If I tap my nose with my finger I only register a single touch, yet the sensation from nerves in my nose has only a few centimetres to travel to my brain, while the one from my fingertip has to travel about a metre up my arm and shoulder. Is this an illusion arranged by my brain, or is the brain unable to distinguish between two events so close together in time? Can anybody explain?
鈥� You do register two touches: the touch of your finger on your nose, and the touch of your nose on your finger. The illusion arises because you have many more sensory receptors on your fingertips than on your nose, and they are more sensitive in different ways. As an aside, look up 鈥渟omatosensory homunculus鈥� on the internet.
If you stub your toe, the first thing you do is grab hold of the injured part with your fingers to see if it鈥檚 OK. You use your fingers because they provide better information about that part of your body than the body part itself.
Advertisement
So you do feel two sensations. Your brain just chooses to disregard the sensory stimulation of the skin on your nose because it is a much less rich source of information.
Roy Hunter, Helensburgh, Strathclyde, UK
鈥� The brain copes admirably with time lags thanks to its internal workings that deal with the different reception times of incoming signals. It is so good at this that in some circumstances it is difficult for people to appreciate a time lag exists at all. The brain seems to do this by tagging stimuli and thoughts with a time-stamp, though nobody knows how or what form this takes in the brain.
Additionally, interacting with other people requires making predictions of elements or events that you cannot yet have heard, seen or sensed. The ability to sing in unison is a good example. You are committed to singing your note before hearing the same note from the other singer. Yet you believe the thought and the action coincide.
Experiments on response times within the brain show that the thinking and planning required for actions like singing a particular note start long before anyone can report they have decided to sing that note. The brain tags the thoughts into a single instance, so you 鈥� the internal observer of your own behaviour 鈥� think that your thought, planning and execution of the action all happened at the same instance.
To see that this is true try very hard to work out when you planned the mouth and throat actions you need to utter the lyrics of a song you are singing. You鈥檒l conclude it all happened at the same time.
Christopher Cradock, Bicester, Oxfordshire, UK