What so the experts say?The scientists who study the body and its systems are called physiologists. If we ask a human physiologist about the senses they will probably list vision, hearing, smell, taste, touch, body position, body movement, muscle force, and balance. That’s 9 senses. Physiologists who specialise in the senses, movement or exercise might also mention a sense of effort, sense of body ownership, and sense of body size. Now we have twelve. Psychologists study the mind and how we behave, and they will tell you about our sense of agency, which is a sense of whether we have control over something. That’s 13 senses so far.
So how many senses do we have?Well actually it depends upon how you count them. Physiologists often group the senses of body position, body movement, body size, force, effort into one sense called proprioception (pro-PREE-o-SEP-SHUN). The collection of senses that gives us our sense of balance is usually called the vestibular (ves-tib-YOOL-ar) sense. Agency is often treated separately, and neither physiologists or psychologists have decided how to classify the sense of body ownership. The sense of body ownership could probably be included in proprioception, but at the moment experts are generally treating it separately. So even the experts in the senses don’t agree completely on how many senses there are, but there seem to be at least 7 or 8.
Why don’t I know about the other senses?Generally, we don’t learn about them at school. I didn’t learn that there were more than 5 senses until I started studying physiology at university. Have extra senses been discovered recently? No. Physiologists have been writing about proprioception for over 100 years1. Similar for the vestibular sense2, 3. The idea of a sense of effort has been around since at least 18674. It is true that physiologists are still learning how these ‘extra’ senses work, but that is also true of the 5 that everyone knows about. Perhaps we are not taught about these extra senses because it is hard to show that they are there. It is also hard to think what it would be like without them.
Losing a senseIn the case of vision, it is easy to demonstrate that it is there, partly because we all know that it can be taken away. We get a feeling of this when we close our eyes and try to walk around. Being permanently blind is much harder, but we all know that there are people who cannot see very well, or at all. Our vision tends to get worse as we age, and we have glasses to correct it when it doesn’t work properly. Hearing is similar. By blocking our ears we can temporarily take away that sense. Our hearing deteriorates through age or injury, and we know people who need hearing aids. What about proprioception and the sense of balance? How do we know they are there, and what happens when we take them away? Are there people who don’t have proprioception? Does it deteriorate as we age? Yes, there are people who have damaged proprioception, or none at all5. It is very difficult for those people to control the movement of their body. Walking, standing, typing, eating, etc. can all be difficult without fully working proprioception. Stroke and spinal injury can also affect proprioception. However, it is difficult for healthy people to feel what it is like to have disrupted proprioception. For that we usually need special tools or experiments.
Measuring the extra sensesThis is probably part of the reason why most people don’t know that there are more than 5 senses. They don’t have the tools to measure them, or now how to do experiments to prove they are there. Fortunately there are some easy experiments to demonstrate proprioception, body ownership, sense of balance, and the others I’ve mentioned. Future blog posts will look at these ‘extra’ senses. What they do, how they work, scientific evidence for them, and how to measure them with simple tools.
- C.S. Sherrington first referred to the ‘proprio-ceptive sense’ and ‘proprio-ception’ in his book The Integrative Action of the Nervous System, published in 1906 by Yale University Press (New Haven). In 1900 Sherrington also published a chapter called ‘The muscular sense’ in the Textbook of Physiology, (London, editor Schafer E., pages 1002-1025) which is about proprioception, but does not use that word.
- The oldest reference I could find referring to the vestibular sense is a paper by Abercrombie & McKenzie called Hysterical deafness with active vestibular, published in the Proceedings of the Royal Society of Medicine in 1910 (Vol 3 (Otal Sect), pages 74-76).
- The origins of vestibular science (2015) by G. Wiest (Ann NY Acad Sci, Vol 1343, pages 1-9) states that vestibular function was discovered in 1873.
- In his Treatise on Physiological Optics (1867; Vol 3, Optical Society of America, Menasha Wisconsin) von Helmholtz referred to a feeling of innervation and an effort of will, which have the same features as the modern ‘sense of effort’.
- Jonathon Cole’s book Pride and a Daily Marathon (1995, MIT Press), describes the case of a man who lost proprioception below the neck, but learned how to move around again despite that loss.
Dr Lee Walsh the founder and director of Platypus Technical Consultants. Lee is an electrical and biomedical engineer, physiologist, technical consultant and science communicator. He has over a decade of experience in measurement, instrumentation and analysis, particularly in clinical settings, physiology and medical device testing.