How does TMS work?


Uses inductance to get electrical energy across
the scalp
Coil of wire gets changing currents run through it



Magnetic field created at scalp with figure-8 coil


Rapid magnetic field changes >> electric current
About 2T
Strength of magnetic field depends on the # of turns
of the wire and the magnitude of the current
First TMS study Barker, Jalinous, & Freeston,
1985
What does TMS do?

Electric current induced in neurons in cortex



Adds noise, disrupts coordinated activity
Temporary “lesion”
 Without the kind of compensation that develops w/ longterm lesions
Apply to different areas of scalp to disrupt function



Disruption does NOT mean brain regions directly under coil
responsible for function
Only that it’s involved somehow in the function
OR connected to regions involved in the function
 Get distal effects through connections (“diaschisis”)
Principles of TMS
http://www.biomag.hus.fi/tms/Thesis/dt.html
Repetitive TMS (rTMS)

Rapidly repeated trains of magnetic pulses




Because single pulses weren’t found to have much
effect on gross measures of behavior early on
Longer lasting effects compared to single pulse
rTMS is thought to effect long-term potentiation
between neurons
Two repetition rates


Slow= below 1 kHz
Fast= above 1 kHz
TMS Coil
Maximum magnetic field at
center of figure-8
http://www.bu.edu/naeser/aphasia/
http://www.icn.ucl.ac.uk/ExperimentalTechniques/Transcranial-magnetic-stimulation/TMS.htm
Frameless Stereotaxy
Therapeutic Uses









OCD
Seizures
Tinnitus
ALS
Chronic pain
Depression
Stroke
Phantom limb pain
Migraine
Drawbacks of TMS

Possible risk of side effects



Seizure, particularly with rTMS
Headache and/or muscle aches caused by
activation of neck and shoulder muscles
The equipment is loud, about 100 dB

Loud enough to cause hearing loss
http://www.biomag.hus.fi/tms/Thesis/dt.html
A Mediating Role of the
Premotor Cortex in
Phoneme Segmentation
Marc Sato, Pascale Tremblay, &
Vincent Gracco (2009)
Auditory theory vs. Motor
theory of speech perception

Speech perception
driven by auditory
mechanisms



This is based on invariant
properties of the acoustic
signal
Not mediated by the motor
system
Speech sounds perceived
by same mechanism for
audition and perceptual
learning

The perception of
speech is a
sensorimotor process


Perception of articulatory
gestures
Speech gestures are
represented as motor
control structures
 Marianna’s question
Support for the Motor Theory
from Imaging Studies
Passive auditory, visual and AV speech perception



Posterior part of left inferior frontal gyrus (Ojanen et al., 2005)
 Broca’s area
Ventral premotor cortex
Single pulse TMS stimulating left primary premotor
cortex
(Fadiga et al., 2002)


Lip or tongue MEP’s enhanced during passive speech
listening and viewing
 Increased activity in Broca’s area and ventral premotor
cortex
Motor facilitation stronger when the muscle activity
and auditory stimuli are for the same articulator
(Fadiga et
al., 2002; Roy et al., 2008)

Similar patterns of motor activity in ventral premotor cortex
while listening to or producing lip/tongue phonemes
Do speech motor centers
contribute to speech perception?

The use of rTMS and electrocortical
stimulation can help to answer questions
about causality which cannot be answered
through passive speech perception
experiments


Creation of a transient ‘virtual lesion’ (Boatman, 2004)
Possible functional role of Broca’s area and
the superior ventral premotor cortex (svPMC)
for auditory speech processing has not bee
determined
Evidence from rTMS studies

Temporary disruption of the left inferior frontal gyrus
doesn’t impair ability on auditory speech
discrimination tasks
(Boatman, 2004; Boatman & Miglioretti, 2005)


Judgments require WM and subvocal rehearsal
 Lucy’s Question
rTMS stimulation of left svPMC (active in syllable
production and perception) resulted impaired ability
to identify auditory syllables
(Meister, Wilson, Deblieck, Wu & Iacoboni, 2007)


Interpretation: premotor cortex contributes to top-down
modulation of the auditory cortex
Note that this study was done with masking noise in the
background
Goal of the Present Study

Extend/refine results of Meister, et al., (2007),
presentation of auditory stimuli without background
noise


Phoneme identification


Solely auditory, no motor system needed
Syllable identification


1 kHz rTMS, frameless stereotaxy to disrupt the svPMC
Similar to phoneme identification
Phoneme discrimination


Segment initial phonemes to make same/different judgment
This task would see the strongest effect of rTMS on accuracy
and reaction time
Participants






