Plack Chapter 4 Reading Questions
What is an auditory filter? In what way does the basilar membrane perform a spectral analysis of an incoming sound
A band-pass filter reflecting the frequency selectivity of a single place in the cochlea. The peripheral auditory system behaves as an array of auditory filters, each passing a narrow range of frequency components. Each place on the basilar membrane has a particular characteristic frequency, a bandwidth, and an impulse response. When a complex sound enters the ear, the higher frequency components of the sound excite the basilar membrane toward the base and the lower frequency components excite the basilar membrane toward the apex
What is adaptation
Auditory nerve fibers also show a characteristic change in firing rate with time from the onset of a sound. When a sound is turned on, the fibers produce a peak of activity (the onset response) that declines with time. In addition, when the sound is turned off, the activity in a neuron falls below its spontaneous activity for 100 milli- seconds or so (see Figure 4.16).The neuron is said to be adapted.Adaptation may be the result of the depletion of neurotransmitter from the inner hair cell
What is the principle function of the cochlear nucleus
Auditory nerve fibers synapse with neurons in the cochlear nucleus, and hence the cochlear nucleus is the first port of call, and first processing center, for neural signals from the cochlea.The signal from the ear is divided at the cochlear nucleus into two "streams" (Pickles, 2008).The more ventral part of the nucleus at the front of the brain- stem (ventral means toward the belly, which is the underside for quadrupeds such as guinea pigs, but toward the front for humans as we stand upright) contains neurons whose axons project mainly to the superior olivary complexes on both sides of the head, and this ventral stream carries the information (time of arrival and intensity) that is used for sound localization. The more dorsal part of the cochlear nucleus toward the back of the brainstem (dorsal means toward the back; think dorsal fin) contains neurons whose axons project mainly to the nuclei of the lateral lemniscus and the inferior col- liculus on the opposite side of the head (contralateral to the cochlear nucleus from which they originate).This pathway may be involved in sound identification.
What are action potentials or spikes, and what is their firing rate? What does the author mean when he writes that "...the connection between two neurons can be excitatory or inhibitory?" In what way is neuronal communication electrochemica
Axons can be quite long (almost a meter in length for some "motor" neurons involved in the control of muscles).They carry information in the form of electri- cal impulses called action potentials or spikes. The magnitude of every spike is the same (about 100 mV) so that information is carried by the firing rate (number of spikes per second) or pattern of spikes, not by variations in the magnitude of the electric potential for each spike. If neurotransmitter is detected by the receiving neuron, then this may trigger—or inhibit—the production of spikes in that neuron. In other words, the connection between two neurons can be excitatory or inhibitory. Hence, neural communication is electrochemical in nature: Electrical impulses in one neuron lead to the release of a chemical that influences the production of electrical impulses in another neuron.
What is the principle function of the superior olive in terms of the neural signal and perception
Each superior olive receives connections from the ventral parts of the cochlear nuclei on both sides of the head (i.e., both ipsilaterally and contralaterally). It is here that the signals from the two ears are combined and neurons in the superior olive respond to sounds presented to either ear. The superior olive is divided into two main parts: the medial superior olive (toward the midline of the brainstem) and the lateral superior olive (toward the sides of the brainstem).The superior olive is thought to use the information from the two ears to determine the direction of sound sources.
What is phase locking, and how might it represent frequency in the auditory nerve
If the basilar mem- brane is vibrating happily up and down in response to a low-frequency pure tone, the stereocilia will bend from side to side, but the hair cells will only depolarize when the stereocilia are bent in one direction—that is, at a particular phase of the vibration. This, in turn, means that neurons in the auditory nerve will tend to pro- duce spikes at a particular phase of the waveform.This property is called phase locking, because the response of the neuron is locked to a particular phase of the stimula- tion, or more accurately, a particular phase in the vibration of the basilar membrane.
