Introduction

TMS introduction

Transcranial Magnetic Stimulation (TMS) is a non-invasive technique that relies on electromagnetic induction to stimulate neurons in specific regions of the brain. By placing a magnetic coil near the scalp, TMS generates a magnetic field that penetrates the skull and induces an electric current in the underlying cortical tissue. This current can depolarize or hyperpolarize neurons, thereby altering their activity.

The shape of the magnetic field and of the induced current depends on the shape of the coil. Usually, the shape is an eight, as this produces a more focused magnetic field at the center, where the two coils meet, and is considered efficient for targeting specific brain regions while minimizing stimulation of surrounding areas. 

Stimulation

The stimulation effect of transcranial magnetic stimulation (TMS) gradually dissipates with distance, which is why TMS is mostly used for superficial brain areas like the motor cortex (M1), involved in controlling finger movements, or Broca’s area, linked to speech production. Although some coil designs can stimulate deeper brain structures, this often reduces spatial precision. Spatial precision is the ability to target specific brain areas accurately. 

To use TMS, a current is applied to the M1 (the primary motor cortex) area, and the energy level required to induce an involuntary movement (a motor twitch) in the patient’s finger is determined. This energy threshold is called the motor threshold (MT). Researchers then typically use 80% of this threshold to stimulate the area of interest.

Frequency

TMS has a spatial resolution of 1cm, but a very good temporal resolution of around 20ms, making it suitable for studying rapid brain processes. The effect of TMS depends on its frequency:

• Low-frequency TMS ( ≤1 Hz): tends to inhibit neural activity by inducing prolonged inhibition by disrupting the natural firing patterns of neurons, reducing their overall activity. This is thought to occur through mechanisms like long-term depression (LTD) of synaptic transmission.

• High-frequency TMS (≥5 Hz): tends to increase neuronal excitability, facilitating the firing of action potentials. The rapid oscillations stimulate neurons to a point where they can reach their threshold for activation.

The two different responses are possible because neurons respond in different ways to different patterns of stimulation. However, as the electric field can propagate through cortical circuits, it can potentially inhibit or activate various connected areas of the brain.

rTMS

In repetitive TMS, multiple pulses are applied over a period, increasing the strength and duration of effects compared to single pulses. However, rTMS must be used cautiously, as it has been associated with seizures in rare cases. 

A suggested protocol is to first apply rTMS to establish a causal relationship with a brain region, and then use single or double-pulse TMS to investigate this relationship with higher temporal precision.

Example study 

Pitcher et al. (2008) explored the role of the occipital face area (OFA, involved in facial recognition) in emotion recognition. Since the OFA is near the skull's surface, it can be targeted with TMS. The researchers used MRI and fMRI to locate the OFA in participants, then:

1. Phase 1: Applied rTMS (10 Hz, 500 ms) to the right OFA. This reduced participants’ ability to recognize emotions from faces, demonstrating a causal link.

2. Phase 2: Applied double-pulse TMS to the right OFA. Here, emotion recognition was only disrupted when TMS was applied 60-100 ms after stimulus onset, suggesting that the OFA’s role in recognizing emotions occurs during early stages of visuaal processing.

This two-step process demonstrates how combining rTMS and single/double-pulse TMS can pinpoint both causal relationships and the timing of neural processes.