Every human has limits. You can only run so fast, jump so high, and go for so long without water. But what about restrictions upon our five senses, those tools that we use to perceive and understand our surroundings? Here are ten limitations on human perception that have a direct impact on how we understand the world.
About one quarter of the human brain is involved in visual processing - more than any other sense. Arguably the most closely studied of the five main senses, the Society for Neuroscience claims that more is known about vision than any other vertebrate sensory system.
10. Field of View
A pair of healthy human eyes has a total field of view of approximately 200 degrees horizontally — about 120 degrees of which are shared by both eyes, giving rise to what's known as binocular vision — and 135 degrees vertically, (though these values tend to decrease with age). This is due to the fact that both of our eyes are positioned more or less on the front of our heads, as opposed to the sides.
Having eyes positioned on the sides of one's head is common in prey species, and while it certainly increases an animal's total field of view, it's often at the expense of sharper binocular vision (see the helpful chart featured here, which shows the differences in vision between pigeons, whose eyes reside on the sides of their heads, and owls, who, like humans, sport front-facing peepers). Then again, if one of your primary concerns as an animal is to avoid being eaten (as opposed to seeing what, precisely, is trying to eat you), that's a pretty reasonable tradeoff.
9. Angular Resolution
Angular resolution is one of the terms used to describe an optical device's ability to distinguish very small details. If you want to talk about the smallest thing perceivable by the human eye, it makes sense to do so in terms of angular resolution.
Angular resolution is commonly measured in units known as arc minutes and arc seconds, which correspond to 1/60th and 1/3600th of a single degree in your field of view, respectively. The typical set of human eyes has an angular resolution on the order of one arc minute, give or take a few arc seconds. If you were to draw a line measuring a third of a millimeter wide on a piece of paper and hold it at arm's length, the line would cover about 1 arc minute of your vision.
8. The Blind Spot
The human eye is lined with photoreceptor cells that it uses to perceive light. Visual information received by these photoreceptor cells is relayed to the brain via the optic nerve. The only problem is that the optic nerve actually passes through part of photoreceptors lining the inside of the eye, creating a small, receptor-less patch where it's impossible to detect light.
Normally this isn't an issue. We've got two eyes, and our brains are incredibly good at using the visual information gathered from each eye to fill in the gaps left by the other's blind spot. But things get screwy when you have to rely on just one eye. Try this optical illusion on for size to see what happens when your brain can't find the visual information lost to your eye's blind spot.
7. The "Visible" Spectrum
Probably the most well-known of human sensory limitations, the typical human eye is only capable of perceiving light at wavelengths between 390 and 750 nanometers. Of course, calling it the "visible" spectrum is a bit of a misnomer, as plenty of animals are capable of perceiving light with frequencies outside this relatively narrow band of electromagnetic radiation.
Commonly listed alongside vision as one of the most important of the human senses, hearing is is a vital part of everything from communication to risk-avoidance.
6. Hearing Range
Among young, healthy humans, the range of frequencies that can be picked up by the human ear is usually cited as 20 — 20,000 Hz; however, the upper limit on that range tends to decrease pretty steadily with age.
5. Absolute Threshold of Hearing
Your absolute threshold of hearing is the quietest sound your ears are capable of picking up when there are no other sounds around to mask its perception. This threshold varies from person to person, changes with age, and is largely dependent on the frequency of the noise being perceived. It's also quieter than you might think.
This chart, borrowed from a comparative analysis of published threshold data, shows how the lower limit of a person's hearing threshold (measured in decibels) changes as a function of frequency. In this particular graph, data has been collected on test subjects between the ages of 18 and 55, tested at frequencies between 125 and 12,000 Hz, to show that the lower limit of detectable sound is not zero decibels (as it is commonly depicted in figures like the one below), but around -5 decibels.
That being said, as the percentiles listed on the right hand side of the chart make clear, the ability to hear noises as quiet as -5 dB is pretty rare (among males and females alike, only around one person in ten will be able to hear sounds quieter than zero decibels). In actuality, the average hearing threshold is, in fact, between 0 and 5 decibels.
