One notable exception is a special resting EEG phenotype, frontal EEG asymmetry (FA), which is expressed as the difference in alpha power between the left and right anterior scalp regions. A good test-retest stability of individual differences in FA was reported (Tomarken et al., 1992). Davidson et al. suggested that greater left than right alpha-band power presumably indicating lower left than right level of prefrontal activation is associated with stronger withdrawal motivation and increased vulnerability to depression, while the opposite pattern of frontal EEG asymmetry is associated with stronger approach motivation and low risk for depression (Davidson, 1998; Davidson and Fox, 1982). Over the past two decades, this attractive hypothesis has generated an extensive literature, however, evidence for the association between FA, depression, and relevant personality traits and behavioral measures remains somewhat mixed (Allen and Kline, 2004; Davidson, 1998; Hagemann, 2004), Nevertheless, FA is still considered by many researchers as an indicator of affective style and risk for internalizing psychopathology (Allen et al., 2004; Allen and Kline, 2004; De Pascalis et al., 2013; Gatzke-Kopp et al., 2012).
