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On the Causal Mechanisms of Stuttering
by Alm, Per (1963), Department of
Clinical Neuroscience
Date published: 2005
Publisher: Department of Clinical
Neuroscience, Lund University
Thesis defended: 2005-02-25 13
location: Aulan Universitetssjukhuset
Lund
The faculty's opponent: Professor
Espen Dietrichs, Oslo Universitet, Nevrologisk avdelning,
Rikshospitalet, Oslo
Classification:
Neurology, neuropsychology, neurophysiology
Keywords:
stuttering;
basal ganglia;
dystonia
Abstract: Stuttering is one of the
most common speech disorders. However, the etiology is poorly
understood, and is likely to be heterogeneous. The aim of this
thesis was to clarify causal mechanisms, focusing the brain. The
project included theoretical development based on published data,
and a broad approach of explorative studies and testing of
hypotheses.
The theoretical work focused the basal ganglia, leading to a model
based on the dual premotor systems hypothesis (G. Goldberg, 1985,
1991), which defines two parallel premotor systems: the medial (the
basal ganglia and the SMA), and the lateral (the lateral premotor
cortex and the cerebellum). Stuttering is suggested to be caused by
a disturbance of the medial system, in most cases in the basal
ganglia. The core dysfunction is proposed to be impaired "go-signals"
from the medial system, supposed to trigger the next motor segment
in speech. According to this model, under some conditions speech
control is shifted from the medial to the lateral system, thereby
bypassing the dysfunction and resulting in fluent speech. The
lateral system is suggested to be active when speech is combined
with sensory input, like chorus speech or metronome. Also the effect
of altered auditory feedback in reducing stuttering is proposed to
be based on this mechanism. It seems as the lateral system is able
control speech timing without sensory input, but that this demands
increased attention to some particular aspect of speech, as occurs
in imitation of dialects, exaggerated rhythm, reduced speech rate,
or role play. Also singing is suggested to be based on the lateral
system.
Superfluous muscular activation accompanying stuttering may be a
type of dystonia: involuntary contractions related to the basal
ganglia disturbance. The high prevalence of stuttering at age 2.5 to
3 years is proposed to reflect a normally occurring peak in the
number of dopamine receptors at this age.
A total of 35 stuttering adults participated. Two studies, of copper
metabolism and "startle prepulse inhibition", did not indicate any
significant differences in comparison with matched controls.
It has previously been reported that stuttering may be associated
with increased neuromuscular reactivity, measured as exaggerated
eye-blink in response to noise. This aspect was investigated. The
stuttering group showed somewhat stronger eye-blink, though not
statistically significant. Strong eye-blink was not related to
anxiety, but was clearly related to low calcium, which is known to
increase the excitability of the nervous system. The stuttering
group showed somewhat lower calcium, and a weak tendency towards
more severe stuttering in case of low calcium. It is possible that
low calcium can increase the severity of stuttering in some cases.
A subgroup reported some traits of childhood ADHD, and this group
typically also reported neurological incidents before the onset of
stuttering. The subgroup without traits of ADHD typically reported
having stuttering relatives but no neurological incident.
List of papers in this dissertation
Per Alm. 2004. Stuttering, emotions, and heart rate during
anticipatory anxiety: a critical review. Journal of Fluency
Disorders vol 29pp 123-133. Elsevier.
Per Alm. 2004. Stuttering and the basal ganglia circuits: a
critical review of possible relations. Journal of Communication
Disorders vol 37pp 325-369. Elsevier.
Per Alm. 2004. Copper in developmental stuttering: a study of
plasma copper, ceruloplasmin, and estimated free copper. Folia
Phoniatrica et Logopaedica S. Karger AG, Basel. (inpress)
Per Alm. 2004. Stuttering and sensory gating: a study of acoustic
startle prepulse inhibition. . (submitted)
Per Alm and Jarl Risberg. 2004. Stuttering in adults: the
acoustic startle response, temperamental traits, and biological
factors. . (submitted)
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