Many people are ticklish when someone else tickles them. However, tickling yourself is difficult because your brain filters out the expected sensations. (Photo by kupicoo/Getty Images)
For many of us, the response to tickling is paradoxical—the playfulness it evokes is often enjoyable, but the jittery nerves and sense of loss of control can feel unpleasant. Whether you find it pleasant, unpleasant, or somewhere in between, you can’t tickle yourself. But why is that?
Experts explained to Live Science that this is because the brain is already aware of the expected, predictable sensation of self-tickling and dampens it.
“This is because the brain is always predicting the future,” David Eagleman, a neuroscientist at Stanford University, told Live Science. “The brain is not just reacting; it’s trying to predict what’s going to happen next.”
Every time you perform an action, the primary motor cortex—the part of the brain responsible for initiating signals—sends a copy of the command, called the “effective copy,” to many areas of the brain to prepare for the sensory information expected as a result of your action.
For example, when you want to pick up a pencil, your brain sends a signal to your hand and fingers, telling them to grab the pencil and lift it. But it doesn’t just send the command to the muscles that will carry out the movement. It simultaneously sends copies to your somatosensory cortex, the part of the brain that processes incoming sensory data, and to your visual cortex, the part of the brain that processes visual information.
Konstantina Kilteny, a neuroscientist at the Karolinska Institute in Stockholm, explains that the brain uses the signals it sends to muscles to predict how the action we initiate will feel before we experience it.
Kilteny runs a somatosensory and gargalesis lab called the Touch and Tickle Lab. Using brain imaging techniques such as fMRI and magnetoencephalography, she and her team are investigating whether the brain perceives self-administered touch differently than touch from another person.
Kilteney told Live Science that people consistently perceive the intensity of their own touch as less intense than external touch. It’s not just a feeling; neuroimaging confirms that the brain responds less strongly to self-initiated touch.
Because these sensations are predictable, the brain attenuates them. Scientifically speaking, we reduce self-generated sensations. However, if these predictions do not match reality, your brain notices.
David Schneider, an assistant professor of neuroscience at New York University who studies acoustic self-awareness, gave an example to Live Science. “When you close a car door, you expect to hear a predictable ‘thud,’” he wrote in an email. “If you hear a ‘clunk,’ instead, your brain instantly recognizes this as a mistake, and you turn around and pull your seatbelt out of the door frame.”
Humans are alert to external stimuli—those perceived by all of our senses—because noticing them can be critical to survival. Imagine you’re walking. Your footsteps make a sound. Hearing your own footsteps isn’t essential, so your brain reduces the noise. But someone walking behind you could be dangerous, so it’s important to notice.
The phenomenon isn't unique to humans. Schneider can't ask the mice he studies whether they hear their own footsteps, but he can record neural activity in the auditory-processing areas of their brains. When he does, he finds that the neurons barely
Sourse: www.livescience.com
