A critical examination of the spectral contrast account of compensation for coarticulation.

Vocal tract gestures for adjacent phones overlap temporally, rendering the acoustic speech signal highly context dependent. For example, following a segment with an anterior place of articulation, a posterior segment's place of articulation is pulled frontward, and listeners' category boundaries shift appropriately. Some theories assume that listeners perceptually attune or compensate for coarticulatory context. An alternative is that shifts result from spectral contrast. Indeed, shifts occur when speech precursors are replaced by pure tones, frequency matched to the formant offset at the assumed locus of contrast (Lotto & Kluender, 1998). However, tone analogues differ from natural formants in several ways, raising the possibility that conditions for contrast may not exist in natural speech. When we matched tones to natural formant intensities and trajectories, boundary shifts diminished. When we presented only the critical spectral region of natural speech tokens, no compensation was observed. These results suggest that conditions for spectral contrast do not exist in typical speech.