Assessing Corticospinal Excitability During Goal-Directed Reaching Behavior

Summary

Reaching is a fundamental skill that allows humans to interact with the environment. Several studies have aimed to characterize reaching behavior using a variety of methodologies. This paper offers an open-source application of transcranial magnetic stimulation to assess the state of corticospinal excitability in humans during reaching task performance.

Abstract

Reaching is a widely studied behavior in motor physiology and neuroscience research. While reaching has been examined using a variety of behavioral manipulations, there remain significant gaps in the understanding of the neural processes involved in reach planning, execution, and control. The novel approach described here combines a two-dimensional reaching task with transcranial magnetic stimulation (TMS) and concurrent electromyography (EMG) recording from multiple muscles. This method allows for the noninvasive detection of corticospinal activity at precise time points during the unfolding of reaching movements. The example task code includes a delayed response reaching task with two possible targets displayed ± 45° off the midline. Single pulse TMS is delivered on the majority of task trials, either at the onset of the preparatory cue (baseline) or 100 ms prior to the imperative cue (delay). This sample design is suitable for investigating changes in corticospinal excitability during reach preparation. The sample code also includes a visuomotor perturbation (i.e., cursor rotation of ± 20°) to investigate the effects of adaptation on corticospinal excitability during reach preparation. The task parameters and TMS delivery can be adjusted to address specific hypotheses about the state of the motor system during reaching behavior. In the initial implementation, motor evoked potentials (MEPs) were successfully elicited on 83% of TMS trials, and reach trajectories were recorded on all trials.

Introduction

Goal-directed reaching is a fundamental motor behavior that allows humans to interact with and manipulate the external environment. The study of reaching in the fields of motor physiology, psychology, and neuroscience has produced rich and extensive literature that includes a variety of methodologies. Early studies of reaching used direct neural recordings in non-human primates to investigate neural activity at the level of single neurons^1,2. More recent studies have investigated reaching using behavioral paradigms that employ sensorimotor adaptation to explore the nature of motor learning and control^3,4,5. Such behavioral tasks combined with functional magnetic resonance imaging and electroencephalography can measure whole brain activity during reaching in humans^6,7. Other studies have applied online TMS to investigate various features of reach preparation and execution^{8,9,10,11,12,13,14}. However, there remains a need for an open-source and flexible approach that combines the behavioral assessment of reaching with TMS. While the utility of combining TMS with behavioral protocols is very well established¹⁵, here, we specifically examine the application of TMS within the context of reaching using an open-source approach. This is novel in that other groups who have published using this combination of methods have not made their tools readily available, prohibiting direct replication. This open-source approach facilitates replication, data sharing, and the possibility of multi-site studies. Additionally, should others wish to pursue novel research questions with similar tools, the open-source code can act as a launch pad for innovation, as it is readily adaptable.

TMS offers a noninvasive means of probing the motor system at precisely controlled time points¹⁶. When applied over the primary motor cortex (M1), TMS can elicit a measurable deflection in the electromyogram of a targeted muscle. The amplitude of this voltage wave, known as the motor evoked potential (MEP), provides an index of the momentary excitability state of the corticospinal (CS) pathway-a resultant analog of all excitatory and inhibitory influences on the CS pathway¹⁷. In addition to providing a reliable within-subject measurement of intrinsic CS excitability, TMS can be combined with other behavioral or kinematic metrics to investigate the relationships between CS activity and behavior in a temporally precise manner. Many studies have utilized a combination of TMS and electromyography (EMG) to address a variety of questions about the motor system, particularly since this combination of methods makes it possible to investigate MEPs under a vast array of behavioral conditions¹⁵. One area where this has proven particularly useful is in the study of action preparation, most often through the study of single-joint movements¹⁸. However, there are comparatively fewer TMS studies of naturalistic multi-joint movements such as reaching.

The current goal was to design a delayed-response reaching task that includes behavioral kinematics, online single-pulse TMS administration, and simultaneous EMG recording from multiple muscles. The task includes a two-dimensional point-to-point reaching paradigm with online visual feedback using a horizontally oriented monitor such that visual feedback matches reach trajectories (i.e., a 1:1 relationship during veridical feedback and no transformation between visual feedback and motion). The current design also includes a set of trials with a visuomotor perturbation. In the provided example, this is a 20° rotational shift in the cursor feedback. Previous studies have used a similar reaching paradigm to address questions about the mechanisms and computations associated with sensorimotor adaptation^{19,20,21,22,23,24,25}. Furthermore, this approach makes it possible to assess motor system excitability dynamics at precise time points during online motor learning.

