Friday, January 26, 2018

Persistence versus recovery of stuttering

In this post, I want to sketch some ideas about the development of stuttering. Today stuttering is often called a neurodevelopmental disorder, which might be correct, however, the brain is plastic, especially in young children, and its development interacts with learning and with the development of behavioral habits and routines.

A part of behavior often overlooked is the allocation of attention, that is, of perceptual and processing capacity during motor tasks, including speaking. In my theory of stuttering, I assume that just this mostly unconscious aspect of behavioral control is the crucial causal factor. The figure below shows the hypothesized development from stuttering onset to recovery (persistent stuttering is not included in this figure). The colors mean: blue = developmental steps, red = causal factors, yellow = triggers or secondary negative factors, green = positive factors.

Transient developmental stuttering
Devekopment of transient stuttering and recovery

I think the cause of childhood stuttering, in most cases, is an imbalance in the development of sensorimotor integration, namely to the favor of action and to the detriment of perception and feedback processing. This becomes problematic in the change to connected speech and sentence production which requires a stronger involvement of auditory feedback and the feedback of breathing in speech control – the brain must perceive and keep in memory which constituents of a sentence have already been produced in order to correctly complete the sentence, and breathing times must be included in the speech sequence.

Of course children are not aware of changing behavior when they start forming sentences. Empirical findings suggesting such an imbalance in the attention system are
  • a high number of dopamine D2 receptors at age 2.5 to 3 (Alm 2004, Fig. 2; see also Rothmond et al. 2012, Fig. 3), associated with a tendency towards generally high motor activity;
  • lower fractional anisotropy (FA) in the left arcuate fasciculus, probably indicating delayed fiber maturation (see Fig. 1, Cluster 1 and 2 in Chow and Chang, in press – interestingly, these clusters of lower FA were much larger in the recovered group than in the persistent group). The structural deficits might be the manifestation (rather the result than the cause) of poor involvement of sensory feedback in motor control.
  • atypical network connectivity in children who stutter (persistent as well as recovered) compared to controls. See Fig. 5 in Chang et al (2018), especially the reduced connectivity between Dorsal Attention Network and Default Mode Network and the hyperconnectivity between Ventral Attention Network and Default Mode Network, indicating an imbalance in the control of attention. Interestingly, there was even found a reduced connectivity in the visual network in stuttering children (Fig. 4), suggesting a general deficit in the involvement of sensory input in the control of behavior.  
The idea that the onset of stuttering in most cases is related to the beginning of connected speech and sentence forming is not new. Bloodstein (2006) pointed to the facts that “early stuttering occurs only on the first word of a syntactic structure; stuttering does not appear to be influenced by word-related factors; early stuttering seldom occurs on one-word utterances; the earliest age at which stuttering is reported is 18 months, with the beginning of grammatical development; the age at which most onset of stuttering is reported, 2-5 years, coincides with the period during which children acquire syntax; considerable spontaneous recovery takes place at the time most children have mastered syntax; incipient stuttering is influenced by the length and grammatical complexity of utterances...”
 
A further suggestion to an imbalance in development as a causal factor comes from the observation that not a few children with early-onset stuttering show syntactic abilities and length of utterances well above the norm for their age (Watkins, Yairi, and Ambrose, 1999). Alm(2004) writes about that: “The group with early onset stuttering, who entered the study at age 2-3, showed syntactic abilities and length of utterances well above what was expected for their age. In fact, in some aspects the language abilities in this group were on a level with the norms for 2 years older children. This was true both for children who recovered and for children who persisted to stutter.” Too much attention (capacity) may be taken by sentence planning, and too little attention may remain for perception and feedback processing in these children.

Late-onset stuttering and the so called ‘psychogenic stuttering’ was included in the above figure because I think that cause and pathomechanism are the same as in developmental sttutering, but affected individuals seem to have no strong predisposition for stuttering, especially not for persistent stuttering. But in a person whose attention system is vulnerable, strongly negative emotions, distress, fear, or the aftermath of a trauma may result in a misallocation of attention during speech and by that in stuttering. Complete recovery is often reached in such cases by a supporting environment and/or by therapy, including psychotherapy that helps coping with traumatic experiences (see Table 1 in Chang et al.,2010).

Spontaneous recovery in general may be caused by a kind of unconscious learning effect: Children eventually learn to adapt their attentional allocation, that is, the allocation of their perceptual and processing capacity to the new demands of connected speech and sentence production. Such a learning effect manifests in brain structure: See, e.g., the upward trajectories of FA for the recovered group in Fig. 1 (Cluster 3 and 5), and Fig. 2 (Cluster 6) in Chow andChang (in press). I interpret those changes in brain structure as consequences of learning since several studies have shown that even a few weeks of practice (e.g., in reading, juggling) result in changes of gray or white matter structure (see Section 4.1 on my website).

