Vision: The Visual System, the Eye, and Color Vision
In the human visual system, the eye receives physical stimuli in the form of light and sends those stimuli as electrical signals to the brain, which interprets the signals as images.
Summarize the process by which visual information is communicated to the brain
- Human vision is one of the most complex visual systems among animals.
- The main sensory organ of the visual system is the eye, which takes in the physical stimuli of light rays and transduces them into electrical and chemical signals that can be interpreted by the brain to construct physical images.
- The eye has three main layers: the sclera, which includes the cornea; the choroid, which includes the pupil, iris, and lens; and the retina, which includes receptor cells called rods and cones.
- The human visual system is capable of complex color perception, which is initiated by cones in the retina and completed by impulse integration in the brain.
- Depth perception is our ability to see in three dimensions and relies on both binocular (two-eye) and monocular (one-eye) cues.
- phototransduction: The process whereby the various bodies in the retina convert light into electrical signals.
- retina: The thin layer of cells at the back of the eyeball where light is converted into neural signals sent to the brain.
- photoreceptor: A specialized neuron able to detect and react to light. Includes both cones (daytime and color) and rods (nighttime).
The human visual system gives our bodies the ability to see our physical environment. The system requires communication between its major sensory organ (the eye) and the core of the central nervous system (the brain) to interpret external stimuli (light waves) as images. Humans are highly visual creatures compared to many other animals which rely more on smell or hearing, and over our evolutionary history we have developed an incredibly complex sight system.
Vision depends mainly on one sensory organ—the eye. Eye constructions vary in complexity depending on the needs of the organism. The human eye is one of the most complicated structures on earth, and it requires many components to allow our advanced visual capabilities. The eye has three major layers:
- the sclera, which maintains, protects, and supports the shape of the eye and includes the cornea;
- the choroid, which provides oxygen and nourishment to the eye and includes the pupil, iris, and lens; and
- the retina, which allows us to piece images together and includes cones and rods.
The Process of Sight
All vision is based on the perception of electromagnetic rays. These rays pass through the cornea in the form of light; the cornea focuses the rays as they enter the eye through the pupil, the black aperture at the front of the eye. The pupil acts as a gatekeeper, allowing as much or as little light to enter as is necessary to see an image properly. The pigmented area around the pupil is the iris. Along with supplying a person’s eye color, the iris is responsible for acting as the pupil’s stop, or sphincter. Two layers of iris muscles contract or dilate the pupil to change the amount of light that enters the eye. Behind the pupil is the lens, which is similar in shape and function to a camera lens. Together with the cornea, the lens adjusts the focal length of the image being seen onto the back of the eye, the retina. Visual reception occurs at the retina where photoreceptor cells called cones and rods give an image color and shadow. The image is transduced into neural impulses and then transferred through the optic nerve to the rest of the brain for processing. The visual cortex in the brain interprets the image to extract form, meaning, memory, and context.
The left hemisphere of the brain controls the motor functions of the right half of the body, and vice versa; the same is true of vision. The left hemisphere of the brain processes visual images from the right-hand side of space, or the right visual field, and the right hemisphere processes visual images from the left-hand side of space, or the left visual field. The optic chiasm is a complicated crossover of optic nerve fibers behind the eyes at the bottom of the brain, allowing the right eye to “wire” to the left neural hemisphere and the left eye to “wire” to the right hemisphere. This allows the visual cortex to receive the same visual field from both eyes.
Human beings are capable of highly complex vision that allows us to perceive colors and depth in intricate detail. Visual stimulus transduction happens in the retina. Photoreceptor cells found in this region have the specialized capability of phototransduction, or the ability to convert light into electrical signals. There are two types of these photoreceptor cells: rods, which are responsible for scotopic vision (night vision), and cones, which are responsible for photopic vision (daytime vision).
Generally speaking, cones are for color vision and rods are for shadows and light differences. The front of your eye has many more cones than rods, while the sides have more rods than cones; for this reason, your peripheral vision is sharper than your direct vision in the darkness, but your peripheral vision is also in black and white.
