Understanding the Place Theory of Hearing
The place theory of hearing proposes that the basilar membrane of the ear is divided into different regions which are stimulated by the frequency of a sound. This theory aims to explain how the human brain differentiates between different levels of pitch. Pitch is the highness or lowness of a sound, determined by the frequency of vibrations in the sound. By distinguishing different pitches, human beings refine their understanding of the sounds they hear; pitch is an important factor in communication.
Cross-section of the cochlea, highlighting its structure and components.
The Structure of the Ear
An explanation of how the ear functions allows for a better understanding of the place theory of hearing. The ear is divided into three parts: the outer ear, the middle ear, and the inner ear.
- Outer Ear: The outer ear is known as the auricle (the pinna). The auricle collects sound waves and transfers them to the ear canal (the external auditory meatus). As they move through the ear canal, sound waves cause the eardrum to vibrate.
- Middle Ear: The eardrum (the tympanic membrane) separates the outer ear from the middle ear. In the middle ear, the vibrations from the eardrum activate the ossicles, which are three small bones: the malleus, the incus, and the stapes. The ossicles connect the middle ear to the inner ear.
- Inner Ear: When vibrations from sound waves reach the inner ear, they move to the cochlea. The cochlea, a small organ filled with fluid, responds to the vibrations and stimulates nerve endings. These nerve endings transform the vibrations into electrical impulses that travel to the brain. The brain interprets these electrical impulses, allowing humans to hear.
Definition of Place Theory
The definition of place theory in psychology proposes an explanation of how human beings perceive pitch. According to place theory, the hair cells and nerve fibers of the cochlea are divided into different regions that detect specific sound frequencies. The areas which are closest to the opening of the cochlea respond to higher tones, while the areas at the opposite end of the cochlea respond to lower tones. This means that when a high tone travels to the auditory nerve, the area or region closest to the cochlea is stimulated, allowing the brain to determine the pitch. This same principle is applied when a low tone travels through the auditory nerve; the area or region near the narrow tip of the cochlea is stimulated, and the brain distinguishes the low sound. In place theory, different regions of the cochlea respond to different frequencies of sound.
The anatomy of the human ear, illustrating the outer, middle, and inner ear components.
Examples of Place Theory in Action
As mentioned, place theory can be used to explain how humans perceive high-pitched sounds, those which are above 1,000 hertz. Some examples of these sounds are crashing cymbals and chirping birds. Both examples are close to 10,000 hertz. The place theory of hearing suggests that once these sound waves travel through the auditory nerve and reach the basilar membranes, the region which can detect this high frequency is activated.
Place Theory vs. Frequency Theory
Two theories of hearing consider how pitch is distinguished: the place theory of hearing and the frequency theory of hearing. To have a better understanding of these theories, one must understand the structure of the ear and how the ear processes sound. The outer ear, consisting of the pinna and the external ear canal, is responsible for collecting sound waves and transferring them to the middle ear. The tympanic membrane, which separates the outer ear from the middle ear, vibrates upon the arrival of sound waves. The ossicular chain, which consists of the malleus, incus, and stapes, transmits the vibrations to the cochlea, which is part of the inner ear. The vibrations created in the cochlea then travel to the brain, and the brain interprets the impulses.
The place theory of hearing suggests that the cochlea has specialized regions that respond differently to determine different frequencies. The area near the narrow tip of the cochlea is stimulated by low frequencies, whereas the area near the opening is stimulated by high frequencies. Place theory cannot account for very low frequencies such as 1,000 hertz and below. The frequency theory of hearing proposes that the whole basilar membrane is put into motion once it receives the frequency of the tone. Further, it states that nerve impulses correspond to the frequency of the tone.
What Is the Place Theory of Hearing?
The place theory of hearing is used to explain how we distinguish high-pitched sounds that possess a frequency that exceeds 5,000 hertz. According to the place theory of hearing, we can hear different pitches due to specific sound frequencies causing vibrations in specific parts on the basilar membrane of the cochlea. In other words, different parts of the cochlea are activated by different frequencies. Each location on the basilar membrane possesses a particular characteristic frequency. For example, a sound that measures 6,000 hertz would stimulate the spot along the basilar membrane that possesses a characteristic frequency of 6,000 hertz. The brain detects the pitch based on the position of the hair cells that transmitted the neural signal.
