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10.3 Intensity discrimination

The smallest detectable change in intensity has been measured using a variety of psychophysical methods and various stimuli. Although the difference threshold depends on several factors including duration, intensity and the kinds of stimuli on which the measurement is made, Weber's law holds for most stimuli. In other words, the smallest detectable change is a constant fraction of the intensity of the stimulus. Expressed in dB, the minimum change in intensity that produces a perceptual differ
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9.2.2 Method of constant stimuli

This method is similar to that described above but has two advantages over the method of limits. The first is that it's designed to overcome bias inherent in presenting stimuli in a set order. This is done by randomising the order of presentation of stimuli. The subject therefore has no way of anticipating the intensity of the next stimulus (it could be softer or louder than the preceding one). In the table, the stimuli would be presented in a random order: for example, 13 dB SPL, 17 dB SPL,
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9.2.1 Method of limits

To determine an auditory threshold using the method of limits, one would begin with an undetectable stimulus and then gradually increase the intensity until the subject detects it. Results from a hypothetical method of limits study are shown in Table 1. Stimulus intensity is shown in the first column and the subject's response to each stimulu
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9.2 Absolute thresholds

The absolute threshold or absolute limen is the smallest value of a stimulus that an observer can detect. The concept of an absolute threshold assumes there is a precise point on the intensity or energy dimension that, when reached, becomes just perceptible to the observer and he or she responds ‘yes – I can detect the stimulus’. It follows that when the stimulus is one unit weaker it will not be detected. If this were the case then some form of hypothetical curve, like th
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7.3 The descending auditory pathway

The auditory system transmits information from the cochlea to the auditory cortex. Another system follows a similar path, but in reverse, from the cortex to the cochlear nuclei. This is the descending auditory pathway. In general, the descending pathway may be regarded as exercising an inhibitory function by means of a sort of negative feedback. It may also determine which ascending impulses are to be blocked and which are allowed to pass to other centres in the brain. The olivocochlear bundl
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7.2.2 The ‘where’ pathway

The ‘where’ pathway involves the ventral cochlear nuclei, the superior olivary complex and the inferior colliculus. The superior olivary complex is composed of the lateral superior olive (LSO) and the medial superior olive (MSO).

The neurons in the superior olivary complex are the first brainstem neurons to receive strong inputs from both cochleae and are involved in sound localisation.

The MSO receives excitatory inputs from the cochlear nuclei on both sides and is tonotopica
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7.2 Coding of information in the higher auditory centres

We have seen that in the cochlear nerve, information about sound intensity is coded for in two ways: the firing rates of neurons and the number of neurons active. These two mechanisms of coding signal intensity are found throughout the auditory pathway and are believed to be the neural correlates of perceived loudness. The tonotopic organisation of the auditory nerve is also preserved throughout the auditory pathway; there are tonotopic maps within each of the auditory nerve relay nuclei, the
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7.1 The ascending auditory pathway

Up till now we have dealt with the anatomy of the auditory periphery and how the basic attributes of sound are coded within the auditory periphery. A great deal of additional processing takes place in the neural centres that lie in the auditory brainstem and cerebral cortex. Because localisation and other binaural perceptions depend on the interaction of information arriving at the two ears, we need to study the central auditory centres, since auditory nerves from the two cochleae interact on
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6.2 Number of neurons hypothesis

In addition to an increase in firing rate of neurons with differing dynamic ranges, the inclusion of discharges from many fibres whose CFs are different from those of the stimulus may also help to account for the wide dynamic range of the ear. You know from Section 3.3 that in response to a pure tone stimulus the basilar membrane vibrates maximally at a g
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3.8 Revision questions

Question 1

Discuss the two ways in which the middle ear increases the effectiveness with which sound is transmitted from the external ear to the inner ear.

Answer
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3.6 Synaptic transmission from hair cells

In addition to being sensory receptors, hair cells are also presynaptic terminals. The membrane at the base of each hair cell contains several presynaptic active zones, where chemical neurotransmitter is released. When the hair cells are depolarised, chemical transmitter is released from the hair cells to the cells of the auditory nerve fibres. Excited by this chemical transmitter, the afferent nerve fibres contacting the hair cells fire a pattern of action potentials that encode features of
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3.5.3 Mechano-electrical transduction is rapid

Many other sensory receptors, such as photoreceptors and olfactory neurons, employ second messengers in the transduction process. This is not true for hair cells. The rapidity with which they respond makes this impossible. In order to deal with the frequencies of biologically relevant stimuli, transduction must be rapid. The highest frequency humans can hear is about 20 000 Hz. This in effect means that hair cells must be able to turn current on and off 20 000 times per second (200 000 tim
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3.5 Neural transduction

The critical event for the transduction of sound into a neural signal is the bending of the stereocilia of the hair cells. In this section we will examine how the flexing of the basilar membrane leads to the bending of the stereocilia and the production of a neural signal.


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3.4 The organ of Corti and hair cells

We have established that the vibration patterns of the basilar membrane carry information about frequency, amplitude and time. The next step is to examine how this information is converted or coded into neural signals in the auditory nervous system. To do so, we must look at the organ of Corti in some detail since it is here that the auditory receptor cells that convert mechanical energy into a change in membrane polarisation are located.

As we saw in
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Summary

The ear is made up of the outer, middle and inner ears. The outer ear consists of the pinna, the external auditory canal and the tympanic membrane. The middle ear is air-filled and contains the middle ear ossicles. The inner ear is fluid-filled and contains the cochlea, the semicircular canals and the vestibule.

Sound in the external environment is channelled into the auditory meatus by the pinna and impinges on the tympanic membrane causing it to vibrate. These vibrations are transmitt
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3.3 The role of the basilar membrane in sound reception

So far we know that sound-induced increases and decreases in air pressure move the tympanum inwards and outwards. The movement of the tympanum displaces the malleus which is fixed to its inner surface. The motion of the malleus and hence the incus results in the stapes functioning like a piston – alternately pushing into the oval window and then retracting from it. Since the oval window communicates with the scala vestibuli, the action of the stapes pushes and pulls cyclically on the fluid
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3.1 Introduction

The inner ear (Figure 3) can be divided into three parts: the semicircular canals, the vestibule and the cochlea, all of which are located in the temporal bone. The semicircular canals and the vestibule affect the sense of balance and are not concerned with hearing. However, the cochlea, and what goes on inside it, provides
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1 Sound reception: the ear

In order to hear a sound, the auditory system must accomplish three basic tasks. First it must deliver the acoustic stimulus to the receptors; second, it must transduce the stimulus from pressure changes into electrical signals; and third, it must process these electrical signals so that they can efficiently indicate the qualities of the sound source such as pitch, loudness and location. How the auditory system accomplishes these tasks is the subject of much of the rest of this block. We will
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Learning outcomes

By the end of this unit you should be able to:

  • distinguish between the major anatomical components of the outer, middle and inner ear;

  • describe the function of the outer, middle and inner ear;

  • describe the structure of the cochlea;

  • describe the structural arrangements of the organ of Corti and the function of the basilar membrane;

  • explain the difference between the four coding mechanisms used in order to transmit inform
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Richard Feynman

Richard P. Feynman (1918–1988)

Figure 36
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