Color vision is a critical component of human vision and plays an important role in both perception and communication. Color sensors are found within cones, which respond to relatively broad color bands in the three basic regions of red, green, and blue (RGB). Any colors in between these three are perceived as different linear combinations of RGB. The eye is much more sensitive to overall light and color intensity than changes in the color itself. Colors have three attributes: brightness, based on luminance and reflectivity; saturation, based on the amount of white present; and hue, based on color combinations. Sophisticated combinations of these receptors signals are transduced into chemical and electrical signals, which are sent to the brain for the dynamic process of color perception.
Depth perception refers to our ability to see the world in three dimensions. With this ability, we can interact with the physical world by accurately gauging the distance to a given object. While depth perception is often attributed to binocular vision (vision from two eyes), it also relies heavily on monocular cues (cues from only one eye) to function properly. These cues range from the convergence of our eyes and accommodation of the lens to optical flow and motion.
Audition: Hearing, the Ear, and Sound Localization
The human auditory system allows us to perceive and localize sounds in our physical environment.
Outline the processes and structures involved in audition
- The human sense of hearing is attributed to the auditory system, which uses the ear to collect, amplify, and transduce sound waves into electrical impulses that allow the brain to perceive and localize sounds.
- The ear can be divided into the outer ear, middle ear, and inner ear, each of which has a specific function in the process of hearing.
- The outer ear is responsible for the collection and amplification of sound. The air-filled middle ear transforms sound waves into vibrations, protecting the inner ear from damage. The fluid-filled inner ear transduces sound vibrations into neural signals that are sent to the brain for processing.
- The cochlea is the major sensory organ of hearing within the inner ear. Hair cells within the cochlea perform the transduction of sound waves.
- Humans are capable of estimating a sound’s origin through a process called sound localization, which relies on timing and intensity differences in sound waves collected by each of our two ears.
- afferent: Leading to the brain.
- interaural: Describing the differences between the reception of sound (especially timing and intensity) by each ear.
The human auditory system allows the body to collect and interpret sound waves into meaningful messages. The main sensory organ responsible for the ability to hear is the ear, which can be broken down into the outer ear, middle ear, and inner ear. The inner ear contains the receptor cells necessary for both hearing and equilibrium maintenance. Human beings also have the special ability of being able to estimate where sounds originate from, commonly called sound localization.
The ear is the main sensory organ of the auditory system. It performs the first processing of sound and houses all of the sensory receptors required for hearing. The ear’s three divisions (outer, middle, and inner) have specialized functions that combine to allow us to hear.
The outer ear is the external portion of the ear, much of which can be seen on the outside of the human head. It includes the pinna, the ear canal, and the most superficial layer of the ear drum, the tympanic membrane. The outer ear’s main task is to gather sound energy and amplify sound pressure. The pinna, the fold of cartilage that surrounds the ear canal, reflects and attenuates sound waves, which helps the brain determine the location of the sound. The sound waves enter the ear canal, which amplifies the sound into the ear drum. Once the wave has vibrated the tympanic membrane, sound enters the middle ear.
The middle ear is an air-filled tympanic (drum-like) cavity that transmits acoustic energy from the ear canal to the cochlea in the inner ear. This is accomplished by a series of three bones in the middle ear: the malleus, the incus, and the stapes. The malleus (Latin for “hammer”) is connected to the mobile portion of the ear drum. It senses sound vibrations and transfers them onto the incus. The incus (Latin for “anvil”) is the bridge between the malleus and the stapes. The stapes (Latin for “stirrup”) transfers the vibrations from the incus to the oval window, the portion of the inner ear to which it is connected. Through these steps, the middle ear acts as a gatekeeper to the inner ear, protecting it from damage by loud sounds.
Unlike the middle ear, the inner ear is filled with fluid. When the stapes footplate pushes down on the oval window in the inner ear, it causes movement in the fluid within the cochlea. The function of the cochlea is to transform mechanical sound waves into electrical or neural signals for use in the brain. Within the cochlea there are three fluid-filled spaces: the tympanic canal, the vestibular canal, and the middle canal. Fluid movement within these canals stimulates hair cells of the organ of Corti, a ribbon of sensory cells along the cochlea. These hair cells transform the fluid waves into electrical impulses using cilia, a specialized type of mechanosensor.
