Effects of preceding context on discrimination of voice onset times

When discriminating pairs of speech stimuli from an acoustic voice onset time (VOT) continuum (for example, one ranging from /ba/ to /pa/), English-speaking subjects show a characteristic performance peak in the region of the phonemic category boundary. We demonstrate that this "category boundary effect" is reduced or eliminated when the stimuli are preceded by /s/. This suppression does not seem to be due to the absence of a phonological voicing contrast for stop consonants following /s/, since it is also obtained when the /s/ terminates a preceding word and (to a lesser extent) when broadband noise is substituted for the fricative noise. The suppression is stronger, however, when the noise has the acoustic properties of a syllable-initial /s/, all else being equal. We hypothesize that these properties make the noise cohere with the following speech signal, which makes it difficult for listeners to focus on the VOT differences to be discriminated.