- (a) What is sound?
Sound waves: stimulus for audition
-undulating displacement of molecules caused by changing pressure
- (b) What are the 3 primary qualities of sound?
3 primary qualities: frequency, amplitude, and complexity
Soundwaves have two physical attributes—-frequency and amplitude— produced by the displacement of air molecules.
–>The combination of these qualities produces complexity.
-The auditor system analyzes each property separately (just as the visual system analyzes color and form separately) to give us three perceptions: pitch, loudness, and timbre
Frequency and pitch perception: The rate at which sound waves vibrate is measured as cycles per second, or Hertz (Hz)
-low pitch, low frequency; fewer cycles/second
-high pitch, high frequency; many cycles/second
-differences in frequency are heard as differences in pitch. Example: each note in a musical scale has a different frequency
Amplitude and perception of loudness: Intensity of sound is usually measured in decibels (dB)
-high amplitude; loud sound
-low amplitude; soft sound
Complexity and timbre (perception of sound quality): Unlike the pure tone of a tuning fork, most sounds are a mixture of frequencies. A sound’s complexity determines its timbre, allowing us to distinguish, for example, a trombone from a violin playing the same note
-simple; pure tone
-complex; mix of frequencies
- What path does sound take to get from the ear to the auditory cortex?
1.Outer Ear:
-Sound waves enter the ear through the pinna (the visible part of the ear) and travel down the ear canal.
-The sound waves strike the eardrum (tympanic membrane), causing it to vibrate.
- Middle Ear:
-The eardrum’s vibrations are transmitted to the three tiny bones in the middle ear: the malleus (hammer), incus (anvil), and stapes (stirrup).
-The stapes bone connects to the oval
window, which leads to the inner ear.
3.Inner Ear:
-The oval window’s vibrations create fluid movement in the cochlea, a spiral-shaped structure.
-Inside the cochlea, specialized hair cells
(receptors) convert the fluid movement
into electrical signals.
-These electrical signals are transmitted
via the cochlear nerve (part of the
vestibulocochlear nerve) to the
brainstem.
- Brainstem:
-The cochlear nerve fibers synapse in the cochlear nuclei of the brainstem.
-From here, the information is relayed to
various brainstem nuclei involved in
sound processing. - Midbrain and Thalamus:
-The auditory pathway continues to the inferior colliculus in the midbrain.
-Next, the medial geniculate nucleus (MGN) in the thalamus receives the auditory signals.
- Cerebral Cortex:
-Primary Auditory Cortex (Auditory Area):
Finally, the MGN projects the auditory information to the primary auditory cortex (also known as Heschl’s gyrus) in the temporal lobe.
-The primary auditory cortex is responsible for conscious perception of sound.
in short:
-inner hair cells synapse on bipolar cells that form the auditory nerve
-to cochlear nucleus
-to olives
-to thalamus
-to auditory cortex (temporal lobe)
Auditory inputs cross to the hemisphere opposite the ear in the hindbrain and midbrain, then recross in the thalamus. In this way information from each ear reaches both hemispheres—multiple nuclei process inputs en route to the auditory cortex
- What are the four sound-related parts of cortex we discussed? What do they do?
- Wernicke’s Area: The cortex of the left planum forms a speech zone
-contains sound images of words
- processes the speech sounds that we hear and decodes them into words and concepts.
-helps to plan out what a person is going to say.
-Damage to Wernicke’s area can impair the ability to understand speech
spoken word –> A1 –> Wernicke’s area –> comprehend word heard
- Broca’s Area: Anterior speech area in the left hemisphere that functions with the motor cortex to produce the movements needed for speaking
-plays a central role in speech generation, language processing, and syntactical aspects of communication.
-stores/contains motor programs for speech
thoughts –> Wernicke’s area –> Broca’s Area
–> Facial area of motor cortex –> cranial nerves –> speak
- Heschl’s Gyrus: Cortex of the larger right hemisphere and has a role in analyzing music
- Insula:
-located within the lateral fissure; multifunctional cortical tissue containing regions related to language, the perception of taste, and the neural structures of underlying social cognition
-injury to the insula can produce disturbances of both language and taste
- What does it mean to say sound is lateralized?
