The authors thank Dr. Jesse C. Dean for helping with methodological development and providing feedback on a draft of the manuscript. This work was supported by a VA Career Development Award-2 RR&#38;D N0787-W (MGB), an Institutional Development Award from the National Institute of General Medical Sciences of the NIH under grant number P20-GM109040 (SAK) and P2CHD086844 (SAK).&#160;The content does not represent the views of the Department of Veterans Affairs or the United States Government.

All experimental procedures presented in this protocol have been approved by the local Institutional Review Board and are in accordance with the Declaration of Helsinki.
1. Consent Process and Safety Questionnaires
<ol>
	<li>Prior to any experiment, explain to each subject the aim(s) of the study, the main experimental procedures, and any potential risk factors associated with participating in the study. After answering any questions or concerns that subjects may have, ask subjects to acknowledge the consent process and sign the informed consent form.</li>
	<li>Administer MRI34 and TMS35 safety-screening questionnaires to ensure subjects&#8217; safety and qualification for both MRI and TMS testing. Exclude all subjects who don&#8217;t meet all safety criteria from both MRI and TMS assessments.</li>
</ol>
2. MRI and Preparation of the Neuronavigation System
<ol>
	<li>Administer the MRI assessment prior to TMS assessment32. Have subjects lie in a supine position with a cushion placed under their knees to ensure a comfortable posture. Instruct subjects to keep still in the scanner.
	<ol>
		<li>Provide ear protection to the subjects to attenuate the loud noise of the scanner. Preferentially use ear plugs over ear muffs due to the use of bilateral supratragic notch for subject-image registration in the neuronavigation system (see 5.2).</li>
		<li>Obtain high-resolution T-1 weighted anatomical brain images (minimum requirements: 1 mm slice thickness and full brain and cerebellar coverage), either as NFTI or DICOM files. Ensure that nose is fully included in the images due to the use of subject&#8217;s tip of the nose for subject-image registration in the neuronavigation system (see 5.2).</li>
	</ol>
	</li>
	<li>Upload MRI files into a neuronavigation system. Co-register manually each subject&#8217;s MRI to anterior and posterior commissures, so the subject&#8217;s MRI can be mapped using the Montreal Neurological Institute atlas.
	<ol>
		<li>Reconstruct the skin and full curvilinear brain model by adjusting the bounding box around the skull and brain tissue, respectively. Identify four anatomical landmarks (tip of the nose, nasion - bridge of the nose, and supratragic notch of the right and left ear) using the skin model (see Figure 1A).</li>
		<li>Place a rectangular grid over leg motor cortical area at each hemisphere using the reconstructed curvilinear brain (see Figure 1B). Position the centered row of the grid at the center and over the gyrus of the leg motor cortical area where the corticospinal tracts that innervate leg motor pools originate36. Position the medial column of the grid parallel and adjacent to the medial wall of the ipsilateral hemisphere.</li>
		<li>Use a cortex-based approach in which error in orientation has a negligible effect on the stimulation site37 instead of using a scalp-based target approach in which any error in orientation can alter the stimulation site. Use this grid to find the hot spot. For motor mapping, use larger grids either by adding more spots and/or increasing the distance between spots (e.g., 10 mm).</li>
	</ol>
	</li>
</ol>
3. Subject Preparation and Placement
<ol>
	<li>Measure the electrophysiological responses by single pulse TMS using a total of 4 surface EMG electrodes. For the preparation and placement of the electrodes, use published guidelines38,39 and complete placement while the subject is in a standing position.
	<ol>
		<li>Prepare the area over which each electrode would be placed by shaving and lightly exfoliating any dead skin cells and oils using alcohol swabs. 
		CAUTION: For subjects on blood thinners (e.g., people post-stroke), use caution during skin preparation due to the potential&#160;risk of bleeding.</li>
		<li>Attach electrodes bilaterally on TA. While in the standing position, ask subjects to lift their toes upwards and then place the electrode at the upper third of the line between the head of the fibula and medial malleolus (i.e., muscle belly immediately lateral to the tibial crest).</li>
		<li>Attach electrodes bilaterally on SOL. While in the standing position, ask the subject to perform heel raise and then place the electrode at the lower third of the line between the lateral femoral condyle and lateral malleolus.</li>
		<li>Attach the ground reference passive electrode either on the patella or lateral malleolus. Depending on the EMG acquisition unit, place the ground electrodes either bilaterally or unilaterally.</li>
	</ol>
	</li>
	<li>Test the electrodes&#8217; placement and quality of the signal.
	<ol>
		<li>Test the electrodes&#8217; placement (e.g., for clear visually detectable EMG bursts) by asking the subject to either dorsiflex or plantarflex the ankle in an upright posture while displaying the raw EMG signal of all muscles tested on a computer screen. In the case of a misplaced electrode, remove and replace it until there is clear visually detectable EMG bursts with minimal background noise. An adequate signal to noise ratio is critical in detecting a motor response (&#62; 50 &#181;V).</li>
