Psychophysical and Physiological Evidence for a Precedence Effect in the Median Sagittal Plane

Ruth Y. Litovsky 1 , Brad Rakerd 2 , Tom C. T. Yin 1 , and William M. Hartmann 3 1 Department of Neurophysiology, University of Wisconsin-Madison, Madison, Wisconsin 53706; and 2 Department of Audiology and Speech Sciences and 3 Department of Physics and Astronomy, Michigan State University, East Lansing, Michigan 48824 Litovsky, Ruth Y., Brad Rakerd, Tom C. T. Yin, and William M. Hartmann. Psychophysical and physiological evidence for a precedence effect in the median sagittal plane. J. Neurophysiol. 77: 2223-2226, 1997. A listener in a room is exposed to multiple versions of any acoustical event, coming from many different directions in space. The precedence effect is thought to discount the reflected sounds in the computation of location, so that a listener perceives the source near its true location. According to most auditory theories, the precedence effect is mediated by binaural differences. This report presents evidence that the precedence effect operates in the median sagittal plane, where binaural differences are virtually absent and where spectral cues provide information regarding the location of sounds. Parallel studies were conducted in psychophysics by measuring human listeners' performance, and in neurophysiology by measuring responses of single neurons in the inferior colliculus of cats. In both experiments the precedence effect was found to operate similarly in the azimuthal and sagittal planes. It is concluded that precedence is mediated by binaurally based and spectrally based localization cues in the azimuthal and sagittal planes, respectively. Thus,models that attribute the precedence effect entirely to processes that involve binaural differences are no longer viable.