What is the spontaneous activity of fibers in the auditory nerve? What is the relation between the level of a tone at characteristic frequency and firing rate for auditory nerve fibers with high (60 spikes/second) and low (<=10 spikes/second) spontaneous firing rates? What does it mean when we say that a neuron's response is saturated
In quiet, most fibers in the auditory nerve show a background level of firing called spontaneous activity. Most fibers (perhaps 90%) have high spontaneous rates, pro- ducing about 60 spikes per second.These fibers tend to be quite sensitive and show an increase in firing rate in response to a low stimulus level. The remaining fibers have low spontaneous rates of less than about 10 spikes per second hen stimulated with a pure tone at its characteristic frequency, a neuron will increase its firing rate as the level of the tone is increased, up to a certain maximum firing rate, at which point the response is saturated: Further increases in level will have no effect on the firing rate.
What is the Organ of Corti, and how is it structured from tectorial membrane to stereocilia to inner and outer hair cells? What are the basic functions of the inner and outer hair cells
Just below this, and sitting on top of the basilar membrane, is the organ of Corti, which contains rows of hair cells and various supporting cells and nerve endings. Cells are the tiny little bags of biochemical machinery, held together by a membrane, that make up most of the human body In each human cochlea there is one row of inner hair cells (closest to the inside of the cochlear spiral) and up to five rows of outer hair cells. Along the length of the cochlea there are thought to be about 3,500 inner hair cells and about 12,000 outer hair cells (Møller, 2013).The tallest tips of the stereocilia of the outer hair cells are embedded in the tectorial membrane, whereas the stereocilia of the inner hair cells are not. The outer hair cells change the mechanical properties of the basilar membrane, as described in Chapter 5.The inner hair cells are responsible for converting the vibration of the basilar membrane into electrical activity.
What is, in order, the process by which sound pressure variations become electrical neural activity
Let's recap what we have found out in our journey so far. Sound enters the ear canal and causes the eardrum to vibrate.These vibrations are transmitted to the oval win- dow in the cochlea by the bones in the middle ear.Vibrations of the oval window cause pressure changes in the cochlea that cause the basilar membrane to vibrate, with different places on the basilar membrane responding best to different frequen- cies.Vibrations on the basilar membrane are detected by the inner hair cells, which cause electrical activity (spikes) in the auditory nerve. From here on in the journey we are following electrical signals through the auditory nervous system.
What is the auditory cortex and what is its principle function in terms of signal analysis and cognitive processes? What caveat does the author provide regarding our knowledge of the human auditory cortex``
Nerve fibers from the inferior colliculus synapse with the medial geniculate body, which is part of the thalamus in the midbrain (just about in the center of the head). The thalamus acts as a sort of relay station for sensory information. Nerve fibers from the medial geniculate body project to the auditory cortex, which is part of the cerebral cortex. The auditory cortex is located at the top of the temporal lobe, hidden in a crease (or fissure) in the cerebral cortex called the Sylvian fissure (see Figure 4.20). The audi- tory cortex consists of a primary field (called "AI") and several adjacent fields. The primary field is located on a "bump" in the folds of the cortex called Heschl's gyrus. The primary field contains a tonotopic representation, in which neurons with simi- lar characteristic frequencies are arranged in strips.
What are neurons? What are the four main structures of a neuron and what are their functions? What is a synapse
Nervous system is composed of cells called neurons. Neurons are responsi- ble for rapid communication between sensory cells, muscle cells, and the brain.The human brain contains over a hundred billion neurons, each of which has hundreds of connections to other neurons. A neuron is composed of four main structures: the dendrites, the soma (or cell body), the axon, and the terminal buttons A connection between a terminal button and a dendrite, or between a sensory cell and a dendrite, is called a synapse.
