10.1 Absolute thresholds

The human ear has incredible absolute sensitivity and dynamic range. The most intense sound we can hear without immediate damage to the ear is at least 140 dB above the faintest sound we can just detect. This corresponds to an intensity ratio of 100 000 000 000 000:1. In this section, we examine how the loudness of a sound can be measured and how the perception of loudness is affected by the intensity and duration of the signal.

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9.3.1 Weber's Law

Pioneering work on the relationship between ΔI and S was done by the German physiologist, Ernst Weber in the 1830s. Weber found that the increment in stimulation required for a JND was proportional to the size of the stimulus. Weber had subjects lift a small ‘standard’ weight (S) and then lift a slightly heavier ‘comparison’ (T) weight and judge which was heavier. He found that when the difference between the standard and comparison weights was small, the subjects found
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9.3 Differential sensitivity

Absolute thresholds represent only one type of threshold; one could also ask whether the subject can detect a difference between two stimuli. The threshold for detection of difference is called a difference threshold or difference limen (DL). The difference threshold is a measure of the smallest detectable difference between two stimuli. Basically it answers the psychophysical question: ‘How different must two stimuli (e.g. two weights, two colours, two sounds) b
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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 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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9.1 Introduction

Psychophysics is the oldest field of the science of psychology. It stems from attempts in the nineteenth century to measure and quantify sensation. It attempts to quantify the relationship between a stimulus and the sensation it evokes, usually for the purpose of understanding the process of perception. Historically, psychophysics has centred around three general approaches. The first involves measuring the smallest value of some stimulus that a listener can detect – a measure of sensitivit
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8.1 Introduction

We have learned so far that physical energy from the environment is transduced into electrochemical messages that affect the nervous system and give rise to psychological experiences, that is, produce sensations and perceptions. Sensation refers to the initial process of detecting and encoding environmental energy. The first step in sensing the world is performed by receptor cells, which in the case of hearing are the hair cells in the cochlea. Perception on the other hand, generally refers t
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7.5 More revision questions

Question 6

Describe how phase locking transmits information about the frequency of a sound (include the volley principle).

Answer

Phase locking is the consiste
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7.4 Summary of Section 7

Fibres of the cochlear nerve synapse on the cells of the cochlear nuclear complex which is the first station of the central auditory pathway. From here signals are sent to the superior olivary complex, the inferior colliculus, lateral lemniscus, medial geniculate nucleus and finally the auditory cortex. The central role of the auditory cortex is the processing of complex sounds.

Each cochlear nuclear complex receives input from only one ear. In the cochlear nuclear complex are several d
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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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5.3 Metabolism

Molecules diffuse more slowly at low temperature: measurements of the rates of diffusion of small molecules such as lactic acid, Ca2+ and analogues of glucose and ATP through fish muscles produced Q10 values of 1.75–2.04 between 5 and 25° C. Nearly all enzyme reactions are slower at low temperatures (although sometimes whole pathways can be faster if an inhibitor is more inhibited by low temperature than are the catalysts). So, in the absence of temperature compensat
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5.2.1 Blood pigments

The solubility of oxygen (and of many other gases) in water increases with decreasing temperature: at 0° C, seawater holds 1.6 times as much oxygen when saturated as at 20° C. This fact, and continual disturbance by frequent storms, mean that the surface waters of polar oceans are very well oxygenated. A family of 17 species of nototheniid fish, the Channichthyidae, have no erythrocytes, no haemoglobin and almost no myoglobin at all stages of the life cycle (Section 1.5).

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4.3 Humans in polar regions

Humans evolved in tropical Africa and gradually colonized colder climates during the Pleistocene ice ages. There have been permanent populations in the Arctic for several thousand years, mostly Inuit (Eskimos) in what are now Canada, Alaska and Greenland, and several groups in northern Europe and Russia, such as the Saami (Lapp) in Scandinavia and the Chukchi in Siberia. Such people do not grow crops and keep only a few domestic animals, mostly for transport (e.g. husky dogs or reindeer), not
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3.5 The structure of adipose tissue

Since food is only available seasonally or intermittently at high latitudes, many arctic birds and mammals, including polar bears, Svalbard reindeer, arctic foxes, seals and walruses, naturally accumulate large stores of fat. The quantity of energy stored and the metabolic control of its use are finely adjusted to the habits and habitat of the species. This section is concerned with the cellular structure and anatomical organization of adipose tissue in such naturally obese species. Most labo
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1.1.1 The chemical structure of DNA

This unit explores the chemical nature of the genome. Genomes are composed of DNA, and a knowledge of the structure of DNA is essential to understand how it can function as hereditary material. DNA is remarkable, breathtakingly simple in its structure yet capable of directing all the living processes in a cell, the production of new cells and the development of a fertilized egg to an individual adult.

DNA illustrates beautifully the precise relationship between molecular structure and b
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Learning outcomes

After studying this unit you should understand:

  • the basic composition and structure of DNA;

  • what is meant by complementary DNA base pairing;

  • how base pairing allows a mechanism for DNA replication;

  • the number of DNA molecules within a chromosome.


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4 From DNA to RNA: transcription

In the process of transcription, the information in a gene, i.e. the DNA base sequence, is copied, or transcribed, to form an RNA molecule. RNA is therefore an intermediary in the flow of information from DNA to protein. Before we consider the details of transcription, we will first look at the structure of RNA.

The name ribonucleic acid suggests that RNA is chemically related to DNA. Like DNA, RNA is a chain of nucleotides.

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3 The flow of information from DNA to RNA to protein

The information flow from DNA to protein is more complex than shown in Figure 1. The genetic information encoded within the DNA of a gene is carried via an intermediary molecule, RNA (ribonucleic acid). Information within a cell can therefore be seen as passing from DNA, via RNA, to a protein. This flow of information can b
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