EEG Signals in Psychiatry: Biomarkers for Depression Management - Quantitative EEG Analysis Methods and Clinical Applications - page 292

Figure 11.1 Reattributional electrode montage.

and R norm ( s,f ), respectively]. Note that these z -scores are based on the average power
in each band for all electrodes within a given qEEG recording, and are not z -scores
referenced to some normative population (e.g., as in the Neurometrics approach
[26]). These z -score-based values reflect whether a given site is above or below the
average power value in each band. The spatial normalization process also places
absolute and relative power values into a common unit (standard deviation or
z -score units), which allows them to be combined.
Third, the cordance values are formed by summing the z -scores for normalized
absolute and relative power

() () ()

Z sf , = A sf , + R sf ,

norm norm

for each electrode site and in each frequency band. Cordance values have been
shown to have higher correlations with regional cerebral blood flow than absolute
or relative power alone [10]; thus, this combination measure can be placed in con-
text with prior work in depression that employed functional measures of regional
brain activity such as PET data.
Other approaches to interpreting qEEG data may offer alternative perspectives.
The qEEG coherence measure [27, 28] has been interpreted as reflecting functional
connectivity [29-31] in pathways linking parts of neural circuits [32-35], and has