EMDR FORUM ARCHIVE, 2000
Re: differential mechanism
Shawn Cahill · 11/02/01 at 9:22 ET
Ulirich: I thank you for being willing to engage in such a detailed exchange. We have covered a lot of ground in a few posts. You raise several points I believe merit further discussion. However, I will have post my response in somewhat piecemeal fashion. Hopefully, this will be the first of a few posts that contintue our dialogue. One of the issues raised in our exchange is the relationship between the dose of exposure to trauma memories and outcome. Specifically, you previously made the assertion that "in contrast to exposure therapy, there does not appear a dose dependant relationship between length of exposure and treatment effect." I questioned the evidential basis for this assertion and you provided two responses, which I will deal with separately and in reverse order in which they appeared in your response. Response 2: You noted procedural differences between EMDR and exposure therapy. Presumably, these procedural differences would create differences between the two therapies in terms of the level of exposure to traumatic memories with exposure therapy involving a greater dose of exposure than EMDR. The problem with this argument is that differences between EMDR and exposure therapy in terms of the "dose" of exposure does nothing to address the issue of whether or not there is a dose-response relationship with EMDR. The only way to find out if there is a relationship between the dose of exposure and outcome with EMDR is to systematically vary the dose of exposure while keeping all other aspects of the treatment constant. Response 1: You assert that it "probably is impossible to systematically vary length of exposure in EMDR, as this would violate the standard treatment protocol." Correct me if I'm wrong, but this appears to be saying that it is impossible in principle to test your hypothesis that there is no dose-dependent relationship between exposure and outcome in EMDR. If this is indeed true, then your earlier statement that there is no dose-dependency in EMDR is a scientifically meaningless statement. Fortunately, I don't think the situation is all that bleak. There seems to be an unspoken assumption in your comment that the "standard treatment protocol" is the most effective variation of EMDR and that any deviations will jeopardize outcome. However, in order to investigate whether or not a particular variable is important, you must necessarily manipulate that variable, which involves altering the "standard treatment protocol." This is not an unsolvable situation, however. It simply requires a somewhat more complex design and/or a systematic series of studies. Let's take for example the hypothesis that longer exposures will be more effective than shorter exposures (this would be one of several possible ways to operationalize dose of exposure). Our study begins with a WL control group and a standard EMDR group. To this, we add at least two additional groups in which we provide EMDR by systematically vary the duration of the time spent focussed on the trauma image. One group would experience short duration exposure and the other would experience long duration exposure. The comparison between standard EMDR and WL allows us to establish whether the treatment worked. The comparison between standard EMDR each of the variations of EMDR allows us to assess whether deviating from EMDR along this parameter makes any difference (note, in principle the variations in EMDR could be associated with better, worse, or no different outcome from the standard variation). Within this context, the comparison between short-duration exposure EMDR and long-duration EMDR then allows us to address the question of whether there is a dose-dependent relationship with EMDR. Additional control groups may be necessary and or/or other operationalizations of "dose of exposure" (e.g., manipulating the number of exposures while equating for duration of individual exposures) may need to be investigated before a clear answer emerges. But in principle, I think it could be done. However, until such studies are done, I must conclude that your assertion about no dose-dependency in EMDR has no empirical basis.
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