A Theory of Stuttering
A Theory of Stuttering
In the last section, I have explained stuttering as caused by invalid error signals in the monitoring system that result in interruptions of speech flow. A similar explanation was proposed by Maraist and Hutton (1957): “the stutterer misevaluates his own speech output at some point in the control system and finds error where, in reality, no error exists. The result of his attempt to correct this non-existent error is stuttering” (p. 385). In contrast, I think that the blockage of speech flow because of an invalid error signal is an automatic brain response and independent of the speaker’s will. The speaker does not attempt to correct an error; he or she is not aware of any error and attempts to overcome the blockage of speech, and just this causes the observable stuttering symptoms.
The view that stuttering occurs because of invalid error signals also appears similar to the Covert Repair Hypothesis (Postma & Kolk, 1993). But they believed that the error signals were valid: a real error in speech planning is detected by pre-articulatory monitoring and is covertly repaired by the Formulator (the theory depends on Levelt’s model of speech processing), and these covert repairs, which are unconscious to the speaker, cause stuttering. By contrast, I do not assume that any real speech error, neither in planning nor in execution, causes stuttering.
Vasic and Wijnen (2001, 2005) proposed a variety of the Covert Repair Hypothesis; in my view, it is rather a variety of Maraist and Hutton’s (1957) hypothesis. They assume that normal speech disfluencies are misevaluated as errors by an oversensitive monitoring system, and the speaker’s attempt to avoid these normal disfluencies causes stuttering. I discuss this hypothesis extensively in Section 2.5.
Hickok, Houde, and Rong (2011) have assumed that stuttering is caused by invalid error signals that result from inaccurate, “noisy” mapping between the internal model of the vocal tract and the sensory system. “This results in a sensory-to-motor ‘error’ correction signal, which itself is noisy and inaccurate. In this way, the system ends up in an inaccurate, iterative predict-correct loop that results in stuttering.” (p. 13). This theory has some similarity with the Covert Repair Hypothesis, as it does not assume a mismatch between sensory prediction and external sensory, i.e., auditory feedback, but between sensory prediction and an internal target representation. That is, they assume an invalid error signal in an internal feedback loop.
Invalid error signals as the cause of stuttering were also proposed by Tian and Poeppel (2012). They assume “that one of the neural mechanisms causing stuttering is a deficit in the motor-to-sensory transformation. That is, the noisy perceptual estimation is mismatched to the external feedback. Such a discrepancy would signal an incorrect error message, and the feedback control system would interpret such an apparent error as the requirement to correct motor action. Hence, unnecessary attempts would be performed to modify the correct articulation, resulting in repetitive/prolonged sound or silent pauses/blocks.” (p. 7). A similar hypothesis had already been proposed by Max et al. (2004).
However, we can assume that auditory predictions and auditory feedback form the basis of the self-monitoring of speech, and supposing inaccurate auditory predictions amounts to the claim that stutterers do not exactly know what their words should sound like. If that were the case, it would not cause invalid error signals, but rather impair the detection of real speech errors.
Moreover, the above theories cannot answer the question of why stuttering does not yet occur in the babbling period or in the period of one-word utterances, when the mapping between the internal model of the vocal tract and the sensory system may in fact still be unstable. As is well known, stuttering typically onsets later, at the time when children start forming sentences.
A further feedback theory of stuttering was proposed by Chang and Zhu (2013). Referring to structural deficits in white matter tracts interconnecting left frontal motor and posterior auditory areas in stutterers, they write: “Insufficient white matter integrity between these regions may lead to subtle inefficiencies in one’s ability to match the auditory target associated with one’s own motor execution (articulation) to actual auditory feedback. If a mismatch occurs between the intended (predicted) auditory target of the speech produced and the actual auditory feedback, the auditory cortex sends corrective signals to the motor system to modify the motor programme for subsequent articulations […].” (p. 14).
What is the difference to my position? First, Chang and Zhu assume that stuttering results from a disruption of the feedback-based online (‘within the flow’) control of speech; they refer to Cai et al. (2012), who examined the online control of speech based on auditory feedback. By contrast, I think that stuttering is elicited by an ‘offline correction’ mechanism that interrupts speech flow to enable the correction of a slip of the tongue (read more).
Second, I think that not insufficient white matter integrity but insufficient attention to auditory feedback and, resulting from this, insufficient processing of auditory feedback causes the mismatch between auditory targets (predictions) and feedback. If stuttering were caused by a relatively invariable structural deficit, it could not be as variable as it is and come and go from one moment to the next.
The above theories show that not a few researchers thought or think in the same direction as I do: that stuttering is not a breakdown of speech control because of weakness or overload, but a response of the control system to error signals that have often to do with auditory feedback in some way. Future research and theoretical modeling must reveal which of the theories is in the best agreement with all relevant data and has the greatest explanatory power regarding the many well-known features of the disorder.
When we notice that we are speaking too loudly or with too high a voice given the situation, we can change the volume or pitch of our voice without interrupting the flow of our speech. Such correction is also made automatically; we spontaneously speak louder in a noisy environment (Lombard effect). By contrast, when we confuse or mispronounce a word or make a grammar mistake, we can repair the error only by interrupting ourselves and speaking the word or phrase again correctly. A premise of my stuttering theory is that these interruptions of speech flow are automatic after structural (non-semantic) speech errors; see Section 2.1.
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