It’s a common experience: the uncontrollable giggles and squirming when someone else tickles you, contrasted sharply with the complete lack of response when you try to tickle yourself. This phenomenon has puzzled many, leading to the question: why can’t we tickle ourselves? The answer lies within the brain’s mechanisms designed to differentiate between self-generated sensations and external stimuli.
Understanding the Tickle Sensation
Tickling is a sensory experience that researchers categorize into two main types. The first, knismesis, refers to a light, feather-like touch that often causes an itching sensation and an urge to scratch, but does not induce laughter. This type of tickle can sometimes be self-evoked. The second, gargalesis, is the more intense, pressure-based tickle that commonly results in laughter and involuntary movements. It is this latter type of tickle that we cannot produce in ourselves.
Both forms of tickling involve mechanoreceptors in the skin. These receptors respond to light touch and pressure. When stimulated, they transmit electrical signals through the nervous system to the brain, particularly to the somatosensory cortex which processes touch sensations, and the anterior cingulate cortex, which analyzes signals as either harmful or playful. For an external tickle to be effective, the brain interprets these incoming signals as novel or unexpected, which triggers the tickle response.
The Brain’s Self-Suppression System
The inability to tickle oneself stems from the brain’s self-suppression mechanism. When an individual attempts to tickle themselves, the cerebellum plays a role. The cerebellum coordinates voluntary movements and receives both sensory input from the body and motor commands from higher cortical areas.
As we initiate a self-tickling movement, the cerebellum generates a predictive signal known as an “efference copy.” This internal copy of the motor command is sent to other brain regions, including the somatosensory cortex. The efference copy informs the somatosensory cortex about the expected sensation from our own action, predicting its timing, location, and intensity.
Upon receiving this prediction, the brain suppresses the sensory feedback generated by the self-initiated movement. This dampening occurs because the brain already “knows” what to expect, removing the element of surprise fundamental to the tickle response. Functional MRI studies have shown reduced activity in the somatosensory cortex when a stimulus is self-produced compared to an identical external stimulus, indicating this suppression. This process, known as reafference, allows the brain to distinguish between sensations caused by our own actions and those originating externally.
The Adaptive Advantage of Sensory Filtering
The brain’s self-suppression mechanism is not unique to tickling but a fundamental aspect of how our brains process sensory information. This sensory filtering system provides an adaptive advantage, allowing us to navigate our environment. By attenuating predictable self-generated sensations, the brain can prioritize and focus its resources on novel or unexpected external stimuli.
Consider the constant sensory input from everyday actions, such as clothing rubbing against the skin, the feeling of our own breathing, or the internal movements of our organs. If the brain did not filter out these self-generated sensations, we would be constantly overwhelmed and distracted. This filtering mechanism, known as sensory adaptation, is important for survival. It enables us to quickly detect important external events, such as a sudden noise, an approaching predator, or another person’s touch.
This ability to differentiate between self-generated and external sensations is central to our sense of self. It allows the brain to maintain a stable perception of the world despite our constant movements. Therefore, the inability to tickle ourselves is not a limitation, but a byproduct of an efficient neural system that ensures our attention is directed towards more significant external information, contributing to our safety and awareness.