10 healthy adults (7 females)
Mean age 27 ± 5 years
9 native speakers of French-Canadian, 1
native speaker of French
All right handed
No history of hearing loss
Corrected-to-normal vision
Stimuli

CVC syllables naturally recorded



Marianna’s question
Spoken by native French-Canadian
Six utterances



/put/ /but/
/pyd/ /byd/
/pon/ /bon/
Procedure




Participants seated 50 cm in front of a computer monitor
Acoustic stimuli presented through loudspeakers
Two experimental sessions
 rTMS session
 Sham session
Experimental tasks
 Phoneme identification


Syllable discrimination


Initial phoneme same /put/ /put/, or not /put/ /but/
Phoneme discrimination


Initial syllable /p/ or /b/
Initial phoneme of syllable pairs same /put/ /put/-/but/ /byt/, or not /pon/
/bon/-/pon/ /byd/
Non-verbal matching control

Letter shown after fixation cross
Experimental Session



All tasks, fixation cross in center of
screen for 250 ms, blank screen for
2500 ms at end
Structural MRI, frameless stereotaxy
TMS stimulation applied with a 70
mm air cooled figure 8 coil



Resting motor threshold (RMT): minimum
stimulus intensity capable of evoking a
motor response
600 pulses applied at 1 kHz with an
intensity of 110% of RMT, inhibition lasts
up to 10 minutes
Sessions separated by 1 hour
Sham Session

Recorded TMS machine noise was presented
through loudspeakers




Ear plugs were worn for both sessions
Same tasks as the experimental session
rTMS coil positioned over svPMC, however
no TMS stimulation was presented
Participants not told which session was the
sham and which one was experimental
Data Analysis

Button press reaction times were examined


RT’s calculated




RT’s slower than 2000 ms considered errors, omitted from
the analysis
Onset of the second fixation cue in control task
Onset of the presented syllable in phoneme identification
task
Onset of the second presented syllable in the phoneme
and syllable discrimination task
Repeated measures ANOVA performed on the
percentage of correct responses and median RT’s
Results

Main effect of task

Lower percent correct for the phoneme
discrimination task



Albert’s Question
Faster reaction times in control task
compared to phoneme discrimination and
other tasks
Main effect of stimulation


Slower RT’s after rTMS compared to sham
Interaction: slower RT’s after rTMS compared to
sham for the phoneme discrimination task
A= percent correct
B= RT
Limitations of rTMS


Inter-participant anatomical differences
Length of inhibitory effects of rTMS



About 10 minutes, task was 6 minutes
Israel’s question
Effect of rTMS on phoneme discrimination
task was not attention or sensory related

No effect observed in the other auditory tasks, or
the visual matching task
Results Compared to Previous
Investigations

No effect in phoneme identification and syllable
discrimination tasks similar to previous work
(Demonet, Thierry, & Cardebat,
2005)



Activation in the left, posterior part of the inferior frontal gyrus
and vPMC along with auditory regions
For phoneme monitoring and discrimination tasks
These areas are active for phoneme recoding and
segmentation, recruited for planning and executing
speech gestures
(Bohland & Guenther, 2006)


Present study supports this and provides evidence for the
participation on the svPMC in the segmentation of the speech
stream
Pawel’s Question
Phoneme Discrimination
Results

Previous work showed rTMS disrupts left
posterior inferior frontal gyrus
(Romero, et al., 2006)


Phoneme discrimination ability effected
The present study and previous work indicate
the inferior frontal gyrus and the svPMC are
important for speech processing when WM
demands are high and articulatory rehearsal
is needed

Also top-down influence on the temporal lobe for
phoneme segmentation needs
Effects of rTMS

rTMS stimulation of the left inferior frontal lobe
or PMC does not impair ability to discriminate
syllable pairs


Phoneme identification and discrimination require
auditory analysis, not influenced by the inhibition of
the rTMS stimulated areas
The phoneme discrimination task was effected
by the stimulation

Suggests that the svPMC plays role in speech
segmentation, especially when WM demands are
high
Which theory is supported?

Dual-stream model





(Hickok & Poeppel, 2001, 2004, 2007)
Dorsal auditory-motor circuit maps sounds on articulatory
based representations
Auditory fields in the superior temporal gyrus are involved in
early stages of speech perception
Later in life the ventral stream projects to the PMC and inferior
frontal gyrus for speech/vocabulary development
Recruitment of motor representation when WM demands are
high
 Results of the phoneme identification and syllable discrimination
tasks do not fit the motor theory
Results support an integrated view of speech
perception