In addition to differences in sample selection and recruitment, EEG-specific methodological issues such as choice of electrode reference may account for differences across studies. Cz has been the most-often utilized reference for asymmetry studies (Coan & Allen, 2003), particularly in infant and child samples, but data from the Cz reference do not correlate highly with that from other references thought to be better suited to capturing regionally specific brain activity (cf. Reid et al., 1998; see Allen et al., 2004a, Hagemann, Naumann, & Thayer, 2001, and Hagemann, 2004 for discussion of additional reference issues). A current source density (CSD) derivation has more recently been advocated as an alternative that reduces the influence of cross-hemisphere and distal volume conduction on the asymmetry score with greater specificity for local electrical sources and sinks (Hagemann, 2004). In simpler terms, the CSD algorithm for a particular electrode location estimates the amount of the brain's electrical current flowing in and out of superficial scalp regions adjacent to that electrode. The CSD-reference might thus be preferred as the reference most likely to link surface recorded frontal
