Hearing in Psychology: The Science of Sound Perception
Hearing, also known as audition, is a crucial sensory function that allows us to perceive and interpret the sounds around us. It is the process by which the ear transforms sound vibrations in the external environment into nerve impulses that are conveyed to the brain, where they are interpreted as sounds. This complex process involves several stages, from the initial capture of sound waves to the final interpretation by the brain.
The Nature of Sound
Sounds are produced when vibrating objects, such as the plucked string of a guitar, produce pressure pulses of vibrating air molecules, better known as sound waves. These waves propagate outward from the vibrating source, reaching the eardrum of a listener and initiating the process of hearing.Physical Attributes of Sound
A sound waveform has three basic physical attributes:
- Frequency: The number of times per second that the vibratory pattern oscillates, measured in hertz (Hz).
- Amplitude: Sound pressure, proportional to sound intensity.
- Temporal Variation: Aspects such as sound duration.
Sound pressure is proportional to sound intensity (in units of power or energy), so sound magnitude can be measured in units of pressure, power, and energy. The common measure of sound level is the decibel (dB), in which the decibel is the logarithm of the ratio of two sound intensities or two sound pressures.
The ear can distinguish different subjective aspects of a sound, such as its loudness and pitch, by detecting and analyzing different physical characteristics of the waves. Pitch is the perception of the frequency of sound waves-i.e., the number of wavelengths that pass a fixed point in a unit of time. The human ear is most sensitive to and most easily detects frequencies of 1,000 to 4,000 hertz, but at least for normal young ears the entire audible range of sounds extends from about 20 to 20,000 hertz. Sound waves of still higher frequency are referred to as ultrasonic, although they can be heard by other mammals.
Loudness is the perception of the intensity of sound-i.e., the pressure exerted by sound waves on the tympanic membrane. The greater their amplitude or strength, the greater the pressure or intensity, and consequently the loudness, of the sound. The intensity of sound is measured and reported in decibels (dB), a unit that expresses the relative magnitude of a sound on a logarithmic scale. On the decibel scale, the range of human hearing extends from 0 dB, which represents a level that is all but inaudible, to about 130 dB, the level at which sound becomes painful.
The Auditory System: A Detailed Look
The ear is a very efficient transducer, changing sound pressure in the air into a neural-electrical signal that is translated by the brain as speech, music, noise, etc. The external ear, middle ear, inner ear, brainstem, and brain each have a specific role in this transformation process.
Outer Ear
The external ear includes the pinna, which helps capture sound in the environment. The external ear canal channels sound to the tympanic membrane (eardrum), which separates the external and middle ear. The large, fleshy structure on the lateral aspect of the head is known as the auricle. The C-shaped curves of the auricle direct sound waves toward the auditory canal. The canal enters the skull through the external auditory meatus of the temporal bone. At the end of the auditory canal is the tympanic membrane, or ear drum, which vibrates after it is struck by sound waves.Middle Ear
The tympanic membrane and the three middle ear bones, or ossicles (malleus, incus, and stapes), assist in the transfer of sound pressure in air into the fluid- and tissue-filled inner ear. The middle ear consists of a space spanned by three small bones called the ossicles. The three ossicles are the malleus, incus, and stapes, which are Latin names that roughly translate to hammer, anvil, and stirrup. The malleus is attached to the tympanic membrane and articulates with the incus. The incus, in turn, articulates with the stapes. The stapes is then attached to the inner ear, where the sound waves will be transduced into a neural signal. The middle ear is connected to the pharynx through the Eustachian tube, which helps equilibrate air pressure across the tympanic membrane.When pressure is transferred from air to a denser medium, such as the inner ear environment, most of the pressure is reflected away. Thus, the inner ear offers impedance to conducting sound pressure to the fluid and tissue of the inner ear. The transfer of pressure in this case is referred to as admittance, while impedance is the restriction of the transfer of pressure. The term “acoustic immittance” is used to describe the transfer process within the middle ear: the word “immittance” combines the words impedance and admittance (im + mittance). As a result of this impedance, there is as much as a 35 dB loss in the transmission of sound pressure to the inner ear. The outer ear, tympanic membrane, and ossicles interact when a sound is present to focus the sound pressure into the inner ear so that most of that 35 dB impedance loss is overcome. Thus, the fluids and tissues of the inner ear vibrate in response to sound in a very efficient manner.
Sound waves are normally transmitted through the ossicular chain of the middle ear to the stapes footplate. The footplate rocks in the oval window of the inner ear, setting the fluids of the inner ear in motion, with the parameters of that motion being dependent on the intensity, frequency, and temporal properties of the signal.
