2.3 Hearing and Vestibular Sense
Sensation and Perception
After Chapter 2.3, you will be able to:
· Identify the structures used to detect linear acceleration and rotational acceleration
· Explain how the structural features of the cochlea and the hair cells are able to transmit information about pitch of an incoming sound to the brain
· List the structures in the auditory pathway
The ear is a complex organ responsible not only for our sense of hearing, but also for our vestibular sense, which is our ability to both detect rotational and linear acceleration and to use this information to inform our sense of balance and spatial orientation. These senses are critically important to our ability to get around in the world, and their associated structures are encased in some of the densest bone of the body to protect these structures from damage.
STRUCTURE AND FUNCTION OF THE EAR
The ear is divided into three parts, as shown in Figure 2.7: the outer, middle, and inner ear. A sound wave first reaches the cartilaginous outside part of the ear, called the pinna or auricle. The main function of the pinna is to channel sound waves into the external auditory canal, which directs the sound waves to the tympanic membrane (eardrum). The membrane vibrates in phase with the incoming sound waves. The frequency of the sound wave determines the rate at which the tympanic membrane vibrates: it moves back and forth at a high rate for high-frequency sounds and more slowly for low-frequency sounds. Louder sounds have greater intensity, which corresponds to an increased amplitude of vibration.
Figure 2.7. Anatomy of the Ear
The tympanic membrane divides the outer ear from the middle ear. The middle ear houses the three smallest bones in the body, called ossicles. The ossicles help transmit and amplify the vibrations from the tympanic membrane to the inner ear. The malleus (hammer) is affixed to the tympanic membrane; it acts on the incus (anvil), which acts on the stapes (stirrup). The baseplate of the stapes rests on the oval window of the cochlea, which is the entrance to the inner ear. The middle ear is connected to the nasal cavity via the Eustachian tube, which helps equalize pressure between the middle ear and the environment.
Remember that sound is a longitudinal wave carried through air (or another medium), which causes displacement of particles parallel to the axis of sound propagation. In other words, when a sound wave hits your eardrum, it literally causes it to oscillate back and forth because of moving air particles. Sound is discussed in Chapter 7 of MCAT Physics and Math Review.
The inner ear sits within a bony labyrinth, which is a hollow region of the temporal bone containing the cochlea, vestibule, and semicircular canals, as shown in Figure 2.8. Inside the bony labyrinth rests a continuous collection of tubes and chambers called the membranous labyrinth. This collection of structures contains receptors for the sense of equilibrium and hearing. The membranous labyrinth is filled by a potassium-rich fluid called endolymph, and is suspended within the bony labyrinth by a thin layer of another fluid called perilymph. Perilymph simultaneously transmits vibrations from the outside world and cushions the inner ear structures.
Figure 2.8. The Membranous and Bony LabyrinthThe membranous labyrinth is filled with endolymph (blue); it is suspended within the bony labyrinth, which is filled with perilymph (purple).
The cochlea is a spiral-shaped organ that contains the receptors for hearing; it is divided into three parts called scalae, as shown in Figure 2.9. All three scalae run the entire length of the cochlea. The middle scala houses the actual hearing apparatus, called the organ of Corti, which rests on a thin, flexible membrane called the basilar membrane. The organ of Corti is composed of thousands of hair cells, which are bathed in endolymph. On top of the organ of Corti is a relatively immobile membrane called the tectorial membrane. The other two scalae, filled with perilymph, surround the hearing apparatus and are continuous with the oval and round windows of the cochlea. Thus, sound entering the cochlea through the oval window causes vibrations in perilymph, which are transmitted to the basilar membrane. Because fluids are essentially incompressible, the round window, a membrane-covered hole in the cochlea, permits the perilymph to actually move within the cochlea. Like the rods and cones of the eye, the hair cells in the organ of Corti transduce the physical stimulus into an electrical signal, which is carried to the central nervous system by the auditory (vestibulocochlear) nerve.
Figure 2.9. Structure of the Cochlea (Cross-Section)
The junction between the stapes and the oval window is extremely similar to a thermodynamic gas—piston system, as described in Chapter 3 of MCAT Physics and Math Review. However, fluids are not as compressible as gases; therefore, the round window must be present to allow the perilymph in the cochlea to actually move back and forth with the stapedial footplate.
The vestibule refers to the portion of the bony labyrinth that contains the utricle and saccule. These structures are sensitive to linear acceleration, so are used as part of the balancing apparatus and to determine one’s orientation in three-dimensional space. The utricle and saccule contain modified hair cells covered with otoliths. As the body accelerates, these otoliths will resist that motion. This bends and stimulates the underlying hair cells, which send a signal to the brain.
While the utricle and saccule are sensitive to linear acceleration, the three semicircular canals are sensitive to rotational acceleration. The semicircular canals are arranged perpendicularly to each other, and each ends in a swelling called an ampulla, where hair cells are located. When the head rotates, endolymph in the semicircular canal resists this motion, bending the underlying hair cells, which send a signal to the brain.
