Sense

• 1. Introduction

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  Close your eyes in a dark room and press gently on the outside corner of one eye through the eyelid. A spot of light appears toward the inside of your visual field, near your nose. No light entered the eye, yet you saw something. This small trick reveals a strange truth about a sense. Perception happens when nerves leading from a sense organ to the brain are stimulated, even if that stimulation has nothing to do with light, sound, or smell. A sense is a biological system an organism uses to gather information about its surroundings by detecting stimuli. Sense organs collect a sound or a smell, then transduce it, transforming the stimulus into a form the brain can read. How many of these systems does a human actually have? Why do some animals feel magnetic fields or hear a plant's distress? And what happens at the molecular gate where the world becomes a nerve impulse? The traditional count is five. The real number is far larger, and the story begins with how the body draws the line between the world outside and the body within.

• 2. Beyond The Five

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  In the time of William Shakespeare, people commonly reckoned five wits or five senses. The words sense and wit were synonyms then, so sight, smell, touch, taste, and hearing were known as the five outward wits. Hindu literature enumerated the same group as the five material faculties. They appear as early as the Katha Upanishad, roughly the 6th century BC, drawn as five horses pulling the chariot of the body, with the mind as chariot driver. Seventeenth-century Dutch and Flemish Baroque painters made the five senses a favorite allegory. In Gerard de Lairesse's Allegory of the Five Senses from 1668, sight is a reclining boy with a convex mirror, hearing is a cupid-like boy with a triangle, and touch is a woman holding a bird. Buddhist philosophy breaks from the count of five. In its idea of Ayatana, or sense-base, the mind itself is treated as a sense organ alongside the other five. Modern science pushes the number much higher. Listing all the sensory modalities can reach as many as 17, and human absolute thresholds have been tabulated for somewhere between nine and 21 external senses. The major senses split into submodalities. Touch alone, known as somatosensation, separates into light pressure, deep pressure, vibration, itch, pain, temperature, and hair movement, each a sensation of its own.

• 3. Inside The Body

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  Interoception is any sense that is normally stimulated from within the body. These rely on sensory receptors buried in internal organs, and they are thought to be atypical in clinical conditions such as alexithymia. Hunger is governed by brain structures including the hypothalamus, which manage the body's energy balance. Peripheral chemoreceptors monitor carbon dioxide and oxygen levels, producing a perception of suffocation when carbon dioxide climbs too high. Chemoreceptors in the circulatory system measure salt and prompt thirst when levels rise, and they can respond to high blood sugar in diabetics. The chemoreceptor trigger zone sits in the medulla and communicates with the vomiting center. Stretch receptors do quiet, constant work the conscious mind rarely notices. Pulmonary stretch receptors in the lungs help control the rate of breathing. Stretch receptors in the gastrointestinal tract sense gas distension that can become colic pain, while sensors in the bladder and rectum register fullness. Baroreceptors relay blood pressure to the brain to hold it steady. Cardioception is the perception of the heart's own activity. The perception of time is also sometimes counted as a sense, though it is tied to no specific receptor at all.

• 4. The Gate Of Transduction

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  Sensory receptors are the cells and structures that detect sensation, and they can be sorted by three criteria: cell type, position, and function. By location, an exteroceptor sits near a stimulus of the external world, like the touch receptors in the skin. An interoceptor reads internal organs, such as the receptors that sense rising blood pressure in the aorta or carotid sinus. By cell type, the receptor may be a neuron with a free nerve ending, a neuron with an encapsulated ending wrapped in connective tissue, or a specialized receptor cell. Pain and temperature receptors in the dermis are free nerve endings. Lamellated corpuscles, also in the dermis, are encapsulated endings that respond to pressure and touch. The light-sensing cells of the retina are specialized receptors called photoreceptors. By function, receptors are named for what they transduce. Mechanoreceptors read pressure, vibration, sound, and body position. Photoreceptors convert visible light into signals. Chemoreceptors interpret taste and smell, osmoreceptors track the solute concentration of body fluids, thermoreceptors sense heat or cold, and nociceptors register tissue damage. A transmembrane protein receptor mediates change in a neuron, often by opening ion channels, and is switched on by chemicals called ligands. A molecule in food, for instance, can act as a ligand for a taste receptor. For humans, the only electromagnetic energy the eyes perceive is visible light. Other organisms carry receptors people lack, such as the heat sensors of snakes, the ultraviolet sensors of bees, and the magnetic receptors of migratory birds.

