Making sense of our senses

Thousands of Filipinos visit their doctors each year for help with sight, hearing, smell, touch, and taste problems.  In some patients, a change in intensity or the absence of one or more of these senses may simply be related to aging.  In others, problems may be caused by certain health conditions, medications or medical treatments.

Although most of these disorders aren’t life-threatening, they can affect your quality of life and potentially have an impact on your health.  For instance, when these senses are impaired, you may eat poorly, socialize less, and be more prone to depression.  A loss of these senses can also make you more vulnerable to accidents because you’re less able to detect common warning signs, such as the smoky smell of a fire or the unusual taste of spoiled food.  If you detect a change in any one of your five senses, it’s important to tell your doctor.  Many types of these disorders can be treated, or possibly reversed, depending on the cause.

To better understand these senses, here is the latest science behind sight, smell, taste, hearing, and touch.

A sight to behold

The human eye is an amazing tool for gathering light.  It can distinguish 500 shades of gray and spot the light of a candle 14 miles away.  But for all its extraordinary ability, seeing is a function of the brain — humans’ visual cortex is more developed than that of any other mammal.

Vision demands that the brain differentiates foreground from background, and edges from lines — skills that even the world’s most powerful computers have been unable to match.  Your brain makes sense of shapes and symbols by putting them together like a jigsaw puzzle, organizing fragments into a coherent whole.  Optical illusions work by exploiting the mind’s tendency to try to find order in patterns.

Here are four things you may not know about your sense of sight:

1. Having a 20/20 vision, the standard for normal acuity, means you can clearly see an image — like the letters of an eye chart — from a distance of 20 feet. Having 20/100 vision means you need to be five times closer, just four feet away, to see the same letter clearly.

2. The world record for human vision was set by Dr. Dennis Levi in 1985. He was able to identify a bright line a quarter of an inch thick from one mile away.

3. Sitting too close to TV may give you a headache, but it won’t ruin your vision. The same is true of reading in dim light.

4. One in 20 men is at least partially color-blind, and color-blindness is 10 times more common in men than in women.  All babies are color-blind at birth.

Do you hear what i hear?

At birth, our ears are pristine organs, capable of discerning among more than 300,000 sounds.  (After years of exposure to loud noises, the hair cells on the cochlea, in the inner ear, flatten, becoming less sensitive.)  Yet even as we age, our eardrums remain finely tuned — they can pick up sounds so faint that the eardrum itself moves a distance less than the diameter of a hydrogen molecule. (This sensitivity developed to protect us from predators.)

Researchers have found that living in loud areas can raise blood pressure by an average of five to eight percent. The word noise comes from nausea, the Latin word for sickness.  But sound can also be a positive force. A study conducted on premature infants, for instance, found that they were able to leave the hospital earlier if soft music was played while they slept.

Here are five things you may not know about your sense of hearing:

1. Even small noises cause the pupil of the eye to dilate.  This may be why surgeons and others who perform delicate manual operations tend to be bothered by uninvited noise:  It subtly blurs their vision.

2. A large meal will temporarily make your hearing less acute.

3. Your ears can determine the direction from which a sound comes quite well, but are less adept at assessing how far away the source is.

4. Ninety percent of a young child’s knowledge is attributed to hearing background conversation.  More than a third of children with even slight hearing loss, researchers estimate, will fail at least one grade.

5. Tinnitus — a buzzing or ringing sound in the ears — affects roughly 15 percent of the population.   The condition is ancient; it’s described on clay tablets from Assyria.

Taste the difference

Taste, designed in part to help us reject harmful foods, has long served as the body’s primary defense against poison.  As befits its role, the system is lightning fast:  The body can detect taste in as little as 0.0015 seconds, compared with 0.0024 seconds for touch and 0.013 for vision.

To be tasted, food molecules must fall into a cluster of cells called a taste bud, more than 10,000 of which are spread over the tongue, palate, and inner cheek.  The taste bud sends a signal to the brain, which then determines everything from whether a food tastes “safe” — which in most instances means it’s not too bitter — to whether we enjoy it.  In a part of the brain known as the anterior cingulate, taste is connected to an emotional response: disgust for rotten meat, say, or delight for a sweet mango.  But while some tastes are innate — nearly all humans are born with a sweet tooth — there’s also evidence that taste can be nurtured.