		<li>Test the quality of the signal (e.g., for baseline noise) by discharging the TMS units for a few times while the TMS coil is held away from the seated subject and with the muscles at rest. Check that the baseline signal for each EMG channel is close to zero (i.e., the peak-to-peak amplitude should be less than 50 &#181;V and there is no baseline noise, such as 50 or 60 Hz power line hum). If baseline noise is present in a channel, remove the corresponding electrode and repeat the skin preparation procedures. If the noise is still present (i.e., peak-to-peak amplitude &#62; 50 &#181;V), adjust the reference electrode&#8217;s position and replace the electrolyte gel .</li>
	</ol>
	</li>
	<li>Secure all electrodes using light foam pre-wrap tape. Periodically throughout the experiment, check to ensure that electrodes are securely attached and that the signal has good quality.</li>
	<li>Seat the subject in a chair. To ensure consistent feet placement across subjects, secure both feet in walking boots (i.e., ankle foot orthosis) that allow the ankle ROM to be adjusted to a specific position and provide resistance during TVA testing. Adjust both hip and knee angles to avoid subject discomfort. Instruct the subject to keep still throughout the experiment. Use a forehead rest attached to the chair to keep subjects still during TMS application, if available.</li>
</ol>
4. TVA Testing
<ol>
	<li>Determine bilaterally the maximum voluntary isometric contraction (MVIC) of each muscle. For each motion (i.e., dorsiflexion and plantarflexion), instruct subjects to maximally contract the contralateral examined muscle (e.g., right TA) 4 times (~5 s contractions separated by 60 s of rest) while subject is seated in the posture described above.</li>
	<li>Calculate the maximum muscle activity value during each MVIC (i.e., the average within a 100 ms window centered around the maximum rectified and smoothed EMG) of the last three trials, the average of the three values, and the 15&#160;&#177;&#160;5% of each muscle&#8217;s average MVIC. 
	CAUTION: A larger % MVIC can be used, but it may not be feasible in clinical cohorts (e.g., people post-stroke).</li>
</ol>
5. Registration in Neuronavigation System
<ol>
	<li>Place the subject tracker, either a headband or glasses, with reflective markers on the subject&#8217;s head at the opposite side from the stimulated hemisphere so the tracker does not obstruct positioning of the coil during the stimulation of each grid spot. 
	CAUTION: In the case that a headband is used, ensure that it is snug on subject&#8217;s head, yet not overly tight because it may cause a headache after an extended period of time.</li>
	<li>Verify the proper position of the motion capture camera by placing the subject tracker, the pointer, and the coil tracker in its capture volume space. Perform the subject-image registration by placing the tip of the pointer on the 4 anatomical landmaks (see Figure 1A).</li>
	<li>Once all anatomical landmarks are sampled, verify whether registration occurred accurately by placing the tip of the pointer on several spots over the subject&#8217;s skull (i.e., validation stage). If the distance from the tip of the pointer to the reconstructed skin is less than 3 mm, proceed to TMS experiment; otherwise, repeat the subject-image registration until the desired error values are obtained. During the experiment, repeat registration if the subject tracker is accidentaly moved.</li>
</ol>
6. TMS
<ol>
	<li>Use the same methodological parameters during rest and TVA.
	<ol>
		<li>Apply single pulse stimuli on the optimal site (i.e., hot spot; see next paragraph for further details) of the examined muscle. Apply each stimulus randomly every 5-10 s to avoid stimulus anticipation and to minimize the carry-over effects of the previous pulse to the subsequent one40.</li>
		<li>In case that two TMS units are simultaneously used, set the units at either the standard or simultaneous mode41. The standard mode applies a weaker pulse than a single unit, whereas the simultaneous mode applies a stronger pulse than a single unit. The use of either one could be based on the needs of the protocol and the total number of stimuli.</li>
		<li>Use a double cone coil to induce a posteroanterior intracranial current. If necessary, use the neuronavigation system to control the coil manually and correct its position in relation to the desired stimulated spot prior to each stimulus.</li>
		<li>Across sessions and subjects, randomize the order of the examined muscle and hemisphere. Always administer the TVA condition after the rest condition to avoid any interference with testing at rest (e.g., fatigue of the descending pathways due to TVA testing).</li>
	</ol>
	</li>
	<li>Determine bilaterally the hot spot of both muscles.
	<ol>
		<li>Find the suprathreshold intensity, which will be used during hot spot hunting, by applying a single stimulus over the centered spot next to the interhemispheric fissure (see blue and red squares in Figure 1B). Use this spot because it is located at the locus of the leg motor area36,42.</li>
		<li>Start at low intensity (e.g., 30% maximum stimulator output; MSO) and gradually increase the TMS intensity by 5% increments, until reaching the intensity that elicits a motor evoked potential (MEP) with a peak-to-peak amplitude greater than 50 &#181;V in all contralateral examined muscles for 3 consecutive stimuli.</li>
		<li>Determine immediately after each stimulus whether a MEP has been elicited based on both the raw waveforms and peak-to-peak amplitudes (search window: 20-60 ms post-TMS onset) of all examined muscles.</li>
		<li>Apply one TMS pulse on each spot of the grid (total 36 stimuli). After the completion of the hot spot protocol, transfer the amplitude and latency values of each spot for all contralateral muscles in a spreadsheet and sort amplitude from high to low and latency from low to high. Identify the hot spot of contralateral TA and SOL as the location in the grid with the largest amplitude and the shortest latency43. 
