Durational shrinkage by noise replacement in quasi-isochronous and hyper-isochronous contexts

When a portion of a sound is replaced by a noise burst, its duration is perceived to be shorter than that of its intact counterpart. To test the robustness of this shrinking effect by noise replacement and to validate the hypothesis that duration can be estimated as a function of accumulated perceptual evidence for the target sound, the shrinking effect was investigated with tonal stimuli in two contextual temporal structures. Two experiments are conducted using (1) a tone with an envelope pattern copied from a naturally spoken word, and (2) an isochronous sequence of four tones. In most cases, the noise replacement causes the perceived duration of the target tone to shrink from that of its intact counterpart. However, a reversal/prolongation tendency by noise was observed for the stimulus with a deviation slightly shorter than an isochronous structure in the second experiment. Although this reversal tendency partially supports the hypothesis that a noise merely enhances a contextual effect (the contextual enhancement hypothesis), the shrinking effect observed under the other conditions was difficult to explain by the contextual enhancement hypothesis. The shrinking effect could be explained in a framework of the traditional neural counting mechanisms with one additional mechanism to control the degree of gate opening depending on the perceptual evidence of the target sound.