Transcranial Magnetic Stimulation in Awake Rhesus macaques: Validation of a Novel Non-invasive Apparatus

This study presents and validates a novel non-invasive head- and arm-fixation apparatus that enables the successful application of human-standard Transcranial Magnetic Stimulation protocols, including adaptive motor threshold determination and short-interval intracortical inhibition measurement, in awake rhesus macaques without surgical intervention.

The Gist

The Big Picture: Bridging the Gap Between Humans and Monkeys

Imagine scientists want to understand how the human brain works, specifically how we control our hands and fingers. They have two main tools:

1. Human Studies: They can zap a human's brain with a magnetic pulse (Transcranial Magnetic Stimulation, or TMS) to see what happens.
2. Animal Studies: They can do the same with animals to get a deeper, more detailed look at the brain's wiring.

The Problem: Rats are great for studying brain wiring, but their brains are very different from ours. Monkeys (specifically Rhesus macaques) have brains that look and work almost exactly like ours. So, they are the perfect "middleman" to translate human findings into animal models.

The Catch: To study a monkey's brain with TMS, the monkey has to sit perfectly still. If it moves, the magnetic pulse misses the target.

• The Old Way: Put the monkey under heavy anesthesia (sleeping gas). Problem: A sleeping brain acts differently than a waking one, so the data isn't very useful for understanding how humans think and move.
• The Surgical Way: Screw a metal bracket into the monkey's skull to hold its head still. Problem: This is invasive, painful, and risky for the animal.
• The New Way (This Paper): The researchers built a high-tech, non-invasive "harness" that holds the monkey's head and arm still without surgery, drugs, or pain.

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The Invention: A Custom-Fitted "Space Suit" for Monkeys

Think of this new apparatus as a custom-tailored, 3D-printed seatbelt system for a monkey.

1. The Head Restraint: Instead of drilling into the skull, they created a mask that fits the monkey's face perfectly.

   • The Analogy: Imagine a dentist's chair, but instead of a hard metal headrest, the monkey wears a soft, custom-molded helmet made of 3D-printed plastic and silicone. It fits their cheekbones and forehead snugly, like a high-end gaming headset, but it's designed to keep their head from bobbing around.
   • Why it matters: The monkey stays awake, alert, and comfortable, just like a human patient in a TMS clinic.

2. The Arm Restraint: To measure the brain's effect on the hand, the monkey's arm needs to be still, but the muscles need to be relaxed.

   • The Analogy: It's like a gentle "arm sling" made of soft foam and Velcro straps. It holds the arm and fingers in place so the monkey can't fidget, but it doesn't squeeze or hurt them.

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The Test Drive: Does It Work?

The researchers put this new "monkey suit" to the test with two different experiments to prove it works just like the human equipment.

Test 1: Finding the "Volume Knob" (Motor Threshold)

In TMS, scientists need to find the exact "volume" (intensity) of the magnetic pulse needed to make a muscle twitch.

• The Human Method: They use a smart algorithm (a computer program) that adjusts the volume up and down rapidly, like a DJ finding the perfect beat, to pinpoint the exact level needed in about 25 tries.
• The Monkey Result: They used this same "DJ algorithm" on the awake monkeys. It worked perfectly. The computer found the exact "twitch threshold" for all four monkeys in the same number of tries as it does for humans. This proved the monkeys were sitting still enough for the machine to get a clean reading.

Test 2: The "Brain Brake" (Short-Interval Inhibition)

This is a more complex test. Scientists send two pulses very quickly: a weak one followed by a strong one.

• The Analogy: Imagine tapping a drum lightly, then immediately tapping it hard. In a healthy brain, the first tap "brakes" the second one, making the second tap weaker than it should be. This is called Short-Interval Cortical Inhibition (SICI). It's a sign that the brain's internal "brakes" (inhibitory circuits) are working.
• The Monkey Result: For the first time, they measured this "brain brake" effect in awake monkeys. The results looked almost identical to human data. The monkeys' brains showed the same "braking" pattern, proving that the non-invasive harness allows for high-quality, human-like brain studies.