		CAUTION: If the largest amplitude and shortest latency are not at the same spot, define the hot spot using the largest amplitude.</li>
	</ol>
	</li>
	<li>Determine bilaterally each muscle&#8217;s resting motor threshold (RMT).
	<ol>
		<li>Select the grid spot in the neuronavigation system that corresponds to the examined muscle&#8217;s hot spot.</li>
		<li>Use an adaptive threshold-hunting method for RMT determination of the examined muscles44. Set the initial intensity and step size at 45 and 6% MSO, respectively32. Run the RMT hunting twice for each muscle and use the average for the subsequent CMR assessment.</li>
	</ol>
	</li>
	<li>Assess bilaterally TA and SOL CMR during rest.
	<ol>
		<li>Select the grid spot in the neuronavigation system that corresponds to the examined muscle&#8217;s hot spot. Apply 10 single TMS pulses at 1.2 RMT of the examined muscle.</li>
		<li>Prior to each stimulus, instruct the subject to stay still and relax the examined muscles bilaterally and monitor the activity of all muscles using a real time visual feedback displaying on a computer screen. In case any muscle is active before or after TMS, discard that trial and apply an additional single pulse. Repeat until 10 waveforms for each contralateral examined muscle at rest have been collected.</li>
	</ol>
	</li>
	<li>Assess bilaterally the TA and SOL CMR during TVA.
	<ol>
		<li>Select the grid spot in the neuronavigation system that corresponds to the examined muscle&#8217;s hot spot.</li>
		<li>Ask subjects to contract the examined muscle at 15&#160;&#177; 5% MVIC and apply 10 single TMS pulses at 1.2 RMT. Instruct subjects to keep the smoothed moving line (root mean square amplitude of 0.165 s) of the examined muscle, either TA or SOL, within the two horizontal cursors (MVIC range: 15 &#177; 5%) and sustain that contraction at that level for few seconds.</li>
		<li>When TA is the examined muscle, ask subjects to pull slightly up against the bootstraps on their contralateral leg (i.e., the leg with the examined muscle contralateral to stimulated hemisphere). When SOL is the examined muscle, ask subjects to push slightly down against the boot on the contralateral leg.</li>
		<li>Monitor the muscle activity of the active examined muscle and the remaining resting muscles using a real time visual feedback display on a computer screen. Discard that stimulus and apply an additional single pulse again in case the examined muscle&#8217;s activity is either below or above the predetermined range or any other muscle is activated. Collect 10 trials while the examined muscle is activated at the predetermined range.</li>
	</ol>
	</li>
</ol>
7. Data Analysis
<ol>
	<li>For all CMR measures except RMT, calculate the value of each measure from each MEP sweep (the total duration should be at least 500 ms with minimum 100 ms pre-stimulus duration) for all muscles and then average these 10 values to get a single value (i.e., mean)32. Amplitude and cortical silent period (CSP) are proxy excitability measures of CMR, whereas latency is a proxy connectivity measure of CMR. For both rest and TVA, normalize latency relative to each subject&#8217;s height, as latency is influenced by distance to the examined muscle45.</li>
	<li>Calculate MEP amplitude and latency during rest.
	<ol>
		<li>Calculate amplitude (&#181;V) from the raw EMG as the largest difference between positive and negative peaks (i.e., peak-to-peak) of the MEP. For these two particular muscles, search for peak-to-peak within a time window of 20-60 ms after TMS onset. 
		CAUTION: Though the MEP search window of 20-60 ms may work for neurologically intact subjects and people post-stroke, wider MEP search windows (e.g., 20-75 ms) might be required for other neurological populations (e.g., multiple sclerosis).</li>
		<li>Calculate latency (ms) from the rectified EMG as the time between TMS onset and MEP onset (i.e., the time when a rectified EMG trace first crosses a predetermined threshold - mean plus three standard deviations of the 100 ms pre-stimulus EMG)32,46.</li>
	</ol>
	</li>
	<li>Calculate MEP amplitude, latency, and CSP during TVA.
	<ol>
		<li>Calculate amplitude (&#181;V) from the raw EMG as the largest difference between positive and negative peaks (i.e., peak-to-peak) of the MEP. For these two particular muscles, search for peak-to-peak within a time window of 20-60 ms after TMS onset.</li>
		<li>Calculate latency (ms) from the rectified EMG as the time between TMS onset and MEP onset.
		<ol>
			<li>Calculate the MEP onset differently in TVA than in rest. Calculate MEP onset and offset by finding the two time points that the rectified EMG trace crosses the predetermined threshold set to the level of 100 ms pre-stimulus mean EMG. Then, find the peaks that are at least greater than the mean of the pre-stimulus EMG plus three standard deviations and between those two time points. Then, search from the first peak to 50 data points (sampling rate of 5000 Hz) before that peak for the time that the rectified EMG trace first crosses the threshold of the mean pre-stimulus EMG. Define that time as the MEP onset32.</li>
		</ol>
		</li>
		<li>Calculate CSP (ms) from the rectified EMG as the time between the MEP offset and EMG resumption (i.e., absolute CSP: exclusion of MEP duration)47. Search from the last peak to 200 data points (sampling rate of 5000 Hz) after that peak for the time that the rectified EMG trace last crossed the threshold of the mean pre-stimulus EMG; define that time as the MEP offset. Then, calculate the resumption of baseline EMG, which is the time that the rectified EMG trace last crosses 25% of the mean pre-stimulus EMG32.</li>
	</ol>
	</li>
</ol>