Inner Ear
The inner ear contains both the vestibular system (underlying the sense of balance and equilibrium) and the cochlea (underlying the sense of hearing). The inner ear is often described as a bony labyrinth, as it is composed of a series of canals embedded within the temporal bone. It has two separate regions, the cochlea and the vestibule, which are responsible for hearing and balance, respectively. The neural signals from these two regions are relayed to the brain stem through separate fiber bundles. However, these two distinct bundles travel together from the inner ear to the brain stem as the vestibulocochlear nerve.The cochlea has three separate fluid compartments; two contain perilymph (scala tympani and scala vestibuli), similar to the body's extracellular fluid, and the other, scala media, contains endolymph, which is similar to intracellular fluids. Sound is transduced into neural signals within the cochlear region of the inner ear, which contains the sensory neurons of the spiral ganglia. These ganglia are located within the spiral-shaped cochlea of the inner ear. The cochlea is attached to the stapes through the oval window. The oval window is located at the beginning of a fluid-filled tube within the cochlea called the scala vestibuli. At the uppermost tip of the cochlea, the scala vestibuli curves over the top of the cochlear duct. The fluid-filled tube, now called the scala tympani, returns to the base of the cochlea, this time travelling under the cochlear duct. The scala tympani ends at the round window, which is covered by a membrane that contains the fluid within the scala. As vibrations of the ossicles travel through the oval window, the fluid of the scala vestibuli and scala tympani moves in a wave-like motion. The frequency of the fluid waves match the frequencies of the sound waves.
A cross-sectional view of the cochlea shows that the scala vestibuli and scala tympani run along both sides of the cochlear duct. The cochlear duct contains several organs of Corti, which transduce the wave motion of the two scala into neural signals. The organs of Corti lie on top of the basilar membrane, which is the side of the cochlear duct located between the organs of Corti and the scala tympani. As the fluid waves move through the scala vestibuli and scala tympani, the basilar membrane moves at a specific spot, depending on the frequency of the waves. Higher frequency waves move the region of the basilar membrane that is close to the base of the cochlea.
Hair Cells and the Organ of Corti
The scala media contains the sensorineural hair cells that are stimulated by changes in fluid and tissue vibration. There are two types of hair cells: inner and outer. Inner hair cells are the auditory biotransducers translating sound vibration into neural discharges. The shearing (a type of bending) of the hairs (stereocilia) of the inner hair cells caused by these vibrations induces a neural-electrical potential that activates a neural response in auditory nerve fibers of the eighth cranial nerve that neurally connect the hair cells to the brainstem. The outer hair cells serve a different purpose. When their stereocilia are sheared, the size of the outer hair cells changes due to a biomechanical alteration. The rapid change in outer hair cell size (especially its length) alters the biomechanical coupling within the cochlea.The structures of the cochlea vibrate in response to sound with a particular vibratory pattern. This vibratory pattern (the traveling wave) allows the inner hair cells and their connections to the auditory nerve to send signals to the brainstem and brain about the sound's vibration and its frequency content. That is, the traveling wave motion of cochlear vibration helps sort out the frequency content of any sound, so that information about the frequency components of sound is coded in the neural responses being sent to the brainstem and brain.
The organs of Corti contain hair cells, which are named for the hair-like stereocilia extending from the cell’s apical surfaces. The stereocilia are an array of microvilli-like structures arranged from tallest to shortest. Protein fibers tether adjacent hairs together within each array, such that the array will bend in response to movements of the basilar membrane. The stereocilia extend up from the hair cells to the overlying tectorial membrane, which is attached medially to the organ of Corti. When the pressure waves from the scala move the basilar membrane, the tectorial membrane slides across the stereocilia. When the stereocilia bend toward the tallest member of their array, tension in the protein tethers opens ion channels in the hair cell membrane. This will depolarize the hair cell membrane, triggering nerve impulses that travel down the afferent nerve fibers attached to the hair cells. When the stereocilia bend toward the shortest member of their array, the tension on the tethers slackens and the ion channels close.
The basilar membrane is the thin membrane that extends from the central core of the cochlea to the edge. A given region of the basilar membrane will only move if the incoming sound is at a specific frequency. Because the tectorial membrane only moves where the basilar membrane moves, the hair cells in this region will also only respond to sounds of this specific frequency.
The cochlea encodes auditory stimuli for frequencies between 20 and 20,000 Hz, which is the range of sound that human ears can detect. Frequencies as low as 20 Hz are detected by hair cells at the apex, or tip, of the cochlea. Frequencies in the higher ranges of 20 KHz are encoded by hair cells at the base of the cochlea, close to the round and oval windows. Most auditory stimuli contain a mixture of sounds at a variety of frequencies and intensities (represented by the amplitude of the sound wave).