The Process of Hearing
Hearing begins with pressure waves hitting the auditory canal and ends when the brain perceives sounds. Sound reception occurs at the ears, where the pinna collects, reflects, attenuates, or amplifies sound waves. These waves travel along the auditory canal until they reach the ear drum, which vibrates in response to the change in pressure caused by the waves. The vibrations of the ear drum cause oscillations in the three bones in the middle ear, the last of which sets the fluid in the cochlea in motion. The cochlea separates sounds according to their place on the frequency spectrum. Hair cells in the cochlea perform the transduction of these sound waves into afferent electrical impulses. Auditory nerve fibers connected to the hair cells form the spiral ganglion, which transmits the electrical signals along the auditory nerve and eventually on to the brain stem. The brain responds to these separate frequencies and composes a complete sound from them.
Humans are able to hear a wide variety of sound frequencies, from approximately 20 to 20,000 Hz. Our ability to judge or estimate where a sound originates, called sound localization, is dependent on the hearing ability of each ear and the exact quality of the sound. Since each ear lies on an opposite side of the head, a sound reaches the closest ear first, and the sound’s amplitude will be larger (and therefore louder) in that ear. Much of the brain’s ability to localize sound depends on these interaural (between-the-ears) differences in sound intensity and timing. Bushy neurons can resolve time differences as small as ten milliseconds, or approximately the time it takes for sound to pass one ear and reach the other.
Gustation: Taste Buds and Taste
The gustatory system, including the mouth, tongue, and taste buds, allows us to transduce chemical molecules into specific taste sensations.
Compare the structural similarities and differences among types of taste buds
- The gustatory system uses a form of chemoreception that allows the human body to interpret chemical compounds in ingested substances as specific tastes.
- There are five main types of taste sensations: bitter, salty, sweet, sour, and umami (savory).
- Taste sensations are transduced by taste cells located in bunches called taste buds. They are found throughout the entire mouth but are most highly concentrated on the tongue, the major sensory organ of the gustatory system.
- While taste buds may differ slightly in location and sensation, they react to all five different types of tastes. Generally speaking, taste serves to create either an appetite for or an aversion to a substance.
- gustducin: A protein associated with the sensation of taste.
- tastant: Any substance that stimulates the sense of taste.
- umami: One of the five basic tastes, the savory taste of foods such as seaweed, cured fish, aged cheeses, and meats.
The gustatory system creates the human sense of taste, allowing us to perceive different flavors from substances that we consume as food and drink. Gustation, along with olfaction (the sense of smell), is classified as chemoreception because it functions by reacting with molecular chemical compounds in a given substance. Specialized cells in the gustatory system that are located on the tongue are called taste buds, and they sense tastants (taste molecules). The taste buds send the information from the tastants to the brain, where a molecule is processed as a certain taste. There are five main tastes: bitter, salty, sweet, sour, and umami (savory). All the varieties of flavor we experience are a combination of some or all of these tastes.
Tongue and Taste Buds
The sense of taste is transduced by taste buds, which are clusters of 50-100 taste receptor cells located in the tongue, soft palate, epiglottis, pharynx, and esophagus. The tongue is the main sensory organ of the gustatory system. The tongue contains papillae, or specialized epithelial cells, which have taste buds on their surface. There are three types of papillae with taste buds in the human gustatory system:
- fungiform papillae, which are mushroom-shaped and located at the tip of the tongue;
- foliate papillae, which are ridges and grooves toward the back of the tongue;
- circumvallate papillae, which are circular-shaped and located in a row just in front of the end of the tongue.
Each taste bud is flask-like in shape and formed by two types of cells: supporting cells and gustatory cells. Gustatory cells are short-lived and are continuously regenerating. They each contain a taste pore at the surface of the tongue which is the site of sensory transduction. Though there are small differences in sensation, all taste buds, no matter their location, can respond to all types of taste.