The authors have nothing to disclose.

Given the emerging interest in how the motor cortex contributes to the motor control of leg muscles during dynamic tasks in various cohorts, a standardized TMS protocol that describes the thorough assessment of these muscles is needed. Therefore, for the first time, the present protocol provides standardized methodological procedures on bilateral assessment of two ankle antagonistic muscles, SOL and TA, during two muscle states (rest and TVA) using a single pulse TMS with neuronavigation.
The ...

Tibialis anterior (TA) and soleus (SOL) are ankle antagonistic muscles located in the anterior and posterior compartment of the lower leg, respectively. Both muscles are uniarticular, while the main function of TA and SOL is to dorsiflex and plantarflex the talocrural joint, respectively1. Furthermore, TA is more functional for long muscle excursions and less important for force production, whereas SOL is an antigravity muscle designed to generate high force with small excursion of the muscle2. Both muscles are especially relevant during upright postural and dynamic tasks (e.g., walking)3,4. Regarding neural control, the motorneuron pools of both muscles receive neural drive from the brain via the motor descending pathways5,6, in addition to varying degrees of sensory drive.
The main motor descending pathway is the corticospinal tract, which originates from the primary, premotor and supplementary motor areas and terminates in the spinal motorneuron pools7,8. In humans, the functional state of this tract (corticomotor response - CMR) can be feasibly assessed using transcranial magnetic stimulation (TMS), a non-invasive brain stimulation tool9,10. Since the introduction of TMS and given their functional significance during upright postural task and walking, CMR of TA and SOL have been assessed in various cohorts and tasks11,12,13,14,15,16,17,18,19,20,21,22,23,24,25,26,27,28,29,30,31,32.
In contrast to the assessment of CMR in upper-extremity muscles33, no universal TMS protocol has been established for the assessment of CMR in lower-extremity muscles. Due to the lack of an established protocol and the large methodological variability across the previous studies (e.g., type of coil, use of neuronavigation, level of tonic activation, testing side and muscle, use and calculation of CMR measures, etc.), the interpretation of results across studies and cohorts can be cumbersome, complicated, and inaccurate. As the measures are functionally relevant in various motor tasks, an established TMS protocol specific to lower extremity CMR assessment will allow motor neuroscientists and rehabilitation scientists to systematically assess the CMR in these muscles across sessions and various cohorts.
Therefore, the objective of this protocol is to describe the bilateral assessment of TA and SOL CMR using single pulse TMS and neuronavigation system. In contrast to previous work, this protocol aims to maximize rigor of the experimental procedures, data acquisition, and data analysis by employing methodological factors that optimize the validity and duration of the experiment, and standardize the CMR assessment of these two lower extremity muscles. Given that the CMR of a muscle depends on whether the muscle is fully relaxed or is partially activated, this protocol describes how the TA and SOL CMR can be assessed during rest and tonic voluntary activation (TVA). The following sections will thoroughly describe the present protocol. Finally, representative data will be presented and discussed. The protocol described here is derived from that in Charalambous et al. 201832.

