Have you ever tried to tickle yourself? Try it; you will find that the feeling will be nothing like the sensation you get when someone else tickles you. But why is this the case?
The simplest answer to this question is to assume that when you tickle yourself you’re expecting the sensation, so are less likely to react. However, functional magnetic resonance imaging (fMRI) has shown that activity in an area of the brain known as the somatosensory cortex is comparable both when subjects are tickled unexpectedly and when they are warned that they are about to be tickled. This provides evidence that the brain responds to an expected sensation in the same way as it does to an unexpected sensation. Meaning that expectation alone cannot be the explanation for our inability to tickle ourselves.
The brain is constantly receiving sensory input (information about our experiences communicated by our physical senses) from everything we touch, see, hear, taste and smell. This constant barrage of information must be sorted and processed by the sensory systems of the brain in order for us to make sense of the world around us. Arguably, the most important feature of normal brain processing is the ability to identify and extract information about externally-induced changes in our environment. Therefore, in order to differentiate between spontaneous environmental changes and those we cause ourselves, the brain categorizes self-produced movements as being less significant than those initiated external to our bodies. Indeed, fMRI scans have identified increased activity in the somatosensory cortex in response to externally produced tickling (as used in the above study) compared to little or no change in activity seen when participants tickle themselves. This data suggests that activity in the brain differs in response to externally and internally produced stimuli, reinforcing the neurological basis for our ability to consciously distinguish between the two.
Research suggests that this ability to recognise a self-initiated movement may depend on a structure at the back of the brain known as the cerebellum. Circuits within the cerebellum have been termed the bodies ‘central monitor’ and may be the key to distinguishing between self-produced sensations and external stimuli. Neurons of the cerebellum have the capacity to calculate strong and accurate predictions about the sensory consequence of self-tickling. This system takes predictions about our movements and compares them with actual sensory feedback produced by the action. The difference between the two is known as an ‘error signal’. If you attempt to tickle yourself, your internal ‘central monitor’ will accurately predict the sensory consequence because the movement is self-produced and there will be little or no difference in error signal. In contrast, when someone else tickles you (even if you are aware it is going to happen), you will not be able to predict exactly what the sensory stimulation will feel like; that is, its position or strength. Therefore, there will be a difference between your brains prediction and the actual sensory feedback.
So it seems that you can’t tickle yourself? Well, at least this is usually the case. However, research has now stumbled upon a remarkable feature of schizophrenia showing that, unlike the rest of us, schizophrenics actually have the capacity to tickle themselves! It has been suggested that this phenomenon may be a caused by neurological changes in the schizophrenic brain which disable the patient’s ability to detect self-initiated actions. It is possible that biochemical or structural changes in the brain cause a malfunction in the predictive system of the cerebellum. This results in a miscommunication of information concerning internally- vs. externally-generated actions. Essentially this means that, although the patient is able to process the intent to move and is aware the movement has occurred, they cannot then link the resulting sensation (the tickle) with their internal knowledge of making the movement. It is therefore possible that this deficit in self-awareness or monitoring could result in thoughts or actions becoming isolated from the internal appreciation that they are producing them. Consequently, schizophrenic patients may misinterpret internally-generated thoughts and movements as external changes in the environment.
Our ability to control the magnitude of our responses based on prior knowledge of our own actions appears to have numerous advantages. This includes the ability to distinguish between real external threats, such as a poisonous spider crawling up our leg, and those we create ourselves, for example resting our own hand on our leg. Indeed, recognising the difference between an external threat and a self-induced false alarm may, in some situations, be the difference between life and death. The multifactorial basis of the tickling sensation indicates a staggering complexity in central processing in the brain. Science is currently unravelling these complexities and, with luck, this research may lead to both a better understanding of disorders such as schizophrenia and may point the way towards novel treatment strategies.
Post by: Isabelle Abbey-Vital