The Balanced Non-Cephalic Reference

Introduction

Selection of a proper recording reference for EEG recording can be difficult when it is not possible to find a cephalic reference that is not contaminated by the EEG signal of interest—a phenomenon also referred to as an “active reference”.

A non-cephalic reference can be used to address the problem of reference contamination caused by an active reference; however, measures must be taken to reduce the resulting large amplitude EKG artifact introduced. Non-cephalic references are commonly used for evoked potential recordings, where signal averaging effectively removes EKG artifact. Because this is not an option for continuous EEG recordings, a different method for minimizing EKG artifact must be employed.

The Balanced Non-Cephalic Reference

EKG contamination of a non-cephalic reference can be diminished by deriving the reference from a combination of two electrodes. If the electrodes are placed to present R-waves of opposite polarity they can be summed to cancel each other—removing the EKG artifact from the non-cephalic reference.

This technique was first introduced in 1951 by Stephenson and Gibbs, who used a potentiometer to “balance” the relative contributions of electrodes on the cervical spine (C7 or C5) and over the suprasternal notch—eliminating or reducing the amplitude of the R-wave.  The output from the potentiometer was then fed to Input II of the differential EEG amplifier.

One of the practical obstacles to using a balanced non-cephalic reference is the potentiometer itself: The technician needs to make careful adjustments prior to acquisition to reduce the EKG artifact as much as possible. During acquisition, further adjustments may be required if there are impedance changes, or more often, the patient shifts position during the recording.

A digitally balanced non-cephalic reference simplifies the procedure by eliminating the need for a potentiometer altogether—eliminating fine adjustment of the balance during acquisition. Instead, a simple slide control is used to digitally balance the reference in review. The advantages of a digitally balanced non-cephalic reference are:

• It is not necessary to change the customary amplifier reference placement (e.g., between CZ and PZ), so the EEG can be viewed using the standard array of montages: The non-cephalic reference electrodes are placed in addition to the standard amplifier reference.
• The non-cephalic reference electrodes are balanced in review, so hook up is simplified without the need for special reference leads/potentiometer connections—the non-cephalic leads can be connected to the amplifier’s numbered inputs, or unused cephalic inputs and labeled accordingly.
• It is not necessary to re-adjust any balance potentiometer during the recording—any readjustment can be done digitally during review.
• The technician can check proper placement of the non-cephalic reference electrodes from Insight’s review screen during acquisition.

The ability to make adjustments in review reduces the difficulty in employing this type of reference by reducing adjustments before acquisition, or requiring refinement of those adjustments throughout the recording.

Demonstration of the Balanced Non-Cephalic Reference

The left panel in Figure 1 shows an example of the use of the balanced non-cephalic reference to clearly depict the dipolar field of a central mid-temporal spike in a patient with benign rolandic epilepsy.  In this case, EKG artifact has been completely eliminated.   In contrast, Figure 2 shows the same segment of EEG displayed with the balanced non-cephalic reference adjusted improperly to demonstrate EKG contamination of the reference. In each Figure, the right panel displays the same segment of EEG in a longitudinal bipolar montage using Insight’s new “Split Montage” display feature (described on page 4).

Figure 1.

Figure 2.

Setting Up a Balanced Non-Cephalic Reference in Insight

The balanced non-cephalic reference (BNR) is essentially a two-channel average reference, with the addition of a “balance slider” to equalize the contribution of the EKG. The slider is adjusted to the point where the EKG is eliminated (or minimized) from the EEG traces.

Because the BNR is a two-channel average reference, it is created in Insight’s Average Reference Editor. Select Montage | Edit Avg Ref from Insight’s menu. The non-cephalic reference electrodes in this recording are amplifier inputs 31 and 32, so 31 and 32 are selected in the Edit Average Reference dialog. Type “BNR” as the Name for the new reference, select Save, then Done.

To select the new BNR reference, select all of the cephalic channels by selecting Montage | Select Subset | 10-20 Channels (or multiple-select channels by holding down the “Ctrl” key while selecting channels in the channel bar to the left). Next, right-click on the channel bar, select Change Reference | All, then select BNR from the list.

Now that the BNR reference is selected, the balance slider can be displayed. Select Montage | Balance Ref… from the Insight menu. Move the Balance Ref slider to the point where the EKG is minimized in the EEG traces.

Conclusion

The balanced non-cephalic reference is useful for resolving questions of reference contamination in clinical EEG interpretation, although its ability to fully eliminate EKG artifact varies considerably between patients. In practice, other electrode locations, including right and left wrists, may also be used instead of the traditional “neck-chest” locations. If the amplifier has additional unused numbered inputs, applying more than the two non-cephalic electrodes (e.g., neck, wrists, or other chest locations) will allow the best combination of reference electrodes to be selected during review.

New Split-Montage View in Insight

Insight’s new Split-Montage display can be used to display EEG in two different montages simultaneously. Both sides of the split-screen display can be time-locked by selecting Sync Split View Times so that they page together. The EEG traces on either side of the screen can be selected to adjust gain, filters, or montage simply by clicking on the traces.

Setting Up a Split-Montage View

Insight’s trace display can be split by using the Split Box in the bottom-left corner of the Insight display, at the end of the scroll bar.

Locate the Split Box on the Scroll Bar.

Place the mouse pointer over the Split Box and hold down the left mouse button.

Drag the Split Box to the right. The split-screen can be resized by dragging the border left or right as shown.

De-select Link Split View from the Insight tool bar.

Select Sync Split View Times from the Scroll menu.

Each side of the trace display can now be configured with individual montage/gain/filter settings. Click on either side of the split screen to display or change settings the settings for that side.