Lateralization: Process whereby functions become localized on one side of the brain
-Analysis of speech takes place largely in
the left hemisphere
-Analysis of musical sounds takes place
largely in the right hemisphere
Left-handed people:
-about 70% are similar to right-handers, having language in left hemisphere
-remaining 30%, speech is represented either in the right hemisphere or bilaterally
- What does it mean when we say that there is a Tonotopic organization of the auditory cortex?
(Greek tono = frequency and topos = place)
The tonotopic organization of the auditory cortex refers to how sounds of different frequencies are spatially processed in the brain.
-Hearing pitch: Tonotopic representation
-Tones close in frequency are represented in neighboring regions, forming tonotopic maps. This organization begins at the cochlea, where different regions of the basilar membrane vibrate at distinct frequencies. Nerves then transmit this frequency information to the primary auditory cortex, maintaining a linear arrangement based on each neuron’s preferred frequency
- Regarding the cochlea and hair cells: how are the hair cells arranged according to frequency? So, if a low frequency sound enters the ear canal, what part of the basilar membrane will respond (apex versus base, etc.)?
In the cochlea is the basilar membrane. This membrane is where sound waves travel along and is responsible for transforming these waves into electrical impulses, which are then transmitted to the brain.
The membrane starts narrow and thick called the base, then ends wide and thin called the apex.
-the base is tuned for fast/high
frequencies
-medium frequencies cause peak bending
of the basilar membrane in the middle
section
-the apex is tuned for slow/low frequencies
- Generally speaking, how do cochlear implants work?
Cochlear implant: is an electronic device implanted surgically in the inner ear to transduce sound waves to neural activity and allow deaf people to hear
-sound waves are converted to battery-driven electrical stimulation which is sent through the port to wires that terminate at appropriate places in the cochlear where they can directly excite the cochlear nerve
-the procedure destroys the organ of Corti, so external sound can no longer be perceived beyond the sound available from the cranial nerve simulator.
-However, the brain will hear as long as appropriate signals are transmitted to the cochlear nerve from the correct locations within the cochlea (high frequencies at the base close to the oval window and low frequencies at the apex).
- How do we sense the amplitude of sound? In other words what process is important in perceiving sounds as louder or quieter?
The perception of sound loudness depends on amplitude. When sound waves enter our ears, their amplitude (height) determines how much energy they carry. Larger waves are perceived as louder
-The greater the amplitude of the incoming sound waves, the higher the firing rate of bipolar cells in the cochlea
-More intense sound waves trigger more intense movements of the basilar membrane, causing more shearing action of the hair cells, which leads to more neurotransmitter release onto bipolar cells
——-In the auditory system, bipolar neurons in the cochlea receive signals generated by hair cells in response to sound waves. These neurons transmit the auditory information to higher brain regions, where it is processed and interpreted, leading to our perception of sound.
- What roles do the Inner Hair Cells (IHC) versus Outer Hair Cells (OHC) play?
Inner Hair Cells (IHC): Hear
-An influx of calcium ions leads the ICH to release neurotransmitters, stimulating increased action potentials in auditory neurons
Outer Hair Cells (OCH): Change tectorial membrane stiffness
-movement of the basilar membrane produces a shearing force in the cochlear fluid that bends the cilia, leading to the opening or closing of calcium channels in the OCH
- When sound enters the cochlea, tectorial and basilar membranes move causing the cilia of the inner hair cell to bend toward the tallest, which in turn causes the mechanical opening of receptors as tip links pull channels open. What happens next?
-Movement of cilia toward the TALLEST cilia DEPOLARIZES the cell, causing the calcium INFLUX and RELEASE of neurotransmitters, which STIMULATES cells that form the auditory nerve.
—> Nerve impulses INCREASE
-Movement of the cilia toward the SHORTEST cilia HYPERPOLARIZES the cell, resulting in LESS neurotransmitters release
—> Activity in auditory neurons DECREASE
- What cues do we use for sound localization?
-Interaural Time Difference (ITD): Neurons in the brainstem compute the difference in a sound wave’s arrival time at each ear
-Interaural Intensity Difference (IID): Another mechanism for source detection is relative loudness on the left and right