Traditionally, humans were thought to have just four main tastes: bitter, salty, sweet, and sour. Recently, umami, which is the Japanese word for “savory,” was added to this list of basic tastes. (Spicy is not a basic taste because the sensation of spicy foods does not come from taste buds but rather from heat and pain receptors.) In general, tastes can be appetitive (pleasant) or aversive (unpleasant), depending on the unique makeup of the material being tasted.There is one type of taste receptor for each flavor, and each type of taste stimulus is transduced by a different mechanism. Bitter, sweet, and umami tastes use similar mechanisms based on a G protein-coupled receptor, or GPCR.
There are several classes of bitter compounds which vary in chemical makeup. The human body has evolved a particularly sophisticated sense for bitter substances and can distinguish between the many radically different compounds that produce a bitter response. Evolutionary psychologists believe this to be a result of the role of bitterness in human survival: some bitter-tasting compounds can be hazardous to our health, so we learned to recognize and avoid bitter substances in general.
The salt receptor, NaCl, is arguable the simplest of all the receptors found in the mouth. An ion channel in the taste cell wall allows Na+ ions to enter the cell. This depolarizes the cell and floods it with ions, leading to a neurotransmitter release.
Like bitter tastes, sweet taste transduction involves GPCRs binding. The specific mechanism depends on the specific molecule flavor. Natural sweeteners such as saccharides activate the GPCRs to release gustducin. Synthetic sweeteners such as saccharin activate a separate set of GPCRs, initiating a similar but different process of protein transitions.
Sour tastes signal the presence of acidic compounds in substances. There are three different receptor proteins at work in a sour taste. The first is a simple ion channel which allows hydrogen ions to flow directly into the cell. The second is a K+ channel which has H+ ions in order to block K+ ions from escaping the cell. The third allows sodium ions to flow down the concentration gradient into the cell. This involvement with sodium ions implies a relationship between salty and sour tastes receptors.
Umami is the newest receptor to be recognized by western scientists in the family of basic tastes. This Japanese word means “savory” or “meaty.” It is thought that umami receptors act similarly to bitter and sweet receptors (involving GPCRs), but very little is known about their actual function. We do know that umami detects glutamates that are common in meats, cheese, and other protein-heavy foods and reacts specifically to foods treated with MSG.
Olfaction: The Nasal Cavity and Smell
The olfactory system gives humans their sense of smell by collecting odorants from the environment and transducing them into neural signals.
Summarize the structure and function of the olfactory system
- Olfaction is a type of chemoreception. Like gustation, this sensory system uses the molecular chemical compounds in substances (in this case, in odorants ) to discern information about the environment.
- The main sensory organ responsible for the human sense of smell is the nasal cavity, which contains olfactory receptors that perform the transduction of odors into neural impulses.
- Human beings can detect a large and diverse number of smells due to the vast number of features and combinations of odor molecules.
- Olfaction is the sense most closely tied to memory because of its close neural connections to areas of the brain responsible for emotion and place memory.
- orbitofrontal: Located in the frontal lobes above the eyes.
- pheromone: A chemical secreted by an animal, especially an insect, that affects the development or behavior of other members of the same species; functions often as a means of attracting a member of the opposite sex.
- odorant: Any substance that has a distinctive smell, especially one added to another substance (such as household gas) for safety purposes.
- mucosa: The membrane where olfactory receptor cells are located.
The olfactory system gives humans their sense of smell by inhaling and detecting odorants in the environment. Olfaction is physiologically related to gustation, the sense of taste, because of its use of chemoreceptors to discern information about substances. Perceiving complex flavors requires recognizing taste and smell sensations at the same time, an interaction known as chemoreceptive sensory interaction. This causes foods to taste different if the olfactory system is compromised. However, olfaction is anatomically different from gustation because it uses the sensory organs of the nose and nasal cavity to capture smells. Humans can identify a large number of odors and use this information to interact successfully with their environment.
The Nose and Nasal Cavity
Olfactory sensitivity is directly proportional to spatial area in the nose—specifically the olfactory epithelium, which is where odorant reception occurs. The area in the nasal cavity near the septum is reserved for the olfactory mucous membrane, where olfactory receptor cells are located. This area is a dime-sized region called the olfactory mucosa. In humans, there are about 10 million olfactory cells, each of which has 350 different receptor types composing the mucous membrane. Each of the 350 receptor types is characteristic of only one odorant type. Each functions using cilia, small hair-like projections that contain olfactory receptor proteins. These proteins carry out the transduction of odorants into electrical signals for neural processing.
