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11.2 Frequency discrimination

Some findings indicate that, for moderate loudness levels, humans can detect a frequency change of about 1 to 3 Hz for frequencies up to about 1000 Hz. Figure 37 shows a plot of the smallest frequency difference for which two tones can be discriminated for a number of reference tones. You can see from the figure that up to about 1000 Hz, the D
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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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6.1 Firing-rate hypothesis

Information about stimulus intensity is encoded in two ways: the firing rates of neurons and the number of active neurons.

Intensity is assumed to be encoded by an increase in discharge rate of action potentials within the auditory system. As the stimulus gets more intense, the basilar membrane vibrates at a greater amplitude causing the membrane potential of activated hair cells to be more depolarised and this causes the nerve fibres that synapse onto the hair cells to fire at a greate
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4 Neural processing of auditory information

In this section we will look at how the frequency selectivity found along the basilar membrane is preserved or modified by the auditory nerve and how information about the intensity of the signal is encoded in the response of the auditory nerve fibres.

The nerve that communicates with or innervates the hair cells along the basilar membrane is called the vestibulocochlear nerve or VIIIth cranial nerve. It enters the brainstem just under the cerebellum and conveys information from
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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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3.7.1 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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3.5.2 Mechanical force directly opens and closes transduction channels

It is believed that tip links aid in causing ‘channels’ to open and close near the top of the hair cell (Figure 16). Tip links are filamentous connections between two stereocilia. Each tip link is a fine fibre obliquely joining the distal end of one stereocilium to the side of the longest adjacent process. It is thought that each l
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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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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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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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2.5.2 Quantum fields and unification

From its inception, quantum physics was concerned not just with particles such as electrons, but also with light and other forms of electromagnetic radiation. In 1900 Planck discovered the quantum in the transfer of energy from matter to radiation, and in 1905, Einstein's explanation of the photoelectric effect assumed that the transfer of energy from radiation to matter occurred in a similarly quantised fashion. It is therefore hardly surprising that the development of quantum mechanics was
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Faraday and Maxwell

Michael Faraday (1791–1867)

Figure 21
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2.4.1 Electromagnetism and fields

When Newton wrote about ‘The System of the World’ in Part 3 of Principia, the only forces he could discuss in any detail were the contact forces that arose when one object touched another, and gravity, which acted at a distance. Even so, Newton thought that there were other forces at work in the world, and hoped they might eventually be brought within his overall scheme just as gravity had been. In fact, Newton wrote:


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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 The outcomes of the public debate

Box 2 contains an edited version of the Executive Summary of the document GM Nation? Findings of the Public Debate. This is a lengthy summary, but it is worth exploring in some detail. The unedited version can be found on http://www.gmnation.org.uk/.

Box 2:
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4.1 Introduction

Pusztai and his team were attempting to develop suitable tests to assess the safety of GM potatoes. Typically, testing the safety of GM food involves comparing its composition and/or its effects with that of the conventionally produced food it most closely resembles. We have seen that such comparisons were at the heart of Pusztai's work.

The comparison of GM and conventional crops and food has led to the so-called principle of substantial equivalence, which has been used extensiv
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3.3 Drawing conclusions

Sections 2.1 and 2.2 have summarised some of the major aspects of the Pusztai affair, but it should be said that almost every detail has been the subject of prolonged and heated dispute. Our purpose is not to attempt to denigrate individuals or institutions. Rather, the hope is that the tale carries some general messages of value about how science is undertaken and communicated, which can sometimes become clearer when things go wrong.

Communicating preliminary scientific information via
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4 Questions

Question 7

In what ways, if any, does the distance to a star influence its position on an H–R diagram?

Answer

The distance to a star does not influence it
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2.5 Using stars to probe the interstellar medium

The effects of interstellar material on starlight can be used to probe the properties of the interstellar medium itself. A few examples are:

  • The presence of particular interstellar atoms or molecules may be determined by identifying the observed spectral lines or bands.

  • The temperature of the gas may be determined from the relative strengths of different lines or bands produced by different energy state changes of the same atom or mol
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