SECOND YEAR COURSE AUTUMN TERM 1994

PERCEPTION
Hearing Lecture Notes (2): Ear and Auditory Nerve
1 THE EAR
2 AUDITORY NERVE
3 WHAT YOU SHOULD KNOW.

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1 THE EAR

There are three main parts of the ear: the pinna (or external ear) and meatus, the middle ear, and the cochlea (inner ear).

1.1 Pinna and meatus

The pinna serves different functions in different animals. Those with mobile pinnae (donkey, cat) use it to amplify sound coming from a particular direction, at the expense of other sounds. The human pinna is not mobile, but serves to colour high frequency sounds by interference between the echoes reflected off its different structures (like the colours of light produced by reflection from an oil slick). Only frequencies that have a wavelength comparable to the dimensions of the pinna are influenced by it (> 3kHz). Different high frequencies are amplified by different amounts depending on the direction of the sound. The brain interprets these changes as direction.

The meatus is the tube that links the pinna to the eardrum. It resonates at around 2kHz so that frequencies in that region are transmitted more efficiently to the cochlea than others. This frequency region is particularly important in speech.

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1.2 Middle ear: tympanic membrane, malleus, incus and stapes

The middle ear transmits the vibrations of the ear drum (tympanic membrance) to the cochlea. The middle ear performs two functions.

(i) Impedance matching - vibrations in air must be transmitted efficiently into the fluid of the cochlea. If there were no middle ear most of the sound would just bounce off the cochlea. The middle ear helps turn a large amplitude vibration in air into a small amplitude vibration (of the same energy) in fluid. The large area of the ear-drum compared with the small area of the stapes helps to achieve this, together with the lever action of the three middle ear bones or ossicles (malleus, incus, stapes).

(ii) Protection against loud low frequency sounds - the cochlea is susceptible to damage from intense sounds. The middle ear offers some protection by the stapedius reflex, which tenses muscles that stiffen the vibration of the ossicles, thus reducing the extent to which low frequency sounds are transmitted. The reflex is triggered by loud sounds; it also reduces the extent of upward spread of masking from intense low-frequency sounds (see hearing lecture 3).

Damage to the middle ear causes a Conductive Hearing Loss which can usually be corrected by a hearing aid. In a conductive hearing loss, absolute thresholds are elevated. These thresholds are measured in an audiological test and shown in an audiogram. Appropriate amplification at different frequencies compensates for the conductive loss.

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1.3 Inner ear: cochlea

The snail-shaped cochlea, unwound, is a three-chambered tube. Two of the chambers are separated by the basilar membrane, on which sits the organ of Corti. The tectorial membrane sits on top of the organ of Corti and is fixed rigidly to the organ of Corti at one end only. Sound produces a travelling wave down the basilar membrane that is detected by shearing movement between the tectorial and basilar membranes bending the hairs on top of inner hair cells that form part of of the organ of Corti. Different frequencies of sound give maximum vibration at different places along the basilar membrane.

When a low frequency pure tone stimulates the ear, the whole basilar membrane, up to the point at which the travelling wave dies out, vibrates at the frequency of the tone. The amplitude of the vibration has a very sharp peak. The vibration to high frequency tones peaks nearer the base of the membrane than does the vibration to low frequency sounds. The characteristic frequency (CF) of a particular place along the membrane is the frequency that peaks at that point. If more than one tone is present at a time, then their vibrations on the membrane add together (but see remarks on non-linearity).



More intense tones give a greater vibration than do less intense:



A brief click contains energy at virtually all frequencies. Each part of the basilar membrane will resonate to its particular frequency.



It is a useful approximation to note that each point on the basilar membrane acts like a very sharply tuned band-pass filter. In the normal ear these filters are just as sharply tuned as are individual fibers of the auditory nerve (see below).

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1.3.1 Non-linearity

In normal ears the response of the basilar membrane to sound is actually non-linear - there is significant distortion.

* If you double the input to the basilar membrane, the output less than doubles (saturating non-linearity).

* If you add a second tone at a different frequency, the response to the first tone decreases (Two-tone suppression)

* If you play two tones (say 1000 & 1200 Hz) a third tone can appear (at 800 Hz) - the so-called Cubic Difference Tone.

1.3.2 Sensori-neural hearing loss (SNHL)

Sensori-neural hearing loss can be brought about by exposure to loud sounds (particularly impusive ones like gun shots), or by infection or by antibiotics. It usually arises from loss of outer hair cells. It is likely that outer hair cells act as tiny motors; they feed back energy into the ear at the CF. In ears with a sensori-neural hearing loss,(SNHL) this distortion is reduced or disappears. So, paradoxically, abnormal ears are more nearly linear.

1.3.3 Forms of deafness

There are two major forms of deafness: conductive and sensori-neural. 
                          Conductive                Sensori-neural            
Origin                    Middle-ear                Cochlea (OHCs)            
Thresholds                Raised                    Raised                    
Filter bandwidths         Normal                    Increased                 
Loudness growth           Normal                    Increased (Recruitment)
Bold symptoms are not alleviated by a conventional hearing aid.

