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1.3. CAR (Common Average Reference)

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Purpose: Removal of common artifacts present in all channels.

Mathematical formula:

avgElectrodes = (F8 + AF4 + AF3 + F7) / 4
for each channel i: signal_i = signal_i - avgElectrodes

How it works:

  • The average value of the first four EEG electrodes (F8, AF4, AF3, F7) is calculated.
  • This average value is subtracted from each channel.
  • Components common to all channels are removed (for example, line noise and eye movements).

Use case: A standard EEG re-referencing method used to reduce the influence of common artifacts.

Physiological explanation:

EEG electrodes record the potential difference between an active electrode and a reference electrode. The problem is that the reference electrode itself (in the typical CleverPoint setup, usually at the center of the forehead or on the earlobe) may contain its own activity or artifacts.

CAR solves this problem:

  • Instead of a single reference electrode, the average of all electrodes is used.
  • This creates a “virtual” reference that reflects overall activity.
  • Components common to all electrodes (line noise, eye movements, general artifacts) are automatically subtracted.
  • Only the local activity of each electrode remains.

Analogy: Imagine measuring mountain heights relative to sea level. If sea level itself fluctuates, all measurements become distorted. CAR is like using the average level of all points for more accurate measurements.