Olfactory transduction is a series of events in which odor molecules are detected by olfactory receptors. These chemical signals are transformed into electrical signals and sent to the brain, where they are perceived as smells.
Once ligands (odorant particles) bind to specific receptors on the external surface of cilia, olfactory transduction is initiated. In mammals, olfactory receptors have been shown to signal via G protein. This is a similar type of signaling of other known G protein-coupled receptors (GPCR). The binding of an odorant particle on an olfactory receptor activates a particular G protein (Gαolf), which then activates adenylate cyclase, leading to cAMP production. cAMP then binds and opens a cyclic nucleotide-gated ion channel. This opening allows for an influx of both Na+ and Ca2+ ions into the cell, thus depolarizing it. The Ca2+ in turn activates chloride channels, causing the departure of Cl–, which results in a further depolarization of the cell.
Interpretation of Smells
Individual features of odor molecules descend on various parts of the olfactory system in the brain and combine to form a representation of odor. Since most odor molecules have several individual features, the number of possible combinations allows the olfactory system to detect an impressively broad range of smells. A group of odorants that shares some chemical feature and causes similar patterns of neural firing is called an odotope.
Humans can differentiate between 10,000 different odors. People (wine or perfume experts, for example) can train their sense of smell to become expert in detecting subtle odors by practicing retrieving smells from memory.
Smell and Memory
Odor information is easily stored in long-term memory and has strong connections to emotional memory. This is most likely due to the olfactory system’s close anatomical ties to the limbic system and the hippocampus, areas of the brain that have been known to be involved in emotion and place memory. Human and animal brains have this in common: the amygdala, which is involved in the processing of fear, causes olfactory memories of threats to lead animals to avoid dangerous situations. The human sense of smell is not quite as powerful as most other animals’ sense of smell, but smell is still deeply tied to human memory and emotion.
Pheromones are airborne, often odorless molecules that are crucial to the behavior of many animals. They are processed by an accessory of the olfactory system. Recent research shows that pheromones play a role in human attraction to potential mates, the synchronization of menstrual cycles among women, and the detection of moods and fear in others. Thanks in large part to the olfactory system, this information can be used to navigate the physical world and collect data about the people around us.
Somatosensation: Pressure, Temperature, and Pain
The somatosensory system allows the human body to perceive the physical sensations of pressure, temperature, and pain.
Summarize the stages of the somatosensory system in which physical stimuli are detected and processed
- The somatosensory system allows the human body to experience pressure, texture, temperature, and pain, and to perceive the position and movement of the body’s muscles and joints.
- The receptor cells in the skin can be broken down into three functional categories: mechanoreceptors that sense pressure and texture, thermoreceptors that sense temperature, and nociceptors that sense pain.
- Mechanoreceptors come in four different types based on their speed of adaptation ( fast or slow) and their receptive field size (large or small).
- Thermoreceptors detect changes in temperature using two types of receptor cells: warm and cold.
- Nociceptors detect pain that ranges from acute and tolerable to chronic and intolerable.
- Proprioceptors allow our bodies to have a sense of kinesthesia, or position and movement in physical space.
- nociception: The physiological process underlying the sensation of pain.
- mechanoreceptor: Any receptor that provides an organism with information about mechanical changes in its environment, such as movement, tension, and pressure.
- thermoreceptor: A nerve cell that is sensitive to changes in temperature.
The human sense of touch is known as the somatic or somatosensory system. Touch is the first sense developed by the body, and the skin is the largest and most complex organ in the somatosensory system. By gathering external stimuli and interpreting them into useful information for the nervous system, skin allows the body to function successfully in the physical world. Touch receptors in the skin have three main subdivisions: mechanoreception (sense of pressure), thermoreception (sense of heat) and nociception (sense of pain). Receptor cells in the muscles and joints called proprioceptors also aid in the somatosensory system, but they are sometimes separated into another sensory category called kinesthesia.
