by Dr. Patrick Britz1 & Dominik Dietze2
1 Marketing Director (Brain Products)
2 Product Manager (Brain Products)
If you are doing source analyses or considering doing so, then this article is for you. Brain Products partnered with BESA to come up with the Source Analysis Bundle. To do EEG source analysis you want to have the best possible EEG data, precise electrode localizations and a highly versatile and user-friendly software. In this article we will introduce our Source Analysis Bundle and state why we think it is the best solution available.
The conditions of an experimental paradigm result in brain activity caused by several generators. Source analysis disentangles the different components and gives you a hint where and when it happened. Information pathways in the brain can be studied by using either the reconstructed activation waveforms, or time-frequency analysis. Source analysis can identify the brain regions involved in different tasks and, depending on data quality and model quality, yield a precise localization of the generators in both space and time.
Time and again over the last years we have found ourselves repeating the words, there is no substitute for good data. Of course, we have better and better methods to fix data, but all those fixes come at a price. For filters, Steve Luck puts it this way, “filters are like medicine”. Filters can be useful but they have side effects. Those side effects can range from mild and tolerable to severe. So let us say it one more time, there is no substitute for good data and everything gets easier with good data. This is particularly true for source analysis.
The bulk of recent publications point towards gel based active electrodes becoming the standard for EEG / ERP research, particularly if good data quality is important. How many electrodes would you like to have to do state of the art source analyses? There are some (white-) papers that suggested to use at least 256. But then, basically all of those papers are not based on active gel based electrodes and we just settled that we want to have those. So how many gel based active electrodes would you like to have?
This topic is not yet settled and the findings vary a bit depending on the source analyses algorithms used. Having more information helps. There is consensus that for most applications 32 is not enough and we learn substantially more with 64 channels. However the advantage of going from 64 to 128 seems to be small. The closer two electrodes get to each other the more they capture the same information. Also it increases the chances of conductive water or gel bridges and conductive bridges are really bad for what we want to achieve (because we want to know where the signal was coming from).
One way to counter this, is to distribute the electrode positions over a wider area and to keep the distances between electrodes for all electrodes similar (equidistant layout). Both requirements are also beneficial to source analyses for further reasons but it would be too much to go into this in detail here so we will write more about this in the next newsletter.
So 64 channels with a wider and potentially equidistant distribution would be a very nice start. In a way, 128 channels would be superb from the data side but comes with additional challenges. So in conclusion, we recommend an active 64 channel amplifier alongside caps with a wide and equidistant layout with the option to maybe upgrade it to 96 or 128 channels later on. In short, if you want to do source analyses, then you certainly want good data, then you want to have an actiCHamp 64, right?
The actiCAPs for BESA from EASYCAP GmbH have an equidistant layout and cover areas from 10% or 20% below the current used standard for EEG caps. Depending on your needs, required electrode density and area of interest we offer caps with either 64 or 96 channel layouts. The caps from EasyCap are amongst the most precise caps on the market. If placed correctly, those caps will position the electrodes within less than 1 cm of the theoretical position. However, there will be no information about the head shape and the potential resulting wrap/distortion of the cap. Still it is accepted that the precision is sufficient for standard EEG analyses like ERPs or such. For group studies on ERP data, digitized electrode positions can prove advantageous and are a nice to have. Electrodes placed in different positions can be subsequently interpolated to standard positions for group averages and statistical analyses. For source analyses however, a higher precision is desired.
For structures that lie in inferior or frontal regions of the brain, the standard spherical head models utilizing spherical electrode coordinates no longer yield a good approximation of the electric fields that are measured on the scalp; realistic head models with realistic head shapes and correct individual electrode positions are essential for precise results of source analysis in these cases.
There are many commercial solutions available to digitize spatial positions. Most of them are not precise enough for our purpose or are time consuming and inconvenient to use since they require that every electrode position is measured individually. When we introduced CapTrak in 2014, we were not the first to offer a photometric solution (measuring electrode positions with cameras), but our aim and focus was to eliminate the downsides of current solutions in the field. We figured out that from an user’s point of view, the two most important aspects of an electrode digitation system, are speed and precision!
Our system benefits from the possibility to software control and enlighten the built-in LED’s of our active electrodes. In order to have the fastest possible localization we enlighten multiple LED’s (electrodes) in patterns along a pre-defined pathway around the subjects head.
This translates into an easy to use, very fast and accurate electrode localization system, based on proprietary triangulation algorithms, which makes it possible to determine the individual positions of, for example, 128 electrodes in less than 2 minutes.
The significant advantages of using individual electrode positions for source localization can be seen by comparing figures 1 and 2 below.
In Fig. 2 (right) you can see how nicely the individual electrode positions fit together with the corresponding individual MR Scan. In Fig. 1 (left) you can basically see the same MR-Scan with placed electrodes using just the standard coordinates from the electrode cap.
Comparing the 2 figures above it can be seen how big the increase in precision for the source localization will be when using individual electrode positions vs. standard positions.
As you will have read in our last newsletter, since the latest CapTrak Software release, actiCHamp and CapTrak are fully integrated. In short once again, you want to do source analyses? Then you want an electrode localization system that combines ease of use, speed and accuracy. Then you want to have CapTrak.
So what we have now is individual electrode positions, high quality EEG data and we will combine these two with an easy to use and powerful software. Apart from spatio-temporal dipole modelling, BESA offers numerous proven source imaging methods that work in the brain volume or on the cortex, in the time domain or in the time-frequency domain. All this is available with standard realistic head models available for various age groups, or with superior individual finite element models that are generated from the subjects’ MRI data.
BESA offers all tools to define experimental paradigms, process the data through the analysis pathway and perform cross-subject statistics of the results. In summary, if you want to do source analyses, you will love doing it with BESA.
So far we have outlined which hard and software you would like to have for source analysis. The Source Analyses Bundle is more than just the sum of its parts. It is just not enough if all the elements of your lab are the best solutions by themselves, they also need to work seamlessly together. Of course, all the parts from Brain Products are optimized to work together. The news is that now BESA natively reads the actiCHamp EEG data and the CapTrak determined individual electrode coordinates. Also BESA imports preprocessed data out of BrainVision Analyzer 2. This allows you to do your data cleaning in Analyzer 2 in the easy and convenient way you are used to.
In case you do not know already, Analyzer 2 has a convenient BESA export: click Export > BESA.
We want you to be able to focus on your experiments instead of having to make different components from different manufacturers work together. Therefore our goal was that our product portfolio would flawlessly integrate with BESA. If you are starting from scratch with EEG and source analysis, then we are happy to offer you the entire package so you can start immediately without complications. The full package consists of CapTrak, actiCHamp, BrainVision Recorder, optimized electrode caps for source localization and a BESA Research Standard or BESA Research Complete first license and this comes at a very attractive bundle price.
If you already have an amplifier, then we can bundle together our CapTrak electrode localization system with an active electrode system (depending on your amplifier) and a BESA Research Standard or BESA Research Complete first license. Again this comes with a convenient bundle discount.
Please contact your local distributor to get a package tailored to your needs and benefit from our current market introductory discounts.