Transcranial Magnetic Stimulation (TMS)

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Transcranial Magnetic Stimulation (TMS)

• Rapid magnetic field changes gtgt electric current
• Magnetic field created at scalp with figure-8
  coil
• 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 see what
  functions disrupted
• 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)

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TMS Coil
Naeser et al. (2004), Fig 2, p 100
Maximum magnetic field at center of figure-8
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Rapid-onset brief electrical current generated
in coil
Produces rapid-onset brief magnetic field pulse
(up to 2 Tesla)
Induces rapid-onset brief electrical field
Induces rapid-onset brief electrical current in
brain (mostly cortex)
Which has an effect on some task
Walsh Cowey (2000), Fig 2, p 76
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Repetitive TMS (rTMS)

• Most studies so far have used rapidly repeated
  trains of magnetic pulses
• Because single pulses werent found to have much
  effect on gross measures of behavior early on
• But more recently, single pulse studies have
  found effects when have constrained hypotheses
  more sensitive behavioral measures
• Can time single pulse at different steps in a
  process to see when it has the most effect
• Very little so far
• Mostly studying vision motor processes

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TMS Language

• Stewart et al. (2001) Stewart, Walsh, Frith,
  Rothwell (2001), Neuroimage, 13, 472-478.
• rTMS during speech
• Monitored speech, EMG, videotaped
• rTMS at a more posterior frontal region of both
• the LH RH produced both
• Speech arrest
• EMG in mentalis muscle
• At a more anterior frontal region, only LH
  stimulation produced
• Speech arrest
• But no EMG in mentalis
• So, probably different causes of the 2 kinds of
  speech arrest, only LH stim leads to the
  non-muscular type

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More TMS Language

• Knecht et al. (2002)
• Knecht, Floel, Drager, Breitenstein,
  Sommer, Henningsen, Ringelstein, Pascual-Leone
  (2002), Nature Neuroscience, 5, 695-699.
• Goal Evaluate the functional significance of
  varying degrees of language lateralization
• Evaluated lateralization in large sample during
  silent word production task w/
• Functional transcranial Doppler sonography (fTCD)
  
• fMRI
• Grouped subjects into 5 categories from very
  strongly left lateralized to very strongly right
  lateralized
• Many more subjects w/ L than w/ R lat
• Task Picture-word verification

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Knecht et al. contd

• rTMS applied during task to
• L R language areas (CP5 CP6, Wernickes
  area)
• Midline occipital region (Oz)
• Control, not expected to affect task performance
• 1 Hz for 10 min, w/ 30 min rest between trains
• Produces disruption lasting up to several minutes
• Significant correlation between degree of
  slowdown inaccuracy in task w/ degree of
  lateralization
• More strongly L-lateralized, more disruption w/
  LH stim
• More strongly R-lateralized, more disruption w/
  RH stim
• Less lateralized, less disruption w/ either H
  stim
• Notice corrections since in-class presentation

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TMS vs Wada Test

• Neurosurgeons routinely use pre-surgical Wada
  Test to determine gross lateralization of
  language
• So can spare language-related tissue
• Risky occasionally causes stroke
• TMS less risky, some proponents argue it should
  replace Wada test
• But others argue that Wada TMS on the same
  person disagree too often to rely on TMS yet

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TMS as Therapeutic Tool

• Already used in treatment of depression
• Naeser et al. (2004)
• Naeser, Martin, Nicholas, Baker, Seekins,
  Kobayashi, Theoret, Fregni, Maria-Tormos,
  Kurland, Doron, Pascual-Leone (2004), Brain
  Language, 93, 95-105.
• Functional imaging studies on non-fluent aphasics
  often show more activation in RH homologues of LH
  language areas than normals do
• Does this represent some kind of adaptive
  strategy?
• Is it actually maladaptive?
• Could TMS suppress this activity, lead to
  better language??

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Naeser et al. contd
4 non-fluent patients, 5-11 years post-stroke
Naeser et al. (2004), Fig 1, p 99 (Radiological
convention L R reversed)
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Naeser et al. contd

• rTMS over RH homologue of Brocas area
• Daily for 10 days, 20 min each time
• Tested picture naming speed accuracy
• Immediately after 10th session
• All patients reliably faster more accurate than
  their pre-treatment baseline measures
• 2 months later 8 months later
• Effects decreased over time, but continued
  through 8 mos for 3 of 4 patients

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The Right Hemisphere (RH) Language

• The RH
• Retrieves holds onto infrequent contextually
  inappropriate meanings of ambiguous words
• Long after the LH settles on what seems to be the
  contextually appropriate meaning
• Plays critical role in understanding non-literal
  language
• Puns, idioms, metaphors, sarcasm, etc.
• Maybe its the RHs job to hold onto stuff that
  might be relevant, just in case things dont turn
  out as the LH thought they would
• LHs job to make quick decisions, which means
  its sometimes going to be wrong need what RH
  has kept

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TMS Study Id Like to See

• What would happen if apply TMS to RH during the
  comprehension of ambiguous words or idioms or
  sarcasm?
• How much would effects depend on timing of
  pulse(s)?
• How would pulse(s) earlier vs later during the
  processing of ambiguous words influence whether
  get priming for targets related to contextually
  irrelevant meanings of ambiguous words?
• How would pulse(s) earlier vs later during idiom
  comprehension influence whether you get the
  idiom?