What is the approximate frequency limit of phase locking? How can phase locking yet occur with an amplitude modulated signal that has a carrier frequency above the normal frequency limit of phase locking
Neurons cannot fire at rates greater than about 200 spikes per second, and this would seem to limit the usefulness of phase locking to a frequency of around 200 Hz. However, even if an individual fiber cannot respond at a sufficiently high rate to represent every cycle of the incoming waveform, information may be com- bined across neurons to represent the frequency of a high-frequency tone. If pure tones with frequencies of 100 and 500 Hz are presented, some neurons will phase lock to 100 Hz and some neurons will phase lock to 500 Hz, reflecting the separation of these components on the basilar membrane.
What is transduction, at least in terms of what happens to acoustic vibrations in the cochlea? By what means is this task accomplished
The different frequencies in the sound wave are separated onto different places on the basilar membrane. This is all a pointless exercise if the ear cannot now tell the brain which parts of the membrane are vibrating and by how much.The ear must convert the mechanical vibrations of the basilar membrane into electri The ear must convert the mechanical vibrations of the basilar membrane into electrical activity in the auditory nerve.This task is accomplished by the inner hair cells.
What causes motion of the stereocilia (inner hair cells on the Organ of Corti)? What is the effect of this motion? What are tip links? What is depolarization, and what process does it cause? What is the relationship between the magnitude of basilar membrane movement, the opening of tip links, the electrical charge in the inner hair cell, the amount of neurotransmitter released by depolarization, and the resulting activity in the auditory nerve
Recall that on top of each hair cell are rows of stereocilia, which look like tiny hairs. When the basilar membrane and the tectorial membrane move up and down, they also move sideways relative to one another. This "shearing" motion causes the stereocilia on the hair cells to sway from side to side (see Figure 4.10).The movement of the ste- reocilia is very small (the figures in this section show huge exaggerations of the actual effect). The stereocilia are connected to one another by protein filaments called tip links. When the stereocilia are bent toward the scala media (i.e., toward the outside of the cochlea), the tip links are stretched. The stretching causes them to pull on tiny molecular "trapdoors." These trapdoors block channels in the membranes of the stereocilia Because the "resting" electric potential of the inner hair cell is negative (about -45 mV), the increase in potential is called depolarization. Depolarization causes a chemical neurotransmitter (glutamate) to be released into the tiny gap (or synaptic cleft) between the hair cell and the neuron in the auditory nerve The larger the movement of the basilar membrane, the more tip links are opened, the greater the electrical change in the hair cell, the more neurotransmitter is released, and the greater the resulting activity in the auditory nerve.
What is a place code or a rate-place code, and how might it represent spectral information
Representation of spectral information in this way is called a place code or a rate-place code, because the spectral information is represented by the pattern of activity across the array of neurons.
What is an ascending auditory pathway? What is the cochlear nucleus? What is the brainstem? What is the central auditory system? What do ipsilateral and contralateral mean, and, above the cochlear nucleus, what is the laterality (mostly) of the main auditory pathways
The auditory nerve carries the information about incoming sound from the cochlea to the cochlear nucleus, a collection of neurons in the brainstem.The brainstem is the part of the brain on top of the spinal cord. The auditory centers in the brainstem and cerebral cortex are called the central auditory system.The information from the cochlear nucleus is passed (via synapses) to a number of other nuclei that are arranged in pairs, one on each side of the brainstem: the superior olive (or superior olivary complex), the nuclei of the lateral lemniscus, and the inferior colliculus (see Figure 4.19).
What is the auditory, or cochlear, nerve? About how many neurons are in the human auditory nerve? About how many dendrites of auditory nerve fibers are contacted by each inner hair cell on the Organ of Corti? What is a tuning curve and what does it show regarding the sensitivity of each neuron as the frequency of a stimulus is moved away from the characteristic frequency
The auditory, or cochlear, nerve is a bundle of axons or nerve fibers that are connected to (synapse with) the hair cells. The auditory nerve and the vestibular nerve (which carries information about balance from the vestibular system) together constitute the eighth cranial nerve (also called the vestibulocochlear nerve). In total, there are about 30,000 neurons in the human auditory nerve. Each inner hair cell is contacted by the dendrites of approximately 10-20 auditory nerve fibers. Tuning curves are essentially inverted versions of the filter shapes we discussed in this chapter The figure shows that each neuron becomes progressively less sensitive as the frequency of stimulation is moved away from the characteristic frequency, as does the place on the basilar membrane to which the neuron is connected.