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Why This Matters (The "So What?")

Before this paper, studying the awake monkey brain with TMS was like trying to take a high-resolution photo of a hummingbird while it's flying in a hurricane. You either had to freeze the bird (anesthesia) or glue it to the table (surgery).

This new apparatus is like a high-speed camera that can capture the hummingbird mid-flight without touching it.

• Better Science: Because the monkeys are awake and natural, the data is much more relevant to how human brains actually work.
• Better Ethics: No surgery, no pain, and no drugs. The monkeys are treated like patients, not lab subjects.
• Future Potential: Now that we have a way to do this, scientists can test new drugs for brain disorders (like Parkinson's or depression) on monkeys using the exact same protocols they use on humans. This makes the path from "lab discovery" to "human cure" much faster and safer.

In short: The researchers built a comfortable, custom-fit chair and mask that lets them zap awake monkeys' brains safely. They proved it works by showing that the monkeys' brain responses are identical to humans', opening the door for better, more ethical medical breakthroughs.

Technical Summary

1. Problem Statement

Transcranial Magnetic Stimulation (TMS) is a critical tool for probing and modulating cortical excitability in both basic neuroscience and clinical practice. While Non-Human Primates (NHPs), particularly rhesus macaques, are ideal translational models due to their anatomical and functional homology with humans (including direct cortico-motoneuronal projections), applying TMS in awake NHPs presents significant technical hurdles:

• Anesthesia Limitations: Conducting TMS under anesthesia alters the cortical state, limiting the validity of translational data.
• Invasive Headposts: Standard awake protocols often require surgically implanted headposts. These carry risks of infection, perioperative stress, and can physically obstruct coil positioning.
• Lack of Standardized Non-Invasive Solutions: Existing non-invasive head-fixation methods are rare, often lab-specific, and struggle with varying cranial morphologies or comfort, leading to movement artifacts during stimulation and Electromyography (EMG) recording.

The authors aimed to develop and validate a genuinely non-invasive, adjustable head- and arm-fixation apparatus that allows for reliable TMS and EMG recording in awake, surgically intact rhesus macaques, enabling the direct application of human TMS protocols.

2. Methodology

Apparatus Design

The researchers developed a custom, modular apparatus consisting of two main units:

• Non-Invasive Head-Fixation Unit:
  • Base: A 3D-printed (PETG) static base mounted to a primate chair neck plate, adjustable via screws for posture.
  • Back Piece: 3D-printed (TPU) with a head cushion, adjustable horizontally.
  • Front Piece (Mask): Composed of a handle (TPU) and a custom face mold made of high-tensile strength silicone (Shore 30A).
  • Customization: Face molds were fabricated in three sizes (small, medium, large) based on MRI-derived 3D face reconstructions of the subjects to ensure a snug fit around the cheekbones without surgical intervention.
• Arm-Fixation Unit:
  • Designed to restrain the arm and fingers while leaving hand muscles accessible for EMG.
  • Features an aluminum alloy base with 3D-printed ankle holders, foam platforms for the lower arm, and polyamide fastening bands (Velcro) to secure the upper arm, lower arm, and fingers.

Experimental Protocol

• Subjects: 4 adult male rhesus macaques (Macaca mulatta).
• Habituation: Subjects underwent positive reinforcement training to acclimate to the apparatus over 9–31 sessions.
• TMS Setup:
  • Navigation: Individual T1-weighted MRI scans were used to create stereotaxic maps. The Brainsight™ frameless neuronavigation system tracked coil position relative to the head-fixation apparatus.
  • Target: Primary Motor Cortex (M1) "hotspot" for the Abductor Pollicis Brevis (APB) muscle.
  • Stimulation: MagPro x100 stimulator with a Cool-B65 figure-eight coil.
  • Recording: Surface EMG electrodes on the right APB muscle; data sampled at 3 kHz.

