Today is Celebration of the Senses Day – a time to consider your amazing sensory abilities and how they interrelate. We all learn as children humans have five basic senses: touch, sight, hearing, smell and taste. Science has done a fantastic job describing the organs associated with each, how they send information to the brain to help us understand and perceive the world around us, and has gone even further to uncover how they uniquely cross-talk for heightened awareness.
Touch is thought to be the first sense humans develop, according to the Stanford Encyclopedia of Philosophy. Touch consists of several distinct sensations communicated to the brain through specialized neurons in the skin. Pressure, temperature, light touch, vibration, pain and other sensations are all part of the touch sense and are all attributed to different receptors in the skin.
Touch isn’t just a sense used to interact with the world; it also seems to be very important to a human’s well-being. For example, touch has been found to convey compassion from one human to another.
Touch can also influence how humans make decisions. Texture can be associated with abstract concepts, and touching something with a texture can influence the decisions a person makes, according to six studies by psychologists at Harvard University and Yale University, published in the June 24, 2010, issue of the journal Science.
“Those tactile sensations are not just changing general orientation or putting people in a good mood,” said Joshua Ackerman, an assistant professor at the Massachusetts Institute of Technology. “They have a specific tie to certain abstract meanings.”
Sight, or perceiving things through the eyes, is a complex process. First, light reflects off an object to the eye. The transparent outer layer of the eye – the cornea – bends the light that passes through the hole of the pupil. The iris (the colored part of the eye) works like the shutter of a camera, retracting to shut out light or opening wider to let in more light.
The cornea focuses most of the light. Then, it passes through the lens, which continues to focus the light. The lens of the eye then bends the light and focuses it on the retina, which is full of nerve cells. These cells are shaped like rods and cones and are named for their shapes. Cones translate light into colors, central vision and details. The rods translate light into peripheral vision and motion. Rods also give humans vision when there is limited light available, like at night. The information translated from the light is sent as electrical impulses to the brain through the optic nerve.
People without sight may compensate with enhanced hearing, taste, touch and smell, according to numerous studies. Their memory and language skills may be better than those with sight, as well.
“Even in the case of being profoundly blind, the brain rewires itself in a manner to use the information at its disposal so that it can interact with the environment in a more effective manner.” That’s according to Dr. Lotfi Merabet, senior author of a 2017 study and the director of the Laboratory for Visual Neuroplasticity at Schepens Eye Research Institute of Massachusetts Eye and Ear.
This sense works via the complex labyrinth that is the human ear. Sound is funneled through the external ear and piped into the external auditory canal. Then, sound waves reach the tympanic membrane, or eardrum. This is a thin sheet of connective tissue that vibrates when sound waves strike it.
The vibrations travel to the middle ear. There, auditory ossicles — three tiny bones called the malleus (hammer), incus (anvil) and stapes (stirrup) — vibrate. The stapes bone, in turn, pushes a structure called the oval window in and out, sending vibrations to the organ of Corti. This spiral organ is the receptor organ for hearing. Tiny hair cells in the organ of Corti translate the vibrations into electrical impulses. The impulses then travel to the brain via sensory nerves.
People retain their sense of balance because the Eustachian tube, or pharyngotympanic tube, in the middle ear equalizes the air pressure there with the air pressure in the atmosphere. The vestibular complex, in the inner ear, is also important for balance, because it contains receptors that regulate a sense of equilibrium. The inner ear is connected to the vestibulocochlear nerve, which carries sound and equilibrium information to the brain.
Humans may be able to smell over 1 trillion scents, according to researchers. They do this with the olfactory cleft, which is found on the roof of the nasal cavity, next to the “smelling” part of the brain – the olfactory bulb and fossa. Nerve endings in the olfactory cleft transmit scents to the brain, according to the American Rhinologic Society.
Dogs are known as great smellers, but research suggests humans are just as good as man’s best friend. Research published in the May 11, 2017, issue of the journal Science suggests humans can discriminate among 1 trillion different odors; it was once believed humans could discern only about 10,000 different smells.
“The fact is the sense of smell is just as good in humans as in other mammals, like rodents and dogs,” John McGann, a neuroscientist at Rutgers University-New Brunswick in New Jersey and the author of the new review, said in a statement. The Rutgers study backs up a previous study at the Rockefeller University in New York, whose findings were published in the March 2014 issue of the journal Science.
