One of the anxiety disorders discussed in Chapter Sixteen is obsessive-compulsiv

An obsession is a recurrent, persistent, unwished-for thought or image, while a compulsion is a repetitive, ritualized, stereotyped behavior that the person feels must be carried out to avoid disaster.
The prefrontal cortex is depleted of serotonin in many people with OCD, causing cognitive rigidity.
In addition, several other parts of the brain are hyperactive, sending continuing alarm signals of impending danger. 
Biologically based disorders do not necessarily require biological treatments.
In studies using PET scans of people with OCD who received medication alone (Prozac) or cognitive-behavior therapy alone, both groups showed similar improvement in metabolism of glucose in the brain.
Changing your behavior and thoughts through psychotherapy or other new experiences can change the way your brain functions as demonstrated in this study.