Identification of the relative onset time of a two-tone complex: auditory sensitivity or language based?

Categorical perception occurs when equal physical differences on a continuum sometimes cause no changes in the identification of a stimulus, while at other points on the same continuum, that difference causes an abrupt change in perception. One of the unresolved issues regarding this phenomenon is what determines categorical perception: a special speech mode or "natural" psychophysical boundaries. One way to answer this question is by investigating categorical perception with non-speech stimuli. An example is the identification of the relative onset of a two-tone complex (TOT) analogous to voice-onset time (VOT), the acoustic cue to voicing in initial position. Studies in English found similar category boundaries for TOT and VOT (at +20 ms) supporting the non-speech specific theory. The purpose of this study was to investigate TOT in Hebrew-speaking listeners whose language uses very different VOT values from those reported in English. Twenty Hebrew-speaking young adults participated in this study. Stimuli consisted of a two-tone complex that varied in the relative onset time of the lower tone from a lead of -50 ms to a lag of +50 ms in 10 ms steps. Results show that: (1) All subjects were able to identify the lag conditions from the simultaneous ones but only half of them were also able to identify the lead from the simultaneous ones. This was explained in terms of prominent pitch cues available when shifting from simultaneous to lagging stimuli. (2) Hebrew category boundaries (CBs) for TOT are shorter than those of VOT, and both are shorter than the respective English ones. Nonetheless, all CBs fell into the range of 10 to 30 ms. The data support the hypothesis that "natural" psychophysical boundaries determine categorical perception but behavioral measures may be influenced by speech.