May 30, 2024

Normal speakers don't ignore auditory feedback

A comment to Lazzari et al. (2024)

The core of my stuttering theory is that stuttering occurs because too little attention is directed to the auditory feedback of speech, and too much to speech planning, motor control (the sensory feedback of articulatory movements), emotions, and other things. The theory implies that the attention system in non-stutterers is more stable and ensures that auditory feedback is always automatically involved in speech-motor control.

It is thus not surprising that I was happy to read the article “Subtle patterns of altered responsiveness to delayed auditory feedback during finger tapping in people who stutter” by Giorgio Lazzari, Robert van de Vorst, Floris von Vugt, and Carlotta Lega. Their most important finding is that stutterers could, but non-stutterers could not disregard auditory feedback in a finger-tapping task.

In the study, stutterers and normally fluent controls performed a synchronization-continuation finger-tapping task. They heard the auditory feedback of their finger tapping through headphones, with various delays. In one experimental condition, the instruction required participants to ignore the feedback, align their physical tap to the clicking of a metronome, and continue in that rhythm even after the metronome signal was switched off.

The result was, that the controls, despite the instruction, could not disregard auditory feedback, even though it was irrelevant to the task. The feedback delay affected their motor performance; they had difficulty maintaining the rhythm. By contrast, the stutterers could better ignore auditory feedback; they maintained consistent motor performance across different delay levels.

The finding sheds light on the (in my view) crucial difference between normal speakers and stutterers when producing sounds by movement. Normal speakers automatically pay attention to auditory feedback—they cannot ignore it. This ensures that auditory feedback is well involved in motor control. By contrast, stutterers can ignore auditory feedback, and they often do so when speaking; they instead focus much on speech planning and/or the sensory feedback of articulatory movements. This results in poor auditory-motor integration and stuttering.

That stutterers ignore auditory feedback when speaking is also suggested by neurological findings: (1) by lack of activation in the posterior superior temporal cortex during speech compared to normally fluent speakers (see Table 1) and (2) by the correlation (found in normal speakers) between attention to auditory speech stimuli and activation in those cortical areas (Jäncke, Mirzazade, & Shah, 1999; Hugdal et al., 2003; Sabri et al., 2008).

The authors conclude that stutterers differ from normal speakers “in specific subcomponents of sensorimotor integration mechanisms.” That’s correct if we consider attention to auditory feedback a subcomponent of the auditory-motor integration mechanism. An important, although not surprising, result of the study is: whether auditory feedback is involved in motor control depends on attention to auditory feedback. Not a few researchers have assumed that stuttering results from poor auditory-motor integration, but the relationship between attention to auditory feedback and auditory-motor integration has been largely overlooked until now.

In an experimental condition, in which participants were instructed to pay attention to auditory feedback and to align it in time with the metronome signal, there was no difference in performance between stutterers and controls; auditory feedback was involved in motor control. By contrast, when the stutterers ignored auditory feedback, it did not affect motor control. We can thus conclude, at least for the stuttering group, that attention to auditory feedback determined auditor-motor integration.

I think that, in the synchronization-continuation finger-tapping task, the delay of auditory feedback played no role other than to help discover that the controls couldn’t ignore auditory feedback (delayed or not), but the stutterers could; there is no direct relationship to the (seemingly opposed) effects of DAF on stutterers and non-stutterers when speaking (see below).

To summarize, the evidence that (1) non-stutterers cannot ignore auditory feedback, but stutterers can, and that (2) attention to auditory feedback determines whether it is involved in motor control are valuable pieces of the puzzle that we must put together to reveal the causes of stuttering.

The effect of DAF on speaking

The delay time best for reducing stuttering is 50–75 ms (Armson & Kiefte, 2008; Kalinowski et al., 1996; Lincoln, Packman, & Onslow, 2006). Such a short delay of auditory feedback hardly evokes disfluencies in normal speakers (Foundas et al., 2004a, 2013; Stuart et al., 2002); even a delay of 120 ms has little effect on normal speakers (Foundas et al., 2004a). However, it sounds unfamiliar and attracts attention; stutterers no longer ignore the auditory feedback of their speech. This improves auditory-motor integration and reduces stuttering.

DAF can also cause disfluencies in normal speakers, the so-called Lee effect (Lee, 1951). The delay time maximally disrupting for normal speakers is about 200 ms (Fairbanks & Guttman, 1958; Stuart et al., 2002). Such a long delay probably disturbs self-monitoring and auditory-motor integration, which manifests in prolonged speech, repetitions, and other, not stuttering-like, errors. This is the case not only with normal speakers, but also with stutterers (Hayden, Scott, & Addicott, 1977). Stutterers may be better able to ignore such adverse DAF and instead focus on the kinesthetic and tactile feedback of their articulatory movements. But in those studies, they were not instructed to ignore the DAF.

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