Human Characteristics of Sound Localization under Masking for the Early Detection of Dementia

Human Characteristics of Sound Localization under Masking for the Early Detection of Dementia

Kouji Nagashima (Graduate School of Natural Science and Technology, Okayama University, Japan), Jinglong Wu (Graduate School of Natural Science and Technology, Okayama University, Japan) and Satoshi Takahashi (Graduate School of Natural Science and Technology, Okayama University, Japan)
DOI: 10.4018/978-1-60960-559-9.ch008

Abstract

Sound localization ability differs among people, such as between a young person, a senior citizen, and a dementia patient. Therefore, it is possible to detect dementia at an early stage by measuring a difference in this ability. Experiments for sound source localization in the horizontal plane show that the ability is improved by separating the presented locations between the signal and a masker. However, there are few data regarding sound localization in the vertical plane. The threshold in the perpendicular plane has been measured, but only experiments in the median plane regarding sound localization have been reported, and its characterization in other aspects has not been clarified. Previous studies about localization ability in the vertical plane have reported contradictory results. One is that the sound source from an upper direction is perceptually superior for a subject, and the other is that a lower direction is superior. The purpose of this study in this chapter is to clarify sound localization ability in the vertical plane and to detect dementia in the early stage using the aging tendency of aural characteristics.
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Sound Localization

Sound localization is an ability that allows a person to judge the direction of a sound source from the information of the sound. Figure 1 shows an example of masking. The cues for sound localization are interaural time and level differences and changes of the spectra. Interaural time and level differences are important in sound localization in the horizontal plane, and changes of the spectra are important in sound localization in the vertical plane.

Figure 1.

Example of sound localization

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Masking

Masking refers to the inability to hear a signal because of a masker. There are various kinds of masking, but this study focuses on simultaneous masking. Figure 2 shows an example of masking.

Figure 2.

Example of masking

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Experiment

Subjects

Ten subjects ranging from 21 to 24 years in age were paid for their participation. All subjects had normal hearing as measured by pure-tone audiometry.

Stimuli

The experimental stimuli used were similar to a previous study. Table 1 shows the parameters of the signals and the maskers.

The signal was a 500-Hz or 4000-Hz pure tone. The signal was 1000 ms in duration. The signal that was measured at the position of the subject’s head was constant at 60 dB. The masker was a 500-Hz pure tone, a 4000-Hz pure tone, or white noise (WN; 125–16000 Hz). The masker was always presented during an experiment. The pure-tone maskers of 500 Hz and 4000 Hz that were measured at the position of the subject’s head were constant at 50 dB and 55 dB, and the white noise masker was constant at 50 dB.

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