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Editorial Focus
). In this issue, Tollin and Yin
(2003) explore the phenomenon
behaviorally in cats. They confirm in cats the phenomenon that is well known
in humans in the horizontal plane, and they demonstrate that summing
localization also occurs in the vertical midline plane. Moreover, they
demonstrate a rather unexpected vertical localization illusion that is
produced by sound sources on the horizon. The latter illusion is explained
well by a model that attributes vertical localization to recognition of
spectral shape cues.
The most novel observation in the Tollin and Yin study is that, when cats
were presented with simultaneous sounds located in the horizontal plane (i.e.,
0° elevation), they oriented to a location on the vertical midline about
8° above the horizontal plane. The midline response in the horizontal
dimension is precisely what is predicted by any model of horizontal
localization based on interaural differences between the sounds at the two
ears. It is the vertical response that is remarkable. Most present-day models
of elevation in the vertical dimension assume that vertical localization cues
result from elevation-dependent filtering of sounds by the listener's external
ears. Based on such a model, one would assume that the cats were somehow
hearing sound spectra that they associated with an elevated source. That
assumption was validated by a careful analysis by Tollin and Yin. They used
measurements of the filter functions of a cat's external ears and inferred the
sounds that would have resulted from the addition of two sounds arriving at
each eardrum. The spectra resulting from the interaction of two simultaneous
sound sources located in the horizontal plane were marked by a conspicuous
spectral notch centered near 12 kHz. Examination of the cats' filter functions
for single sounds showed that the filters for 0° elevation showed a notch
centered near 11 kHz, whereas filter functions for elevations near 8°
showed a notch near 12 kHz. This result is consistent with a popular model in
which vertical localization is attributed entirely to identification of the
frequency of a spectral notch (Rice et al.
1992), although the result cannot exclude the possibility that the
cats were basing their location judgments on some other feature of the
spectrum.
The beauty of Tollin and Yin's observation of a vertical localization
illusion in cats is that it provides us with a useful animal model of vertical
localization. A previous study has demonstrated in the cat's auditory cortex a
correlate of a vertical localization illusion
(Xu et al., 1999), but that
study used a narrowband sound that probed vertical localization mechanisms
rather selectively. The new result opens a door to physiological study of
vertical localization of broadband sounds that will permit us to examine the
neural substrates that underline the computations that listeners perform
routinely to localize everyday environmental and communication sounds.
Kresge Hearing Research Institute, University of Michigan, Ann Arbor, Michigan 48109-0506
REFERENCES
Mickey BJ and
Middlebrooks, JC. Responses of auditory cortical neurons to pairs of
sounds: correlates of fusion and localization, J
Neurophysiol 86:
1333–1350, 2001.
Rice, JJ. May BJ, Spirou GA, and Young, ED. Pinna-based spectral cues for sound localization in cat. Hearing Res 58: 132–152, 1992.[ISI][Medline]
Tollin, DJ, and
Yin, TCT. Spectral cues explain illusory elevation effects with stereo
sounds in cats. J Neurophysiol
90: 525–530,
2003.
Xu L, Furukawa S, and Middlebrooks, JC. Auditory cortical responses in the cat to sounds that produce spatial illusions, Nature 399: 688–691, 1999.[Medline]
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