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12.7 Summary of Section 12

For precise localisation of a sound source, binaural cues are required.

Two types of binaural cue are used to localise non-continuous sounds in the horizontal plane: interaural time differences, which are most efficient for low-frequency sounds (20–1500 Hz) and interaural intensity cues, which are important for high-frequency sounds (1500–20 000 Hz). The frequency responses in the superior olive reflect these differences. The medial superior olive includes neurons that are responsiv
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12.6 Distance cues

There are two main cues available that allow us to judge the distance to a sound source. The first of these is the sound pressure level. Sound pressure level drops by 6 dB each time the distance that a sound travels doubles. In other words, if the sound pressure level of a sound is 60 dB SPL when its source is 1 m from you, then it will be 54 dB SPL if you move back another metre so that you are now 2 m away from its source. Therefore lower sound pressure levels indicate a greater distance. A
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12.4 Interaural intensity differences

The brain has another process for localizing high-frequency sounds (above 1500 Hz): interaural intensity differences.

Where does processing of interaural intensity differences take place?


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12.3 Interaural time delays: continuous tones

Coincidence detectors and delay lines cannot be used to localise a continuous tone.

Why?

Answer

Because, a continuous tone is always present at both ears and if we
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12.2 Interaural time delays: non-continuous sounds

The average distance between human ears is about 20 cm. Therefore, if a sudden noise comes at you from the right, perpendicular to your head, it will reach your right ear 0.6 ms before it reaches your left ear. For a sound coming from directly in front of you there will be no delay, and at angles between, the delay will be between 0 and 0.6 ms. Therefore there is a simple relationship between the location of the sound source and the interaural delay. It is this delay that enables us to locali
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12.1 Localisation of sound in the horizontal plane

While information about frequency and intensity is essential for interpreting sounds in our environment, sound localisation can be of critical importance for survival. For example, if you carelessly cross the street, your localisation of a car's horn may be all that saves you. Our current understanding of the mechanisms underlying sound localisation suggests that we use different techniques for locating sources in the horizontal plane and vertical plane.

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11.4 Signal duration

Since hearing is largely a matter of stimulus reception over time, we would expect time to influence the perception of sound. It has been known for many years that both absolute thresholds and the loudness of sounds depend upon signal duration. The studies of absolute threshold described earlier were all carried out with tone bursts of relatively long duration. For durations exceeding 500 ms, the sound intensity at threshold is roughly independent of duration. However for durations of less th
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11.3 Frequency selectivity

In preceding sections we examined two ways in which the auditory system may code frequency information: the place theory and phase locking. In this section we will look at the psychophysical evidence for place coding on the basilar membrane by examining the ability of the auditory system to resolve the components of sinusoidal waves in a complex sound – a phenomenon known as frequency selectivity.

The perception of a sound depends not only on its own frequency and intensity but also o
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3.7.1 Summary of Sections 3.4 to 3.7

Hair cells are found in the organ of Corti and run the length of the basilar membrane. They transform mechanical energy into neural signals.

When the basilar membrane vibrates in response to sound, hair cells located at the site of maximal vibration on the basilar membrane are stimulated. This means that the mechanical properties of the membrane allow the auditory system to distinguish one frequency from another by the location on the membrane that is maximally excited by a particular f
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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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3.2 The anatomy of the cochlea

The cochlea has a spiral shape resembling the shell of a snail (Figure 4a). You can approximate the structure of the cochlea by wrapping a drinking straw 2.5 times around the tip of a sharpened pencil. The hollow tube, represented by the straw, has walls made of bone and the central pillar of the cochlea, represented by the pencil, is a conical
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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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2.6.2 End-of-unit questions

Question 8

Express the following numbers using scientific (powers of ten) notation:

  • (a) 2.1 million

  • (b) 36 000

  • (c) 1/10

  • (d) 0.00005


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2.3 The irreversible Universe

‘Science owes more to the steam engine than the steam engine owes to Science.’

L.J. Henderson (1917)

From the time of Newton until the end of the nineteenth century the development of physics consisted essentially of the refinement and extension of the mechanical view of the Universe. There were many stages in this process but one of the most interesting came towards its end with the re
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2.2.2 Energy and conservation

Newtonian mechanics is concerned with explaining motion, yet it contains within it the much simpler idea that some things never change. Take the concept of mass, for example, which appears throughout Newtonian mechanics, including the law of gravitation. In Newtonian mechanics, mass is conserved. This means that the mass of the Universe is constant and the mass of any specified collection of particles is constant, no matter how much rearrangement occurs within the system. A chemist might take
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5.3 GM Nation? The public debate

The key objective of the national dialogue on GM was to allow the exchange of views and information – members of the public would presumably learn more about the issues; experts and policy makers would learn more of the reasoning behind the public's concerns.

Question 16


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2.3.1 Applying the principles

Trying to use ‘guiding principles’ of this type does not make assessment straightforward. For example, such principles can't be rigidly applied in an abstract way, reflecting absolutes such as what is ‘right’ or ‘wrong'; their operation depends on context.

We can explore this further by attempting to apply the third of these principles. Justice might be considered to involve directing the benefits of a new technology to those who need it most. At the same time, the role of pol
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Introduction

This unit is an adapted extract from the course Science in context (S250)

In recent years, scientists have made huge gains in their understanding of how genes can be altered and transferred from one organism to another – but that knowledge has been acquired amidst controversy and concern. The deep ethical concerns that have resulted from the emergence of genetic manipulation are explor
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1.5 Star clusters and stellar evolution

Detailed observations of star clusters suggest that they occur because the stars in them form at about the same time. Moreover, the compositions of the stars are similar. Isolated stars (including isolated binary stars) result from the later partial or complete dispersal of a cluster.

The crucial points for us here are that all the stars in a cluster formed at about the same time, and all have similar compositions.

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7 Conclusion

In this unit we have studied animals in the context of their own habitat rather than using the traditional comparative physiology approach of comparing organ systems in different species. Although we have looked at extreme habitats, specifically deserts, it has become clear that, for many species, extreme physiological adaptations are not present and that even endotherms, birds and mammals rely on behavioural strategies, thereby reducing the need for physiological strategies that are costly i
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