Structure of the Ear
In order for us to truly understand the place theory of hearing, we must first have basic knowledge about the structure of the ear. We absorb sound into the outer ear, which includes the external auditory canal and the auricle, or pinna. The sound transforms into an acoustical signal after it is absorbed. The tympanic membrane, commonly known as the eardrum, is the part of the ear that separates the outer ear from the middle ear. Once the acoustical signal has reached the middle ear, the motion of the ossicular chain, which is made of the malleus, incus, and stapes, causes the acoustical signal to become mechanical. The ossicular chain also carries the acoustical signal to the inner ear, the location where the sound enters the cochlea.
Cochlea and Organ of Corti
Housed within the cochlea is the Organ of Corti, also known as the hearing organ, which houses sensory hair cells. Once the sound enters the cochlea, it causes the hair cells of the Organ of Corti to move. The sound is then converted into nerve impulses that are carried to the brain through the auditory nerve.
Frequency-to-place relationship in the uncoiled cochlea.
The ear-brain system is a complex instrument. Currently there are two overlapping theories of how we hear; the place theory of hearing and the temporal theory of hearing. We know from the structure of the cochlea that different parts resonate at different frequencies; the end closest to the stapes resonates at high frequencies and the end furthest from the ossicles resonates at low frequencies. Nerves are connected to hairs located along the cochlea which are stimulated when vibrations are present. A logical conclusion is that each place in the cochlea corresponds to the perception of a given frequency.
How the Place Theory Works
A given frequency presented to the cochlea only causes motion in one part of the cochlea. A problem with the place theory is that the resonance curves turn out to be very broad and they overlap. In other words the sections of the cochlea are low Q-factor resonators. This would seem to make it very difficult for the ear to pick out frequencies which are close together but we know that the just noticeable difference in frequency is about \(1\text{ Hz}\) for frequencies lower than \(1000\text{ Hz}\) for most people. If the place theory of hearing was correct we would expect that changing the frequency from, say \(250\text{ Hz}\) to \(240\text{ Hz}\) would shift the region of the basilar membrane that vibrates and trigger different nerve cells. But what actually happens is that the region that vibrates at \(250\text{ Hz}\) overlaps with the region that vibrates at \(240\text{ Hz}\) to such an extent that pretty much the same nerves are firing.
We also know from the uncertainty principle that a sharp resonance would mean less information about the duration of the sound. If sharp resonance peaks (high Q-factor for the cochlea) were the mechanism that enabled us to hear frequencies that are close together, we would not be able to hear sudden changes in frequency. A possible explanation to save the place theory might be that neighboring nerves are inhibited by the nerve firing at the center of the excited region.
Place Theory vs. Temporal Theory
Place theory is a theory of hearing that states that our perception of sound depends on where each component frequency produces vibrations along the basilar membrane. The main alternative to the place theory is the temporal theory, also known as timing theory. These theories are closely linked with the volley principle or volley theory, a mechanism by which groups of neurons can encode the timing of a sound waveform. In all cases, neural firing patterns in time determine the perception of pitch.
Experiments and Cochlear Implants
Experiments to distinguish between place theory and rate theory are difficult to devise, because of the strong correlation: large vibrations with low rate are produced at the apical end of the basilar membrane while large vibrations with high rate are produced at the basal end. The two can be controlled independently using cochlear implants: pulses with a range of rates can be applied via electrodes distributed along the membrane. Experiments using implant recipients showed that, at low stimulation rates, ratings of pitch on a pitch scale were proportional to the log of stimulation rate, but also decreased with distance from the round window.
5 Key Facts About Place Theory
- Place theory was first proposed by Hermann von Helmholtz in the 19th century and has since been a foundational concept in understanding auditory perception.
- In place theory, high-frequency sounds are thought to stimulate hair cells at the base of the cochlea, while low-frequency sounds activate hair cells toward the apex.
- This theory works well for explaining pitch perception for higher frequencies but is less effective for lower frequencies, where other theories like frequency theory apply.
- The ability to distinguish between different pitches relies on the precise location of activation along the basilar membrane in response to varying sound frequencies.
- Place theory supports the understanding of how complex sounds, like music, can be analyzed and interpreted based on their frequency components and spatial distribution in the cochlea.