DLs) for detecting pure-tone frequency differences are up to 20 times greater than human DLs (Sinnott et al., 1987; see also Pfingst, 1993, Prosen et al., 1990, Sinnott et al., 1985). However, monkey and human DLs for vowel formant frequency discriminations are quite similar (see Sinnott and Kreiter, 1991). This observation was explored by Sommers et al. (1992), who explored the acoustic cues (i.e. phase- and amplitude-based cues) used in pure-tone frequency discriminations and formant frequency discriminations in single formant and vowel-like (multi-formant) stimuli. Sommers et al. (1992) found that macaque subjects were more sensitive to frequency increments in the formant discriminations than in the corresponding pure-tone discriminations. They further reported that the increased sensitivity to the changes in the formant stimuli was a function of the relative level changes in the stimulus harmonics at or near the formant peaks. Since then, Sommers and Kewley-Port (1996) have shown human formant frequency discrimination of steady state vowels to be similarly mediated by selective attention to harmonic level variation at harmonics close to the formant frequency (see also Assmann and Neary, 1987). The present experiments were designed to extend the observations of Sommers et al. (1992) to frequency components distant from the formant peaks. These experiments directly test the prediction that relative level changes at harmonics at or near the formant peaks are more salient than relative level changes at harmonics distant from these peaks. The results provide insight into the nature of spectral shape processing in non-human primates.

In natural speech signals, the frequency and level of the fundamental frequency component vary over time. In these experiments component frequency is fixed and we systematically vary component amplitude. In Experiment I, we test the hypothesis that intensity increments added at or near vowel formants can be detected at lower levels than increments added at components distant to the formant frequencies. By manipulating the relative levels of the components to be equal (‘flat’), rather than vowel-like, we further examine the manner in which removal of formant intensity peaks affects increment detection thresholds. This is further examined in Experiment II, in which we manipulate the number of stimulus components and thus alter the vowel-like spectral shape. Finally, in Experiment III, we test the hypothesis that dynamically changing intensity increments, which more closely mimic the intensity changes in communication signals, result in lower increment detection thresholds than the comparable manipulations without dynamic change across the stimulus duration.

Section snippets

Experiment I: vowel spectrum, flat, and expanded spectrum

The present study was designed to determine whether spectral shape discrimination by macaque monkeys is enhanced when the manipulated component is within one of the formants of a vowel-like (/ae/) stimulus. Evidence for such an enhancement would be provided by observation of increment detection thresholds that are higher for components between formants than at formants. Such a result would be similar to data collected from human subjects (Kakusho et al., 1971), and would be consistent with the

Experiment II: high fundamental

Thresholds for detecting increments at formant peaks were not lower than thresholds for detecting increments within a spectrally flat stimulus. However, the results from the /ae/ spectrum condition in the first experiment were consistent with the predictions of Sommers et al. (1992), as well as data collected from human subjects (Kakusho et al., 1971)). Specifically, sensitivity to amplitude increments was greatest when increments were added at formant peaks rather than other non-formant

Experiment III: dynamic increments

Speech contains a rich harmonic structure, and both rapid and slow fluctuations in amplitude are evident. Variations in both harmonic structure and amplitude are produced as a function of vocal tract properties, including positioning of the teeth, tongue, and lips (see Lieberman, 1977). Macaque monkey coo calls constitute one of the macaque’s primary communication call classes (see Green, 1975). As in speech signals, characteristic properties of the vocal tract influence coo call acoustics (see

General discussion

Sinnott and Kreiter (1991) suggested several explanations for the similarities in monkey and human vowel formant DLs in light of the large differences between monkey and human pure-tone DLs. One suggestion was that mechanisms other than frequency discrimination, such as ‘profile analysis’, might contribute to vowel encoding. Profile analysis is a strategy for comparing changes in the pattern of energy across an acoustic stimulus (for reviews, see Green, 1983, Green, 1988). The use of a profile

Acknowledgements

The authors thank Catherine Thompson, who had the primary responsibility for the daily care and testing of these animals. We also thank Mitchell Sommers for his contributions to the design of these experiments, and John Middlebrooks and Jennifer Lentz for valuable comments on earlier versions of this manuscript. Finally, we thank David Green for permitting reproduction of the human data depicted in Fig. 6, and we thank Zekiye Onsan for providing these data. This research was supported by the

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    Present address: Department of Psychology, Kenyon College, Gambier, OH 43022-9623, USA.

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