Validation Experiments

1. Motor Threshold (MT) Determination:
   • Performed on all 4 subjects.
   • Used an adaptive stepping algorithm (DCS-HA) via the Stochastic Approximator of MT (SAMT) software.
   • Goal: Determine the minimum intensity yielding a 100 µV peak-to-peak MEP in 50% of trials within ~25 pulses.
2. Short-Interval Intracortical Inhibition (SICI):
   • Performed on 2 subjects ('Sp' and 'Sz').
   • Paired-Pulse Protocol: A subthreshold conditioning stimulus (80% MT) followed by a suprathreshold test stimulus (120% MT).
   • Interstimulus Intervals (ISI): 1 ms, 2.5 ms, and 5 ms.
   • Analysis: Compared conditioned MEP amplitudes against unconditioned single-pulse controls.

3. Key Contributions

• Novel Apparatus: Introduction of a fully non-invasive, 3D-printable, and adjustable head-and-arm fixation system that accommodates individual cranial morphologies without surgery.
• Protocol Translation: Successful implementation of human-standard TMS protocols (Adaptive MT and SICI) in awake NHPs for the first time using a non-invasive setup.
• Algorithm Validation: Demonstration that the human-derived DCS-HA adaptive algorithm (SAMT) converges reliably in rhesus macaques, validating its use for rapid MT determination in NHPs.
• First SICI in Awake NHPs: The first reported measurement of robust short-interval cortical inhibition in awake, non-surgically restrained macaques.

4. Results

Habituation

All 4 subjects were successfully habituated to the full apparatus. The average training duration was 16 sessions (range: 9–31), with subjects sitting calmly for up to 20 minutes.

Motor Threshold (MT)

• Convergence: The adaptive algorithm successfully converged to valid MT estimates in all 4 subjects within 25 pulses.
• Metrics: The "n-of-ten" criterion (3–7 suprathreshold responses in the last 10 pulses) was met for all subjects.
• Values: Final MT estimates were 41.6%, 46.3%, 23.4%, and 30.3% of Maximum Stimulator Output (%MSO) for the four subjects.
• Validation: The proportion of responses above the 100 µV threshold at perithreshold intensities was approximately 50%, confirming the validity of the MTs according to International Federation of Clinical Neurophysiology (IFCN) standards.

Short-Interval Intracortical Inhibition (SICI)

• Aggregated Data: A significant inhibition effect was observed across all ISIs compared to unconditioned controls.
  • 2.5 ms ISI: Strongest inhibition (MEP amplitude dropped to 65% of control; 95% CI: 58–72%).
  • 1 ms ISI: Moderate inhibition (76% of control).
  • 5 ms ISI: Weakest but significant inhibition (83% of control).
• Individual Variability: While the group average showed peak inhibition at 2.5 ms (consistent with human data), individual subjects showed variability: Subject 'Sp' peaked at 1 ms, while 'Sz' showed minimal 1 ms inhibition and strong 2.5 ms inhibition.
• Consistency: Within-subject results were highly consistent across sessions, indicating the apparatus provides stable recording conditions.

5. Significance and Impact

• Translational Bridge: This study bridges the gap between rodent models (limited by anatomy) and human studies by providing a method to test human-specific TMS protocols in a primate model with similar cortical organization.
• Animal Welfare: By eliminating the need for surgical headposts, the apparatus reduces perioperative risks, infection potential, and long-term discomfort, aligning with the "3Rs" (Replacement, Reduction, Refinement) of animal research.
• Ecological Validity: The non-invasive nature allows for longitudinal studies and the combination of TMS with cognitive, pharmacological, or social behavioral experiments in a naturalistic, awake state.
• Future Directions: The validated apparatus enables the mapping of cortical excitability differences between NHPs and humans, facilitating the development of bidirectionally translatable neuromodulation therapies for neurological and psychiatric disorders.

In conclusion, the authors have successfully validated a novel, non-invasive platform that enables high-fidelity TMS and EMG recording in awake rhesus macaques, proving that human-standard protocols can be directly translated to non-invasive NHP models.