Spectral contrast effects produced by competing speech contexts

The long-term spectrum of a preceding sentence can alter the perception of a following speech sound in a contrastive manner. This speech context effect contributes to our ability to extract reliable spectral characteristics of the surrounding acoustic environment and to compensate for the voice characteristics of different speakers or spectral colorations in different listening environments to maintain perceptual constancy. The extent to which such effects are mediated by low-level "automatic" processes, or require directed attention, remains unknown. This study investigated spectral context effects by measuring the effects of two competing sentences on the phoneme category boundary between /I/ and /e/ in a following target word, while directing listeners' attention to one or the other context sentence. Spatial separation of the context sentences was achieved either by presenting them to different ears, or by presenting them to both ears but imposing an interaural time difference (ITD) between the ears. The results confirmed large context effects based on ear of presentation. Smaller effects were observed based on either ITD or attention. The results, combined with predictions from a two-stage model, suggest that ear-specific factors dominate speech context effects but that the effects can be modulated by higher-level features, such as perceived location, and by attention.