Human brain invasive techniques: microelectrode recording and microstimulation
Human brain invasive techniques: microelectrode recording and microstimulation
The techniques discussed in this chapter allow recording under local anesthesia of the activity of cortical and subcortical structures such as the thalamus, basal ganglia, and cephalic brainstem neurons, and while recording, the patient can be asked to perform different motor and cerebral activities and can be made to report on the effects produced by the intracerebral stimulation.
This chapter describes techniques for obtaining recordings and implementing microstimulation and assumes that the operator has some basic knowledge and experience with extracellular recording methods in experimental animals.
Operation method
Microelectrode recording and microstimulation experiments
Principle
This chapter will describe the instrumentation and steps necessary to obtain microelectrode recordings and implement stimulation in stereotactic surgery. Many of the steps are similar to those employed in animal studies and will therefore be described only briefly, with emphasis placed on those aspects of human trials that require special attention. Examples of the kinds of information that may be obtained by applying this technique will also be listed.
Materials and Instruments
Microelectrode CT MR Signal Processing Unit Move move The following, after a brief description of the whole operation, describes the electrodes and instrumentation in more detail, especially some of the issues related to human recordings, such as safety, aseptic operation, electrical noise and stability. Under local anesthesia, the patient's head is placed on a stereotaxic machine and the relevant stereotaxic coordinates of the anterior-posterior coalition (AC, PC) and/or brain targets are localized by MR or X-ray (CT) imaging techniques. Then, under local anesthesia, a small hole is drilled in the skull and a catheter is inserted into the target, but stopping l0 mm or more above the suspected target. A microelectrode mounted in a thin protective catheter is inserted into the main guide tube and then slowly leaned against the target with the help of a microdriver. The electrodes are connected to a preamplifier, and the signals from the electrodes are further amplified, filtered, displayed on an oscilloscope, and fed into an acoustic amplifier so that the signals can be heard through speakers or headphones. Stimulus strings were intermittently applied to selected points through the microelectrodes, and the effects of various arbitrary movements were observed, and the patient was asked how he or she felt after being stimulated. The equipment selected for the purpose of the study is described below. The microelectrodes used for human recordings are metallic microelectrodes, usually made of tungsten or cypress-iridium alloys, covered with glass or other insulating layers (see Chapter V). We currently use tungsten microelectrodes encased in polyparaxylene (Micro probe, e.g., WE300325A). Since subcortical structures such as the thalamus or basal ganglia are usually recorded during stereotactic surgery, the electrodes have to be very long, much longer than those used in animal experiments. Since long electrodes are difficult to etch and insulate, we usually insert a short microelectrode (such as those used in animal experiments) into a non-sterile steel tube, which is then insulated with polyimide tubing (Kapton). The tip of the microelectrode is l5-4(Vm or less in length and has an initial impedance of l-2 MO. The electrode shank has the insulation removed and is bent into a thin stainless steel tube (e.g., 25-gauge, SmallParts), which is then insulated with polyimide tubing (use 23-gauge insulated tubing for the 25-gauge tubing described above, MicroML). Under a dissecting microscope, the insulating tube is pushed down towards the electrode so that it overlaps and covers the insulating stem of the electrode, and the joints are sealed and insulated with epoxy resin. Electrodes made in this way can be used without further treatment, but we have found that gold and cobalt plating on the tip of the crane electrode records better results (MerrillandAinsworth1972). Plating reduces the impedance by a factor of 5?10, resulting in a final impedance of only a few hundred Kft. To ensure that the insulation is not broken, the impedance of the electrode can be checked by immersing the tip in saline and slowly moving the electrode down until the part of the electrode where it meets the polyimide tubing is submerged below the surface of the liquid, and as the electrode is immersed further and further there should be no appreciable change in the impedance. In addition to this, we usually use a low DC voltage (3?5V) when the electrode is immersed in the liquid, so that air bubbles are generated only at the tip of the electrode. The prepared electrodes are inserted into small, labeled, protective tubes for portability. Several companies now sell long electrodes prepared for human recordings (ApolloMirosurgical, ARS, FrederckHaer Corporation, Radionics). We also sometimes use electrodes with longer tips, which are made of 25-gauge, non-sterile steel tubing insulated with polyimide tubing, leaving a 1.5-mm tip, beveled, and with the tip polished to remove sharp edges. a This type of electrode is mainly used for large stimuli and/or for microinjections of lidocaine, as we described earlier in the case of the thalamus section (Dostrovskyetal.1993 b). Any amplifier designed for extracellular neuronal microelectrode recording can be used (see Chapter 5 and Safety Matters later in this chapter). We used either the WPIDAM80 preamplifier (WorldPrecision Instruments) or the GuidelineSystem3000 system (Axon Instruments). When using the WPI amplifier, the probe is mounted on a curved cart adjuster with a hydraulic microdrive cylinder built into the adjuster. If the Axon Instruments system is used, the leads on the probe can be relatively long and the probe should be placed on a table near the patient's head. Many extracellular recording amplifiers have low-pass and high-pass filters, and sometimes band-pass filters, and can be effectively amplified for further filtering or amplification. However, in some cases it is necessary to connect the output of the amplifier to