Because reaching has proven to be a fruitful behavior for investigating learning/adaptation, assessing CS excitability in the context of this behavior has enormous potential to shed light on the neural substrates involved in these behaviors. These may include local inhibitory influences, changes in tuning properties, the timing of neural events, etc., as have been established in non-human primate research. However, these features have been more difficult to quantify in humans and clinical populations. Neural dynamics can also be investigated in the absence of overt movement in humans using the combined TMS and EMG approach (i.e., during the preparation of movement or at rest).

The tools presented are open-source, and the code is easily adaptable. This novel paradigm will produce important insights into the mechanisms involved in the preparation, execution, termination, and adaptation of reaching movements. Moreover, this combination of methods has the potential to uncover relationships between electrophysiology and reaching behavior in humans.

Protocol

All methods detailed here were performed in compliance with IRB protocol and approval (University of Oregon IRB protocol number 10182017.017). Informed consent was obtained from all subjects.

1. Reaching apparatus

1. Place a large graphics tablet flat on a desktop.
2. Use an adjustable 80-20 aluminum frame to position the task monitor 6-8 in above the tablet in parallel, with the screen facing upward (for a blueprint, check here: https://github.com/greenhouselab/Reach_TMS and Supplementary Figure 1).
   NOTE: This setup allows for participants to reach across the tablet and acquire targets presented on the task monitor while occluding vision of their reaching arm.
3. Use the setup described in Kim et al.³ as a reference.

2. Machine interfaces

1. Connect the tablet to the computer via a USB port. Connect the task monitor to the computer via the HDMI port. Connect the rear TMS port to the computer via a DB-9 cable.
2. Connect the EMG system to the computer via a PCI-6220 card DAQ. Connect the photodiode to the EMG system via a BNC cable.

3. Photodiode sensor

1. Attach a photodiode sensor to the BNC cable. Secure the photodiode sensor with tape to the top-right corner of the task monitor, with the sensor facing the screen, ≤ 1 cm away.
   NOTE: This will record the timing of stimuli presented on the task monitor as analog data in an independent input channel.

4. Software

1. Download VETA Toolbox²⁶ (https://github.com/greenhouselab/Veta) for MATLAB 2018 to interface with the hardware for data collection.
2. Download the reaching task code (https://github.com/greenhouselab/Reach_TMS) developed for the control of experimental parameters and interfacing with the tablet.

5. Participant screening and informed consent

1. Screen the subject for contraindications to TMS. Exclusion criteria include a personal or family history of seizure, headache, brain trauma, fainting, chronic stress or anxiety, problems with sleep, and any neuroactive medication. Additional exclusion criteria include any metal implants in the brain or skull and any recreational drug or alcohol use in the 24 h prior to testing. Inclusion criteria included right-handedness and age between 18 and 35 years.
2. Provide a written explanation of the procedure and associated risks, clarifying any further questions the participant may have.
3. Obtain informed consent from participants.

6. Subject setup

1. Position the subject in a comfortable chair facing the tablet. Ensure that the knees are flexed to 90° with the legs under the desk.
2. Prepare the skin and place EMG electrodes.
   1. Use fine-grain sandpaper to gently abrade the skin at the site of the right first dorsal interossei (FDI), extensor carpi radialis, and anterior deltoid muscles, as well as the C4 prominence at the base of the neck, to detect electrical artifacts produced by the TMS pulse.
      NOTE: Muscle recording sites can be adapted based on user needs.
   2. Swab each abraded area with an alcohol prep pad once per electrode site to clean.
   3. Place one EMG electrode on each site. Ensure that the electrodes run perpendicular to the muscle fibers. Place the ground electrode on the bony prominence of the right elbow.
   4. Secure each electrode with medical tape.
3. Check the quality of the EMG recording. Use the VETA toolbox to visualize all EMG traces and ensure they are free of artifacts. If EMG traces are noisy, ensure the ground is properly placed and that all electrodes make proper contact with the skin.

7. Transcranial magnetic stimulation

1. Turn on the TMS machine.
2. Find the TMS hot spot of the right FDI muscle via stimulation of the left M1.
   1. Place the coil ~5 cm lateral and 2 cm anterior to the vertex of the head, oriented ~45° off the midline.
   2. Administer TMS pulses once every 4 s while repositioning the coil in increments of approximately 5 mm in the anterior-posterior and medial-lateral plane.
   3. Beginning at 30% maximum stimulator output, gradually increase the TMS intensity by 2% increments until MEPs are observed.
   4. Once the optimum location is identified, at which MEPs can be reliably elicited on the majority (~75%) of pulses at the lowest possible stimulator intensity, determine the resting motor threshold (RMT) by finding the intensity level that produces MEPs with a peak-to-peak amplitude of >50 µV on five out of 10 pulses.
   5. Mark the position by gently placing thin strips of reflective tape on the participant's head along the perimeter of the coil. Maintain coil positioning either by manually holding the coil or using a stand to support it.

