It’s a common, often perplexing, experience: you try to tickle yourself, but nothing happens. The playful sensation that erupts into giggles when someone else tickles you simply vanishes when your own fingers are involved. This everyday mystery reveals fascinating insights into how our brains distinguish between our own actions and external events.
How Your Brain Anticipates Movement
Our brains predict the sensory outcomes of our actions. When you move, your brain doesn’t just send a command to your muscles; it also generates an internal prediction of the sensations that movement will create. This predictive process helps us differentiate between self-generated sensations and those from our environment.
This internal prediction involves an “efference copy,” a duplicate of the motor command sent from the brain to the muscles. This copy is relayed to sensory processing areas, such as the somatosensory cortex and cerebellum, before the movement occurs. The cerebellum, a brain region coordinating movement, compares the expected sensory input from the efference copy with the actual sensory feedback. This comparison allows the brain to anticipate and account for self-produced sensations.
The Science of Sensory Suppression
Building on this predictive capability, the brain actively suppresses sensory information expected from our own actions. This phenomenon is known as sensory attenuation or neural filtering. When the brain predicts a sensation based on an efference copy, it reduces the intensity of the incoming sensory signal from that self-generated touch. This makes the sensation less pronounced and less ticklish.
Functional brain imaging studies show that the somatosensory cortex, which processes body sensations, exhibits a lower response to self-tickling than to external tickling. This reduced activity indicates the brain actively suppresses the sensation. The brain interprets these self-generated sensations as non-threatening and expected, allowing it to focus attention on unexpected external stimuli. Without this suppression, we would be overwhelmed by our own movements, making it difficult to perceive important external events.
Researchers have demonstrated this suppression: introducing a delay of less than a second between a person’s action to tickle themselves and the actual delivery of the tickling stimulus can make the self-tickle effective. This suggests the brain’s prediction window is very precise; disrupting this timing prevents full sensory suppression. The brain’s ability to distinguish between self-generated and external sensations is fundamental to how we perceive our body and interact with the world.
Why External Touch is Different
When another person tickles you, your brain does not have a predictive model or efference copy of that action. The sensation is unexpected. This lack of predictability prevents the brain’s sensory suppression mechanism from activating.
The unexpected nature of external touch means the full sensory signal reaches your brain without being attenuated. This element of surprise, combined with lack of control over the incoming stimulus, contributes to the ticklish sensation. The brain’s response to an unexpected touch is different because it hasn’t sent the motor command that would prepare it for the sensation. Consequently, the sensory input is perceived with its full intensity, leading to the characteristic ticklish response.