an additional filter (Krohn_Hite, Model 3700) and amplifier (see also the section on electronic safety below) y. The output of the amplifier is displayed on an oscilloscope and/or a computerized data collection system equipped with a spike2 (CED1401 with spike2, DataWave. The output of the amplifier is displayed on an oscilloscope and/or a display with a computerized data collection system (CED1401 with spike2, DataWave, GuidelineSystem 3000). The signal is also fed into an acoustic system, preferably with baseline noise (Grassam8) suppression. In some cases, a window discriminator can be connected to the amplifier output so that different individual units can be recognized and their discharge frequency can be displayed (Winston Electronics, GuidelineSystem3000). The output signal of the window discriminator can also sometimes be used to identify individual neural units in response to active or passive movement when fed to an acoustic monitor and/or an on-line display of the neuron's firing rate.Microelectrode recording systems for use in functional stereotactic surgery are also sold by Radionics Corporation and ARS. In order to give microstimulation using a microelectrode, the output of the stimulator must be fed into the microelectrode ^ Either the amplifier wires to the electrode can be artificially disconnected and the output lead of the stimulator can be connected to the electrode (cathode to tip of electrode); or a specially designed amplifier or loop can be used, using which stimulation can be performed without changing the leads (GuidelineSystem3OOO, the ARS system). The stimulator must be capable of generating unidirectional or bidirectional bursts of ^100f into 0.O5^li0ms duration. The most commonly used is the 3,000 Hz sequential stimulus ls. In the GuideUneSystem3000, the stimulator is housed within the recording system as a single unit, whereas in the WPIAnapulsemodelA310, the stimulator p: is made up of a separate stimulus generator and a constant-current stimulus isolator (A360). In order to estimate the effect produced by the injury, a microinjection of local anesthetic can be administered at the location of the target point. Although we have observed the effects of such injections on movement and tremor in patients with movement disorders, it must be emphasized that systematic studies applying this method to produce the expected value are still lacking. Instead of a microelectrode, a non-sterile steel tube of the same diameter as the microelectrode (see Microelectrode section above) is filled with preservative-free 2% lidocaine prior to insertion into the brain and connected to a 25, Hamilton syringe through a high-quality polyethylene tube (PE50). We usually start with an injection and then give additional doses if no effect is seen within a few minutes. See Dostrovsky et al. (1993b) for details. To determine whether the electrodes reach the visual beam (for pale bulb localization), a flash lamp can be used. A flashlight or even turning the room light on and off is usually sufficient, but a flashlight is necessary if you want to record visual evoked potentials (slow waves) from the optic bundle. EMG electrodes and/or accelerometers may be attached to the limb primarily for research purposes. When estimating the effect of microstimulation or strong stimulation on movement (usually tremor) to determine the optimal target for deep brain stimulation or injury sites, there B is only a need to assess stimulation effects by observing the output of the EMG or accelerometer. This requires an on-line display of the recorded results, for which a computerized terminal display is most convenient (Spike2, CED), and suitable amplifiers and filters are necessary to process the EMG and/or accelerometer signals. For offline analysis, the unit recordings and any associated signals from the sensors, such as EMGs, must be stored, which can be recorded to videotape or high-capacity digital storage media such as CDs, ZIP disks, etc., using a digital video recorder (InstmtechCorp, VR-100-B). Alternatively, a standard video camera can be used to record the entire process on videotape, which can then be fed directly into a standard hi-fi VCR. Neuronal activity (output of the microelectrode amplifier) can be recorded using the unison channel in both of the above. The uniqueness of human research and the associated safety are two major technical issues in human experimentation. The first is the impact of electroshock on patient safety. Although most equipment designed for animal experiments may be safe and reliable and preclude the possibility of dangerous electroshock due to malfunction, most hospitals or countries still have very strict regulations for any electrically related instrumentation used on patients. Instrumentation designed for animal experiments usually does not meet these regulations, or they have not been validated and permitted for human application. Therefore, if a physician intends to use an unauthorized device for human use, he or she must undergo special instrumentation tests and be approved by the local authorities. In order to improve the safety of use, it is common to connect all instruments to the mains through a separate transformer and/or a grounded earth leakage circuit protector, although this may not be required locally. In addition, battery-operated terminal amplifiers and stimulus isolators are recommended. Ideally, the amplifier, even if it is battery operated, should pass through a medically permitted stand-alone amplifier before it can be connected to other instruments. To the author's knowledge, although several other instrument manufacturers have introduced similar devices, to date, the Complete Recording System from Axon is the only one developed specifically for human trials and approved for human use in the United States. Obviously, microelectrodes must be sterilized. Gas sterilization is optional. The electrodes are placed in perforated boxes that are sterilized prior to the procedure and must be prepared well in advance. If multiple surgeries are required in a single day, then multiple electrode boxes must be prepared because once a box is opened for one patient, the remaining electrodes in that box are not allowed to be applied to another patient. Depending on the construction of the electrodes, it is also possible