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

Litovsky, Ruth Y., Brad Rakerd, Tom C. T. Yin, and William is symmetrical with respect to the two ears. Although microM. Hartmann. Psychophysical and physiological evidence for a phone measurements normally find some binaural differprecedence effect in the median sagittal plane. J. Neurophysiol. ences for sources in the sagittal plane (Searle et al. 1975; 77: 2223–2226, 1997. A listener in a room is exposed to multiple Wightman and Kistler 1989), the preponderance of evidence versions of any acoustical event, coming from many different direcshows that these differences are not reliable enough to serve tions in space. The precedence effect is thought to discount the as localization cues (Asano et al. 1990; Hebrank and Wright reflected sounds in the computation of location, so that a listener 1974; Middlebrooks and Green 1991). Instead, sources in perceives the source near its true location. According to most audithe median sagittal plane are localized on the basis of spectral tory theories, the precedence effect is mediated by binaural differshape cues, peaks and valleys introduced by direction-depenences. This report presents evidence that the precedence effect operates in the median sagittal plane, where binaural differences dent filtering performed by the external ears, head, and torso are virtually absent and where spectral cues provide information (Blauert 1983; Gardner and Gardner 1973; Roffler and Butregarding the location of sounds. Parallel studies were conducted ler 1968). in psychophysics by measuring human listeners’ performance, and The present study is a search for a precedence effect in the in neurophysiology by measuring responses of single neurons in median sagittal plane, a precedence effect that is mediated by the inferior colliculus of cats. In both experiments the precedence spectral shape cues. Early evidence that such an effect likely effect was found to operate similarly in the azimuthal and sagittal exists was found in a front-back competition experiment by planes. It is concluded that precedence is mediated by binaurally Blauert (1971). The present search was conducted on two based and spectrally based localization cues in the azimuthal and levels: one psychophysical in humans, the other physiologisagittal planes, respectively. Thus,models that attribute the prececal in cats. dence effect entirely to processes that involve binaural differences are no longer viable.