What is the role of the basilar membrane? What is a characteristic frequency, and how does it relate to location on the basilar membrane
The basilar membrane is very important to mammalian hearing. The basilar membrane separates out the frequency components of a sound. At the base of the cochlea, near the oval window, the basilar membrane is narrow and stiff. This area is most sensitive to high frequencies. The other end of the membrane, at the tip or apex of the cochlea, is wide and loose and is most sensitive to low frequencies. The properties of the membrane vary continuously between these extremes along its length so that each place on the basilar membrane has a particular frequency of sound, or characteristic frequency, to which it is most sensitive.
What is the cochlea, and how is it structured from base to apex? What are the scala vestibuli, scala media, scala tympani and helicotrema
The cochlea is the most important part of the story because this is where transduction occurs. It is here that acoustic vibrations are converted into electri- cal neural activity. Notice that the tube is divided along its length by two membranes, Reissner's membrane and the basilar membrane. This creates three fluid-filled compartments: the scala vestibuli, the scala media, and the scala tympani.The scala vestibuli and the scala tympani are connected by a small opening (the helicotrema) between the basilar membrane and the cochlea wall at the apex (see Figure 4.3). The scala media, however, is an entirely separate compartment that contains a different fluid composition (endolymph) from that in the other two scalae (perilymph) . At the base of the cochlea, near the oval window, the basilar membrane is narrow and stiff. This area is most sensitive to high frequencies. The other end of the membrane, at the tip or apex of the cochlea, is wide and loose and is most sensitive to low frequencies
What is the eardrum? What are the ossicles and what is their function in the middle ear (pp. 60-61)? What does the author mean when he writes that the middle ear "...acts as an impedance-matching transformer
The eardrum is a thin, taut, and easily punctured membrane that vibrates in response to pressure changes in the ear canal On the other side of the eardrum from the ear canal is the middle ear.The middle ear is filled with air and is connected to the back of the throat by the Eustachian tube. Although the middle ear is filled with air, the acoustic vibrations are carried from the eardrum to the cochlea (where transduction takes place) by three tiny bones—the smallest in the body—called the malleus, incus, and stapes (literally, "ham- mer," "anvil," and "stirrup"). These bones are called, collectively, the ossicles. Their job is to transmit the pressure variations in an air-filled compartment (the ear canal) into pressure variations in a water-filled compartment (the cochlea) as efficiently as possible. The middle ear as a whole acts as an impedance-matching transformer.
What is the principle function of the inferior colliculus in terms of the neural signal and perception
The inferior colliculus is a vital processing stage in the auditory pathway, and almost all ascending nerve fibers synapse here. The information streams concerning sound localization and sound identity converge at this nucleus. Neurons in the central nucleus of the inferior colliculus are tonopically arranged in layers, each layer containing neurons with the same characteristic frequency. Nerve fibers from different nuclei, but with the same characteristic frequency, converge on the same isofrequency layer in the inferior colliculus.
What is the principle function of the lateral lemniscus in terms of the neural signal and perception
The lateral lemniscus is a tract of nerve fibers running from the cochlear nucleus and superior olive to the inferior colliculus. However, some neurons synapse with nuclei that are located in this region. The ventral nucleus of the lateral lemniscus receives input from the contralateral cochlear nucleus and may be part of the "sound identification" stream.
What is the main function of sensory systems?
The main function of sensory systems is to get information about the outside world to the brain, where it can be used to build representations of the environment and to help plan future behavior.