The somatosensory system uses specialized receptor cells in the skin and body to detect changes in the environment. The receptors collect and convert physical stimuli into electrical and chemical signals through the transduction process and send these impulses to the nervous system for processing. Sensory cell function in the somatosensory system is determined by location.
The receptors in the skin, also called cutaneous receptors, tell the body about the three main subdivisions mentioned above: pressure and surface texture (mechanoreceptors), temperature (thermoreceptors), and pain (nociceptors). The receptors in the muscles and joints provide information about muscle length, muscle tension, and joint angles.
Mechanoreceptors in the skin give us a sense of pressure and texture. These receptors differ in their field size (small or large) and their speeds of adaptation (fast or slow). Thus, there are four types of mechanoreceptors based on the four possible combinations of fast vs. slow speed and large vs. small receptive fields. The speed of adaptation refers to how quickly the receptor will react to a stimulus and how long that reaction will be sustained after the stimulus is removed. Rapidly adapting cells allow us to adjust grip and force appropriately. Slowly adapting cells allow us to perceive form and texture. The receptive field size refers to the amount of skin area that responds to the stimulus, with smaller areas specializing in locating stimuli accurately.
Thermoreceptors detect changes in temperature through their free nerve endings. There are two types of thermoreceptors that signal temperature changes in our own skin: warm and cold receptors. Our sense of temperature is a result of the comparison of the signals from each of the two types of thermoreceptors. These receptors are not good indicators of absolute temperature, but they are very sensitive to changes in skin temperature.
Nociceptors use free nerve endings to detect pain. Functionally, nociceptors are specialized, high-threshold mechanoceptors or polymodal receptors. They respond not only to intense mechanical stimuli but also to heat and noxious chemicals—anything that may cause the body harm. Their response magnitude, or the amount of pain you feel, is directly related to the degree of tissue damage inflicted.
Pain signals can be separated into three types that correspond to the different types of nerve fibers used for transmitting these signals. The first type is a rapidly transmitted signal with a high spatial resolution, called first pain or cutaneous pricking pain. This type of signal is easy to locate and generally easy to tolerate. The second type is much slower and highly affective, called second pain or burning pain. This signal is more difficult to locate and not as easy to tolerate. The third type arises from viscera, musculature, and joints; it is called deep pain. This type of signal is very difficult to locate, and often it is intolerable and chronic.
Proprioceptors are the receptor cells found in the body’s muscles and joints. They detect joint position and movement, and the direction and velocity of the movement. There are many receptors in the muscles, muscle fascia, joints, and ligaments, all of which are stimulated by stretching in the area in which they lie. Muscle receptors are most active in large joints such as the hip and knee joints, while joint and skin receptors are more meaningful to finger and toe joints. All of these receptors contribute to overall kinesthesia, or the perception of bodily movements.
Somatic System Disorders
A somatic system disorder (formerly called a somatoform disorder) is a type of psychological disorder related to the somatosensory system. Somatic system disorders present symptoms of physical pain or illness that cannot be explained by a medical condition, injury, or substance. The patient must also be excessively worried about his symptoms, and this worry must be judged to be out of proportion to the severity of the physical complaints themselves. This class of disorders includes:
- Conversion disorder: A somatic symptom disorder involving an actual loss of bodily function such as blindness, paralysis, or numbness due to excessive anxiety.
- Illness anxiety disorder: A somatic symptom disorder involving persistent and excessive worry about developing a serious illness. This disorder has recently been reviewed and expanded into three different classifications.
- Body dysmorphic disorder: The afflicted individual is concerned with body image and is excessively concerned about and preoccupied with a perceived defect in his or her physical appearance.
- Pain disorder: Chronic pain experienced by a patient in one or more areas that is thought to be caused by psychological stress. The pain is often so severe that it prevents proper body function. Duration may be as short as a few days or as long as many years.
- Undifferentiated somatic symptom disorder – only one unexplained symptom is required for at least 6 months.
Additional Sensory Systems
Two additional sensory systems are proprioception (which interprets body position) and the vestibular system (which interprets balance).
