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ACQ
Volume 12, Number 1 2010
ACQ
uiring knowledge in speech, language and hearing
parameters. TMS has been described as “a sensitive
technique for investigating the corticobulbar tract, which
is difficult to study by other methods” (Pouget, Trefouret,
& Attarian, 2000, p. 182). It has an advantage over
neuroimaging techniques in that it can directly interact with
the brain and has a high degree of temporal precision.
Neuroimaging techniques are based on measures of
regional cerebral blood flow and glucose metabolism and,
therefore, can only provide insights into the neural activity
that is correlated with a given behaviour but can give no
indication of whether the activity is excitatory or inhibitory in
nature. Observational neuroimaging techniques also do not
provide a means by which alterations in brain activity can be
modulated like with TMS.
TMS analyses and parameters
The types of evaluations and functions that can be performed
with TMS are introduced below, together with a review of how
TMS has been applied in the study of speech and motor
speech disorders to date, with a focus on tongue function.
Mapping cortical regions, function
and plasticity
Single pulse TMS can be used to produce an anatomical
map of the motor cortex by recording the sites over the
scalp (and underlying cortex), which, when stimulated with
TMS, activate the muscles of interest. By mapping the size
of the stimulation sites at different time intervals, cortical
plasticity associated with the learning of motor skills, disease
progression, and/or recovery of function following injury can
be examined (George et al., 1999; Pascual-Leone, Grafman,
Cohen, Roth, & Hallett, 1997).
The site of the tongue motor cortex has been mapped
(Rödel, Laskawi, & Markus, 2003) and plasticity of the tongue
motor cortex region induced by tongue training tasks in healthy
speakers has been investigated (Svensson, Romaniello,
Arendt-Neilson, & Sessle, 2003), as have changes in corti-
cobulbar pathway organisation and tongue cortical motor
maps associated with disease (e.g., unilateral peripheral facial
paralysis, Rödel, Tergau, Markus, & Laskawi, 2004) and recovery
following stroke (Muellbacher, Artner, & Mamoli, 1999).
Evaluating the integrity of the
corticobulbar pathways
Testing whether the corticobulbar pathways are intact or
damaged can be achieved with single pulse TMS through
measures of the latency and size of the motor evoked
response (MEP) in the muscle of interest. Prolonged
latencies may be indicative of demyelination of the pathways,
whereas reduced amplitude responses may be suggestive of
a loss of neurons or axons (Kobayashi & Pascual-Leone,
2003). The corticohypoglossal pathways of healthy speakers
and speakers with disorders including stroke, amyotrophic
lateral sclerosis, myotonic dystrophy, Guillian-Barré
syndrome, and brainstem lesions, have been examined
through measures of the size and latency of the MEP (e.g.,
Muellbacher, Mathis, & Hess, 1994; Urban, Hopf, Fleischer,
Zorowka, & Müller-Forell, 1997). A noted benefit of TMS has
been its utility in identifying early, subclinical upper motor
neuron deficits (Pouget et al., 2000).
Determining levels of cortical excitability
Various measures can be used to determine cortical
excitability or responsiveness to stimulation.
•
Motor threshold:
lowest intensity of TMS stimulation
required to produce a consistent motor evoked potential
(MEP) in the muscle of interest. An increased motor
threshold for a given individual indicates reduced
excitability (i.e., greater stimulation required to activate
the cortical region), whereas a decreased motor threshold
indicates increased excitability.
•
Motor evoked potential (MEP) amplitudes:
the size of the
electrical potential (in microvolts) recorded in the muscle
of interest when the corticobulbar/ spinal pathways are
stimulated. Increased amplitudes indicate increased
excitability.
•
Input–output responsiveness curves:
This procedure involves
taking a set of MEP amplitude recordings (output) at different
levels of input (i.e., different voluntary contraction or TMS
stimulation levels). Regression curves are applied to the
MEP amplitudes, with measures of y-intercept and slope
derived, representing sensitivity and gain (i.e., degree of
facilitation associated with increased inputs), respectively.
TMS has been used to examine tongue motor cortex
excitability in healthy speakers on the basis of MEP amplitudes
(Fadiga, Craighero, Buccino, & Rizzolatti, 2002) and motor
thresholds (Muellbacher, Boroojerdi, Ziemann, & Hallett, 2001).
Investigating inhibitory and facilitatory
intracortical circuits
Intracortical inhibition and facilitation can be studied using
TMS presented as a paired-pulse and with silent periods.
Figure 1. a) Photograph of a figure-of-eight TMS coil being held against the head over the motor cortex; b) Illustration of the TMS
coil, activation of the corticohypoglossal pathway, and the resultant motor evoked potential (MEP) recorded in the tongue using
electromyography (EMG)