to autoclave them prior to application. Another method of sterilization is to soak the electrodes in a disinfectant solution (2% glutaraldehyde) and then rinse them with sterile water. Any objects attached to the stereotactic holder and electrode holder must also be sterile. We gas sterilize the wires and catheters connecting the electrodes to the holder for fixation together with the microelectrodes. Pure metal electrode/microdriver holders must be autoclaved just prior to operation. Microdrivers and preamplifiers are more difficult to sterilize. In our procedure, we use hydraulic microdrive cylinders, both of which are fixed to the operating stand, and they are sterilized by immersing them in a glutaraldehyde solution prior to use and then immediately rinsed with sterile water. If the preamplifier is hermetically sealed, it can be gas sterilized; if not, it and its cable can be sterilized by covering it with a sterilizable sleeve. In the past, it was common to use a small probe (DAM80,WPI) that was inserted into a small hole in the electrode holder or carrier, thus covering most of the unsterilized probe. The foremost part of the output cable is covered with sterile sticky plastic. The system we use now does not require the amplifier's probes to be very close to the electrodes, so only the leads need to be gas sterilized. Because of the strong gain and relatively high electrode impedance used in the recording of individual extracellular neural units, electrical noise interference is a common problem, and sometimes it becomes downright difficult to troubleshoot electrical noise. This can be an even more problematic issue in the operating room environment, where there are all different types of electrical appliances running either in the operating room or in the next room. It is possible that the worst of the noise is coming from the unipolar cautery in the next operating room, and it is almost impossible to completely eliminate it. Ideally, an operating room equipped with a shield would be ideal, but it is often impractical due to the high cost of construction. Heart rate monitors and/or other monitors attached to the patient can also sometimes cause interference, so it is important to disconnect these connections during the recording process. Fluorescent lamps are another source of interference and may need to be shielded by a screen connected to ground or turned off. Qualified incandescent and surgical lamps do not usually cause any problems, and the use of Humbug filters (Quest Corp.) effectively eliminates noise caused by oscillations in the trunk line (power cord). In some cases there is a feedback oscillation from the sound monitor speaker to the stent/microelectrode. It is usually caused by vibration of the microelectrode inside the tube, and sometimes a loose bracket can also trigger this problem. Turning down the volume or switching to headphones will eliminate it. Sometimes replacing the electrodes will also solve the problem. If the patient is talking during the recording, it often generates a recording perturbation, which can sometimes be solved by changing the electrodes. Because the stent is tightly fixed to the skull surface, there should be no problems with electrode movement relative to the skull. However, in some patients, displacement of the brain relative to the skull due to heart and/or respiration can reduce stability and cause fluctuations in the amplitude of action potentials. In addition, large active or passive movements of the legs, arms, and/or shoulders can sometimes cause displacement of the brain to the point of loss of study results, and coughing almost always triggers displacement of the brain resulting in loss of recorded results. In order to accurately set the coordinates of the electrode tracks, it is necessary to use MR or CT imaging techniques (before MR and CT became common, contrast was injected into the ventricles, a method that is still used today) to determine the target directly from the image. However, since it is often difficult to see the target accurately from the image, we always apply the anterior combined (AC) and posterior combined (PC) coordinates and their frame coordinates, which are added to standard brain stereotactic mapping coordinates using a procedure on the frame coordinates. The AC and PC stereotactic coordinates can be calculated using MRI or CT scanning computer software. The position of the patient's AC and PC coordinates on the standard atlas can be sketched with the aid of a computer program that enlarges or reduces the atlas according to Schaltenbrand and Wahren's standard atlas (Schaltenbrandand and Wahren 1977), on which the electrode trajectories can also be traced (Fig. 42-1). A very noteworthy possible complication is the possibility of brain displacement, which can occur during brain scanning with insertion of electrodes and also during electrode recording. The main cause of this phenomenon may be the loss of cerebrospinal fluid at the cranial drill hole. Using the technique described above, our research group has successfully completed more than 500 recordings involving localized targets in the motor or sensory thalamus, subthalamic nuclei, periventricular gray matter, pallidum, and anterior cingulate cortex. The various types of information obtained by the application of this technique are as follows: Fig. 42-2 records the activity of two individual neural units in the ventral caudate nucleus of the thalamus excited by tactile stimulation applied to a finger. Figure 42-3 shows a recording of the activity of a "tremor" cell in the pallidum. Figure 42-4 shows the effect of increasing the microstimulation current intensity on the size of the projection area (sensory area) and the intensity of sensation. Figure 42_5 shows the recordings and microstimulation parameters obtained from the microelectrode trace when the microelectrode was inserted into the thalamus of an amputee patient. Figure 42-6 shows the effect of electrical stimulation and lidocaine injection on tremor within the thalamus of a patient with Parkinson's disease. Figure 42-7 shows a histogram of the frequency of one neuron firing recorded in the anterior cingulate cortex after administration of noxious thermal stimulation to the upper limb. For more product details, please visit Aladdin Scientific website.
