A second linked-reference issue: possible biasing of power and coherence spectra.

The effect of a linked-ears reference on scalp-recorded potentials was investigated in six subjects during an eyes-closed counting task. EEG was recorded from the midline and surrounding electrodes with a linked-ears reference, and from Fz, Cz, and Pz with an active reference three cms anterior (close-bipolar). The linked-ears data were also converted to current source densities by applying the Laplacian. Coherence and power spectra were calculated for the three data sets. Linked-ears frontal relative power was significantly higher than current source density or close-bipolar power at 7-Hz--the frequency of peak parietal power. Also, the linked-ears coherence spectrum was significantly higher than current source density or close-bipolar coherence spectra, and tended to have a smaller range of values. These data replicate and extend earlier reports of distortion of coherence spectra by a common reference.