When I assume that most stuttering children eventually learn to adapt their attentional allocation to the demands of connected speech. then this doesn’t mean they all learn to behave completely in the same way as children who have never stuttered do. The results depicted in Fig. 6 in Chang et al. (2008) suggest that the left brain hemisphere played a minor and the right one a greater role in speaking in the children who had recovered from stuttering, compared to the normal fluent controls. The cause could be that speech control in the recovered children, on average, was not as automatic (i.e., more volitional) as in normal fluent children. 

I don’t like to speak of compensation in the brain here, because there is no little manager sitting in the brain who notices a dysfunction and bids another part of the brain to sub. It is rather the child who becomes aware of a problem and tries to cope with it, and this behavior, over time, results in a structural change in the brain.

Persistent developmental stuttering


Development of persistent stuttering
 
If my above ideas about transient stuttering are true, then the question arises: Why does a minority of the children affected by stuttering not learn to adapt their attention to the demands of connected speech? Is there an additional causal factor impairing the allocation of attention in these children? In fact, there seems to exist such a factor: a subtle deficit in central auditory processing. Many empirical findings point in this direction (see Section 3.3.1 for an overview). Unfortunately, we have data about auditory processing in persistent stuttering only, but not in individuals who recovered. 

 Unfortunately, there are few data comparing auditory processing in those who persisted in stuttering and in those who recovered. The only study I know is by Howell, Davis, and Williams (2006) who compared children of both groups in a backward-masking task the performance or which is assumed to reflect the operation of central auditory processing, especially of temporal structure. They found an appropriately 10 decibel higher mean backward-masking threshold in the persistent group. The difference was statistically significant, but there was a high variability in the persistent group, thus the authors conclude that an auditory deficit may be sufficient, but not necessary, for the disorder to persist.
 
Further, Usler and Weber-Fox (2015) and Mohan and Weber (2015) found differences between children with persistent stuttering and those who recovered in the processing of auditorily presented verbal stimuli. Furthermore, Chow and Chang (in press) found a structural deficit (lower FA) in fibers of the splenium, i.e., the posterior part of the corpus callosum in children who eventually persisted in stuttering, but not in those who eventually recovered (see Fig. 1, Cluster 4 in the study). The affected fibers probably connect bilateral temporal regions (Kuvazeva, 2013), and lower FA may be related to a less effective labor division between hemispheres in auditory processing.

The difference in FA in the mentioned cluster is great already with the youngest children, and there is no much overlap between the persistent group, on one hand, and the recovered and control group, on the other hand. Therefore, the deficit can hardly be a consequence of stuttering. Interestingly, Chow, Liu, Bernstein Ratner, and Braun found a strong relation between the FA in the splenium and stuttering severity in adults who stutter (unpublished study; the results were presented at the 2014 ASHA Convention).

A further finding by Chow and Chang (in press) which distinguished the persistent from the recovered group of stuttering children is an initially higher FA value and an abnormal developmental trajectory in the thalamic radiation (see Fig. 2, Cluster 8, 9, and 10 in the study). These findings as well can hardly be explained as a consequence of stuttering. As the thalamus plays a central role in attention regulation, the findings may be related to an abnormal development of the attention system, possibly increasing the imbalance described above as the causal Factor 1. This assumption is supported by findings of anomalous, mainly decreased functional connectivity between Dorsal as well as Ventral Attention Network and Default Mode Network especially in children who eventually persisted in stuttering (Fig. 3 in Chang et al., 2018).

The hypothesis that transient and persistent developmental stuttering are in core the same disorder, and  that persistence is caused by an additional factor has already been proposed by Ambrose, Cox, and Yairi (1997) in a study of the genetic basis of persistence and recovery. They wrote: “ It was found that recovery or persistence is indeed transmitted, and further, that recovery does not appear to be a genetically milder form of stuttering, nor do the two types of stuttering appear to be genetically independent disorders. Data are most consistent with the hypothesis that persistent and recovered stuttering possess a common genetic etiology, and that persistence is, in part, due to additional genetic factors.”

The proportion of the two factors may differ between individuals, however, Factor 2 seems to be necessary for stuttering to become persistent. Factor 1 may be differently caused in persistent stuttering than in transient stuttering – as mentioned above, the clusters of reduced FA in the arcuate fasciculus (Cluster 1 and 2 in Fig. 1 in Chow and Chang, in press) were larger and the FA value in the anterior cluster was lower in the recovered than in the persistent group. However, in the persisten group, it was found an abnormal, stagnant or downward developmental trajectory of FA in the radiation of the thalamus (Fig. 2, Clusters 8, 9, 10). The thalamus is a part of the brain which plays a central role in the control of attention. We can assume that the male-to-female ratio in persistent stuttering has to do with Factor 1, namely with the fact that hyperactivity and impulsivity are more prevalent in males. 
 

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