Contrast the roles of proprioception and the vestibular system
- In addition to our five basic senses of vision, audation, gustation, olfaction, and the somatosensory systems, recent advances have expanded our defined senses to include proprioception and the vestibular system.
- Proprioception is the sense of the relative position of neighboring parts of the body and the strength of effort employed in movement.
- Kinesthesia is the awareness of the position and movement of the parts of the body using sensory organs, which are known as proprioceptors, located in joints and muscles. Kinesthesia is a key component in muscle memory and hand-eye coordination.
- While the terms proprioception and kinesthesia are often used interchangeably, they actually have many different components.
- The vestibular system, situated in the inner ear, is the sensory system that contributes to balance and the sense of spatial orientation.
- Proprioception and the vestibular system both contribute to “a sense of balance,” but in different ways.
- proprioception: The sense of the position of parts of the body, relative to other neighboring parts of the body.
- vestibular system: The sensory system that contributes to balance and the sense of spatial orientation.
- kinesthesia: Proprioception or static position sense; the perception of the position and posture of the body; also, more broadly, including the motion of the body as well.
No matter what your level of experience with psychology is, you have probably heard of the five basic senses, which consist of the visual, auditory (hearing), gustatory (taste), olfactory (smell), and somatosensory (touch) systems. However, recent advances in science have expanded this canonical list of five sense systems to include two more: proprioception, which is the sense of the positioning of parts of the body; and the vestibular system, which senses gravity and provides balance.
Proprioception and Kinesthesia
Proprioception is the sense of the relative positioning of neighboring parts of the body, and sense of the strength of effort needed for movement. It is distinguished from exteroception, by which one perceives the outside world, and interoception, by which one perceives pain, hunger, and the movement of internal organs. A major component of proprioception is joint position sense (JPS), which involves an individual’s ability to perceive the position of a joint without the aid of vision. Proprioception is one of the subtler sensory systems, but it comes into play almost every moment. This system is activated when you step off a curb and know where to put your foot, or when you push an elevator button and control how hard you have to press down with your fingers.
Kinesthesia is the awareness of the position and movement of the parts of the body using sensory organs, which are known as proprioceptors, in joints and muscles. Kinesthesia is a key component in muscle memory and hand-eye coordination. The discovery of kinesthesia served as a precursor to the study of proprioception. While the terms proprioception and kinesthesia are often used interchangeably, they actually have many different components. Often the kinesthetic sense is differentiated from proprioception by excluding the sense of equilibrium or balance from kinesthesia. An inner ear infection, for example, might degrade the sense of balance. This would degrade the proprioceptive sense, but not the kinesthetic sense. The affected individual would be able to walk, but only by using the sense of sight to maintain balance; the person would be unable to walk with eyes closed. Another difference in proprioception and kinesthesia is that kinesthesia focuses on the body’s motion or movements, while proprioception focuses more on the body’s awareness of its movements and behaviors. This has led to the notion that kinesthesia is more behavioral, and proprioception is more cognitive.
The Vestibular System
The vestibular system is the sensory system that contributes to balance and the sense of spatial orientation. Together with the cochlea (a part of the auditory system) it constitutes the labyrinth of the inner ear in most mammals, situated within the vestibulum in the inner ear.
There are two main components of the vestibulum: the semicircular canal system, which indicates rotational movements; and the otoliths, which indicate linear accelerations. Some signals from the vestibular system are sent to the neural structures that control eye movements and provide us with clear vision, a process known as the vestibulo-ocular reflex. Other signals are sent to the muscles that control posture and keep us upright.
Proprioception vs. Vestibular System
While both the vestibular system and proprioception contribute to the “sense of balance,” they have different functions. Proprioception has to do with the positioning of limbs and awareness of body parts in relation to one another, while the vestibular system contributes to the understanding of where the entire body is in space. If there was a problem with your proprioception, you might fall over if you tried to walk because you would lose your innate understanding of where your feet and legs were in space. On the other hand, if there was a problem with your vestibular system (such as vertigo), you might feel like your entire body was spinning in space and be unable to walk for that reason.