by Dr. Pedro M. R. Reis
Scientific Consultant (Brain Products)
Accurate source localization needs accurate electrode positions. With the CapTrak users can accurately and quickly acquire positions of all Brain Products active electrodes. However, a few things should be considered to ensure a quick and successful electrode localization. By understanding the basic concepts behind the CapTrak’s functions and by following adequate scanning procedures, CapTrak users can experience a better acquisition process. In this article, we introduce you to the essential elements that are needed for the proper functioning of the system. Afterwards we present several tips on how you can improve your scanning process using the CapTrak. These include optimal electrode cable arrangement, camera settings, ambient light conditions and others. We also include a video as an example of scanning motion. Finally, we suggest how to maintain the CapTrak in good working condition.
CapTrak is the electrode localization system from Brain Products which provides its users with precise and quick results. The system is easy to use but experience has told us that understanding some basic principles and measuring procedures helps to make the most out of the CapTrak. Even though the CapTrak is robust and easy to use, sometimes not everything goes to plan and a little push in the right direction can make the difference.
In this Support Tip, we provide you with some practical suggestions to help you improve the digitization of your active electrode set when using the CapTrak. These suggestions are useful for both beginners and experienced users.
2. Basic Understanding of the CapTrak
The CapTrak consists of a handheld scanner which integrates two cameras with an angle of 75° at a distance of 195 mm between them and the CapTrak software. The scanner specific “CameraCalibrationParameters” file gives the software the required information about the lens distortion and actual alignment of the cameras within the camera housing. That is the calibration information file which needs to be loaded only once. The software controls which electrodes light up and with which color, creating light patterns. The cameras capture these light patterns and send them back to the software. Based on the previously loaded electrode position file, the patterns allow the software to identify electrodes and their precise position. Finally, the software uses the landmarks (blue electrodes), which are placed at specific sites on the subject’s head, to bring together the scanned electrodes and the subject’s head.
The following elements are crucial for the proper functioning of the system:
2.1. Electrodes: Based on their integrated three-color LEDs all types of Brain Products active electrodes can be detected by the CapTrak. Without the LEDs the electrodes would not be detectable. The total number of electrodes is also important. The higher the electrode density, the easier it is for the system to digitize the electrodes. This is due to the smaller distance between single electrodes, which makes it easy for detection patterns to be formed.
2.2. Landmarks: The landmarks are used to create points to define the head coordinate system which is used as a reference base for all electrodes. If the landmarks are moved during the scanning process, then the reference base will not be correctly established.
2.3. Electrode Position File (*.BVEF): This file contains information about electrode numbers, names and their expected standard position. This information is used by the software to create the LED patterns and sequences for the scanning. If the loaded file does not correspond to the used cap, then the scan will fail because the software will expect the electrodes to be in other positions and will not be able to create LED patterns.
2.4. Calibration: After the CapTrak’s final assembly, the in-house calibration determines the optimal parameters (depending on camera positions, lens distortion) which are saved and shipped with the CameraCalibrationParameters file. These optimal parameters are then used by the CapTraks scanning and pattern matching algorithms in order to accurately determine electrode positions. The system is aligned through calibration so that epipolar geometry triangulation can provide accurate results. As long as the patterns to be detected are inside the field of view of the cameras, the acquired electrode positions can be accurately determined. However, if the device becomes de-calibrated, for example after a transport damage, there will be a mismatch between the cameras’ positions and the provided calibration parameters. This mismatch can result in unsuccessful or inaccurate electrode scanning.
2.5. Light: Electrodes must be visible to both cameras at the same time so that the software can recognize the electrodes through the light created by their integrated LEDs. However, the system is susceptible to other sources of light. Every area of a surface that is producing a visible reflection or emitting light can potentially interfere with the scanning.
3. Suggestions for preparing a scanning session with the CapTrak
In this section we provide suggestions from our experience in using the CapTrak. These are complementary to the operating instructions.
3.1. Calibration Check
A calibration check verifies the correct calibration state of the system, it is not the calibration procedure itself. A de-calibrated device can fail to scan the electrodes, or it can take too long. During the calibration check you need to match the precision checkerboard, captured by the camera, with an example displayed by the software as seen in Figure 1. The images do not need to be exactly the same, as in the example, but it facilitates the calibration check when you try to match them as well as possible.
3.2. Electrode cable arrangement
For the system to determine the electrode positions, both cameras must see the LEDs that comprise each of the scanning patterns. If cables or other objects are covering the LEDs, then the system is not able to determine the electrode’s coordinates. Cable arrangements that cover the LEDs, as seen in Figure 2, will prevent a quick and smooth scanning.