Humans have 400 smelling receptors. While this isn’t as many as animals that are super smellers have, the much more complicated human brain makes the difference. In fact, poor smelling ability in people may be a symptom of a medical condition or aging. For example, the distorted or decreased ability to smell is a symptom of schizophrenia and depression. Old age can also lessen the ability to smell properly. More than 75 percent of people over the age of 80 years may have major olfactory impairment, according to a 2006 paper published by the National Institutes of Health (NIH).
The gustatory sense is usually broken down into the perception of four different tastes: salty, sweet, sour and bitter. There is also a fifth taste, defined as umami or savory. There may be many other flavors not yet discovered. Also, spicy is not a taste. It is actually a pain signal, according to the National Library of Medicine (NLM).
The sense of taste aided in human evolution, according to the NLM, because taste helped people test the food they ate. A bitter or sour taste indicated that a plant might be poisonous or rotten. Something salty or sweet, however, often meant the food was rich in nutrients.
Taste is sensed in the taste buds. Adults have 2,000 to 4,000 taste buds. Most of them are on the tongue, but they also line the back of the throat, the epiglottis, the nasal cavity and the esophagus. Sensory cells on the buds form capsules shaped like flower buds or oranges. The tips of these capsules have pores that work like funnels with tiny taste hairs. Proteins on the hairs bind chemicals to the cells for tasting.
It is a myth that the tongue has specific zones for each flavor. The five tastes can be sensed on all parts of the tongue, although the sides are more sensitive than the middle. About half of the sensory cells in taste buds react to several of the five basic tastes. The cells differ in their level of sensitivity. Each has a specific palette of tastes with a fixed ranking, so some cells may be more sensitive to sweet, followed by bitter, sour and salty, while others have their own rankings. The full experience of a flavor is produced only after all of the information from various parts of the tongue is combined.
The other half of the sensory cells are specialized to react to only one taste. It’s their job to transmit information about the intensity — how salty or sweet something tastes.
Other factors help build the perception of taste in the brain. For example, the smell of the food greatly affects how the brain perceives the taste. Smells are sent to the mouth in a process called olfactory referral. This is why someone with a stuffy nose may have trouble tasting food properly. Texture, translated by the sense of touch, also contributes to taste, and recent studies even show color and shape can affect the way we perceive a properly attribute taste to a food.
The sense of space
In addition to the traditional big five, other senses help us translate a myriad of inputs into how we perceive and relate to the physical world. One deals with how your brain understands where your body is in space. This sense is called proprioception.
Proprioception includes the sense of movement and position of our limbs and muscles. For example, proprioception 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. People with poor proprioception may be clumsy and uncoordinated.
Researchers at the NIH found that people who have particularly poor proprioception through mechanosensation — the ability to sense force, such as feeling when someone presses down on your skin — may have a mutated gene that is passed down from generation to generation. That comes from a September 2016 study in the New England Journal of Medicine. “The patient’s version of [the gene] PIEZO2 may not work, so their neurons cannot detect touch or limb movements,” said Alexander Chesler, a principal investigator at the National Center for Complementary and Integrative Health and the lead author of the study.
Additional senses & variations
There are more-subtle senses that most people never really perceive. For example, there are neuron sensors that sense movement to control balance and the tilt of the head. Specific kinesthetic receptors exist for detecting stretching in muscles and tendons, helping people to keep track of their limbs. Other receptors detect levels of oxygen in certain arteries of the bloodstream.
While our five basic sense seem to operate independently, as distinct modes of perceiving the world, in reality, they collaborate closely to enable the mind to better understand our surroundings. We can become keenly aware of this collaboration under special conditions.
In some cases, a sense may covertly influence another we think is dominant. When visual information clashes with that from sound, sensory crosstalk can cause what we see to alter what we hear. When one sense drops out, another can pick up the slack. For instance, people who are blind can train their hearing to play double duty. Those who are both blind and deaf can train touch to step in—even to help them interpret speech.
Our senses must also regularly meet and greet in the brain to provide accurate impressions of the world. Our ability to perceive the emotions of others relies on combinations of cues from sounds, sights and even smells. Perceptual systems, particularly smell, connect with memory and emotion centers to enable sensory cues to trigger feelings and recollections, and are incorporated within them.
