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Hearing
Hearing, or audition, depends on the presence of sound waves, which travel much more slowly than light waves. Sound waves are changes in pressure generated by vibrating molecules. The physical characteristics of sound waves influence the three psychological features of sound: loudness, pitch, and timbre.
Loudness depends on the amplitude, or height, of sound waves. The greater the amplitude, the louder the sound is perceived. Amplitude is measured in decibels. The absolute threshold of human hearing is defined as 0 decibels. Loudness doubles with every 10-decibel increase in amplitude. The loudness of normal human conversation is about 60 decibels. A whisper is about 20 decibels. A shout right into someone’s ear is about 115 decibels. Being exposed to sounds that are over 120 decibels, even for brief periods, can damage the auditory system.
Pitch refers to the perceived frequency of a sound, determining how “high” or “low” a sound seems to a listener. Pitch, though influenced by amplitude, depends most on the frequency of sound waves. Frequency is the number of times per second a sound wave cycles from the highest to the lowest point. The higher the frequency, the higher the pitch. Frequency is measured in hertz (Hz), or cycles per second. Frequency also affects loudness, with higher-pitched sounds being perceived as louder. Amplitude and frequency of sound waves interact to produce the experiences of loudness and pitch. Humans can hear sounds that are between 20 and 20,000 hertz.
Timbre, or the particular quality of a sound, depends on the complexity of a sound wave. A pure tone has sound waves of only one frequency. Most sound waves are a mixture of different frequencies.
The Structure of the Ear
Knowing the basic structure of the ear is essential to understanding how hearing works. The ear has three basic parts: the outer ear, the middle ear, and the inner ear.
The visible part of the ear is the pinna, which collects sound waves and passes them along the auditory canal to a membrane called the eardrum. When sound waves hit the eardrum, it vibrates. The eardrum transmits the vibration to three bones, or ossicles, in the middle ear, which are called the hammer, the anvil, and the stirrup. The diagram of the ear shows how they got these names: they actually look like a hammer, an anvil, and a stirrup. In response to the vibration, these ossicles move one after another. Their function is to amplify the sound vibrations.
From the ossicles, vibrations move through a membrane called the oval window to the cochlea of the inner ear. The cochlea is a coiled, fluid-filled tunnel.
Inside the cochlea are receptors called cilia, or hair cells, that are embedded in the basilar membrane. The basilar membrane runs along the whole length of the coiled cochlea. Vibrations that reach the inner ear cause the fluid in the cochlea to move in waves. These waves in turn make the hair cells move.
The movement triggers impulses in neurons that connect with the hair cells. The axons of these neurons come together to form the auditory nerve, which sends impulses from the ear to the brain. In the brain, the thalamus and the auditory cortex, which is in the temporal lobe of the cerebrum, receive auditory information.
Pitch Perception
Three main theories explain how people distinguish the pitch of different sounds: place theory, frequency theory, and the volley principle. Each theory addresses how the auditory system processes sound waves of varying frequencies to perceive different pitches. Place theory focuses on where sounds are detected along the cochlea. Frequency theory emphasizes the rate at which auditory nerves fire. The volley principle bridges the gap for high-frequency sounds that frequency theory alone cannot account for.
Place theory explains how people discriminate high-pitched sounds with a frequency greater than 5000 Hz. It states that sound waves of different frequencies trigger receptors at different places on the basilar membrane. High-pitched sounds stimulate the hair cells near the base of the cochlea, while low-pitched sounds stimulate the hair cells near the apex. The brain figures out the pitch of the sound by detecting the position of the hair cells that sent the neural signal.
Frequency theory explains how people discriminate low-pitched sounds that have a frequency below 1000 Hz. According to frequency theory, sound waves of different frequencies make the whole basilar membrane vibrate at different rates and therefore cause neural impulses to be sent at different rates. Pitch is determined by how fast neural signals move along to the brain.
The detection of moderately pitched sounds, with a frequency between 1000 and 5000 Hz, is explained by both place theory and frequency theory. To discriminate among these sounds, the brain uses a code based both on where the neural impulses originated (place theory) and how quickly neural impulses move (frequency theory).
The volley principle builds on frequency theory by explaining how the brain processes frequencies above 1000 Hz. At these higher frequencies, individual neurons cannot fire fast enough to match the wave frequency due to the refractory period. Instead, groups of neurons fire in a staggered, coordinated pattern, with each neuron firing at a different point in the sound wave cycle. This “volley” of impulses allows the combined firing rate of the neurons to match the sound wave’s frequency, allowing for accurate pitch perception. The volley principle fills the gap between frequency and place theories, enabling the brain to process pitches in the mid-range between 1000 and 5000 Hz, where neither theory alone is sufficient.
Locating Sounds
In the same way that people use two eyes to perceive depth, people use two ears to locate the origin of sounds. The left ear receives sound waves coming from the left slightly faster than the right ear does. The signal received by the left ear may also be a little more intense than that received in the right ear, because the signal has to go around the head to enter the right ear.
Locating a sound is difficult if both ears receive a signal of exactly the same intensity at exactly the same time, as when a sound originates from directly in front, directly behind, or directly above. Turning the head or cocking it to one side can help circumvent this difficulty
Hearing Difficulties
Hearing difficulties can result from aging or damage to various parts of the auditory system, affecting one’s ability to detect or process sound. There are two primary types of hearing loss: conduction deafness and sensorineural deafness.
Conduction deafness occurs when there is a problem with the physical transmission of sound through the outer ear or middle ear, such as blockages, damage to the eardrum, or issues with the ossicles. Causes include earwax buildup, infections, otosclerosis (a condition affecting the ossicles), injuries, or tumors. This type of hearing loss can often be treated with surgery or hearing aids.
