PSY396 Lecture Week 3: Introduction to transcranial magnetic stimulation (TMS)

What is TMS?

A technique that works through the principle of magnetic field induction

• Discovered by British physicist Michael Faraday in the late 1800s

• Faraday’s law of induction

Faraday’s law of induction

• When coil A (electromagnet) moves through coil B, this creates a change in magnetic field

• This induces an electromagnetic field (EMF), which will generate current in a nearby conductor B (observed in G)

Key takeaway: a moving electric current induces a change in the magnetic field, and a change in the magnetic field can induce current in a conductor (like the copper wire here)

How TMS works

A brief, high current, electric pulse travels through to a handheld coil Induces a magnetic field

Magnetic field passes through scalp

• Induces current flow in cortical tissue

• Neurons depolarise

Action potentials!

How is TMS used in cog neuro?

1. To investigate the dynamic cortical processes underpinning human behaviour

2. To investigate the causal involvement of cortical regions in human behaviour

3. To induce neuroplasticity (for research and clinical purposes)

Techniques compared

Investigating dynamic cortical processes

TMS can activate cortical neurons

Which neurons are activated depends on…

• Where the coil is

• Coil geometry (c.f. Talebinejad and Musallam 2010)

• Stimulus intensity

• Brain state (i.e., at rest, or engaged in a task) •

When we apply stimulate the cortical neurons before/during/after a task, we can observe the dynamic cortical processes underpinning the task

• Excitation

• Inhibition

Investigating dynamic cortical processes - Measuring excitation

• Deliver single-pulse TMS to scalp

• Activates local neurons (excitatory and inhibitory)

Example: primary motor cortex (M1)

• Specific M1 regions control specific muscles

• Place coil over a region → stimulate at a sufficient intensity → net excitation of corticospinal neurons → signal travels along corticospinal tract toward muscle → produces muscle activity (measured by EMG)

• The muscular response to a TMS pulse = motor evoked potential (MEP)

• MEP = measure of corticospinal excitability; bigger MEP = more excitability

Investigating dynamic cortical processes - Measuring excitation - Action selection & execution

• Go/No-Go task

• Tells us about underpinning dynamics of action selection in healthy YAs

• Question: Why do we get slower at action selection & execution with age?

Young:

• Generally greater excitability

• Dip

• Gradual increase

Older:

• Generally less excitability

• No dip

• Limited increase

Investigating dynamic cortical processes - Measuring inhibition

Deliver paired-pulse TMS to scalp

Different protocols, common = short-interval intracortical inhibition (SICI)

• Selectively targets GABA-A mediated inhibitory circuit by applying two TMS pulses with a short interval

Investigating dynamic cortical processes - Measuring inhibition - How SICI works

→ Apply single-pulse TMS

• → Produces an MEP of a certain size, giving us a measure of corticospinal excitability

→ Apply paired-pulse TMS

• → Single-pulse preceded by weaker pulse

• → Weaker conditioning stimulus selectively activates inhibitory circuit (lower threshold)

• → Stronger test stimulus is applied 2.5-3ms later, while the inhibitory circuits are still active

• → As a result, the response (i.e., MEP) produced by the test stimulus is smaller than when we give the test stimulus alone

→ We compare the size of the conditioned MEP to the unconditioned MEP

• → The greater the suppression of the MEP, the greater the SICI

Investigating dynamic cortical processes - Measuring inhibition - Action selection & execution

Q: Why do we get slower at action selection & execution with age?

• Task: Go/No-Go

Young:

• Generally greater release of inhibition

• No-Go: Inhibition increased

• Go: Inhibition reduced

Older:

• Generally lower release of inhibition

• Time-course of SICI modulation preserved was in healthy aging

Investigating causal cortical involvement

Schulter et al. 1998 investigated the involvement of different motor regions in movement selection

• Task: choice RT

TMS to induce neuroplasticity (for research and clinical purposes)

TMS can induce lasting changes in neuroplasticity through repetition → repetitive TMS (rTMS)

• Utilises Hebbian rule

• → ‘When an axon of cell A is near enough to excite a cell B and repeatedly or persistently takes part in firing it, some growth process or metabolic change takes place in one or both cells such that A's efficiency, as one of the cells firing B, is increased’ - Donald Hebb

• ‘What fires together, wires together’ - Carla Shatz

• Many types of rTMS

rTMS can be used to induce neuroplasticity

Different protocols for different purposes

• High-frequency rTMS (>1Hz) → upregulates

• Low-frequency rTMS (≤1Hz) → downregulates

To induce neuroplasticity (for research and clinical purposes) - Treatment of depression

• In depression, a part of the central executive network (DLPFC) is hypoactive

• High-frequency rTMS to the DLPFC helps to upregulate this activity & ameliorate depressive symptoms

• Treatment response is associated with an increase in BOLD activity

• This rTMS protocol was approved by Medicare (2021) as a subsidised treatment

Effect of high-frequency rTMS (>1 Hz) over the left DLPFC (Kedzior et al., 2015) on depression: