Cortical Plasticity and Rehabilitation: TMS and NIBS Comprehensive Study Guide

Learning Objectives of Cortical Plasticity and Rehabilitation

• Examine various methods used to alter brain excitability in human subjects.

• Evaluate relevant evidence demonstrating that non-invasive brain stimulation (NIBS) induces long-term potentiation (LTP)-like plasticity.

• Understand the underlying reasons for the high variability observed in human responses to non-invasive brain stimulation.

• Analyze how non-invasive brain stimulation techniques can be strategically applied to improve motor function and assist in rehabilitation.

Transcranial Magnetic Stimulation (TMS) Fundamentals

• Definition and Mechanism: TMS is a primary technique in motor system research used to measure changes in the excitability of the motor cortex and spinal neurons before and after interventional treatments.

  • Hardware: It consists of a magnetic stimulator connected to a specialized coil.

  • Application: The coil is placed over the specific motor cortical representation for a target muscle (e.g., the hand region of the motor cortex).

  • Induction: The coil discharges a magnetic field that passes unattenuated through the scalp and skull, generating an electrical current in the underlying motor cortex neurons.

  • Signal Propagation: Action potentials are generated and travel down corticospinal neurons to the spinal cord, through motor neurons, and eventually reach the target muscle.

• Motor Evoked Potential (MEP):

  • The resulting muscle response is measured using Electromyography (EMG) electrodes placed over the muscle.

  • The MEP amplitude reflects the excitability of the corticospinal neurons activated by the stimulus.

  • Excitability Increases (LTP-like): Revealed as a larger MEP (often color-coded red in diagrams).

  • Excitability Decreases (LTD-like/Injury): Revealed as a smaller MEP (often color-coded blue in diagrams).

• TMS Modalities:

  • Single Pulse TMS: Produces a single MEP; used as a baseline measure of corticospinal excitability.

  • Paired Pulse TMS: Uses a conditioning stimulus to modify the size of a test stimulus delivered shortly thereafter to measure intracortical dynamics.

    • A conditioning stimulus given $12\,ms$ before the test stimulus identifies intracortical excitability (larger MEP).

    • A conditioning stimulus given $2\,ms$ before the test stimulus identifies intracortical inhibition (smaller MEP).

  • Repetitive TMS (rTMS): Involves trains of stimuli delivered over time (e.g., $10\,minutes$) to actively alter or modulate cortical excitability.

Non-Invasive Brain Stimulation (NIBS) Techniques

• Overview: NIBS is increasingly utilized by clinicians and researchers to alter the functional state of the human brain through neuroplasticity processes.

• Repetitive TMS (rTMS):

  • Stimulation at different frequencies, intensities, and durations can induce effects lasting up to one hour.

  • Low-Frequency rTMS: Delivered below $2\,Hz$ for at least $10\,minutes$; produces a long-lasting decrease in cortical excitability.

  • High-Frequency rTMS: Delivered above $5\,Hz$; produces a long-lasting increase in cortical excitability.

• Theta Burst Stimulation (TBS):

  • A newer, patterned approach using short bursts of high-frequency stimulation ($50\,Hz$ bursts repeated every $200\,ms$).

  • Uses lower stimulus intensities and shorter durations, making it more suitable for patient populations.

  • Intermittent TBS (iTBS): Consists of $2\,s$ of stimulation repeated every $10\,s$ for a total of $190\,s$. Results in increased MEP amplitude lasting approximately $15\,minutes$.

  • Continuous TBS (cTBS): Consists of continuous stimulation for $40\,s$. Results in a decrease in MEP amplitude lasting nearly $60\,minutes$.

• Paired Associative Stimulation (PAS):

  • Pairs peripheral nerve stimulation with TMS of the motor cortex repeatedly ($100$ to $200$ pairs over $10$ to $15\,minutes$).

  • The timing between stimuli determines the plastic effect (Spike-timing dependent plasticity):

    • Interstimulus interval of $25\,ms$: Signals reach the cortex simultaneously, resulting in LTP-like increases in excitability.

    • Interstimulus interval of $10\,ms$: Signals occur out of phase at the cortex, resulting in LTD-like decreases in excitability.

• Transcutaneous Direct Current Stimulation (tDCS):

  • Involves passing a weak electrical current through pads/electrodes on the scalp for $10$ to $20\,minutes$.

  • Anodal Stimulation: Current flows through the anode placed over the motor cortex, increasing excitability.

  • Cathodal Stimulation: Current flows through the cathode placed over the motor cortex, decreasing excitability.

  • Advantages: Portable and inexpensive for rehabilitation.

  • Disadvantages/Side Effects: Potential for itching or burning sensations on the skin if safety guidelines are not followed.

