How does TMS work?

1
How does TMS work?

• Uses inductance to get electrical energy across
  the scalp
• Coil of wire gets changing currents run through
  it
• Rapid magnetic field changes gtgt electric current
• About 2T
• Magnetic field created at scalp with figure-8
  coil
• 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

2
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/
  long-term lesions
• Apply to different areas of scalp to disrupt
  function
• Disruption does NOT mean brain regions directly
  under coil responsible for function
• Only that its involved somehow in the function
• OR connected to regions involved in the function
• Get distal effects through connections
  (diaschisis)

3
Principles of TMS
http//www.biomag.hus.fi/tms/Thesis/dt.html
4
Repetitive TMS (rTMS)

• Rapidly repeated trains of magnetic pulses
• Because single pulses werent 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

5
TMS Coil
Maximum magnetic field at center of figure-8
http//www.bu.edu/naeser/aphasia/
http//www.icn.ucl.ac.uk/Experimental-Techniques/T
ranscranial-magnetic-stimulation/TMS.htm
Frameless Stereotaxy
6
Therapeutic Uses

• OCD
• Seizures
• Tinnitus
• ALS
• Chronic pain
• Depression
• Stroke
• Phantom limb pain
• Migraine

7
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
8
A Mediating Role of the Premotor Cortex in
Phoneme Segmentation

• Marc Sato, Pascale Tremblay, Vincent Gracco
  (2009)

9
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
• Mariannas question

10
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)
• Brocas area
• Ventral premotor cortex
• Single pulse TMS stimulating left primary
  premotor cortex (Fadiga et al., 2002)
• Lip or tongue MEPs enhanced during passive
  speech listening and viewing
• Increased activity in Brocas 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

11
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 Brocas area and the
  superior ventral premotor cortex (svPMC) for
  auditory speech processing has not bee determined

12
Evidence from rTMS studies

• Temporary disruption of the left inferior frontal
  gyrus doesnt impair ability on auditory speech
  discrimination tasks (Boatman, 2004 Boatman
  Miglioretti, 2005)
• Judgments require WM and subvocal rehearsal
• Lucys 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

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Goal of the Present Study

• Extend/refine results of Meister, et al., (2007),
  presentation of auditory stimuli without
  background noise
• 1 kHz rTMS, frameless stereotaxy to disrupt the
  svPMC
• Phoneme identification
• Solely auditory, no motor system needed
• Syllable identification
• 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

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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

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Stimuli

• CVC syllables naturally recorded
• Mariannas question
• Spoken by native French-Canadian
• Six utterances
• /put/ /but/
• /pyd/ /byd/
• /pon/ /bon/

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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
• Initial syllable /p/ or /b/
• Syllable discrimination
• Initial phoneme same /put/ /put/, or not /put/
  /but/
• Phoneme discrimination
• 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

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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

18
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

19
Data Analysis

• Button press reaction times were examined
• RTs slower than 2000 ms considered errors,
  omitted from the analysis
• RTs calculated
• 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 RTs

20
Results

• Main effect of task
• Lower percent correct for the phoneme
  discrimination task
• Alberts Question
• Faster reaction times in control task compared to
  phoneme discrimination and other tasks
• Main effect of stimulation
• Slower RTs after rTMS compared to sham
• Interaction slower RTs after rTMS compared to
  sham for the phoneme discrimination task

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A percent correct B RT
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Limitations of rTMS

• Inter-participant anatomical differences
• Length of inhibitory effects of rTMS
• About 10 minutes, task was 6 minutes
• Israels 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

23
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
• Pawels Question

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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

25
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

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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