Elsevier

Cognition

Volume 82, Issue 3, January 2002, Pages B113-B122
Cognition

Brief article
Auditory stream segregation in Japanese monkeys

https://doi.org/10.1016/S0010-0277(01)00161-5Get rights and content

Abstract

Japanese monkeys were examined to determine whether they perceptually segregate tone sequences. Monkeys were required to discriminate two sequences of tones (target sequences) differing in frequency contours. Distractor sequences were presented simultaneously with the target sequences. Monkeys could discriminate the sequences when the frequency ranges of the target and distractor sequences did not overlap, but they could not when the ranges overlapped. Subsequent probe tests confirmed that the discrimination depended on cues other than the local pitch of the component tones regardless of the presence of the distractor sequence. The results suggest that monkeys segregate tone sequences based on frequency proximity, and they perceive global characters of the segregated streams.

Introduction

Perceptual grouping is important in human auditory perception. Perceivers resolve a sequence of sounds into elements and reconstruct them into streams, which are sequences of auditory events the elements of which are perceptually related to each other. In this process, it is necessary to select which elements are grouped into the same stream and which are not. Bregman (1990) argued the importance of such a process of auditory scene analysis.

Comparing the properties of auditory scene analysis between humans and other species may help us understand the evolution of the auditory system. As for humans, it seems highly beneficial for nonhumans to extract significant information embedded in noises in their auditory world. Recent studies have demonstrated that several nonhuman species possess the ability of stream segregation (European starlings: Braaten and Hulse, 1993, Hulse et al., 1997, MacDougall-Shackleton et al., 1998, Wisniewski and Hulse, 1997; zebra finches and Bengalese finches: Benney & Braaten, 2000; goldfish: Fay, 1998). For example, Hulse et al. (1997) showed that European starlings identify birdsongs of one species even if the songs are embedded in other songs. These studies with nonhumans implied that stream segregation is common in vertebrates; however, it is still unclear to what extent these animals share the process of streaming with humans.

From the early stages of development, humans perceive frequency contours (i.e. frequency up-and-down patterns) of tone sequences regardless of the simultaneous presence of distractor sequences (Demany, 1982, Dowling, 1973). This means that humans can perceive frequency contours of a stream that is segregated from other tones. Nonhuman species have been shown to perceive the frequency contours of tone sequences (monkey: Izumi, 2001; avian: Braaten et al., 1990, Cynx, 1995, Cynx et al., 1986, Hulse and Cynx, 1985, Hulse et al., 1984, MacDougall-Shackleton and Hulse, 1996, Page et al., 1989; dolphin: Ralston & Herman, 1995). However, there has been no study to determine whether they can perceive global features of a segregated stream. Japanese monkeys have been reported to possess two types of contact calls that are distinguished by the temporal position of the frequency inflection from rising to falling (e.g. Green, 1975, May et al., 1989). To discriminate these calls embedded in environment noise, it seems to be important to segregate the calls from noise and to attend to the frequency relationships only within the calls. There is, however, a possibility that the processes of stream segregation in nonhumans differ from those in humans, and that nonhumans ignore a global feature such as frequency contour for the perception of segregated auditory streams.

In monkeys, the discrimination of the duration of the gap between two tones deteriorates as the frequency separation between tones increases (Izumi, 1999). Many studies have shown a similar perceptual effect in humans (e.g. Divenyi & Danner, 1977). These results have been discussed as a result of the difficulty in integration between frequency channels, and may relate to the process of stream segregation (see Bregman, 1990).

In the present study, Japanese monkeys were trained to discriminate two sequences of tones (target sequences) that differed in their frequency contours. Distractor sequences were presented simultaneously with the target sequences under some conditions. I predicted that the presence of a distractor would have little effect on the performance if monkeys' discrimination were based on the segregated target sequences. Subsequent probe tests were planned to determine whether the monkeys discriminated the stimuli by cues other than the local pitch of a component tone.

Section snippets

Experiment 1: the effect of frequency proximity between target and distractor sequences

Monkeys were examined to determine whether they perceive target sequences (“figure”) mixed with distractor sequences (“ground”). The frequency ranges of the target and distractor sequences overlapped each other in one condition, but they did not in the other condition. If the discrimination performances of the monkeys are maintained when the frequency ranges do not overlap, this suggests that monkeys segregate tone sequences based on frequency proximity as do humans.

Experiment 2: probe test with new starting points of target sequences

This experiment was intended to test whether the monkeys discriminated the sequences by cues other than the local pitch of the component tones. For comparison stimuli, the starting positions of the target sequences were altered. If monkeys use relatively global cues (e.g. frequency contour) for their discrimination, the alteration would have little effect on their performance. On the other hand, if the discrimination depends on local cues, the performance is predicted to deteriorate because the

General discussion

In Experiment 1, monkeys discriminated target sequences regardless of the simultaneous presentation of the distractor sequence when the frequency ranges of the target and the distractor sequences did not overlap, although the discrimination was lost when the frequency ranges overlapped. The probe tests in Experiment 2 suggested that the discrimination of the monkeys depended on cues other than the local pitch of a component tone. The results suggested that monkeys perceptually segregate tone

Acknowledgements

The author wishes to express special thanks to Dr Shozo Kojima for his encouragement. Thanks are also due to Mr Sumiharu Nagumo for his technical support.

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