What is the ear canal (external auditory meatus)? What effect does the ear canal have on sound waves and, in turn, on perception
The opening in the pinna, the concha, leads to the ear canal (external auditory meatus) that is a short and crooked tube ending at the eardrum (tympanic membrane). The tube is about 2.5 cm long and has resonant properties like an organ pipe that is open at one end. ecause of the resonance of the ear canal and the concha, we are more sensitive to sound frequencies between about 1000 and 6000 Hz. The pinna, concha, and ear canal together make up the outer ear. The propagation of sound down the ear canal is the last stage in hearing in which sound waves are carried by the air.
What is tonotopic organization and how does it relate to the basilar membrane, auditory nerve and auditory cortex
The organization of frequency in terms of place is called tonotopic organization and is preserved right up to the primary auditory cortex, part of the cerebral cortex of the brain.There are cross-connections between neurons in the brain with different characteristic frequencie
What are the relative patterns of response to high and low frequency pure tones on the basilar membrane? Does a particular place on the basilar membrane respond to a range of frequencies or to one frequency only `
The peak of the traveling wave traces an outline, or envelope, which shows the overall region of response on the basilar membrane. Although there is a peak at one place on the basilar membrane, the region of response covers a fair proportion of the total length, especially for low-frequency sounds (see Figure 4.9). This is because each place acts as a band-pass filter and responds to a range of frequen- cies. It is clearly not the case that each place on the basilar membrane responds to one frequency and one frequency only (although the response will be maximal for stimulation at the characteristic frequency). Indeed, in response to very intense low-frequency sounds, every place on the membrane produces a significant vibra- tion, irrespective of characteristic frequency.
What are the three conceptual divisions of the peripheral auditory system
The peripheral auditory system is divided into the outer ear, middle ear, and inner ear.
What is the pinna? What effect does the pinna have on sound waves and, in turn, on perception
The pinna is the external part of the ear—that strangely shaped cartilaginous flap that you hook your sunglasses on. The pinna is the bit that gets cut off when someone has his or her ear cut off, although the hearing sense is not greatly affected by this amputation. Our pinnae are too small and inflexible to be very useful for collecting sound from a particular direction, for example. They do, however, cause spectral modifications (i.e., filtering) to the sound as it enters the ear, and these modifications vary depending on the direction the sound is coming from.The spectral modifications help the auditory system determine the location of a sound source
What is a descending auditory pathway? According to the author, what might be the principle functions of descending auditory pathways
There are also descending auditory pathways carrying information from higher auditory centers to lower auditory centers, even as far as the cochlea itself. The olivocochlear bundle contains fibers that originate in the ispilateral (same side) and contralateral (opposite side) superior olives.These efferent (i.e., from the brain) fibers travel down the auditory nerve and synapse in the cochlea. Some synapse on the axons of the afferent (i.e., to the brain) fibers innervating the inner hair cells, and others synapse directly on the outer hair cells.Those that synapse on the outer hair cells can control the motion of the basilar membrane, to some extent. It seems that the auditory sys- tem is designed so that higher auditory, and perhaps cognitive, centers can exert control on the activity of lower auditory centers, and thus influence the processing of sound. In particular, these descending neural pathways may be involved in perceptual learning. Signals from higher centers may cause changes in the ways neurons in the brainstem process sounds, to enhance their ability to extract relevant stimulus characteristics and to adapt to changing conditions.
What did von Békésy observe regarding a pure tone played to the ear? What is a traveling wave
Von Békésy observed that if a pure tone is played to the ear, a characteristic pat- tern of vibration is produced on the basilar membrane. This pattern of vibration is called a traveling wave, as illustrated in Figure 4.8. If we follow the wave from the base to the apex, we can see that it builds up gradually until it reaches a maximum (at the place on the basilar membrane that resonates at the fre- quency of the tone) before diminishing rapidly. Similar to a water wave on a pond, the traveling wave does not correspond to any movement of material from base to apex.
