Inhibition of stuttering from second speech signals: An evaluation of temporal and hierarchical aspects

Stuttering is an intermittent and involuntary speech disorder overtly characterized by syllable repetitions, phoneme prolongations and postural fixations that disrupt the natural flow of speech. Overt stuttering is reduced by 60–100% as the person who stutters produces speech while perceiving an ongoing second speech signal. The purpose of the current investigation was to further examine mechanisms of stuttering inhibition during perception of second speech signals. To do this the researcher conducted two experiments examining the level of inhibition during temporal-spatial alignment alterations and level of inhibition during hierarchically difficult scripted telephone conversations under combined altered auditory feedback signals. The first study examined temporal-spatial alignments of speakers during choral and shadow speech. Choral speech is when two speakers talk in approximate simultaneity. This effect is believed to be the most powerful inhibitor of stuttering, reducing it 90–100%. A slightly less powerful inhibitor of stuttering is shadow speech, which is historically defined as the person who stutters lagging or shadowing behind a fluent speakers utterance. Reductions under shadow speech typically range from 80–90%. Interestingly, prior to the current investigation, empirical analysis of output from people who stutter (PWS) when maintaining the lead speaker position during shadow speech, had yet to be evaluated. This temporal-spatial alignment most mimics delayed auditory feedback with a second speaker. Experiment I included four conditions: 1) choral speech, 2) shadow speech with the person who stutters maintaining the lead speaker position, 3) shadow speech with the person who stutters maintaining the lag speaker position, and 4) baseline. Nine participants who stutter verbally read 300 syllable passages while a second fluent speaker read the same passage and maintained close temporal-spatial alignments during choral conditions and three to four word separations during shadow speech conditions. Stuttering frequency was significantly reduced 95% during choral speech and approximately 80% during both shadow speech conditions. Results challenge notions put forth by previous hypotheses regarding reductions in stuttering during perception of second signals. Experiment II examined stuttering inhibition during scripted telephone conversations under altered auditory feedback. As with the lag shadow speech condition, altered auditory feedback and more specifically delayed auditory feedback, presents a second speech signal along with ongoing speech. Delayed auditory feedback and frequency-altered feedback generate second speech signals from the speakers' initial speech productions. Reductions in stuttering frequency under altered auditory feedback typically range from 60–80%. These reductions occur during the presentation of one signal and one combination of signals across a variety of settings; specifically, telephone conversations, which are judged to be one of the most hierarchically difficult situations for people who stutter. The second experiment examined nine people who stutter during 15 scripted telephone conversations under baseline, one combination of DAF and FAF (i.e., 50 ms delay and ½ octave shift up respectively; 1 COMBO), and two combinations of DAF and FAF (i.e., 1 COMBO plus 200 ms delay and ½ shift down respectively; 2 COMBO). Stuttering was significantly inhibited during both altered feedback conditions (i.e., 63% during 1 COMBO and 74% during 2 COMBO). Furthermore, significant reductions in stuttering frequency during the 2 COMBO conditions as compared to the 1 COMBO indicated that presentation of increased gestural information enhances the inhibitory effects. Results from both studies challenge notions put forth by previous models of stuttering reduction during the perception of second signals. The findings that stuttering was reduced to similar extents during both lead and lag conditions in Experiment I and that more robust stuttering inhibition occurred during the COMBO II condition in Experiment II, challenge fundamental notions from previous reduction theories during the perception of second signals. Furthermore, these findings support the flexible and dynamic gestural percepts hypothesized in the Gestural Model of Stuttering Inhibition. It is likely that increased gestural information alters mirror neuron system activation patterns, which enables a more efficient and effective release of the central neural block that is stuttering, therefore increasing inhibitory effectiveness from the perception of second signals