The effects of temporal modification of second speech signals on stuttering inhibition at two speech rates in adults.

The recovery of 'gestural' speech information via the engagement of mirror neurons has been suggested to be the key agent in stuttering inhibition during the presentation of exogenous second speech signals. Based on this hypothesis, we expect the amount of stuttering inhibition to depend on the ease of recovery of exogenous speech gestures. To examine this possibility, linguistically non-congruent second speech signals were temporally compressed and expanded in two experiments. In Experiment 1, 12 participants who stutter read passages aloud at normal and fast speech rates while listening to second speech signals that were 0, 40, 80% compressed, and 40 and 80% expanded. Except for the 80% compressed speech signal, all other stimuli induced significant stuttering inhibition relative to the control condition. The 80% compressed speech signal was the first exogenously presented speech signal that failed to significantly reduce stuttering frequency by 60--70% that has been the case in our research over the years. It was hypothesized that at a compression ratio of 80%, exogenous speech signals generated too many gestures per unit time to allow for adequate gestural recovery via mirror neurons. However, considering that 80% compressed signal was also highly unintelligible, a second experiment was conducted to further examine whether the effects of temporal compression on stuttering inhibition are mediated by speech intelligibility. In Experiment 2, 10 participants who stutter read passages at a normal rate while listening to linguistically non-congruent second speech signals that were compressed by 0, 20, 40, 60, and 80%. Results revealed that 0 and 20% compressed speech signals induced approximately 52% stuttering inhibition. In contrast, compression ratios of 40% and beyond induced only 27% stuttering inhibition although 40 and 60% compressed signals were perceptually intelligible. Our findings suggest that recovery of gestural information is affected by temporal compression before intelligibility starts to decrease.