<table>
	<tbody>
		<tr>
			<td>2 Magstim stimulators (Bistim module)</td>
			<td>The Magstim Company Limited; Whitland, UK</td>
			<td></td>
			<td>Used to elicit bilateral motor evoked potentials in tibialis anterior and soleus muscles.</td>
		</tr>
		<tr>
			<td>Adaptive parameter estimation by sequential testing (PEST) for TMS</td>
			<td>http://www.clinicalresearcher.org/software.htm</td>
			<td></td>
			<td>Used to determine motor thresholds.</td>
		</tr>
		<tr>
			<td>Amplifier</td>
			<td>Motion Lab Systems; Baton Rouge, LN, USA</td>
			<td>MA-300</td>
			<td>Used to amplify EMG data.</td>
		</tr>
		<tr>
			<td>Data Aqcuisition Unit</td>
			<td>Motion Lab Systems; Baton Rouge, LN, USA</td>
			<td>Micro 1401</td>
			<td>Used to aqcuire EMG data.</td>
		</tr>
		<tr>
			<td>Double cone coil</td>
			<td>The Magstim Company Limited; Whitland, UK</td>
			<td>PN: 9902AP</td>
			<td>Used to elicit bilateral motor evoked potentials in tibialis anterior and soleus muscles.</td>
		</tr>
		<tr>
			<td>Polaris</td>
			<td>Northen Digital Inc.; Waterloo, Ontario, Canada</td>
			<td></td>
			<td>Used to track the reflectiive markers located on subject tracker and coil tracker.</td>
		</tr>
		<tr>
			<td>Signal</td>
			<td>Cambridge Electronics Design Limited; Cambridge, UK</td>
			<td>version 6</td>
			<td>Used to collect motor evoked potentials during rest and TVA.</td>
		</tr>
		<tr>
			<td>Single double differential surface EMG electrodes</td>
			<td>Motion Lab Systems; Baton Rouge, LN, USA</td>
			<td>MA-411</td>
			<td>Used to record EMG signals.</td>
		</tr>
		<tr>
			<td>TMS Frameless Stereotaxy Neuronavigation Sytem</td>
			<td>Brainsight 3, Rouge Research, 
			Montreal, Canada</td>
			<td></td>
			<td>Used to navigate coil position during TMS assessment.</td>
		</tr>
		<tr>
			<td>Walker boot</td>
			<td>Mountainside Medical Equipment, Marcy, NY</td>
			<td></td>
			<td>Used to stabilize ankle joint.</td>
		</tr>
	</tbody>
</table>

bilateral assessment of the corticospinal pathways of the ankle muscles using navigated transcranial magnetic stimulation

Distal leg muscles receive neural input from motor cortical areas via the corticospinal tract, which is one of the main motor descending pathway in humans and can be assessed using transcranial magnetic stimulation (TMS). Given the role of distal leg muscles in upright postural and dynamic tasks, such as walking, a growing research interest in the assessment and modulation of the corticospinal tracts relative to the function of these muscles has emerged in the last decade. However, methodological parameters used in previous work have varied across studies making the interpretation of results from cross-sectional and longitudinal studies less robust. Therefore, use of a standardized TMS protocol specific to the assessment of leg muscles&#39; corticomotor response (CMR) will allow for direct comparison of results across studies and cohorts. The objective of this paper is to present a protocol that provides the flexibility to simultaneously assess the bilateral CMR of two main ankle antagonistic muscles, the tibialis anterior and soleus, using single pulse TMS with a neuronavigation system. The present protocol is applicable while the examined muscle is either fully relaxed or isometrically contracted at a defined percentage of maximum isometric voluntary contraction. Using each subject&#39;s structural MRI with the neuronavigation system ensures accurate and precise positioning of the coil over the leg cortical representations during assessment. Given the inconsistency in CMR derived measures, this protocol also describes a standardized calculation of these measures using automated algorithms. Though this protocol is not conducted during upright postural or dynamic tasks, it&#160;can be used to assess bilaterally any pair of leg muscles, either antagonistic or synergistic, in both neurologically intact and impaired subjects.

Figures 2-4 present data from a representative neurologically intact 31 year old male with height and weight of 178 cm and 83 kg, respectively.
Figure 2 presents the bilateral hot spots and RMT of each ankle muscle. Using the spot located on the center of the leg area in each hemisphere (see squares in Figure 1B), the intensity of 45% MSO was bilaterally used for the hot spot hunting. The hot spot location for each muscle differed between hemi...

Watch this Scientific Journal Video about Bilateral Assessment of the Corticospinal Pathways of the Ankle Muscles Using Navigated Transcranial Magnetic Stimulation at JoVE.com

Bilateral Assessment of the Corticospinal Pathways of the Ankle Muscles Using Navigated Transcranial Magnetic Stimulation

The present protocol describes the simultaneous, bilateral assessment of the corticomotor response of the tibialis anterior and soleus during rest and tonic voluntary activation using a single pulse transcranial magnetic stimulation and neuronavigation system.

Distal leg muscles receive neural input from motor cortical areas via the corticospinal tract, which is one of the main motor descending pathway in humans ...

bilateral-assessment-corticospinal-pathways-ankle-muscles-using

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8.4K Views.  New York University School of Medicine. The present protocol describes the simultaneous, bilateral assessment of the corticomotor response of the tibialis anterior and soleus during rest and tonic voluntary activation using a single pulse transcranial magnetic stimulation and neuronavigation system.