Sensorineural deafness results from damage to the inner ear structures, such as the hair cells in the cochlea, or damage to the auditory nerve. It can occur due to aging, prolonged exposure to loud sounds, certain medications (like antibiotics or chemotherapy drugs), head injuries, genetic conditions, infections, or Meniere’s disease, which involves fluid buildup in the inner ear. This type of hearing loss is typically permanent, but cochlear implants or other strategies can help manage it in some cases.
Taste and Smell
Taste and smell are chemical senses. As light waves stimulate vision and sound waves stimulate sound, chemicals stimulate taste and smell.
Smell
Smell, or olfaction, happens when odor molecules in the air enter the nose during the breathing process. Smell receptors, located in the olfactory epithelium at the top of the nasal passage, detect these odor molecules. When odor molecules bind to the receptors, they send impulses along the olfactory nerve to the olfactory bulb located at the base of the brain. Researchers theorize that there are a great many types of olfactory receptors. People perceive particular smells when different combinations of receptors are stimulated.
The olfactory bulb sends information directly to brain regions like the amygdala and hippocampus, bypassing the thalamus—making it the only sensory pathway that skips this brain structure. This direct pathway explains the strong link between smell, emotion, and memory. Additionally, pheromones, which are chemical signals released by other individuals, influence behavioral and physiological responses. Pheromones play a significant role in social interactions, including mating behaviors, territory-making, and parental bonding.
Remembrance of Smells Past
The sense of smell is closely connected with memory. Most people have had the experience of smelling something, maybe a certain perfume or spice, and suddenly experiencing a strong emotional memory. Researchers don’t know exactly why this happens, but they theorize that smell and memory trigger each other because they are processed in neighboring regions of the brain.
Taste
Taste, or gustation, happens when chemicals in food and drinks stimulate receptors in the tongue and throat, on the inside of the cheeks, and on the roof of the mouth. These receptors are inside taste buds, which in turn are inside little bumps on the tongue called papillae. Taste receptors have a short life span and are replaced about every 10 days.
For a long time, researchers believed in the existence of four tastes: salty, sweet, sour, and bitter. Recently, researchers have expanded this list to include umami, a savory taste found in foods such as monosodium glutamate (MSG) and many protein-rich foods, and oleogustus, the distinct taste of fat. Taste is also strongly influenced by smell, which plays a critical role in how flavors are perceived. This interaction explains why food may taste bland or muted when one’s sense of smell is impaired.
Taste Sensitivity
The number of taste receptors varies between individuals, influencing how strongly flavors are perceived. People can be classified as supertasters, medium tasters, or nontasters based on the density of these receptors. Supertasters have more taste receptors and therefore experience a heightened sensitivity to bitter or spicy foods. Nontasters have fewer receptors, requiring more intense flavors to perceive taste. Medium tasters fall in the middle, experiencing a moderate sensitivity to flavors.
The Interaction of Taste and Smell
Taste and smell work together to create the full experience of flavor, with smell contributing essential nuances that enhance taste perception. As odor molecules travel from the mouth to the nose during eating or drinking, the brain combines this information with taste signals, producing complex flavor sensations. Without smell, the perception of taste can become muted or incomplete, making food seem bland. This interaction is why people with a cold often find it difficult to enjoy food.
Touch
The sense of touch is really a collection of several senses, encompassing pressure, pain, cold, and warmth. The senses of itch and tickle are related to pressure, and burn injuries are related to pain. Touch receptors are stimulated by mechanical, chemical, and thermal energy.
Pressure seems to be the only kind of touch sense that has specific receptors. Interestingly, the sensation of “hot” is not detected by a single receptor but results from the combined activation of both warm and cold receptors in the skin. These different touch stimuli are transmitted to the brain, where they are interpreted to help us respond to our environment, such as avoiding injury or seeking comfort.
The Gate Control Theory of Pain
Researchers don’t completely understand the mechanics of pain, although they do know that processes in the injured part of the body and processes in the brain both play a role.
In the 1960s, Ronald Melzack and Patrick Wall proposed an important theory about pain called the gate control theory of pain. Gate control theory states that pain signals traveling from the body to the brain must go through a gate in the spinal cord. If the gate is closed, pain signals can’t reach the brain. The gate isn’t a physical structure like a fence gate, but rather a pattern of neural activity that either stops pain signals or allows them to pass. Signals from the brain can open or shut the gate. For example, focusing on pain tends to increase it, whereas ignoring the pain tends to decrease it. Other signals from the skin senses can also close the gate. This process explains why massage, ice, and heat relieve pain.
The phenomenon of phantom limb sensation—an experience of feeling or pain in a limb that has been amputated—illustrates the brain’s role in pain perception. Even though the physical limb is no longer present, the brain continues to generate sensory experiences, sometimes resulting in persistent pain in the area where the limb once was.
Position, Movement, and Balance
Kinesthesis is the sense of the position and movement of body parts. Through kinesthesis, people know where all the parts of their bodies are and how they are moving. Receptors for kinesthesis are located in the muscles, joints, and tendons.
The sense of balance or equilibrium provides information about where the body exists in space. The sense of balance tells people whether they are standing up, falling in an elevator, or riding a roller coaster. The sensory system involved in balance is called the vestibular system. The main structures in the vestibular system are three fluid-filled tubes called semicircular canals, which are located in the inner ear. As the head moves, the fluid in the semicircular canals moves too, stimulating receptors called hair cells, which then send impulses to the brain.
