Making Sense of Senses

Aristotle (384-322 BCE) is credited with first numbering the senses in his work De Anima, which translates to Of the Soul.  Those are the same five senses that we are all taught as children: sight/vision, taste/gustation, touch/tactition, smell/olfaction, and hearing/audition.  But you may have been misinformed.  A broadly acceptable definition of a sense from neurologists is "a group of sensory cells that responds to a specific physical phenomenon, and that corresponds to a particular region of the brain where the signals are received and interpreted."  In addition to the basic five senses, most neurologists count and agree upon four additional senses: thermoception (sense of temperature), nociception (perception of pain), equilibrioception (perception of balance), and proprioception (body awareness).  

As scientists learn more about these senses, new research avenues have opened up.  Proprioception, which includes the sense of movement and position of our limbs and muscles, is one such example.  Proprioception is possible thanks to receptors in our muscles known as spindles, which tell the brain about the current length and stretch of the muscles.  This sense enables a person to touch their finger to the tip of their nose, even with their eyes closed; it enables a person to climb steps without looking at each one.  In September of 2016, researchers at the National Institutes of Health (NIH) published a study in the New England Journal of Medicine that found that people who have particularly poor proprioception through mechanosensation (the ability to sense force, such as feeling when someone is pressing down on your skin) may have a mutated gene that is passed down from generation to generation.  Alec Nickolls, a postdoctoral fellow in the Intramural Research Program at the NIH, studies an extremely rare condition that causes the loss of this proprioception sense.  His research could not only provide insights into how proprioception works, but could also potentially lead to new treatments with individuals with chronic pain.

Why is there so much disagreement on how many senses we have?  Some neuroscientists put the number of human senses as high as 33!  Shouldn't it be easy to count something as basic as a sense?  One reason for the discrepancy is that there is a lot of cross-talk between sensory systems, which makes it tricky to determine where one ends and another one begins.  An example of this is the vestibular system.  We know that the vestibular system, which is located in the inner ear, is an integral part of how we balance ourselves.  But it also plays a critical role in vision, allowing us to keep our two eyes focused on things even while our heads are moving around.  Another reason for the discrepancy is that some scientists split senses while others don't.  For example, think about thermoception - the ability to sense temperature.  Some doctors split thermoception into "sense of heat" and "sense of cold" as two different senses.  Also consider the sense of taste.  Taste can be divided into sweet, sour, salty, bitter, and umami.  Should taste then be divided and counted as five senses, or just the one overarching sense of taste? 

Other defined senses we have are chronoception (ability to sense the passing of time), kinaesthesia (ability to sense movement), and pruritus (the sense of itch).  Some doctors also include physiological experiences as senses, such as the sensation of hunger or thirst, or the need to empty your bladder or bowel.  

Simply defining what we mean by "sense" can lead you down a slippery slope into philosophy.  Christian Jarrett, author of Great Myths of the Brain, explains, "There is no single logical way to define the senses. In some ways, it makes little sense to draw divisions between them at all, considering that they often seem to blend together. The color of food, the look of a meal's presentation, and even the sounds of a restaurant can influence taste, for instance."  Dr. John M. Henshaw, Chair of the Department of Mechanical Engineering at The University of Tulsa and the author of A Tour of the Senses: How Your Brain Interprets the World, said, "It is harder than it might first appear to put a definitive figure on the total number of senses that humans possess.  At some point, it becomes just a bit arbitrary."