A Clinical Test for Sound Localization and Its Applications

ing a temporal lobectomy is the failure to accurately localize the source of sound in the opposite auditory field by using a simple bedside tuning fork method. Even with this simple test, it is clear that this impairment is frequent and warrants further consideration. This project was started with the idea of designing a simple, readily performed, accurate clinical test for sound localization. Once the test was standardized, a quantitative normal range was established and the defects in a preliminary group of cases were observed. The journals devoted to clinical neurology contain few papers concerning the problems in relation to sound localization, so that a review of the accumulated data is indicated. The world literature is very extensive and diversified, for which reason an attempt is made to group the studies into two categories. One group is concerned with the role of sound physics and peripheral factors in sound localization, and the other deals with the role of the central neural mechanisms. According to Boring,l the psychologists of the nineteenth century, for the most part, doubted the existence of any real auditory space perception. The empiricists, like Berkeley, Mill, and Bain, accepted the hypothesis that past experience was the basis for sound localization. The modern concept of auditory space perception began, perhaps, with the observations of the Webers. E. H. Weber2 made the observation that, when two watches were placed near an observer's ears, they could both be heard simultaneously and each could be referred to its proper side. This perhaps marked the origin of the idea that the primary dimension of auditory localization is left and right. E. F. Weber3 studied sound localization with the observer's head submerged in water, and the fact that a more consistent de P ect followfrom his results concluded that sound could not be localized accurately except from left to right. He erroneously attributed sound localization to tactual differences at the external canal. Bloch,4,5 studying sound localization, found that the source of sound is better localized in front in the median plane, less accurately localized in the back, and least accurately from the sides. He also noted that there is no confusion of left and right. Matsumotos in 1897, using a sound cage for the first time, duplicated Blochs experiments. In attempting to explain the mechanisms involved in sound localization, out of the variety of experiments and works three theories have been postulated: the intensity theory, the phase theory, and the time theory. The intensity theory, which is the oldest of the three, is based on the principle that when two sounds, varying only in intensity, are led to both ears simultaneously, the observer will tend to localize the sound toward the side where it is of greater intensity. The phase theory is based on the observation that when two tones, varying only in phase, are led to the ears of an observer, he will tend to localize the source of sound as coming from the side of the leading phase. The time theory is based on the principle that when two sharp sounds, like two clicks, varying only in time, are led to the ears of the observer, he will localize the sound as coming from the side of the first arrival. The only difference between the time and the phase theory is that the first depends on the time of arrival of a single sound, while the second depends upon the time of arrival of the crest of the wave phase of a sound, like a pure tone. Many reports in the literature favor the intensity theory. '-20 Lord Rayleighs-11 definitely established the theory and the fact that voices are localized easier than pure tones. Thompson15-17 built his pseudophone with an attached artificial pinna, and his experiments favor the intensity theory. He also demonstrated the effect of the external ear in the localization of sound.