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TMS & Combining Methods - Coggle Diagram
TMS & Combining Methods
TMS
What TMS Is
Magnetic field applied close to scalp
Interfere w. normal activity in neurones
Act like artificial neurone
How Works
Electric current in coil induces magnetic field
Change in magnetic field induce electric field in tissue directly below coil
Drives current
Interference Technique
Map to brain level
ms-minutes log time
Faraday Law of Induction
Why
Human brain not necessarily the same as that of other animals
Localisation & extent of lesions due to natural causes
Cannot be controlled
Limited no. lesion study samples & may have lost range of functions
Brain can reorganise/redistribute functions
Can study normal & lesioned brain in same subject & w/in short space of time
Study Design
Investigate the what of cortical function
Specificity
Control conditions to provide dissociation
Critical Evaluation
Advantage
Disadvantage
Advantage
Localised activity
Show necessity of activity for task
Disadvantage
Not enough to stimulate region & observe effect
Dissociation
Task
Localisation
Time
Task Dissociation
TMS affect outcome in task X not task Y
Location Dissociation
Affect outcome when applied to location X not Y
Time Dissociation
Affect outcome at time X not Y
Specificity
Task
Location
Time
Specificity of Task
Lesion
TMS
Study: TMS & Posterior Parietal Cortex (Bjoertmot et al., 2002)
A: Effect of TMS over PPC
P: Normal subjects
M: Bisected line judgement before vs after rTMS
R: Judgement dependent on rTMS application
Judged as longer when TMS applied
Effect only observed in near-space
Implication
Parietal neglect can be induced in normal subjects
By disruption to PPC
Opposite bias already known in normal subjects
Pseudo-neglect
Study: Specificity of Task
A: Study of posterior parietal cortex
P: Left unilateral neglect
M: Ps judge bisected line
R: Left unilateral neglect due to posterior parietal damage
Judge line to be longer on right
Distortions greater in near-space
Study: Specificity of Location (Cattaneo et al., 2009)
A: Processing of grammatical gender
P: Subjects w. lesions to Broca's area
M: Single pulse TMS applied at stimulus onset
R: Result in grammatical deficits
Satiation effect still evident when TMS applied over right but not left IFG
Significance
Direct neural representations exist w/in Broca's area for diff classes grammatical gender
Area could be representing sound patterns that indicate gender
Processed either in Broca's area/homologue in right hemisphere
Verbal satiation paradigm of higher level adaption phenomenon
Study: Specificity of Time (Pitcher et al., 2007)
A: When occipital face area processes visual features
M: Face-matching task
R: Diff in % correct at diff times
Sig lower in rOFA vs vertex at 60-100 TMS timing
No sig diff at other timing
A: Areas that Process Visual Features
Right occipital face area
Right fusiform face area
Superior temporal sulcus
Right Occipital Face Area
Process visual features before combination into whole face
Right Fusiform Face Area
Process invariant aspects of faces
Such as identity
Superior Temporal Sulcus
Processes changeable aspects of faces
Such as expression
M: Face-Matching Task
Faces differed in parts
Double-pulse TMS separated by 40ms
6s time window after stimulus onset
TMS over OFA or control vertex site
R: Significance
OFA processes face parts in early & temporally discrete time period
Action Potentials
Cell-to-cell communication in neurones
Rapid change in membrane potential
Results in current (wave of electric charge) travelling along axon
Usually triggered by synaptic input
Action Potential & TMS
Depolarisation & generation of action potential
Axon must be perpendicular to field to be affected
Implication for how coils positioned
Modularity of Brain Function
Phrenology
Lesion studies
Stimulation
Phrenology
Pseudoscience of attributing character traits to external features of skull
Remained predominant view for 30+ years
Pioneers
Gall
Flourens
Gall
Phenology based off his work in late 1700s
Fluorenes
Experiments directly studied brain
Concluded no modularity w/in cortex
Lesion
Study of aphasic patients
Aphasia
Broca
Wernicke
Broca's Aphasia
Identified by Broca (1860)
Broca's area involved in control of speech
Wernicke's Aphasia
Identified by Wernicke
Area involved in understanding of written & spoken language
Stimulation (Fritsch & Hitzig)
Stimulated small regions on brain surface w. electricity
First evidence for fine localisation of function
Concept of Brain Localisation
Established by Ferrier & Goltz at International Medical Congress (1881)
Combining Methods
TMS & fMRI
Common motivation to demonstrate functional connectivity between cortical areas