Here are four things you may not know about your sense of taste:

1. The taste map you learned growing up (the one that showed the tip of the tongue registering sweet flavors; the back, bitter; and the sides, salty and sour) is a myth (see diagram on Page E-1). While receptors for the five basic tastes do exist, they’re not confined to specific areas but are distributed across the tongue.

2. French nutritionist Philippe Besnard recently discovered taste buds that seem to respond specifically to the flavor of fat.

3. It is believed that roughly one-quarter of all people are “super-tasters,” and another quarter are “non-tasters.”  Super-tasters have more taste buds and are often sensitive to bitter foods.  Non-tasters have fewer taste buds and tend to have high tolerance for spicy foods.

4. Your taste buds die off and degenerate every few days.  As you age, the cycle slows, dulling your ability to taste — which explains why older people tend to like their food saltier and spicier.

A touch of class

Of all your senses, touch is the most difficult to fathom without.  With hundreds of nerve endings in every square inch of skin, your body functions like an antenna, receiving a constant stream of information ranging from the firmness of the chair you’re sitting on to the heat of the sun through the window.

Touch is the first sense we develop in utero, and it is crucial to survival.  Babies can die from lack of it, and as adults, touch helps to protect us from harm.  Some nerves are specialized to feel texture, others detect temperature or register pain.  Nerves known as proprioceptors sense the position of our body parts in space — enabling us to cross our arms with ease, or lean out of a window without falling.

Here are five things you may not know about your sense of touch:

1. The skin is your body’s largest organ and contains more than four million sensory receptors.

2. Among the body’s most sensitive areas are the lips, the back of the neck, the finger tips, and the soles of the feet. The least sensitive is the middle of your back.

3. Being touched can reduce stress, by lowering levels of hormones like cortisol.

4. Pain is the body’s warning system, and it’s thorough: People have more receptors for pain than any other sensation.

5. Thermoreceptors perceive sensations related to temperature.  But they stop being stimulated when the surface of the skin drops below 41°F (which is why your skin starts to feel numb in icy temperatures) or rises above 113°F (at which point pain receptors take over to avoid burns).

The nose knows

Smell may be our most evocative sense. Studies have shown that people can recall a scent with 65-percent accuracy after one year; visual memory sinks to 50 percent after just a few months.  And because smells are processed by the same part of the brain that handles memories and emotions — the temporal lobe — we respond to them with rare intensity.  Decades later, a passing scent may summon a memory of a past place or event, one so vivid we seem transported across time and space.

Though not on a par with our canine friends, the human nose is still something of a marvel (see diagram on Page E-1).  An ordinary person can pick up a whiff of skunk when the amount of scent in the air is less than one 10-trillionth of an ounce.  The nose can also determine where a smell is coming from, pointing you — for better or for worse – toward the source.  Still, our sense of smell is deeply individual:  Some people can’t smell mushrooms; others can’t smell roses.  These differences are mostly genetic, but simpler things — small physiological changes and factors like mood and medications (antibiotics, statins, and blood pressure drugs) can affect our sense of smell — enhancing or diminishing our ability to detect odors.  In fact, it’s believed that we never experience a smell the same way twice, since the sensitivity of our nose changes from hour to hour and day to day.

Here are five things you may not know about your sense of smell:      

1. In general, your sense of smell is weakest in the morning and grows stronger as the day wears on.

2. Smells can affect your behavior.  A recent study showed that people sitting in a citrus-scented room cooperated more in trust experiments and even offered more charitable donations.

3. Your sense of smell becomes more acute when you’re hungry.

4. The ability to detect scents is boosted by estrogen, which is why women (and especially pregnant women) tend to have more sensitive noses than men.

5. Astronauts in space often lose their sense of smell and taste.  Because of the lack of gravity, their sinuses fill up with fluid, causing stuffiness like from a cold.

The good news is, many changes in our senses can often be successfully treated.