Recording EOG signals with actiCHamp – active or passive electrodes?
by Mário Bártolo, Ph.D. & Dr. Tracy Warbrick
Technical Consultants (Brain Products)
The artifacts produced by eye movements and blinks are problematic for the EEG researcher and can have a negative impact on results. Advanced algorithms (e.g. Ocular Correction ICA) can be very effective in correcting the ocular related artifacts, however in some cases a dedicated ocular channel is required. Below, we will describe various approaches on how to use an actiCHamp amplifier to record an Electrooculogram (EOG).
The human eye can be perceived as a dipole with the positive pole at the front of the eye and the negative pole at the back. The corneo-retinal potential elicited from this dipole is called the EOG. Any eye movement or blinks will result in a potential spread that will be recorded by all EEG electrodes in the form of signal offsets and transients. While an eye movement changes the direction of the corneo-retinal dipole and elicit an artifact that is roughly linearly dependent with the movement size and direction, a blink creates a “short-circuit” between the cornea and the extra ocular skin and elicits a monophasic deflection of 50-100 µV in the EEG channels.
A typical approach to record an EOG signal is to use a bipolar montage, where one pair of electrodes is used to record horizontal eye movements (HEOG) and a second pair of electrodes is used to record vertical eye movements (VEOG). The respective position of each electrode is illustrated in Figure 1.
Here, HEOG is acquired by placing one electrode at the canthus of one eye (positive site) and the second electrode at the canthus of the other eye (negative site). The placement of one electrode below and another above the eye allows the recording of VEOG.
Based on the above characteristics of the EOG, Brain Products offers different solutions to achieve a good EOG signal. In what follows, we will describe how to record an EOG signal using actiCHamp and how to set up a Recorder workspace based on the method chosen.
Figure 1: Active and passive electrodes placement for recording Horizontal and Vertical EOG, respectively.
EOG signal measured with actiCAP active electrodes
One of the main advantages of the actiCAP is that the electrodes can be used to acquire EOGs, EMGs and ECGs as these are not fixed to the cap itself. Brain Products provides actiCAP holders and adhesive rings to apply the electrodes to the required part of the body. Moreover, in case the standard active electrode is too big and somehow obstructs the subject’s field of view, a flat actiCAP electrode could be the best option. Due to flexibility of electrode changing provided by the actiCAP this is easily done by opening the splitter box, removing one standard electrode, connecting the flat electrode and closing the splitter box.
How to set up a Recorder Workspace
In Figure 1, HEOG is recorded using two flat actiCAP electrodes, one at each outer canthi of the eyes. As previously mentioned, these two flat actiCAP electrodes are replacing two standard active electrodes. Thus, in the Recorder workspace it is necessary to name these channels accordingly and ensure that the correct physical channel number is used. Figure 2 shows how a 32 channel actiCHamp workspace should be set if physical channel number 21 and number 10 are used to record HEOG (please refer to column “Name” and “Phys. Chn.”). It is important to highlight that these EOG channels will have the same reference frame as all the other EEG channels.
Figure 2: Recorder workspace settings to record HEOG with active electrodes.
EOG signal measured with passive electrodes
In the above example, two active electrodes were used to record the EOG signal. In case sacrificing EEG electrodes is not an option to your application or you require a true bipolar EOG signal recorded with a bipolar amplifier, then the BIP2AUX adapter is the right solution for you. The BIP2AUX allows recording of bipolar signals through one of the eight actiCHamp’s AUX inputs. Because the AUX port is intended to accept signals in the Volt range, the BIP2AUX adapter amplifies the incoming bioelectrical signal e.g. EOG, and relays it through the AUX port of the actiCHamp. Importantly, since the input impedance of the BIP2AUX is very high, any electrode that is filled with gel and has good skin contact will record a very good EOG signal. Thus, there is no need to measure electrode impedances which practically means less preparation time.
How to set up a Recorder Workspace
In case you record the EOG signal using two passive electrodes together with the BIP2AUX adapter, you must ensure that you take the bipolar amplifier gain into account when setting up the Recorder workspace. As demonstrated in Figure 3, this is done by selecting the “Diff. Unit” box. In this example, the “Unit” chosen is “µV” while the Gradient is set to 0.1. More details on how to adjust the sensors for the AUX inputs of the actiCHamp can be found in the Recorder User Manual.
Figure 3: Recorder workspace settings to record VEOG with passive electrodes.
Lastly, it is important to highlight that the AUX channels are always the last eight channels of the actiCHamp and are included at the channel table end (see highlighted region of Figure 3). Thus, if one is using an actiCHamp with 2 modules i.e. 64 EEG channels, the physical number of the first AUX channel would be 65.
In order to verify and compare the results of the above mentioned methodologies, we have performed an EOG recording with a home-made setup where active and passive electrodes were superimposed on each other.
BrainVision Analyzer was used to process the recorded signals. Figure 4 shows the results of a VEOG recorded from a healthy subject during consecutive blinking. Here, the red trace corresponds to VEOG recorded with active electrodes, while the black trace corresponds to VEOG recorded using passive electrodes and the BIP2AUX adapter. From Figure 4 it is clear that both signal plots share the same temporal characteristics and have similar amplitudes values.
Figure 4: EOG signal recorded with active electrodes (red trace) and passive electrodes (black trace). Please note the temporal characteristics and amplitude similarity of the EOG signals within a specific time interval (see values inside red and black boxes).
As a conclusion, if recording an EOG signal is important to your research, the actiCHamp amplifier offers you various solutions to this end. Above, we have described how to use two different electrode types (active and passive) to do an EOG recording and how to set up a Recorder workspace accordingly. Importantly, it was demonstrated that both methods i.e. record EOG with active electrodes or record EOG with passive electrodes connected to the BIP2AUX adapter, result in similar EOG signals so you are free to choose the method that is most convenient to your research.