SENSITIVITY

Introduction

The way waveforms appear on your screen when reading an EEG epoch can be manipulated by changing the sensitivity setting in your reading software. Sensitivity in EEG is measured in microvolts per millimeter (μV/mm). Sensitivity directly affects the Y axis (Height) of a given waveform but does not affect the X axis (Length). The standard sensitivity setting is 7 μV/mm, meaning that it takes 7 microvolts to move the waveform 1 millimeter in the vertical direction.

Sensitivity Visualized

The following epochs demonstrate how the sensitivity setting is used to increase or decrease the vertical component of a waveform.

Figure 2.2.1 EEG Epoch at 7μV/mm showing onset of GTC

The image above shows the onset of a generalized tonic-clonic seizure at 7 μV/mm. Notice the spike discharges that are circled in red. These discharges are about 3 millimeters tall, suggesting that the overall voltage of the discharge was 21 microvolts. 

Below we have the same exact EEG epoch, but at a sensitivity of 30 μV/mm. This means that it now takes 30 microvolts to move the waveform a single millimeter thus lowering the sensitivity. The same discharges shown above are circled in red, and as you can see with the decreased sensitivity setting the height of these waveforms is now much lower.

Figure 2.2.2 The same EEG epoch from above at 30 μV/mm

Sensitivity Calculations

Sensitivity may not be intuitive at first, as it can be confusing to understand that increasing the voltage required to move the waveform is actually decreasing the sensitivity, and decreasing the voltage required to move the waveform is increasing the sensitivity. However understanding this concept is essential to answering questions on test day. The following video goes over practice problems that are similar to what you’ll see on test day.