Video: Bilateral Assessment of the Corticospinal Pathways of the Ankle Muscles Using Navigated Transcranial Magnetic Stimulation

A Novel Approach for Documenting Phosphenes Induced by Transcranial Magnetic Stimulation

Stimulation of the human visual cortex produces a transient perception of light, known as a phosphene. Phosphenes are induced by invasive electrical stimulation of the occipital cortex, but also by non-invasive Transcranial Magnetic Stimulation (TMS)1 of the same cortical regions. The intensity at which a phosphene is induced (phosphene threshold) is a well established measure of visual cortical excitability and is used to study cortico-cortical interactions, functional organization 2, susceptibility to pathology 3,4 and visual processing 5-7. Phosphenes are typically defined by three characteristics: they are observed in the visual hemifield contralateral to stimulation; they are induced when the subject s eyes are open or closed, and their spatial location changes with the direction of gaze 2. Various methods have been used to document phosphenes, but a standardized methodology is lacking. We demonstrate a reliable procedure to obtain phosphene threshold values and introduce a novel system for the documentation and analysis of phosphenes. We developed the Laser Tracking and Painting system (LTaP), a low cost, easily built and operated system that records the location and size of perceived phosphenes in real-time. The LTaP system provides a stable and customizable environment for quantification and analysis of phosphenes.

Phosphenes are transient percepts of light that can be induced by applying Transcranial Magnetic Stimulation (TMS) to visually sensitive regions of cortex. We demonstrate a standard protocol for determining the phosphene threshold value and introduce a novel method for quantifying and analyzing perceived phosphenes.

Stimulation of the human visual cortex produces a transient perception of light, known as a phosphene. Phosphenes are induced by invasive electrical ...

Combining Transcranial Magnetic Stimulation and fMRI to Examine the Default Mode Network

The default mode network is a group of brain regions that are active when an individual is not focused on the outside world and the brain is at "wakeful rest."1,2,3 It is thought the default mode network corresponds to self-referential or "internal mentation".2,3
It has been hypothesized that, in humans, activity within the default mode network is correlated with certain pathologies (for instance, hyper-activation has been linked to schizophrenia 4,5,6 and autism spectrum disorders 7 whilst hypo-activation of the network has been linked to Alzheimer's and other neurodegenerative diseases 8). As such, noninvasive modulation of this network may represent a potential therapeutic intervention for a number of neurological and psychiatric pathologies linked to abnormal network activation. One possible tool to effect this modulation is Transcranial Magnetic Stimulation: a non-invasive neurostimulatory and neuromodulatory technique that can transiently or lastingly modulate cortical excitability (either increasing or decreasing it) via the application of localized magnetic field pulses.9
In order to explore the default mode network's propensity towards and tolerance of modulation, we will be combining TMS (to the left inferior parietal lobe) with functional magnetic resonance imaging (fMRI). Through this article, we will examine the protocol and considerations necessary to successfully combine these two neuroscientific tools.

In this article, we examine the methodology and considerations relevant to the combination of TMS and fMRI to examine the effects of brain stimulation on the default network.

The default mode network is a group of brain regions that are active when an individual is not focused on the outside world and the brain is at "wakeful ...

Electrode Positioning and Montage in Transcranial Direct Current Stimulation

Transcranial direct current stimulation (tDCS) is a technique that has been intensively investigated in the past decade as this method offers a non-invasive and safe alternative to change cortical excitability2. The effects of one session of tDCS can last for several minutes, and its effects depend on polarity of stimulation, such as that cathodal stimulation induces a decrease in cortical excitability, and anodal stimulation induces an increase in cortical excitability that may last beyond the duration of stimulation6. These effects have been explored in cognitive neuroscience and also clinically in a variety of neuropsychiatric disorders &#x2013; especially when applied over several consecutive sessions4. One area that has been attracting attention of neuroscientists and clinicians is the use of tDCS for modulation of pain-related neural networks3,5. Modulation of two main cortical areas in pain research has been explored: primary motor cortex and dorsolateral prefrontal cortex7. Due to the critical role of electrode montage, in this article, we show different alternatives for electrode placement for tDCS clinical trials on pain; discussing advantages and disadvantages of each method of stimulation.

Transcranial direct current stimulation (tDCS) is an established technique to modulate cortical excitability1,2. It has been used as an investigative tool in neuroscience due to its effects on cortical plasticity, easy operation, and safe profile. One area that tDCS has been showing encouraging results is pain alleviation 3-5.

Transcranial direct current stimulation (tDCS) is a technique that has been intensively investigated in the past decade as this method offers a ...

The Use of Magnetic Resonance Spectroscopy as a Tool for the Measurement of Bi-hemispheric Transcranial Electric Stimulation Effects on Primary Motor Cortex Metabolism

Transcranial direct current stimulation (tDCS) is a neuromodulation technique that has been increasingly used over the past decade in the treatment of neurological and psychiatric disorders such as stroke and depression. Yet, the mechanisms underlying its ability to modulate brain excitability to improve clinical symptoms remains poorly understood 33. To help improve this understanding, proton magnetic resonance spectroscopy (1H-MRS) can be used as it allows the in vivo quantification of brain metabolites such as &#947;-aminobutyric acid (GABA) and glutamate in a region-specific manner 41. In fact, a recent study demonstrated that 1H-MRS is indeed a powerful means to better understand the effects of tDCS on neurotransmitter concentration 34. This article aims to describe the complete protocol for combining tDCS (NeuroConn MR compatible stimulator) with 1H-MRS at 3 T using a MEGA-PRESS sequence. We will describe the impact of a protocol that has shown great promise for the treatment of motor dysfunctions after stroke, which consists of bilateral stimulation of primary motor cortices 27,30,31. Methodological factors to consider and possible modifications to the protocol are also discussed.