The crosstalking of the senses provides some of the most magnificent material for interesting science, illusions, inventions and just plain art. Here are a few of the best examples of the complex interactions – and extraordinary feats – of our cross-wired senses.
Calling to See
Bats and dolphins, among other animals, emit sounds into their surroundings —not to communicate with other bats and whales — but to “see” what is around them. They read echoes of the sound waves, which bounce off objects, to identify and locate objects.
This sensory system is called echolocation. Although most of us can only imagine the pictures that form from sound, some sightless people have managed to master a form of echolocation. By uttering sounds and clicks, these individuals can use their ears to navigate. Some, such as Daniel Kish, have even taught others to use this form of human sonar. Kish once described human echolocation as “something like seeing the world in dim flashes of light.”
Fingers Do the Hearing
People who are both deaf and blind are incredibly good at using other senses, such as touch, to navigate and understand the world. Some use the Tadoma Speechreading Method to perceive speech by touching the lips of another person as they talk. First taught in the 1920s, lip-reading by touch was a popular form of communication among the deafblind. Helen Keller was one of its early adopters.
If taught early in development, the Tadoma Method can help a deaf-blind child learn to speak as well as to understand others. Those who lose their sight and hearing later in life can use it to read lips.
But because the method is extremely difficult and time consuming to learn, by the 1950s it began to lose ground to American Sign Language as the dominant teaching method. Today, only about 50 people in the world still use of the Tadoma Method.
Still, In ASL, the deaf-blind place their hands over another signer’s hands and follow the motions with their fingers—which is easier because the movements are far less subtle.
Blind baseball seems almost impossible to even imagine, but since 1975, when a few blind Minnesotans invented “beep baseball”, those who lack sight have taken part in America’s favorite pastime. Thanks to a one-pound beeping oversized softball and some tweaks to the game, players can hit a home run without ever seeing the ball. They use the sound the ball emits to orient themselves, make contact using a bat and to field. Special bases make it possible to round the diamond. They might be particularly well-suited to this form of the game, as previous research suggests that blind individuals can more easily localize sounds than sighted people can.
Then there is synesthesia
For a few individuals with a condition called synesthesia, the senses collide dramatically and uniquely to form a kaleidoscope world in which chicken tastes like stars, a symphony smells of fresh baked bread or words are bathed in red, green or purple.
People with synesthesia have a particularly curious cross wiring of the senses in which activating one sense spontaneously triggers another. They see colors when they hear noises, associate particular personalities with days of the week, or hear sounds when they see moving dots.
Synesthesia is thought to be genetic, and recent research even suggests it may confer an evolutionary advantage. Most synesthetes don’t notice anything strange about the way they perceive their environments until it is brought to their attention.
Given that, at any moment in time, we are bombarded by such a diverse combination of sensory experiences, our appreciation of the individual senses can become somewhat muddled. Our taste experience is affected by the smell, texture and temperature of our food. Similarly, our hearing is said to decrease after overeating, and our sight is affected by noises around us. Sight can also be hampered after eating fatty foods.
Here’s another interesting snippet – if a sad, depressed person tells you their world is dull and grey, and flowers have lost their smell, they’re not just speaking metaphorically. Research shows sensory perception can actually be diminished in depressed individuals. So focussing on a renewed appreciation of your senses can actually help one get out of an emotional rut.
Five senses? More than ten!
The categorization of our five primary senses (sight, smell, hearing, taste and touch) is attributed to Aristotle. While this basic list is still valid, humans have a number of additional ‘sensory abilities’ not covered above. These secondary senses include:
- Sense of balance and acceleration– the ability to sense body movement, direction and acceleration, and to maintain balance and equilibrium.
- Temperature sense– the ability to sense heat and the absence of heat (cold).
- Sense of Pain– the sense of pain was previously believed to be an overloading of pressure receptors, but it has since been identified as a distinct phenomenon that intertwines with the other senses, including touch.
- Sense of Time– the ability to perceive the passage of time, both short passages as well as longer time cycles.
We are gifted with a complex system of basic and intertwined senses designed to help us take it all in….remember to enjoy and appreciate them all today!