These two senses rely on different sensory organs, but are very closely related; when someone loses his or her sense of smell, for example, their sense of taste is dramatically diminished.
4. Limitations in Wine Tasting
The sense of taste is arguably the weakest of the human senses. This is something we've talked about before; your ability to "taste" wine, for example, is actually more dependent upon your sense of smell. Here's what we had to say about the limitations of taste back in March:
In what is hands down my favorite experiment on the limitations of the human palate ever performed, researcher Frédéric Brochet invited 57 wine experts to give their opinions on what appeared to be two glasses of wine - one red, and one white. The wines were actually the exact same white wine; the "red" had simply been mixed with red food coloring.
The experts proceeded to describe the "red" wine in language typically reserved for characterizing reds, noting, for example, its "jamminess," or the flavors imparted by its "crushed red fruit." Incredibly, not a single expert noticed that it was, in fact, a white wine.
However, there is also evidence for the existence of so-called "supertasters" — i.e. people who are unusually sensitive to what are known as the "basic tastes," namely bitter, sweet, sour and salty.
These supertasters were discovered by Linda Bartoshuk (a pioneer in the field of psychophysics, the study of how stimuli such as taste lead to subjective experience), who demonstrated that their sensitive tastes were correlated with higher densities of fungiform papillae, the bumps on the tongue containing taste buds. That being said, the ability to objectively gauge intensity of flavor perception has remained a significant challenge in the world of psychophysics, making absolute threshold tests (like those used to determine the limits of human hearing) difficult to perform. [Supertaster image via Science]
2. Odor Detection Threshold
Like taste thresholds, the limits of odor detection have proven difficult to pin down. Writing in the journal Chemical Senses, Dr. Thomas Hummel (an ear nose and throat doctor) describes some of the challenges involved:
Tests for the assessment of olfactory functions are numerous. However, in the clinical practice of otorhinolaryngology [the study of ear, nose and throat function] of neurology few, if any, of them are actually used. The reasons may be found int he inconsistency of some tests, the lack of normative data, the time needed for adnimistration and the limited availability of these tests.
The sense of touch is remarkably complex, and involves the detection of everything from pressure, to itchiness, to temperature. Most of these sensations and their mechanisms remain poorly understood, but are thought to involve a range of nerves in the skin capable of responding to various forms of stimuli. So-called "Merkel disk receptors," for example, are involved in the perception of pressure; while "Rufini corpuscles" are believed to detect the sensation we associate with stretching.
1. Two-point Discrimination
Here's something we do know for sure about the sense of touch: it tends to be keenest in regions of the body that are densely populated with sensory neurons. One of the simplest ways of demonstrating this relationship is with something called the two point discrimination test. Best of all? You can try it out at home. You'll need a partner, but here's a simple experiment designed by neuroscientist Marjorie A. Murray:
Bend a paper clip into the shape of a U with the tips about 2 cm apart. Make sure the tips of the U are even with each other. Lightly touch the two ends of the paper clip to the back of the hand of your subject. Your subject should not look at the area of skin that is being tested. Do not press too hard! Make sure both tips touch the skin at the same time. Ask your subject if he or she felt one or two pressure points. If your subject reported one point, spread the tips of the clip a bit further apart, then touch the back of the subject's hand again. If your subject reported 2 points, push the tips a bit closer together, and test again. Measure the distance at which the subject reports "I feel two points."
Try the two point discrimination test on different parts of the body (fingers, cheek, forehead, foot, stomach, calf, back, etc.), and compare the distances required to elicit the sensation of two distinct points for each body region. When you're finished, your results should wind up being similar to the ones listed here.
"The receptors in our skin are NOT distributed in a uniform way around our bodies," writes Murray.
"Some places, such as our fingers and lips, have more touch receptors than other parts of our body, such as our backs. That's one reason why we are more sensitive to touch on our fingers and face than on our backs."