The effects of delayed auditory feedback depend on both the rhythm and content of the feedback signal

Delayed auditory feedback (DAF) has long been known to disrupt fluent speech, yet the underlying mechanisms remain a matter of debate. Some have hypothesized that interference occurs because of asynchronous sensory and motor rhythms, independent of the specific content of auditory inputs. The goal of this study was to compare the effects of canonical DAF with those of delayed envelope modulated noise (EMN), a manipulation that preserves the speech envelope but removes other speech content. We hypothesized that canonical DAF would disrupt speech more than delayed EMN, revealing an important role for speech-related spectral content in feedback-driven interference of speech.

38 neurologically healthy adults with no history of speech, language, or hearing disorders produced two types of speech materials (non-lexical 3-syllable sequences and sentences) under five auditory feedback conditions (speech feedback with minimal or 200 ms delay, EMN with minimal or 200 ms delay, and constant speech-shaped noise). Primary outcome measures were speech rate and error probability, with secondary measures including fundamental frequency, intensity, and within-trial modulation of each. Audio recordings were transcribed and analyzed using mixed-effects models for each outcome measure and stimulus type.

For each outcome measure and stimulus type, the effect of delay differed significantly between the SPEECH and EMN conditions. Speech feedback with 200 ms delay elicited large reductions in speech rate (~0.8-0.9 syllables/s) and large increases in error probability (~31-44%), which were consistent across stimulus types. EMN with 200 ms delay, on the other hand, elicited much smaller effects, with slight increases in speech rate (~0.13-0.14 syllables/s) and no significant changes in error rate. Delayed speech also caused increases in intensity and intensity modulation and decreases in pitch modulation, whereas results were less consistent for delayed EMN.

These findings suggest that the presence of speech-related spectral content in the auditory feedback signal, above and beyond the asynchrony of speech rhythms, is necessary to elicit the full disruptive effects of DAF. We describe study results within a theoretical framework that includes roles for both rhythmic asynchrony and feedback content in explaining these disruptions.