Investigating the Function of Deep Cortical and Subcortical Structures Using Stereotactic Electroencephalography: Lessons from the Anterior Cingulate Cortex

The overall goal of this procedure is to record electrophysiological signals from deep within the brain. This is accomplished by first placing electrodes in the target regions of interest. The data acquisition rig is then set up by the patient's bedside, and the patient is instructed how to perform a simple cognitive task in the final step.

The electrophysiological signals are recorded through the electrodes while the patient completes the task. Ultimately, multi tapered spectral analysis is used to demonstrate variances in the local field, potential activity in the dorsal anterior cingulate cortex, depending on the difficulty of the cognitive task. The advantage of stereo ence holography over other recording techniques such as subdural grid implantation is that with stereo EEG, we have access to deep structures within the brain.

This is an advantage clinically when we think that seizures may be arising from these deep structures and also from a research perspective, cause it gives us access to areas of the brain that we don't have access to. Otherwise, Patient selection is critically important because this technique is particularly suited to the localization of seizures coming from deep cortical and subcortical structures. Recordings like these can help us answer key questions in the epilepsy field, for example, where do seizures start?

Where do seizures spread, and how do we map these areas? Using standard EEG techniques, Though, this method can provide insight into epilepsy and seizure origination, and we can also apply it to other aspects of neuroscience such as cognition, memory, and social awareness. Generally, individuals new to this method will struggle with the data acquisition component of the technique because acquiring electrophysiological signals can be complex.

Following the CT scan, return the patient to the operating room and prepare the surgical field according to routine sterile methods. Next, using printed stereotactic coordinates from the scan set the coordinates for the first depth electrode on the head frame in the lateral vertical and anterior posterior planes. The coordinates are verified and modified if necessary by a cos surgeon Using a computer workstation in the operating room, use the guide block to identify the insertion site on the skin and mark this position with a marking pen.

Inject one to two milliliters of local anesthesia into the marked incision. Then use a number 11 scalpel to nick the marked incision down to the skull, and use a monopolar cautery directed with a coated opterator to cauterize the dermis and deep tissue using the guide block. To maintain the proper trajectory, create a bur hole using a 2.1 millimeter twist drill bit and a handheld electrical drill.

Open the dura using monopolar cautery guided by a rigid coated opterator probe, still guiding the trajectory with the guide block. Screw an anchor bolt into the hole and place a premeasured stylet probe through the anchor bolt to make a track for the electrode. Then carefully advance the electrode to the pre-calculated depth, tightening the anchor bolt capped down to secure the electrode once it is in place.

To ensure an adequate placement trajectory for all of the electrodes, bring the sterile draped fluoroscope into the surgical field and obtain an AP fluoroscopic image. Then connect the electrodes to the clinical EEG system to verify the appropriate impedances. Place the dressing.

Remove the stereotactic head frame, and wake the patient from anesthesia. For the behavioral task set up, open the appropriate behavioral software and set the conditions file designed to run the multisource interference task. To include all four trial types of equal frequency, press the set conditions button to choose the desired conditions file.

Then click test in the display box to test the behavioral display monitor. The test visual stimulus should appear for two to three seconds. Next, connect the button box to the analog inputs on the data acquisition board and to a power source.

Use a ribbon cable split into nine ribbons to connect eight of the ribbons to port zero through seven on the digital IO portion of the data acquisition board. Connecting the ninth ribbon to the zero port on the digital PFI portion of the board. Then set the desired sampling rate in the neural signal processor software.

For example, here, the desired sampling rate is set to 50, 000 samples per second with an alias and down sample online of 1000 samples per second. Finish the setup by connecting the amplifier to the neural signal processor via fiber optic cable and the neural signal processor to the data streamer and the optical PCI card in the neural data acquisition computer via fiber optic cable. The key to ensuring successful data acquisition is to test the signal processing setup prior to entering the patient's room so that a recording can proceed smoothly.

When the behavioral monitor is ready. Transport the research rig into the patient's room, placing the monitor in front of the patient on a portable table. Connect the monitor to the behavioral control computer with a standard DVI cable and place the recording rig in an unobtrusive location.

Next, connect the research system to the splitter box that separates the research recording from the clinical system. Then hand the button box to the patient and instruct the patient to identify the target by pressing the corresponding button. Finally, click run to run the task and allow the patient to complete two blocks of 150 trials each using the neural signal processor software to control the recording parameters.

Once a patient is selected for stereotactic EEG electrode placement, a volumetric T two and T one contrast enhanced MRI is performed. Stereotactic EEG electrode trajectories are then planned using the stereotactic navigation of the volumetric MRI sequences. This technique facilitates the collection of local field potentials from structures deep within the cortex, such as the dorsal anterior cingulate cortex as shown here, which would not be possible with a typical surface electrode placement.

After an adequate number of multi-source interference task trials, the local field potential data from the stereotactic EEG electrodes in the dorsal anterior cingulate cortex are pre-processed to align the local field potential data to the Q presentation for further downstream analysis. Further, once aligned the local field, potential data can be averaged to examine the changes in the mean electrophysiological response between the trial types. Subsequently, multi taper spectrograms are made to investigate the changes in the frequency bands over time.

Indeed, as scalp, EEG studies have implicated different frequency bands in the activity observed in the dorsal anterior cingulate cortex. Time frequency analysis is an important method for linking the electrophysiological changes observed in the dorsal anterior cingulate cortex with the appropriate corresponding behavior. Once mastered, it takes about two to three hours with minimal complications.

When attempting this procedure, it is important to remember to tailor the cognitive task to the cortical or subcortical region being recorded. Following this procedure, other data analysis methods can be applied, such as time frequency analysis. These allow us to untangle the effects of different types of neuro oscillations at different times.

After watching this video, you should have a good understanding of how to record electrophysiological signals from deep within the brain using stereotactically placed electrodes and a data acquisition rig.