Here is a great link for a study on intrusive thoughts:
“Participants with the highest concentrations of a chemical known as Gaba in their hippocampus were best at suppressing the unwanted thoughts”
“A difficulty with or an inability to break free from intrusive and unwanted thoughts are a reality both for neurotypical people and also for those with various types of mental illness. Conditions ranging from obsessive-compulsive disorder and post-traumatic stress disorder to depression and schizophrenia all count this type of behavior among their symptoms.”