8. Reaching task setup

1. Put a Velcro glove on the right hand of the participant to facilitate a relaxed power grip posture.
2. Attach the stylus to the glove and advise the subject to keep the hand relaxed between reaching movements.
3. Communicate the task instructions, which are as follows: Guide the cursor to the home position on the bottom of the screen. You will see a cue at one of two target locations. When the target fills in with color, reach through the target as fast and as accurately as possible. Then return to the home position. Indicate locations of home positions, cues, and targets (Figure 1A).
4. Coach the participant to slice through targets with the stylus as quickly and as accurately as possible. Turn off the lights in the task room to obscure the participant's vision of arm movements and improve the visibility of the task monitor.

9. Task design

1. Control visual stimulus presentation with Psychtoolbox 3.0 in Matlab 2018 (Supplementary Coding File 1).
2. Use the following parameters to match the current data: 20 practice trials; 270 test trials; TMS on 4/5 of test trials; TMS either coincides with the preparatory cue onset (baseline TMS) or 100 ms before the imperative cue (delay TMS) with equal frequency; 1/10 of total trials are catch trials, in which the imperative cue does not appear; the home position is a circle with a 2 cm radius positioned in the bottom center of the workspace; two circular targets with 1 cm radius are positioned 15 cm from the home position at +45° and -45° away from the midline.
3. Set the event order and durations as follows: preparatory cue at 900 ms and imperative cue at 900 ms.

10. TMS administration

1. The VETA toolbox concurrently administers TMS and records EMG https://github.com/greenhouselab/Veta.
2. Control the timing of the TMS pulses with the VETA toolbox to coincide with the chosen behavioral events (i.e., the onset of the preparatory cue or 100 ms preceding target onset).
3. Deliver TMS with sufficient frequency to ensure a sufficient number of MEPs for analysis.
   NOTE: As written, the task code will deliver a TMS pulse on 4/5 of total trials either at the onset of the preparatory cue-to elicit baseline MEPs-or 100 ms before the imperative cue-to elicit delayed MEPs. Parameters can be adjusted in the code according to user needs. Trials without TMS can be used to evaluate behavioral performance in the absence of TMS. This is useful for determining any possible influence of TMS on performance.

Results

Successful execution of the described methods includes the recording of tablet data, EMG traces, and reliable elicitation of MEPs. An experiment was completed that included 270 test trials with TMS delivered on 4/5 of the trials (216 trials).

Data were collected from 16 participants (eight females; eight males) aged 25 ± 10 years, all of whom self-reported as right-handed. We assessed the effectiveness of the visual perturbation on behavioral performance by deriving a learning function for one...

Discussion

The methods outlined above offer a novel approach to studying motor preparation in the context of reaching behaviors. Although reaching represents a popular model task in the study of motor control and learning, there is a need for precisely evaluating the CS dynamics associated with reaching behavior. TMS offers a noninvasive, temporally precise method of capturing CS activity at discrete time points during reaching. The approach described here combines two independent subfields-TMS and reaching-into a ...

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
2-Port Native PCI Express	StarTech.com	RS232 Card with 16950 UART	Must be compatible with desktop computer
Adjustable 80-20 aluminum frame	any
Alcohol prep pads	any		EMG preparation
Bagnoli Bipolar Electrodes	Delsys	DE 2.1
Bagnoli Reference Electrode	Delsys	USX2000	2” (5cm) Round
Bagnoli-8 EMG System	Delsys
Chair	any
Computer monitor for EMG/TMS	n/a
Desk	any
Desktop Computer	Dell	xps 8930	RAM: 16 GB, Storage: 1TB, Graphics: 1060 6GB
EMG electrodes	Delsys	Sensor Adhesive Interface
Fine grain sandpaper	any		EMG preparation
Graphics tablet	Wacom	Intuos-4 XL
Handle of paint roller	any		to be used as stylus handle, hollowed out center must be large enough for stylus to sit securely inside
Medical tape	any		To secure EMG electrodes
PCI-6220 card DAQ	National Instruments		To interface EMG system
Photodiode Sensor	Vishay	BPW21R	To record timing of task events into EMG trace.
Rear TMS port	Magstim		Included with TMS machine
Right-handed polyethylene glove	any		Cut out thumb and index finger of glove to expose FDI muscle
Sensory Adhesive Interface, 2-slot	Delsys	SC-F01
Stylus	Wacom	Intuos-4 grip pen
Tablet-to-Computer USB cable	any		Included in Tablet purchase
Task Monitor	Asus	VG248
TMS coil	Magstim	D70 Remote Coil	7cm diameter, figure-of-eight coil
TMS machine	Magstim	200-2
TMS-to-Computer DB9 cable	any		Connects to PCIe Serial Card
Velcro	any		To be placed on glove and stylus handle