Evidence for LTP and LTD-Like Plasticity in Humans

• There are five primary lines of evidence suggesting NIBS-induced changes in humans parallel LTP/LTD observed in animal brain slices:

  • History of Stimulation: The existing state of the synapse influences the response to new stimulation (Metaplasticity).

  • Frequency Dependence: Changing the frequency of stimulation changes the direction of the effect (excitatory vs. inhibitory).

  • NMDA Receptor Involvement: The effects are blocked by NMDA receptor antagonists.

  • Neurotrophin Reliance: The magnitude of plasticity is influenced by factors such as Brain-Derived Neurotrophic Factor (BDNF).

  • Behavioral Impact: These cortical changes directly affect motor learning.

Metaplasticity and Regulatory Mechanisms

• Definition: Metaplasticity refers to the phenomenon where the prior history of neuronal activity changes the threshold for inducing subsequent LTP or LTD.

• Priming Experiments:

  • If a test iTBS (normally excitatory) is preceded by priming iTBS, the response actually decreases.

  • If a test iTBS is preceded by priming cTBS (inhibitory), the response increases.

  • Essentially, pairing identical protocols decreases the test response, while pairing opposing protocols enhances it.

• Motor Learning Application: Studies show that motor learning (e.g., rapid thumb flexion) is significantly improved when preceded by protocols that induce LTD-like states (PAS LTD), illustrating that the history of activation dictates the learning capacity of the motor cortex.

Biological and Genetic Mediators of Plasticity

• NMDA Receptors:

  • Studies utilizing memantine (an NMDA receptor antagonist) demonstrate that pharmacological blockade of these receptors completely abolishes the effects of both iTBS and cTBS.

  • The placebo group showed standard MEP facilitation for iTBS and suppression for cTBS, while the memantine group showed no significant change.

• BDNF Polymorphism:

  • Brain-Derived Neurotrophic Factor (BDNF) is a neurotrophin essential for plasticity.

  • In humans, a common polymorphism in the BDNF gene (Val66Met) affects plasticity.

  • Research comparing val-val (control), val-met, and met-met groups shows that training-dependent increases in motor map area are significantly reduced in individuals with the BDNF polymorphism compared to the val-val group.

Clinical Applications: Stroke and Depression

• Stroke Rehabilitation:

  • Pathophysiology: Motor deficits are associated with an imbalance where the healthy hemisphere over-inhibits the damaged (lesioned) hemisphere (interhemispheric inhibition).

  • Application 1: High-frequency (excitatory) rTMS, iTBS, or anodal tDCS applied to the lesioned hemisphere.

  • Application 2: Low-frequency (inhibitory) rTMS, cTBS, or cathodal tDCS applied to the intact/healthy hemisphere to reduce excessive inhibition of the damaged side.

  • Outcome Study: Excitatory TBS to the stroke hemisphere improved reaction time for at least $30\,minutes$, whereas inhibitory TBS to the intact hemisphere showed no improvement in that specific study.

• Drug-Resistant Depression:

  • Application 1: High-frequency excitatory rTMS to the hypoactive left prefrontal cortex.

  • Application 2: Low-frequency inhibitory rTMS to the hyperactive right prefrontal cortex.

  • Treatment Regimen: Usually involves daily application over several weeks.

Variability in Human Responses

• NIBS techniques are characterized by high inter-individual and intra-individual variability.

• Standard Results vs. Real-World Data:

  • In a study of $12$ subjects receiving cTBS, only some showed the expected decrease in MEP; others showed an increase or no change.

  • In a larger study of $52$ subjects, the average effect of iTBS and cTBS was negligible due to the extreme variation between individuals.

  • Repeat sessions (one week apart) also show high variability within the same subject.

• Factors Influencing Response (ordered from least to most influential):

  • Gender: Females may show different plastic responses.

  • Physical Activity: Active lifestyles are beneficial for plasticity.

  • Time of Day: Stimulation in the afternoon is often linked to better responses.

  • Age: Younger brains generally exhibit higher plasticity.

  • Attention/Focus: Being focused on the task during stimulation optimizes outcomes.

  • Synaptic History: The state of the brain (metaplasticity) prior to stimulation.

  • Pharmacology: Specific drugs can enhance the plastic response.

  • Genetic Factors: BDNF profiles and other neurochemical release patterns are the strongest predictors of response.

Future Directions and Cumulative Benefits

• The effects of a single NIBS session are often short-lived and variable.

• Repeated Application: Evidence suggests that performing sessions daily or multiple times within a session increases therapeutic benefits.

• Motor Skill Improvement: In a study over five consecutive days, subjects receiving anodal tDCS during a motor task showed continual and superior performance improvements each day compared to a sham stimulation group.

• Conclusion: Repeated application is considered the most promising approach for achieving substantial rehabilitation outcomes.