Virtual lesions: Transcranial Magnetic Stimulation (TMS)
Background: lesion methods

• Lesion methods - useful to explore the relationship between the brain and behaviour.
• Used in animals & humans
• Often neurologically impaired individuals
• In animals - experimenter can control the size & site of the damage

Transcranial Magnetic Stimulation (TMS)

• offers a way to produce localised stimulation of the brain in humans
• is non-invasive
• Device - tight wire coil (insulated) & connected to powerful electrical capacitors (see fig. 4.25a&b p.146)
• Trigger capacitors --> send electrical current through coil --> generate magnetic field.
• This causes a physiological current that causes the neurons to fire.

Exact method causing neuronal discharge?

• Not well understood
• Could be that action potentials in the soma are generated
• Or - axons may be directly stimulated by current

• The area of neuronal activation will depend on the shape & positioning of the coil. Currently, activation can be restricted to approx. 1.5 cm squared.

TMS used to explore the role of various brain areas.
e.g. (video) when coil is placed on the 'hand area of the motor cortex' the muscles in the fingers & wrist are stimulated. The subject can see their hand twitching yet can do nothing about it.
Direct stimulation of motor cortex - easy way to asses the integrity of teh motor pathways because 20ms after stimulation, muscle activity can be seen.
TMS has been used extensively in research and has shown, for example, that the motor cortex is active when we perceive the actions of others (e.g. waving or throwing a ball) - activity in motor cortex may be related to action comprehension, therefore (although we cannot infer this for sure).
TMS can be used to create 'virtual lesions'
Normal activity in a selected region of the brain is disrupted when it is stimulated (p.146-147). Helps us to shed light on the normal function of the disrupted tissue.
Advantage of 'virtual lesions' created using Transcranial Magnetic Stimulations:

• Noninvasive, safe & produces fairly brief change to neural activity.
• Can compare one person's performance when the brain area is stimulated to their behaviour when it is not (e.g. touch your nose with your finger)

• Stimulation is effective on various brain sites e.g. visual cortex stimulated - may interfere with ability to read a letter
• Patient may not always be aware of effects of stimulation