Hearing

Hearing, also known as auditory perception, is the ability to perceive sounds through an organ like the ear. This process works by detecting vibrations, which are periodic changes in the pressure of a surrounding medium. Sound can travel through solid, liquid, or gaseous matter. In humans and other vertebrates, this is performed by the auditory system. This system detects mechanical waves and transduces them into nerve impulses. These impulses are then perceived by the brain, specifically in the temporal lobe. Because it relies on the movement of molecules, hearing is a type of mechanosensation, much like the sense of touch.

The human auditory system consists of three main parts: the outer ear, the middle ear, and the inner ear. The outer ear includes the pinna, which is the visible part of the ear, and the ear canal. The pinna serves to focus sound waves through the canal toward the eardrum, or tympanic membrane. Because the outer ear of most mammals is asymmetrical, sound is filtered differently depending on where it comes from. This helps animals localize sound vertically. The eardrum is an airtight membrane that vibrates according to the waveform of the sound. To protect this area, ceruminous and sebaceous glands produce cerumen, or ear wax, to prevent physical damage and microbial invasion.

Once the eardrum vibrates, the signal moves into the middle ear. This is a small, air-filled chamber located medial to the eardrum. Inside this chamber are the ossicles, which are the three smallest bones in the human body. These bones include the malleus, the incus, and the stapes. They are also known as the hammer, anvil, and stirrup. The purpose of these ossicles is to provide impedance matching. This helps overcome the impedance mismatch between air waves and cochlear waves. The stapes transmits these vibrations to the inner ear through a flexible membrane called the oval window. A second membrane, the round window, allows for the smooth displacement of the fluid inside the inner ear.

The inner ear is a highly complex organ containing the cochlea. The cochlea is a spiral-shaped tube filled with fluid called endolymph. Inside the cochlea is the organ of Corti, which is the main site for mechanical to neural transduction. A structure called the basilar membrane vibrates as waves move through the cochlear fluid. This membrane is tonotopic, meaning different frequencies have specific places of resonance along it. High frequencies are found at the basal entrance, while low frequencies are at the apex. This motion causes the depolarization of hair cells, which are specialized auditory receptors. While hair cells do not produce action potentials themselves, they release neurotransmitters at synapses with the auditory nerve.

After the hair cells release signals, the information travels through a specific neural pathway. The signal moves from the cochlea via the auditory nerve to the cochlear nucleus in the brainstem. From there, signals are projected to the inferior colliculus in the midbrain. The inferior colliculus integrates auditory input and manages subconscious reflexes, such as the auditory startle response. The signal then moves to the medial geniculate nucleus in the thalamus. Finally, the information reaches the primary auditory cortex in the temporal lobe. This is where sound is believed to first become a conscious experience. Nearby, Wernicke's area helps in interpreting sounds, which is necessary to understand spoken words.

Hearing can be lost through various types of impairment, such as conductive, sensorineural, or mixed hearing loss. The degree of loss is often measured in decibels (dB HL). Mild hearing loss occurs when the quietest sounds heard are between 25 and 40 dB HL. Moderate loss involves difficulty following conversations without aids, with thresholds between 40 and 70 dB HL. Severe hearing loss requires powerful aids, with thresholds between 70 and 95 dB HL. Profound hearing loss occurs at 95 dB HL or more, often requiring sign language. While some causes like noise-induced hearing loss can be prevented with earplugs or earmuffs, neural loss currently cannot be cured. Instead, clinicians use audioprosthetic devices like cochlear implants to mitigate the effects.

Different species have evolved unique hearing ranges to aid survival. Humans typically hear frequencies between 20 Hz and 20,000 Hz, known as the audio or sonic range. Frequencies above this are ultrasonic, while those below are infrasonic. Some animals utilize these extremes; for example, bats use ultrasound for echolocation, and whales use infrasound for communication.

A domestic cat can hear high-frequency sounds up to two octaves higher than a human. Even invertebrates can detect vibrations. While they lack ears, some insects use specialized hairs to detect air vibrations. In 1923, scientist Charles Henry Turner showed that ants could sense these vibrations to navigate. This demonstrates how diverse the biological mechanisms for hearing can be across the animal kingdom.