This article aims to describe a basic protocol for combining transcranial direct current stimulation (tDCS) with proton magnetic resonance spectroscopy (1H-MRS) measurements to investigate the effects of bilateral stimulation on primary motor cortex metabolism.

Transcranial direct current stimulation (tDCS) is a neuromodulation technique that has been increasingly used over the past decade in the treatment of ...

Unilateral Pyramidotomy of the Corticospinal Tract in Rats for Assessment of Neuroplasticity-inducing Therapies

The corticospinal tract (CST) can be completely severed unilaterally in the medullary pyramids of the rodent brainstem. The CST is a motor tract that has great importance for distal muscle control in humans and, to a lesser extent, in rodents. A unilateral cut of one pyramid results in loss of CST innervation of the spinal cord mainly on the contralateral side of the spinal cord leading to transient motor disability in the forelimbs and sustained loss of dexterity. Ipsilateral projections of the corticospinal tract are minor. We have refined our surgical method to increase the chances of lesion completeness. We describe postsurgical care. Deficits on the Montoya staircase pellet reaching test and the horizontal ladder test shown here are detected up to 8 weeks postinjury. Deficits on the cylinder rearing test are only detected transiently. Therefore, the cylinder test may only be suitable for detection of short term recovery. We show how, electrophysiologically and anatomically, one may assess lesions and plastic changes. We also describe how to analyse fibers from the uninjured CST sprouting across the midline into the deprived areas. It is challenging to obtain &#62;90% complete lesions consistently due to the proximity to the basilar artery in the medulla oblongata and survival rates can be low. Alternative surgical approaches and behavioural testing are described in this protocol. The pyramidotomy model is a good tool for assessing neuroplasticity-inducing treatments, which increase sprouting of intact fibers after injury.

The corticospinal tract, one of the major sensorimotor tracts, can be lesioned unilaterally in the rodent brainstem in order to test neuroplasticity-inducing therapies for the central nervous system. This surgical procedure (&#8220;pyramidotomy&#8221;) and postoperative assessments are described in this protocol.

The corticospinal tract (CST) can be completely severed unilaterally in the medullary pyramids of the rodent brainstem. The CST is a motor tract that has ...

Effects of Transcranial Alternating Current Stimulation on the Primary Motor Cortex by Online Combined Approach with Transcranial Magnetic Stimulation

Transcranial Alternating Current Stimulation (tACS) is a neuromodulatory technique able to act through sinusoidal electrical waveforms in a specific frequency and in turn modulate ongoing cortical oscillatory activity. This neurotool allows the establishment of a causal link between endogenous oscillatory activity and behavior. Most of the tACS studies have shown online effects of tACS. However, little is known about the underlying action mechanisms of this technique because of the AC-induced artifacts on Electroencephalography (EEG) signals. Here we show a unique approach to investigate online physiological frequency-specific effects of tACS of the primary motor cortex (M1) by using single pulse Transcranial Magnetic Stimulation (TMS) to probe cortical excitability changes. In our setup, the TMS coil is placed over the tACS electrode while Motor Evoked Potentials (MEPs) are collected to test the effects of the ongoing M1-tACS. So far, this approach has mainly been used to study the visual and motor systems. However, the current tACS-TMS setup can pave the way for future investigations of cognitive functions. Therefore, we provide a step-by-step manual and video guidelines for the procedure.

Transcranial Alternating Current Stimulation (tACS) allows the modulation of cortical excitability in a frequency-specific fashion. Here we show a unique approach which combines online tACS with single pulse Transcranial Magnetic Stimulation (TMS) in order to "probe" cortical excitability by means of Motor Evoked Potentials.

Transcranial Alternating Current Stimulation (tACS) is a neuromodulatory technique able to act through sinusoidal electrical waveforms in a specific ...

Combined Transcranial Magnetic Stimulation and Electroencephalography of the Dorsolateral Prefrontal Cortex

Transcranial magnetic stimulation (TMS) is a non-invasive method that produces neural excitation in the cortex by means of brief, time-varying magnetic field pulses. The initiation of cortical activation or its modulation depends on the background activation of the neurons of the cortical region activated, the characteristics of the coil, its position and its orientation with respect to the head. TMS combined with simultaneous electrocephalography (EEG) and neuronavigation (nTMS-EEG) allows for the assessment of cortico-cortical excitability and connectivity in almost all cortical areas in a reproducible manner. This advance makes nTMS-EEG a powerful tool that can accurately assess brain dynamics and neurophysiology in test-retest paradigms that are required for clinical trials. Limitations of this method include artifacts that cover the initial brain reactivity to stimulation. Thus, the process of removing artifacts may also extract valuable information. Moreover, the optimal parameters for dorsolateral prefrontal (DLPFC) stimulation are not fully known and current protocols utilize variations from the motor cortex (M1) stimulation paradigms. However, evolving nTMS-EEG designs hope to address these issues. The protocol presented here introduces some standard practices for assessing neurophysiological functioning from stimulation to the DLPFC that can be applied in patients with treatment resistant psychiatric disorders that receive treatment such as transcranial direct current stimulation (tDCS), repetitive transcranial magnetic stimulation (rTMS), magnetic seizure therapy (MST) or electroconvulsive therapy (ECT).