Know cortical areas interact
Work together
Apply TMS & observe active areas throughout brain using fMRI
Benefits
Extension of use
Observation of cortical networks
Extension of Use
TMS previously used to demonstrate function of single regions
TMS now used to study consequence of functional interactions
How support perception and cognition
Cortical Networks
Network effects can be context-dependent
Task
Attentional state
Study: Combine TMS & fMRI (Bestmann et al., 2008)
A: Functional interactions of dorsal pre-motor cortex
M: Grip test w. TMS application
R: Regions selectively activated by gripping
TMS affected BOLD signal in PMd & M1 of opposite hemisphere
Effect differed between rest & grip condition
M: Grip Test
Indicator tell subjects whether/not to grip
TMS in PMd & M1
R: Significance
Inter-hemispheric interactions in motor network depend on state of motor system
fMRI & EEG
fMRI good for where not when
EEG goof for when not where
Conducting Research
Performed separately
Data combined later
Performed simultaneously
Where imperative that data refer to exactly same events
Study: fMRI & EEG (Crottaz-Herbette & Menon, 2006)
A: Where & when ACC modulate attentional resources
M: Combine dipole modelling & connectivity analysis of oddball paradigm
R: Large attentional-related component in ACC
Sensory regions generated large signal ~60 ms after feedback
A: ACC
Involved in attentional selection
Thought to modulate responses in higher-order cortical areas
Not known if modulate early sensory processing
M: Measures
Oddball paradigm
Dipole modelling
Oddball Paradigm
Interleave frequent & infrequent stimuli
Button press indicate identification of stimulus
Visual modality
Blue/green circle against black bg
Auditory modality
Tones of 1000/2000 Hz
Dipole Modelling
ERP modelled as being result of multiple charged sources
Inverse problem
fMRI used to constrain locations of dipoles
R: Implication
ACC involved in top-down attentional modulation on sensory processing areas
TMS & EEG
Useful when question is when
Benefit
Allows to locate neuronal activity elicited by TMS as well as how & when activity in one area affects other areas
Study: TMS & EEG (Taylor et al., 2007)
A: Role of frontal eye fields in attentional selection
M: Visual spatial attention task w. FEF TMS
R: FEF TMS affect normal attentional modulation of ERP
A: Frontal Eye Fields
Belong to circuit of areas that control orientating of visual spatial info
Part of pre-motor cortex
During visual attention task activity increase in FEF & visual cortex
TMS disrupt performance in tasks requiring visual spatial attention
TMS activate network areas inc. visual cortex
But no direct evidence for causal interaction between FEF activity & visual cortex activity
M: Visual Spatial Attention Task
P fixes central cross
Cue indicate where task likely to appear
Valid cue
Invalid cue
Target appear & P press button to indicate location of target
RT recorded
R: Implication
FEF exert causal influence over activity in visual cortex
During voluntary orienting of visual-spatial attention
Importance
Diff methods have diff strengths
Logistical considerations
Logistical Considerations
Equipment taken into MRI scanner must be MRI compatible
Not just MRI safe
TMS disturb EEG amplifiers
Operation of MRI scanner disturb EEG amplifiers
Managing Issues
Special types equipment must be used
Software developed to remove artefacts
Critical Evaluation of TMS
Considerations of TMS
Coil
Magnetic field strength
Stimulation concentration
TMS Coil
Electric field weakest at centre of coil & outwards from rim
Not focal but local
Types of Coil
Simple round
Figure-of-eight
Simple Round
Current coil reaches peak within 200us
Zero w/in ~1ms
Induced magnetic field directly related to coil current
Rate of change of magnetic field determines magnitude of electric field in underlying tissue which drives current
Single pulse vs repetitive
Stimulation Concentration
Concentrated stimulation to 2-2.5 cm
Max 5cm
But spread of propagated activity
Stimulation Location
Cannot directly stimulate ventral cortex/deeper structures
Magnetic Field Strength
Max strength of 2-3 Tesla
Anatomical Localisation
External landmarks/measurements/trial & error
Tracked by software if head moves
Coil simultaneously tracked
Methods of Localisation
Navigated TMS
Anchor known landmarks
Anchor Known Landmarks
To subject own MRI
Ear canals
Nose
Safety Issues
Generally considered as safe as other investigative techniques
Contraindications
Personal family of epilepsy
Existing neurological condition
Recommendation of Use
Only stimulate at fixed % of stimulation
Stimulation of superficial nerves can cause tingling/discomfort
Noise can potentially be damaging