OCD May Be a Brain Gate Stuck Open

Think of the brain as running a checkpoint for everything trying to reach your awareness. Sights, sounds, and touch all arrive at a central relay, and most are turned away before you ever notice them. The working theory researchers hold is that in obsessive compulsive disorder, that checkpoint lets too much through, over and over.

The checkpoint is a loop of three brain areas in constant back and forth. The cortex, the wrinkled outer shell of the brain, handles perception and understanding of what is happening. Beneath it sits the striatum, part of a system called the basal ganglia, that controls action selection, the brain's go and no go switch for behavior.

The third part is the thalamus, an egg shaped structure in the middle of the brain that collects what you see and hear and feel and passes it upward to the cortex. Wrapped around it is a thin shell called the thalamic reticular nucleus. It works like a guard at the gate, deciding which signals are allowed through to conscious experience and which are stopped.

This three part system is called the cortico-striatal-thalamic loop. Dysfunction in that loop is thought to underlie OCD. That idea remains a hypothesis, not a proven cause, but it gives researchers a way to explain why unwanted material seems to reach awareness and why unwanted urges are hard to hold back.

Scientists test the idea by provocation. In laboratory studies, people with contamination fears are handed a towel damp with sweat from another person's hand, and sometimes with sweat wiped from the back of their own neck, and placed in a brain scanner. That setup almost always provokes the urgent thought, this is bad, I need to clean. At the same time, scans show extra blood flow and neural activity in the loop.

The same loop quiets with treatment in some people. When patients who benefit take a class of antidepressants called selective serotonin reuptake inhibitors, the distress eases and activity in the loop becomes less intense. The drugs do not help everyone, which is part of why the gate theory matters: it points to where else future treatments might intervene.