The fact that the different frequencies of sound are coded by different auditory nerve fibers is referred to as the place theory of frequency processing, and the auditory nerve is said to be “tonotopically” organized in that each nerve fiber carries information to the brainstem and brain about a narrow range of frequencies. In addition, the temporal pattern of neural responses of the auditory nerve fibers responds to the temporal pattern of oscillations of the incoming sound as long as the temporal variations are less than about 5000 Hz.
In general, the more intense the sound is, the greater the number of neural discharges that are being sent by the auditory nerve to the brainstem and brain. Thus, the cochlea sends neural information to the brainstem and brain via the auditory nerve about the three physical properties of sound: frequency, temporal variation, and level. The biomechanical response of the cochlea is very sensitive to sound, is highly frequency selective, and behaves in a nonlinear manner. A great deal of this sensitivity, frequency selectivity, and nonlinearity is a function of the motility of the outer hair cells.
There are two major consequences of the nonlinear function of the cochlea: (1) neural output is a compressive function of sound level. This means that, at low sound levels, there is a one-to-one relationship between increases in sound level and increases in neural output; however, at higher sound levels, the rate at which the neural output increases with increases in sound level is lower. (2) The cochlea and auditory nerve produce distortion products. For instance, if the sound input contains two frequencies, f1 and f2, distortion products at frequencies equal to 2f1, 2f2, f2-f1, and 2f1-f2 may be produced by the nonlinear function of the cochlea. The distortion product 2f1-f2 (the cubic-difference tone) may be especially strong and this cubic-difference distortion product is used in several measures of auditory function.
At 60 dB SPL the bones of the skull begin to vibrate, bypassing the middle ear system. This direct vibration of the skull can cause the cochlea to vibrate and, thus, the hair cells to shear and to start the process of hearing. This is a very inefficient way of hearing, in that this way of exciting the auditory nervous system represents at least a 60 dB hearing loss.
Auditory Pathway to the Brain
In order for a sound to be transmitted to the central nervous system, the energy of the sound undergoes three transformations. First, the air vibrations are converted to vibrations of the tympanic membrane and ossicles of the middle ear. These in turn become vibrations in the fluid within the cochlea. Finally, the fluid vibrations set up traveling waves along the basilar membrane that stimulate the hair cells of the organ of Corti. These cells convert the sound vibrations to nerve impulses in the fibres of the cochlear nerve, which transmits them to the brainstem, from which they are relayed, after extensive processing, to the primary auditory area of the cerebral cortex, the ultimate centre of the brain for hearing. Only when the nerve impulses reach this area does the listener become aware of the sound.There are many neural centers in the brainstem and in the brain that process the information provided by the auditory nerve. The primary centers in the auditory brainstem in order of their anatomical location from the cochlea to the cortex are: cochlear nucleus, olivary complex, lateral lemniscus, inferior colliculus, and medial geniculate. The outer, middle, and inner ears along with the auditory nerve make up the peripheral auditory system, and the brainstem and brain constitute the central auditory nervous system. Together the peripheral and central nervous systems are responsible for hearing and auditory perception.
Auditory Perception in Daily Life
Hearing allows one to identify and recognize objects in the world based on the sound they produce, and hearing makes communication using sound possible. In the workplace, hearing may allow a worker to:- Communicate using human speech (e.g., communicate with a supervisor who is giving oral instructions).
- Process information-bearing sounds (e.g., respond to an auditory warning).
- Locate the spatial position of a sound source (e.g., locate the position of a car based on the sound it produces).
Listeners can detect the presence of a sound; discriminate changes in frequency, level, and time; recognize different speech sounds; localize the source of a sound; and identify and recognize different sound sources.
The auditory system must often accomplish these workplace tasks when there are many sources producing sound at about the same time, so that the sound from one source may interfere with the ability to “hear” the sound from another source. The interfering sound may make it difficult to detect another sound, to discriminate among different sounds, or to identify a particular sound. A hearing loss may make it difficult to perform one or all of these tasks even in the absence of interfering sounds but especially in the presence of interfering sounds.
Sound Detection
The healthy, young auditory system can detect tones in quiet with frequencies ranging from approximately 20 to 20000 Hz.Table 1. Comparison of Somatosensory Receptors
| Receptor Type | Stimulus | Location |
|---|---|---|
| Merkel cells | Low-frequency vibration, light touch | Skin |
| Tactile (Meissner) corpuscles | Light touch | Skin |
| Lamellated (Pacinian) corpuscles | Deep pressure and vibration | Deep in the dermis, subcutaneous tissue |
| Hair follicle plexus | Movement of hair | Hair follicles |
| Bulbous corpuscles | Stretching of the skin | Skin |
| Muscle spindles | Stretching of muscles | Skeletal muscle tissue |
| Golgi tendon organs | Stretching of tendons | Tendons |