The auditory pathways in the brain are a bit more complex than the visual pathways. Most sound information passes through the vestibulocochlear nerve to the brainstem, where it ascends to the medial geniculate nucleus (MGN) of the thalamus. From there, nerve fibers project to the auditory cortex in the temporal lobe for sound processing. Some information is also sent to the superior olive, which localizes the sound, and the inferior colliculus, which is involved in the startle reflex and helps keep the eyes fixed on a point while the head is turned (vestibulo—ocular reflex).
The lateral geniculate nucleus (LGN) is for light; the medial geniculate nucleus (MGN) is for music.
Hair cells are named for the long tufts of stereocilia on their top surface, shown in Figure 2.10. As vibrations reach the basilar membrane underlying the organ of Corti, the stereocilia adorning the hair cells begin to sway back and forth within the endolymph. The swaying causes the opening of ion channels, which cause a receptor potential. Certain hair cells are also directly connected to the immobile tectorial membrane; these hair cells are involved in amplifying the incoming sound.
Figure 2.10. Stereocilia of a Hair CellMovement of fluid inside the cochlea leads to depolarization of the neuron associated with the hair cell.
The basilar membrane changes thickness depending on its location in the cochlea. The accepted theory on sound perception is place theory, which states that the location of a hair cell on the basilar membrane determines the perception of pitch when that hair cell is vibrated. The highest-frequency pitches cause vibrations of the basilar membrane very close to the oval window, whereas low-frequency pitches cause vibrations at the apex, away from the oval window. Thus, the cochlea is tonotopically organized: which hair cells are vibrating gives the brain an indication of the pitch of the sound.
Behavioral Sciences Guided Example With Expert Thinking
Researchers sought to explain a puzzling phenomenon: that differences in frequency of auditory stimuli sometimes correspond with greater than proportional increases in perceived pitch, even when taking Weber’s law into account.
Translated: even if a noise 'should' sound high (high frequency), it isn't necessarily perceived as high (high pitch).
The researchers hypothesized that differences in pitch specifically for sounds that are familiar, such as human speech, would be amplified perceptually when compared to those of unfamiliar sounds. Participants listened to recordings of both human vocalizations and synthesized tones at 165 Hz and 175 Hz.
There are a couple of independent variables here: type of sound and frequency. The dependent variable must be what the participants report hearing.
Adapted from: Monson BB, Han S, Purves D (2013) Are Auditory Percepts Determined by Experience? PLoS ONE 8(5): e63728. https://doi.org/10.1371/journal.pone.0063728
What results would help to support the researchers’ hypothesis? What effect would these results have on the place theory of sound perception?
When we encounter experimental passages on the MCAT, there are a few things we should always be on the lookout for: what is the hypothesis, and what are the independent and dependent variables? What results are presented, and what effect do they have on the hypothesis? What conclusion did the researchers come to, and is this conclusion reasonable given the results? If any of these pieces are missing, we should anticipate questions asking us to identify possibilities that would fit with the passage and our outside content knowledge. Here, we are given a hypothesis without results, and the question asks us for results that would support the hypothesis. We'll need to dive into the scenario to identify what is being tested and why, and then use that information in conjunction with our content knowledge to answer this question.
The researchers hypothesized that differences in perceived pitch are amplified for more commonly encountered sounds. Take a moment to consider what the researchers should expect if this hypothesis is correct. It would stand to reason that the difference between perceived pitch for the 165 Hz and 175 Hz samples would be larger for voices (which we hear all the time) than for synthesized tones (which we don't hear too often). In other words, a result that would help support the researchers' hypothesis would be experimental evidence that human vocalizations sounded more different in pitch than the synthesized tones, despite the absolute difference in frequency remaining the same.
For the second part of the question, we need to recall that place theory predicts that perceived pitch results directly from the location of the hair cells that are vibrated when exposed to that frequency; higher frequencies vibrate hair cells closer to the oval window, and lower frequencies vibrate hair cells that are farther away. What is important here is that, according to place theory, the type of sound is irrelevant; sounds of the same frequency should be perceived in the same way. However, the hypothesis of this study (and the hypothetical results we've just imagined in support of that hypothesis), call place theory into question as, according to the passage, similar changes in vibrations are not perceived as the same change in pitch. According to this study, the perceived pitch instead depends at least in part on the nature of the sound perceived by the listener. Note that this new finding wouldn't affect a place theory of sound sensation/detection, which could still be accurate: this experiment is specifically about how sounds are perceived, not about how they are sensed.
In sum, to support the researchers, the results must find a greater perceived difference in pitch between the vocal samples as compared to the synthesized audio samples. This finding would contradict the place theory of sound perception.
MCAT Concept Check 2.3:
Before you move on, assess your understanding of the material with these questions.
1. What structures are used to detect linear acceleration? Rotational acceleration?
o Linear acceleration:
o Rotational acceleration:
2. List the structures in the auditory pathway, from where sound enters the pinna to the auditory projection areas in the brain.
3. How does the organization of the cochlea indicate the pitch of an incoming sound?