The overall goal of this experiment is to evaluate regional cortical hyperexcitability in patients with epilepsy using resting-state functional connectivity, MRI-guided, transcranial magnetic stimulation in combination with simultaneous EEG recording. This method can help answer key questions in the field of epilepsy and neurophysiology, such as whether patients with epilepsy have evidence of hyperexcitability in regions that are believed to be part of the epileptogenic network. The main advantage of this technique is that it can be used to assess differences in cerebral excitability as a function of connectivity and can be used to assess cortical reactivity across a variety of different brain regions.
This technique has implications for the diagnosis and treatment of epilepsy, as cortical hyperexcitability can be identified even when the routine EEG is normal, and epileptogenic circuits can be targeted therapeutically. Demonstrating the procedure will be Tamara Gedankian, a research assistant in my laboratory. Prior to testing, determine the two TMS target regions by superimposing each subject's functional connectivity map onto each subject's structural image.
To begin the experimental session, bring the subject into the testing room and have them sit in the chair. Measure the subject's head, and select an Electroencephalography, or EEG, cap, of appropriate size to enable low electrode impedances. Next, thoroughly clean the skin under each electrode using a cotton tip applicator and alcohol.
Add conductive gel to each electrode, and press down on the electrode to ensure good contact between the scalp, gel, and electrode. To minimize charging artifacts, ensure that the gel does not spread outside of the electrode holder. Place the reference and ground electrodes on the forehead, and as far from the stimulation coil as possible, to minimize the possibility of a TMS-induced electrode artifact contaminating the entire recording.
Place these electrodes within a few centimeters of each other to minimize common-mode noise. Then press the measure impedances button on the EEG system. Check electrode impedances by plugging the EEG output cables into the impedance jack of the EEG recording system.
Ensure that the electrode impedance is not greater than five kilomes. Next, prepare the electromyography electrodes on the contralateral hand. Give the subject earplugs to minimize risk of hearing loss and tinnitus.
Then place the infrared detectors on the subject's head, ensuring that the detectors are placed in a way to minimized risk of movement during the experimental session. Coregister the subject's head with the MRI images by identifying the location of the preselected, external anatomic fiducial markers on the subject using the pointer that is included with the neuronavigation equipment. Familiarize the subject with stimulation by applying a pulse elsewhere or by applying a low-intensity stimulation pulse to the scalp.
Determine the resting motor threshold by locating the subject's motor cortex on the hemisphere ipsilateral to the functional connectivity-base targets. Angle the coil perpendicular to the gyrus with the handle pointing occipitally, and begin stimulation at an intensity that is expected to be sub-threshold. Then increase stimulation intensity in steps of 5%max stimulator output until TMS consistently evokes motor-evoked potentials with amplitudes greater than 50 microvolts in each trial.
Decrease stimulation intensity in steps of 1%max stimulator output until less than five positive responses out of 10 are recorded. Finally, set the TMS intensity to the desired value. Apply single pulses of TMS to each of the target regions using the neuronavigation software, with variable intervals between pulses to minimize cortical plasticity and subject expectancy effects.
Begin by performing an initial round of an Independent Component Analysis, or ICA, and remove the one to two components representing the large TMS-induced, initial muscle activation. To do this, run ICA using the fast ICA method with the symmetric approach, and the 10 contrast function, using the command shown here. Identify components consistent with the TMS artifact by selecting Tools, Reject data using ICA, and remove components by map, which will plot topographic maps of all the ICA components.
Then click on the number for each component to plot component details. Next, delete the artifactual components by selecting Tools, Remove components, and entering the relevant component numbers in the field for Component(s)to remove from data. In the confirmation box that pops up, press Plot ERPs to review the event-related potentials, or ERPs, that result from deletion of the selected components.
To review single trial effects, press Plot single trials. After reviewing the ERP, as in single trials, press the Accept button to delete the selected components. Perform a second round of ICA and remove components corresponding to artifacts of decay, blinks, muscle, and electrode noise.
To do this, run ICA using the fast ICA method with the symmetric approach and the tan contrast function just as done for the first round of ICA. Likewise, evaluate component properties just as done with the topographic map in the first round of ICA. Then mark components consistent with residual TMS decay artifacts, blink artifacts, and muscle artifacts.
Additionally, mark components consistent with channel noise based on spatial and temporal distribution. Finally, remove marked components, just as done in the first round of ICA, by selecting Tools, Remove components, and entering the relevant component numbers in the field for Component(s)to remove from data. Resting-state functional connectivity MRI is used to identify regions on the cortical surface with connectivity to the regions of heterotopia.
TMS to these regions produces abnormally increased delayed activity relative to regions that do not have abnormal connectivity and relative to the same sights in healthy controls. Here, source localization of the abnormal late peaks in the TMS-evoked potentials in patients with epilepsy can identify brain regions from which the abnormal activity arises. It may spatially colocalize with the patient's seizure-focus.
After watching this video, you should have a good understanding of how to use resting-state functional connectivity, MRI-guided, TMS EEG to evaluate brain excitability in different regions in patients with epilepsy and other neuropsychiatric disorders. Following this procedure, other methods, like repetitive TMS, can be performed to determine if decreasing the cortical excitability in brain regions that are part of the pathogenic network can modify disease activity.