• 5. The Limits Of Detection

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  Every sense organ needs a minimum amount of stimulation before it registers anything. That minimum is the absolute threshold, defined as the smallest stimulus a subject can detect 50 percent of the time. The classic tabulated examples are startling in their delicacy. Vision can catch a candle flame 48 kilometers, about 30 miles, away on a dark, clear night. Hearing can pick up a watch ticking 6 meters away in silence. Smell can register a single drop of perfume diffused through three rooms, and taste can detect a teaspoon of sugar in 7.5 liters of water. Past the absolute threshold lies the differential threshold, the just noticeable difference, which is the smallest gap between two stimuli that a person can tell apart. Weber's Law states that this difference is a constant fraction of the comparison stimulus, so bigger stimuli demand bigger differences before anyone notices. Signal detection theory looks at how a subject responds to a stimulus buried in noise. Internal noise is static in the nervous system itself. A person with closed eyes in a dark room still sees a blotchy grey pattern with brighter flashes, which is that internal noise made visible. The nervous system sets a criterion, an internal threshold, and judges any signal that clears it as real. Shift that criterion and you change the odds of false positives and false negatives. Not every sense feels the same to everyone. The molecule propylthiouracil, or PROP, tastes bitter to some people, nearly tasteless to others, and somewhere in between to the rest, a difference with a genetic basis that shapes food preferences and health.

• 6. How The Ear And Eye Work

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  The large fleshy structure on the side of the head is the auricle, the visible start of a chain that turns vibrating air into a nerve signal. At the end of the auditory canal sits the tympanic membrane, the eardrum, which trembles when sound waves strike it. Behind it, the middle ear holds three tiny bones called the ossicles: the malleus, incus, and stapes, Latin names that translate roughly to hammer, anvil, and stirrup. The stapes passes the motion to the inner ear, where hair-like fibers detect mechanical motion across a range of about 20 to 20,000 hertz, with hearing at high frequencies declining as a person ages. The eye runs on a different physics. Photoreceptors in the retina come in two forms, rods and cones. Rods are very sensitive to light but read no color, while cones distinguish color yet fail in dim light, which is why a dark room appears in shades of grey. Visible light is electromagnetic radiation with a wavelength between 380 and 720 nanometers, where 380 reads as blue and 720 as dark red. Three types of cone opsins, each tuned to different wavelengths, give color vision, and the brain extracts color by comparing how strongly the three cones fire. Some people lose vision in an unusual way. Those whose visual cortex is damaged but whose eyes still work can react to visual stimuli without conscious awareness, a state called blindsight. On the 14th of February 2013, researchers built a neural implant that let rats sense infrared light, the first time living creatures were given a new ability rather than a repaired one.

• 7. Senses Humans Lack

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  Mantis shrimps carry twelve distinct kinds of color receptors, against three in humans and two in most mammals, and they perceive both polarized light and multispectral images. The animal world is full of such instruments people simply do not have. Pit vipers, pythons, and some boas detect infrared light and sense the body heat of their prey. A blind rattlesnake can target a vulnerable body part of prey at wavelengths between 5 and 30 micrometers. The pit's mechanism is not a photoreceptor at all but a temperature-sensitive ion channel that fires when incoming radiation warms the thin pit membrane. Magnetoception, the ability to read direction from the Earth's magnetic field, guides birds during migration and has been seen in bees. Cattle align themselves north to south, and magnetotactic bacteria build miniature magnets inside their own bodies to find their orientation. Electroreception lets several fish, sharks, and rays sense electric fields nearby, and in cartilaginous fish this works through an organ called the ampullae of Lorenzini. Among mammals only the dolphin and monotreme orders are known to do it, and the platypus has the sharpest version. A dolphin reads electric fields through electroreceptors in vibrissal crypts on its snout, evolved from whisker motion sensors, sensitive to fields as weak as 4.6 microvolts per centimeter, enough to find prey hidden in seafloor sediment. Plants sense light, temperature, humidity, magnetic fields, and tissue damage without any nervous system, responding through hormonal and cell-to-cell pathways. Under stress they may emit airborne sounds like screaming, too high for human ears, that a mouse or bat could hear from as far as 15 feet away.