To avoid cables covering the LEDs, when inserting the electrodes in the EEG cap, do so while routing the cables to the back of the head and also towards the sides through the spaces between the electrodes as seen in Figure 3. By distributing the cabling, you will avoid having too many cables in the back which can occlude the LEDs.
In our experience, it also helps to bind neighboring cables together in groups so that they do not obscure LEDs. You can bind the cables by using hook-and-loop fastener cable binders which are integrated on the cap or can be bought from a hardware store.
3.3. LED check
Performing an LED check before scanning ensures that all LEDs are working properly. The LED check also helps you identify improperly placed electrodes, especially if they are on the wrong hemisphere of the head. To open the LED check, click on File → Start LED Check. You can choose between a single test and test by hemisphere. See Figure 4.
3.4. Adequate ambient light conditions
As explained, the CapTrak is a light sensitive device. It tries to recognize light from the LEDs of the active electrodes, but it is still susceptible to environmental light. This means that other sources of light can also be recognized by the CapTrak and potentially interfere with the scan as in Figure 5.
It is important to reduce the sources of interfering light. You can do this by taking the following steps:
- Make sure that the room that you are in has constant, equally distributed light without reflections or shadows.
- The desks or table surfaces around should not have visible bright reflecting areas. Close sun blinds and turn off all light sources that cause reflection.
- While having the CapTrak computer display at viewing range, avoid it interfering with the scan by not placing the subject in front of the display.
- After finding out and pilot testing the conditions that work for your measurement setting try to keep them the same for the next measurements.
3.5. Camera Settings
Appropriate camera settings can help the software to reduce the influence of unwanted light sources during scanning. In the CapTrak software you can find a camera symbol where you can adjust the camera settings. When you press the camera symbol you can use the sliders to select the settings for shutter and for gain as seen in Figure 6.
Gain amplifies or attenuates the ambient light. You can use this to adjust your system to the environmental light. In most cases, 50% gain works well.
Shutter time is the amount of time that the camera sensor is exposed to light. In most cases, it works well when you have selected a medium shutter time such as 10 ms. If you set a low value, for example, 2 ms, the cameras will not detect all light sources within the exposure time. If you set a high value, the cameras will get too many interfering light sources and the measurement will fail.
Auto gain sets the gain automatically for the selected shutter time. This can help with the amount of light captured by the camera and make the scan easier in some cases.
4. Scanning Session
The user interface shows important information during the scan. It gives you direct visual feedback about what the cameras are seeing. By using the information on the display, you can better adjust the scanner’s position to capture the electrodes’ positions.
Another important factor is the distance between the scanner and the electrodes. The ideal distance is approximately 30 cm. Audio feedback and a distance indicator on the screen tell you if the scanner is too close or too far away.
When considering distance and visual feedback, the user can better coordinate the scanning movements:
- The scanner should be moved in a gentle motion in different directions as if one were cleaning windows with a cloth in circular motions.
- Try pointing the center of the top of the scanner to the electrode being scanned. If there are two electrodes aim to a point between them and look at the screen while aligning the cameras with the imaginary line that passes through the green electrodes.
- Bending and twisting the (Firewire) connection cable will result in damage to it and the system will eventually malfunction. Therefore, avoid bending or twisting the cable while you move around the test subject by keeping the cable straight and without tension.
5. Maintenance of CapTrak
5.1. Computer settings
The handheld device requires an adequate energy supply and processing power which is why the power plan on your CapTrak computer should be set to High Performance. The provided computer is previously configured, however, please check the power plan to make sure that it is properly configured. You can verify and change the power plan settings by opening the Control Panel, select Power Options, and then choose or create a High-performance power plan. See Figure 7 for an example of what the Power Options of the computer should look like.
5.2. Installing new hardware or new software on the CapTrak computer
As the CapTrak computer is preconfigured to work in exclusivity with the CapTrak, any changes in hardware or software can possibly result in compatibility problems. We recommend the following:
- Use the provided computer exclusively for CapTrak.
- Do not install other software or hardware apart from those it already came with.
- Keep the computer offline and do not apply windows updates.
5.3. Make sure the CapTrak software is up to date
Please check regularly our website for new versions of the software. The most recent version has updated parameters for a better recognition of electrodes and better error logging.
5.4. Handling of the CapTrak after scanning
Whenever the CapTrak device is not being used, it should be either placed back to its case or rested on a soft surface and its cable should be kept without tension. In Figure 8 you can see examples of how to coil the cable.
When coiling the Firewire cable do not wrap it around the scanner but wind it in a loose circle without tension. Otherwise, the cable can become damaged. The minimum bending radius is 10 cm. Any diameter lower than this can damage the cable and consequently, the system will no longer work.
©Brain Products GmbH 2018