Asymmetry spectrogram

In the asymmetry spectrogram, the time is plotted on X-Axis, frequency on Y-Axis and the relative asymmetry in power between the two hemispheres is represented on the Z-Axis using a colored scale (Figure 3).

The asymmetry spectrogram Z axis follows the color scheme traditionally associated with the US political system, with the blue associated with left wing politicians and the red color associated with right wing politicians. The blue end of the color spectrum represents higher power in the left hemisphere and the red end of the color spectrum represents higher power in the right hemisphere. (See Figure 2)

Example:

In the Persyst 13 qEEG comrehensive panel (Figure 1) shown below, the asymmetry spectrogram has a blue color for majority of the period indicating a higher power on the left (or a lower power on the right). There is intermittent steretotypical red signal corresponding to electrographical seizures originating from the right hemisphere. The corresponding raw EEG is uploaded on this page under EEGs.

Asymmetry in power can result from various factors, including focal slowing, skull defects, extra-axial fluid collection, focal rhythmic or periodic patterns, and seizures; therefore, it should always be interpreted in the context of the raw EEG findings.