Obsessive-compulsive disorder (OCD) has puzzled artists and scientists for centuries. Afflicting one in 50 people, OCD can take several forms, such as compulsively putting things in just the right order or checking if the stove is turned off 10 times in a row. One type of OCD that affects nearly half of those with the condition entails irresistible washing urges. People with this type can spend hours scrubbing their hands in agitation after touching something as trivial as a doorknob even though they know this makes no sense. There is currently a shortage of effective therapies for OCD: 40 percent of patients do not benefit from existing treatments.

A major issue is that today’s treatments are often too stressful. First-line “nonpharmacological therapies” involve telling patients to repeatedly touch things such as toilet seats and then refrain from washing their hands. But recent work by my colleagues and me has found something surprising: people diagnosed with OCD appear to have a more malleable “sense of self,” or brain-based “self-representation” or “body image”—the feeling of being anchored here and now in one’s body—than those without the disorder. This finding suggests new ways to treat OCD and perhaps unexpected insights into how our brain creates a distinction between “self” and “other.”

In our recent experiments, for example, we showed that people with and without OCD responded differently to a well-known illusion. In our first study, a person without OCD watched as an experimenter used a paintbrush to stroke a rubber hand and the subject’s hidden real hand in precise synchrony. This induces the so-called rubber hand illusion: the feeling that a fake hand is your hand. When the experimenter stroked the rubber hand and the real one out of sync, the effect was not induced (or was greatly diminished). This compelling illusion illustrates how your brain creates your body image based on statistical correlations. It’s extremely unlikely for such stroking to be seen on a rubber hand and simultaneously felt on a hidden real one by chance. So your brain concludes, however illogically, that the rubber hand is part of your body.

After a few minutes of such stroking, we “contaminated” the fake hand (using items such as fake feces). Intriguingly, participants without OCD reported feeling OCD-like disgust, which seemingly arose from the rubber hand. This experiment was later replicated in a large study in Japan, indicating that the finding is robust across cultures. Put differently, beyond feeling like the rubber hand was their own (the standard illusion), the subjects were disgusted by what it was touching.

In a follow-up study my colleague Vilayanur S. Ramachandran of the University of California, San Diego, and I—along with Richard J. McNally, Jason A. Elias and Sriramya Potluri, all then at Harvard University—found that people with OCD felt like the fake hand was theirs even when the experimenter stroked the real and rubber hands out of sync with each other. As noted, the illusion occurs because your brain extracts statistical correlations from sensory inputs: you feel your unseen hand being stroked and see the fake hand being touched the exact same way. The fact that people with OCD experienced a vivid illusion during out of sync stroking suggests they have a more expansive self-representation to the degree that they are willing to seamlessly ignore conflicting sensory inputs— and still accept the rubber hand as their own. Indeed, this is the first study to suggest that OCD involves a more malleable body image⸻in other words, it is the first to indicate that people with the condition construct their sense of self differently than others. Just as in our previous study, inducing the rubber hand illusion and smearing the rubber hand with fake feces provoked disgust—apparently fooling the brain into attributing the disgust to the to the fake hand.

Taken together, these studies indicate that the “self” is more fluid for people with OCD. Their greater susceptibility to the rubber hand illusion might be explained by dysregulation of chemicals such as dopamine (a feature of OCD). The studies also suggest that once the rubber hand trick is induced, contaminating the fake hand might activate brain modules involved in disgust. The experiments illustrate how seemingly unrelated brain centers—for vision, touch and disgust—may interact in a dynamic fashion to weave together perceptual reality. Indeed, just the right kind of physical stimulation for a few minutes can make someone abandon a lifetime of experience that a rubber hand is not a part of their body. Astonishingly, when presented with this scenario, you will make the perceptual decision that a fake hand is yours and experience bona fide contamination sensations arising from it.

In a related study, Ramachandran and I found that college students with OCD symptoms felt disgust while watching an experimenter contaminate himself and relief while watching him wash his hands—suggesting that highly visceral disgust reactions, as experienced in the context of OCD-like contamination aversion, can ultimately break down the barrier between self and other. Surprisingly, we found that to people with OCD symptoms, it didn’t matter whether they or the experimenter was contaminated—they felt equally disgusted! Participants’ verbal reports about how disgusted they felt were the same both when they touched the contaminant and when they merely watched the experimenter do so. Even more intriguingly, once the participants had contaminated themselves, they reported relief from watching the experimenter washing his own hands. Notably, some participants would dictate how the experimenter should wash his hands, saying things such as “Wash more on this side” or “Pour more water between these fingers.” Echoing these results, we recently found that OCD patients at McLean Hospital in Massachusetts reported experiencing handwashing urges arising from watching an experimenter contaminate himself. They also reported feeling equally disgusted and anxious both when watching an experimenter contaminate himself and when they themselves were contaminated. Finally, once the patients had contaminated themselves, they reported feeling relief—reductions in disgust by 22 percent, equivalent to actual handwashing—from watching the experimenter washing his own hands. Overall, these results are counterintuitive. They demonstrate the elusive interface between mind and body and feelings such as disgust. It may be that contamination feelings in OCD have the potential to override logic and the “self-other” barrier.

Our studies may lead to new treatments for OCD. Traditional therapy has patients touch disgusting things, then shows them that nothing bad happens when they don’t wash their hands. These treatments don’t always work well because patients are too anxious to touch contaminated objects. But what if a rubber hand that feels like the patient’s own is contaminated instead? Indeed, such prolonged contamination of a fake hand should eventually lead to desensitization just like traditional therapy. Unlike standard OCD treatment, this novel rubber hand therapy—which we’ve dubbed “multisensory stimulation therapy”—does not require patients to touch highly aversive “contaminants.” Accordingly, patients who are too frightened to engage in traditional therapy because of the direct skin exposure may be more willing accept this technique.

Likewise the observations that feelings of contamination and relief can arise vicariously may pave the way for new treatments. Watching a video of oneself touching disgusting objects should have a desensitizing effect over time. Similarly, patients could repeatedly watch themselves washing their hands to eradicate washing urges. With our colleagues, Barbara J. Sahakian of the University of Cambridge and I found that people with contamination fears improved their OCD symptoms by simply watching a brief video of themselves touching fake feces or washing their hands on a smartphone a few times daily for a week. OCD is a perplexing condition that blurs the boundary between mind and body, reality and illusion. One may have to fool the brain to overcome the condition—combating one illusion with another.