NAB-Transcanial magnetic stimulation (TMS)

TMS:

• Non-invasive, painless, safe stimulation of human brain cortex (through the skull)
• useful in studying behaviour during virtual brain lesions, chronometry and functional connectivity
• could administered as single pulse or repetitive TMS
• stimulater in the shape of 8 placed above the scalp, contains a coil of wire->brief pulse of high electrical current fed through the coil magnetic files in lines of flux formed perpendicular to the plane of the coil, into the skull->magnetic field induces electric filed perpendicular to magnetic filed-> electric field leads to neuronal excitation trans-canially (within the brain)
• effect depends on stimulation site

>>How to measure the effects of TMS:

• Motor cortex stimulation
• activates corticospinal neurons trans-synaptically (occurring or existing across a nerve synapse)
• e.g. TMS coil 5 cm lateral from vertex (highest point)->often contralat' thumb twitches (20 ms post TMS)
• record motor evoke potential (MEPs)* (surface EMG, target muscle relaxed)
• record silent period* in contracted target muscles~150 ms after motor cortex stimulation, cortical mechanisms
• Occipital cortex stimulation:
• excitatory effects: e.g. phosphenes*; inhibitory effects: suppression of motion perception and letter identification
• Somatosensory cortex stimulation
• may elicit tingling, block the detection of peripheral stimuli (tactile, pain)
• can modify somatosensory evoked potentials (SEPs)
• Auditory cortex stimulation: 
• Interpretation of results challenging: loud coil click
•  Frontal cortex stimulation
• Effects on subject's mood? Potential for therapeutic use?

*MEPs=the electromyograph** responses of the peripheral muscles to electrical stimulation to the motor cortex. Changes in the MEP signal have been shown to exhibit significant correlation with neurological deficit and spinal cord injury.
**Electromyograph (EMG)=electrodiagnostic medicine technique for evaluating and recording the electrical activity produced by skeletal muscles, performed with an instrument- electromyograph to produce a record called electromyogram.
*Silent period=MEG silence after MEP
*Phosphenes=a ring or spot of light produced by pressure on the eyeball or direct stimulation the visual system than by light.

TMS application: cross modal plasticity

>Blind people can learn to read Braille, compared to sighted people they have superior tactile perception. Is there any underlying changes in the brain?
>Their visual cortex is known to be activated during Braille reading, is there functional significance of this activation?->TMS
>Chronometry-timing of mental events

• single pulse TMS for mental chronometry
• early blind subjects: blind before 1 year old
• real & nonsensical Braille stimuli presented via tactile stimulator
• interval between tactile stimulus (Braille) and TMS systematically varied
• subjects have to detect stimuli and identify if they are real or nonsensical
• DV: number of correctly detected/identified stimuli
• Conclusion: visual cortex contributes to tactile information processing in early blind subjects=crossmodal plasticity

*Different symbols (square, triangle, circle) representing different subjects.

*Open symbols=detected stimuli; filled=correctly identified stimuli

TMS application: virtual lesions

>Repetitive TMS can create temporary inhibitor of brain areas, fully reversible virtual lesions (for a few minutes, subjects behave as if after a brain lesion)

>errors during Braille reading by early blind and sighted subjects depend on site of virtual lesions

>cross modal plasticity in early blind subjects stated that occipital cortex supports Braille reading

>for sighted controls, max error rate after anterior parietal virtual lesion

>for early blind subjects, max error rate after occipital virtual lesion

TMS Advantages & Disadvantages:

• Advantages:
• temporal resolution in millisecond range
• virtual lesion in subject may be better defined than lesion in patient
• short duration of experiment minimises risk of plasticity
• repeated studies in the same subject
• group studies with standardised experimental setup
• study double dissociations: stimulate or temporal disrupt different cortical regions during one task, one region during different tasks
• Disadvantages:
• spatial undersampling(only one area at a time)
• only cortical area accessible
• auditory cortex stimulation problematic (muscles)
• loud coil click, need 'sham stimulation'