Page 18 - Summer 2008

in vision: opening one eye and then the other made apparent the differences between them. These were remarked upon and examined empirically by Ptolemy in the second century and by Alhazen in the tenth. They were illustrated in the eighteenth century, although the involvement of spatial dif- ferences in the images project- ed to each eye in stereoscopic depth perception was not understood until the nine-
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teenth century. Similarly,
double vision of a single object could readily be experienced by either crossing the eyes or by pressing one of them with a finger; Aristotle described the consequences of the latter. However, phenomenal differ- ences between the ears were more difficult to discern; the most common procedure was to cover one ear or to stop it by means of a finger, and this method was employed in the late eighteenth and early nine- teenth centuries.
Secondly, the mobility of
the eyes in both version and
vergence is not matched in
humans with a corresponding
mobility of the ears. Moreover,
abnormalities of binocular eye
movements, as in strabismus,
were an early topic of medical concern. The consequences of
losing an eye for spatial vision were remarked upon in the sev-
enteenth century, with a corresponding appreciation of the
advantages that accrue to having two functioning and aligned
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The stereoscope revolutionized studies of binocular vision—it provided both a device for con- trolling the stimulus to each eye and a means for establishing the link between retinal disparity and depth perception. With the aid of the stereoscope, disparate patterns could be produced with ease, and the consequences of viewing them could be
determined.
When binaural instruments were introduced in the nine-
teenth century, they were fashioned on binocular instruments
Thirdly, the manipulation of spatial patterns of illumina- tion was much easier than that for temporal differences in sound patterns. Even before the invention of the stereoscope in the 1830s by Charles Wheatstone (1802–1875), several means of presenting different stimuli to the two eyes were available. These consisted of bifixating a close point while presented with two more distant and horizontally displaced patterns, viewing two objects through an aperture, placing a prism before one eye or viewing two patterns on opposite sides of a septum, or look-
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eyes.
ing down two viewing tubes.
such as the stereoscope and pseudoscope. However, it was not until the invention of headphones and precise elec- tronic control of sounds that a detailed investigation of bin- aural hearing became feasible. Nevertheless, studies of binau- ral integration did take place before precise stimulus control became available. In this arti- cle we review these early stud- ies and speculations, and relate them briefly to more recent work in the field.
Early speculations and informal studies
The seeds of the revolu- tion in binaural hearing were sown by an American and an Italian, long before the stethophone was devised, and their studies were both theoretical and empirical. The source of this interest derived from inquiries into binocular integration—more specifically, it was stimulated by studies of binocular color mixing. In contrast to the lawful combination of colors from different regions of the spectrum, it was found that presenting different colors to each eye did not follow simi- lar rules. The colors tended
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combination.
Wells was born in Charlestown, South Carolina, to
Scottish parents; he returned to Scotland for his education and graduated in medicine from Edinburgh University and eventually practiced in London. He is best known for his the- ory of the formation of dew, and he also anticipated Charles
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Among those who later pursued binocular color combination were Wells, Venturi, Wheatstone, Ernst Heinrich Weber (1795–1878) and August Seebeck (1805–1849), and it is notable that all these thinkers were to consider similar aspects of binaural
to engage in rivalry rather than fusion.
A foot- note in his monograph on binocular visual direction (the title page of which is shown in Fig. 2) reads: “From the fact of the two colours being thus perceived distinct from each other, I would infer, by analogy, a mode of argument indeed often fal- lacious, that if it were possible for us to hear any one sound with one ear only, and another sound with the other ear only, such sounds would in no case coalesce either wholly or in part, as two sounds frequently do, when heard at the same time by one ear; that consequently, if the sounds of one musical instru- ment were to be heard by one ear only, and those of another, by
Darwin in proposing a theory of natural selection.
Fig. 2. The title page of Wells’s essay in which he described his thought experiments concerning the perceptual combination of sounds at the two ears.
Binaural Hearing and the Stethophone 17