A number of previous meta-analyses have found that EMDR has sustained and lasting treatment effects for Posttraumatic Stress Disorder (Bisson et al., 2007; Bradley, Greene, Russ, Dutra, & Westen, 2005; Seidler & Wagner, 2006). EMDR is now considered to meet criteria for evidence-based practice in the United Kingdom by the National Institute for Clinical Excellence (2005), in America by the American Psychiatric Association (2004), in Australia by the Australian Centre for Posttraumatic Mental Health (2007), and in the Netherlands by the Dutch National Steering Committee for Guidelines for Mental Health Care (2003).
Although the active processes in EMDR appear to be different to traditional exposure treatments (Lee, Taylor, & Drummond, 2006), the mechanism of action for the success of EMDR remains controversial (Rogers & Silver, 2002; Shapiro, 2012; Smyth & Poole, 2002). There is disagreement as to whether eye movements add anything to the effectiveness of EMDR (Davidson & Parker, 2001; MacCulloch, 2006).
The treatment studies that have attempted to isolate the eye movement component from the full treatment package have produced results ranging from a very large effect size consistent with eye movements enhancing processing (Wilson, Silver, Covi, & Foster, 1996) to findings of no differences (Renfrey & Spates, 1994). On the other hand, non-clinical laboratory studies that investigated the effects of eye movements on autobiographical memories have found decreases in vividness and/or emotionality compared to control conditions such as finger tapping ( van den Hout, Muris, Salemink, & Kindt, 2001), spatial tapping ( Andrade, Kavanagh, & Baddeley, 1997), and no eye movement ( Barrowcliff, Gray, Freeman, & MacCulloch, 2004; Gunter & Bodner, 2008; Kavanagh, Freese, Andrade, & May, 2001). Whilst these laboratory studies show a clear processing effect for eye movements, they did not involve all the procedural elements of EMDR ( Shapiro, 2001: p. 472).
In an attempt to discover any general trends in research that has examined the effects of eye movements on memory, Davidson and Parker (2001) conducted a meta-analysis of published studies investigating effect size differences between EMDR with eye movements and EMDR without eye movements. Their conclusion when looking at pre-versus-post single session measures was that there was no significant additional effect of eye movements. Their measure of effect size was R, which ranges from plus one to minus one; R2 is the amount of variance in the dependent variable accounted for by the independent variable. However there were methodological problems in this meta-analysis. Initially R scores were converted to Z scores. The simple mean of these scores was converted back to R, and then subjected to a t-test using the number of studies to determine the degrees of freedom. The problem with this approach is that it treats all studies as if they are of equal weight. The usual practice in meta-analysis is to weight each study in relation to the number of participants and for the degrees of freedom to be calculated using the total number of participants ( Rosenthal & DiMatteo, 2001). This provides a more appropriate test of significance and provides more power to investigate small magnitude effect sizes ( Rosenthal, 1991).
Since 2001, there have been additional published papers investigating the effects of eye movements on various measures. Therefore, we decided to conduct a new meta-analysis, including all studies published in the past 23 years and adjusting for the sample size in each study.
