Asymmetry Indices

Asymmetry indices compare the power in homologous electrodes in both hemispheres

Absolute asymmetry index EASI: The absolute value of the difference between the hemispheres.

Relative asymmetry index REASI: Assigns a positive or negative value to the difference between the two hemisphere

• Positive value (Above the baseline: Right hemisphere has more power
• Negative value (Below the baseline) : Left hemisphere has more power

The absolute and relative asymmetry indices are often trended together.

In the qEEG example below,

• The EASI is represented by a Yellow trend showing the absolute or total asymmetry between the two hemispheres.
• The REASI is represented by Green trend. The trend when below the baseline (has a negative value), represents a relatively higher power in the left hemisphere and when above the baseline (has a positive value) represents a higher power on the right side. In the example shown, the green trend is seen below the baseline representing higher power in the left. During the seizures, it flips to above the baseline (positive value), indicating a relatively higher power in the right hemisphere during the seizures. The raw EEG in this case is consistent with right sided slowing (hence higher power on the left) and focal seizures originating from the right hemisphere.