Have you ever thought about why people are ticklish? According to scientists, ticklishness is a defense mechanism humans developed against bugs, spiders, and other critters that may be touching our skin. Feeling ticklish is our built-in response to predators or threats, and combines the sensations of touch and pain.

The part of the brain that is associated with tickling is the cerebellum. It monitors our movements, but ignores movements not perceived as a threat. For example, we do not notice our vocal chords moving when we speak, but we may jump if someone touches us on the shoulder. This selective perception is probably the reason why we cannot tickle ourselves.

We only feel ticklish if we detect a sense of invasion or attack. If we try to tickle ourselves, our brains anticipate this "attack" and prevent us from feeling panicked. Strangely enough, even if a person knows that they are about to be tickled, the fear of being touched may cause the same "ticklish" reaction. Some people laugh even before being tickled.

A scientific study was recently done in England to determine whether or not people can tickle themselves. Volunteers were attached to a brain-scanning device and were tickled on their palms with a piece of soft foam . The participants' brain scans were monitored while they were being tickled, and when they tickled themselves. It was found that during self-tickling, the cerebellum alerted another area of the brain about what to expect, so the ticklish feeling was diminished.

Researchers did find a way, however, for people to tickle themselves. When volunteers activated a robot by remote control to tickle them after a short delay, the volunteers felt as if someone else were tickling them. It seems that the cerebellum sends a signal to disregard the forthcoming movement, then moves on to other things. Your brain "forgets" that you are tickling yourself—even with a delay as short as a fifth of a second. So it is possible to tickle yourself—but only by remote control. What will science discover next?

你有没有想过人为什么会怕痒？根据科学家的观点，怕痒是一种防御机制，是人类把它培养来对付那些可能接触我们皮肤的虫子、蜘蛛或其他生物。怕痒是我们对侵扰者或威胁产生的内在反应，它使触觉和痛觉结合起来了。

脑部与怕痒有关的部位是小脑。小脑监控我们的一举一动，但对其认为不具威胁性的活动，往往会忽视。举例来说，我们说话时不会察觉声带的振动，但是如有人触摸我们的肩膀时，我们可能会吓一跳。这种选择性的知觉或许就是我们无法自已哈痒的原因。

我们只会在察觉受到侵犯或攻击时才会觉得痒。如果试着替自己哈痒，脑部会自动预感到这种“攻击”从而使我们感觉不到惊吓。奇怪的是，即使有人知道他要被哈痒了，担心被人触摸，也会引起同样“怕痒”的反应。有些人甚至还没被哈到痒就先笑了。

英格兰近期做了一项关于人是否可以替自己哈痒的科学研究。自愿受测者接上脑部扫描仪，并用一块软泡沫绵给手掌哈痒。参与者在被哈痒和自行哈痒时，脑部扫描状况都受到监视。结果发现，在自行哈痒的过程中，小脑会向脑部的另一部位发出警告，使其预先防范，所以痒的感觉就消失了。

然而，研究人员也发现了可以替自己哈痒的方法。自愿者用遥控器操纵机器人，延迟片刻之后再让机器人为自己哈痒，自愿者就会觉得好像是别人在给他们哈痒。看来小脑在发出信号忽视即将到来的动作，然后将注意力转移到其他事物上。尽管只有零点二秒的时间差，脑部就会“忘记”你在替自己哈痒。所以自己给自己哈痒仍有可能，只不过需要依靠遥控器。谁知道接下来科学还会有什么新发现呢？