I just reread the original post and see that you want to know whether dissociation will affect the outcome of an EEG....since this is a test for physical health problems one of two things will happen if someone dissociates during the test...

1. nothing will happen, since the test is about brain waves dissociation most times shows up as the person fell partially or fully asleep ( lower brain waves, lower electrical impulses being fired)

or

2. Since and EEG is a medical not mental exam the person doing the dissociating will be diagnosed with a physical health problem like seizures, epilepsy, cancers, Alzheimer disease and other brain injury problems like Parkensons disease, MS (multiple Sclerosis) and other physical health diseases where the brain doesnt fuction like normal. (higher than normal brain waves, higher than normal electrical impulses firing and mis firing.

my suggestion is let the doctors know you have a dissociation problem so that they can schedule many follow up EEG's that they can compare to each other to get an accurate diagnosis of whether you have any of the physical health problems that EEG's test for.

on rare occasions another situation can happen..

3. getting an inconclusive, unreliable reading...

Most times in my location when a person has a problem that may interfere with a tests results or they feel someone is trying to mislead or manipulate a test, they do multiple EEG's and long term EEG's in an inpatient setting. then compare all the results to come to a final conclusion\diagnosis of whether the patient is having seizures, epilepsy or other physical health problems that can cause a person to have dissociative like symptoms