The protocol presented here is for TMS-EEG studies utilizing intracortical excitability test-retest design paradigms. The intent of the protocol is to produce reliable and reproducible cortical excitability measures for assessing neurophysiological functioning related to therapeutic interventions in the treatment of neuropsychiatric diseases such as major depression.

Transcranial magnetic stimulation (TMS) is a non-invasive method that produces neural excitation in the cortex by means of brief, time-varying magnetic ...

Stimulation Location Determination using a 3D Digitizer with High-Definition Transcranial Direct Current Stimulation

The abundance of neuroimaging data and rapid development of machine learning has made it possible to investigate brain activation patterns. However, causal evidence of brain area activation leading to a behavior is often left missing. Transcranial direct current stimulation (tDCS), which can temporarily alter brain cortical excitability and activity, is a noninvasive neurophysiological tool used to study causal relationships in the human brain. High-definition transcranial direct current stimulation (HD-tDCS) is a noninvasive brain stimulation (NIBS) technique that produces a more focal current compared to conventional tDCS. Traditionally, the stimulation location has been roughly determined through the 10-20 EEG system, because determining precise stimulation points can be difficult. This protocol uses a 3D digitizer with HD-tDCS to increase accuracy in determination of stimulation points. The method is demonstrated using a 3D digitizer for more accurate localization of stimulation points in the right temporo-parietal junction (rTPJ).

Presented here is a protocol to achieve higher accuracy in determination of stimulation location combining a 3D digitizer with high-definition transcranial direct current stimulation.

The abundance of neuroimaging data and rapid development of machine learning has made it possible to investigate brain activation patterns. However, ...

Measuring and Manipulating Functionally Specific Neural Pathways in the Human Motor System with Transcranial Magnetic Stimulation

Understanding interactions between brain areas is important for the study of goal-directed behavior. Functional neuroimaging of brain connectivity has provided important insights into fundamental processes of the brain like cognition, learning, and motor control. However, this approach cannot provide causal evidence for the involvement of brain areas of interest. Transcranial magnetic stimulation (TMS) is a powerful, noninvasive tool for studying the human brain that can overcome this limitation by transiently modifying brain activity. Here, we highlight recent advances using a paired-pulse, dual-site TMS method with two coils that causally probes cortico-cortical interactions in the human motor system during different task contexts. Additionally, we describe a dual-site TMS protocol based on cortical paired associative stimulation (cPAS) that transiently enhances synaptic efficiency in two interconnected brain areas by applying repeated pairs of cortical stimuli with two coils. These methods can provide a better understanding of the mechanisms underlying cognitive-motor function as well as a new perspective on manipulating specific neural pathways in a targeted fashion to modulate brain circuits and improve behavior. This approach may prove to be an effective tool to develop more sophisticated models of brain-behavior relations and improve diagnosis and treatment of many neurological and psychiatric disorders.

This article describes new approaches to measure and strengthen functionally specific neural pathways with transcranial magnetic stimulation. These advanced noninvasive brain stimulation methodologies can provide new opportunities for the understanding of brain-behavior relations and development of new therapies to treat brain disorders.

Understanding interactions between brain areas is important for the study of goal-directed behavior. Functional neuroimaging of brain connectivity has ...

Employing Transcranial Magnetic Stimulation in a Resource Limited Environment to Establish Brain-Behavior Relationships

Neuroimaging is typically perceived as a resource demanding discipline. While this is the case in certain circumstances, institutions with limited resources have historically contributed significantly to the field of neuroscience, including neuroimaging. In the study of self-deception, we have successfully employed single-pulse TMS to determine the brain correlates of abilities including overclaiming and self-enhancement. Even without the use of neuro-navigation, methods provided here lead to successful outcomes. For example, it was discovered that decreases in self-deceptive responding lead to a decrease in affect. These methods provide data that are reliable and valid, and such methods provide research opportunities otherwise unavailable.&#160;Through the use of these methods, the overall knowledge base in the field of neuroscience is expanded, providing research opportunities to students such as those at our institution&#160;(Montclair State University is a Hispanic-Serving Institute) who are often denied such research experiences.

Transcranial Magnetic Stimulation (TMS) and low-frequency TMS (lfTMS) have&#160;been demonstrated to be major contributors to brain literature. Here we highlight the methods for investigating the cortical correlates of self-deception using TMS.

Bilateral Assessment of the Corticospinal Pathways of the Ankle Muscles Using Navigated Transcranial Magnetic Stimulation

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