1.3.4 Role of outer hair cells

The active feedback of energy by outer hair cells into the basilar membrane is probably responsible for:

(i) the sharp peak in the basilar membrane response -low thresholds and narrow bandwidth;

(ii) oto-acoustic emissions (sounds that come out of the ear);

(iii) the non-linear response of the basilar membrane vibration. The more linear behaviour of the SNHL basilar membrane is probably the cause of loudness recruitment (abnormally rapid growth of loudness).

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2 AUDITORY NERVE

As the hairs of inner hair cells bend, the voltage of the hair cell changes; when the hairs are bent sufficiently in one direction (but not the other) the voltage changes enough to release neurotransmitter in the junction between the hair cell and the auditory nerve synapse, and the auditory nerve fires. This direction corresponds to a pressure rarefaction in the air. After firing, an auditory nerve fibre has a refractory period of around 1 ms. Each hair cell has about 10 auditory nerve fibers connected to it. These fibers have different thresholds.

Inner hair cells stimulate the afferent auditory nerve, outer hair cells generally do not, but are innervated by the efferent auditory nerve. Efferent activity may influence the mechanical response of the basilar membrane via the outer hair cells.

2.1 Response to single pure tones

As the amplitude of a tone played to the ear increases, so the rate of firing of a nerve fibre at CF increases up to saturation. Most auditory nerve fibers have high spontaneous rates and saturate rapidly, but there are others (which are hard to record from) that have low spontaneous rates and saturate more slowly. High spontaneous rate fibers code intensity changes at low levels, and the low spontaneous rate ones code intensity changes at high levels.

2.2 Frequency threshold curves (FTCs)

FTCs plot the minimum intensity of sound needed at a particular frequency to just stimulate an auditory nerve fibre above spontaneous activity. The high frequency slopes are very steep (c. 300 dB/oct), the low frequency slopes generally have a steep tip followed by a flatter base. Damage to the cochlea easily abolishes the tip, and explains some features of Sensori-Neural Hearing Loss: raised thresholds and reduced frequency selectivity.

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2.3 Characteristic frequency (CF)

The CF of an auditory nerve fibre is the frequency at which least energy is needed to stimulate it. Different nerve fibers have different CFs and different thresholds. The CF of a fiber is roughly the same as the resonant frequency of the part of the basilar membrane that it is attached to.

2.4 Phase locking

The auditory nerve will tend to fire at a particular phase of a stimulating low-frequency tone. So the inter-spike intervals tend to occur at integer multiples of the period of the tone. With high frequency tones (> 3kHz) phase locking gets weaker, because the capacitance of inner hair cells prevents them from changing in voltage sufficiently rapidly.

phase locking

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2.5 Coding frequency

How does the brain tell, from the pattern of firing in the auditory nerve, what frequencies are present? There are two alternatives:

(a) place of maximal excitation - fibres whose CF is close to a stimulating tone's frequency will fire at a higher rate than those remote from it. So the frequency of a tone will be given by the place on the membrane from which emerge fibers having the highest rate of firing.

(b) timing information - fibres with a CF near to a stimulating tone's frequency will be phase locked to the tone, provided it is low in frequency (< 3kHz). So, consistent inter-spike intervals across a band of fibers indicate the frequency of a tone.

2.6 Coding intensity

How does the brain tell, from the pattern of firing in the auditory nerve, what are the intensities of the different frequencies present? The dynamic range of most auditory nerve fibres (high spontaneous) is not sufficient to cover the range of hearing (c.100dB). Low spontaneous rte fibers have a larger dynamic range and provide useful information at high levels. So information about intensity is carried in different fibers at different levels.

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2.7 Two-tone suppression

If a tone at a fiber's CF is played just above threshold for that fiber, the fiber will fire. But if a second tone is also played, at a frequency and level in the shaded area of the next diagram, then the firing rate will be reduced. This two-tone suppression demonstrates that the normal auditory system is non-linear. If the system were linear, then the firing rate could only be unchanged or increased by the addition of an extra tone.

Two-tone suppression is a characteristic of the normal ear and may be absent in the damaged ear. It is formally similar to lateral inhibition in vision, but it has a very different underlying cause. Lateral inhibition in vision is the result of neural mechanisms whereas two-tone inhibition is the result of mechanical processes inthe cochlea.

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2.8 Cochlear implants

Implants can be fitted to patients who are profoundly deaf (>90dB loss), who gain very little benefit from conventional hearing aids. In multi-channel implants, a number of bipolar electrodes are inserted into the cochlea, terminating at different places. Electrical current derived from band-pass filtering sound can stimulate selectively auditory nerve fibers near the electrode, giving some crude 'place' coding of frequency.

The best patients can understand careful speech over the telephone, but there is a great deal of variation across patients, which may be partly due to the integrity of the auditory nerve and higher pathways. It is increasingly common to fit cochlear implants to profoundly deaf children, so that they gain exposure to spoken language. This move raises ethical issues, as well as social ones for the signing deaf community, some of whom oppose implants.

3 WHAT YOU SHOULD KNOW.

You should understand the meaning of all the terms shown in italics. You should also be able to explain all the diagrams in this handout. If you do not understand any of the terms or diagrams, first try asking someone else in the class whom you think might understand. If you still don't, then ask me either in the lecture or afterwards.

Chris Darwin

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