- Chronic stimulation of the vagus nerve does not seem to produce significant differences between high frequency and low frequency stimulation groups. Individuals within each group show significant changes between preoperative assessment and after 6-month stimulation. Some subjects showed significant improvement and some showed significant slowing of responses. Subjects who showed improvement are still considerably slower than normals, but all patients have a very long history of complex partial seizures and exposure to multiple medications. Larger homogeneous sample sizes are needed to delineate more clearly the correlation between cognitive performance, medication effects, and stimulation effects.