Task 9 TMS Flashcards
TMS (def)
method to magnetically stimulate focal regions of brain
TMS methodology
- electromagnetic induction: stimulation of a region of cortex beneath current-carrying coil
- change of rate in electrical current –> generates magnetic field –> induces secondary electrical current to flow in nearby wire
- secondary electrical current induces neurons below stimulation site and causes them to fire (produce APs) as if they were responding to environmental stimuli
Virtual lesion
- TMS can function as virtual lesion: temporarily interferes with ongoing cognitive activity in particular region
–> inference of necessity of a region for a particular task - brief pulse (1 ms) can lead to cortex effects lasting for 10ms
- TMS involves neuron activation from internal source (task demand) and external source (TMS)
–> interference between two activations
Advantages of TMS over lesions
- no reorganization / compensation
- reversible effects
- can be used to determine timing of cognition
- focal lesion
- lesion can be moved within same patient
- study functional integration
Disadvantages of TMS
- restriction of sites which can be stimulated
- spatial extent of TMS isn’t fully known –> distant brain regions may also get stimulation
Advantages of lesions over TMS
- subcortical lesions possible
- accurate localization of lesions with MRI
- changes in behaviour/cognition more apparent
- accidents of nature can give insight into new patterns of behaviour
Disadvantages of lesion studies
- cannot be used within-subjects
- brain organization
- brain injury could be larger than area under investigation
Functional specialization
trying to understand functional contribution of particular regions to aspects of cognition
Functional integration
- trying to understand how one region influences another or how cognitive functions influence each other
Practical study designs of TMS: Timing
- timing of pulses is crucial
1. Single-pulse TMS: pulse delivery = variable: gives info on timing of cognition + necessity of brain region
2. rTMS: train of repetitive pulses during task = cannot tell about timing but more powerful in determining necessity
Practical study designs of TMS: Position
- some assumptions about which brain region would be interesting to stimulate
- positions on head can be defined relative to landmarks (EEG system)
- Frameless stereotaxy: use of fMRI to locate candidate regions of stimulation
Control
- Compare performance when region is stimulted in critical vs. non-critical time window
- Compare stimulation in critical and non-critical regions (other hemisphere for lateralized functio, adjacent regions to determine spatial size)
- Sham TMS: coil is held in air
- Task control: same region is stimulated but for another task
Safety
- number + intensity of pulses
- exclude patients with epilepsy, pacemakers, etc.
- protect ears with earplugs
- not test same participant often within short interval
tDCS
method using electrical currents to stimulate the brain
Cathodal tDCS
- decreases cortical excitability = decreases performance: virtual lesion approach
- affects glutamate system (excitatory transmitter)
Anodal tDCS
- increases cortical excitability = increases performance: increases spotaneous firing rate of neurons
- affects GABA system (inhibitory transmitter)
- e.g.,: stimulation over virtual cortex increases N100 ERP => weak visual stimuli are more easily seen
tDCS Methodology
- weak electrical current that flows from pos. side (anode) to negative side (cathode)
- stimulating pad is placed over region of interest and control pad in site of no interest
- cognitive task is performed and compared with sham stimulation
Effects of stimulation (tDCS)
- immediate effects: occur on resting membrane potential
- aftereffects of stimulation: occur due to changes in synaptic plasticity influencing learning and affecting different neurotransmitter systems
Advantages of tDCS
- little discomfort
- ability to facilitate cognitive function (anodal tDCS)
Disadvantages
- poor temporal and spatial resolution than TMS
Types of coils
- Circular coil: maximum current in outer winding –> produces ring-shaped magnetic field around coil
- Figure-of-8 coil: two circular ring-shaped coils: maximum induced current is directly under intersection –> more focused magnetic field distribution than circular coil
Stimulation parameters
describe physical properties of applied magnetic stimulation
Stimulation characteristics
describe induced physiological effect of TMS
spTMS
used in tasks that require integrating sensory input/motor input or to investigate timing of perceptual processes due to precise timing/duration
spTMS is capable of identifying temporal order in which specific visual processes are activated: finds critical functional time point of brain activations
rTMS
- used to investigate higher cognitive functions: creates transient functional lesions to see functional relevance of region identified through functional imaging
TMS interference
- TMS: inducing noise into neural processes –> disrupts coordinated pattern of neural activity
Spatial resolution
magnetic field is not spacially focal: TMS is limited because it cannot compare many sites
- restricted to superficial cortical regions –> stimulating deeper cortical structures is likely to stimulate overlying cortex
Temporal resolution
- TMS simultaneously activates many neurons
- maximal activation: lowest SNR in stimulated ratio
- TMS typcially precedes ERP peaks: very high temporal resolution
