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Hearing is a familiar and important human sense that is a topic naturally of interest...
Hearing is a familiar and important human sense that is a topic naturally of interest to those who are curious about human biology. This unit will enable you to relate what you read to your own sensory experiences – and indeed many of the questions asked have exactly that function. This unit will be best understood by those with some biological understanding.
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 information form the ear to the brain;
- describe the ascending auditory pathway and the funciton of the main nuclei involved;
- describe the basic principles of psychophysics;
- explain the difference between intensity and loudness and between frequency and pitch;
- describe the use of intensity and timing cues in sound localisation;
- decribe the main causes of hearing impairments and the methods used to rehabilitate hearing-impaired individuals.
- Learning outcomes
- 1 Sound reception: the ear
- 2 Structure and function
- 3 The structure and function of the inner ear
- 3.1 Introduction
- 3.2 The anatomy of the cochlea
- 3.3 The role of the basilar membrane in sound reception
- 3.4 The organ of Corti and hair cells
- 3.5 Neural transduction
- 3.6 Synaptic transmission from hair cells
- 3.7 Hair cell tuning
- 3.8 Revision questions
- 4 Neural processing of auditory information
- 5 Frequency coding in cochlear nerve fibres
- 6 Intensity coding
- 7 The central auditory nervous system
- 8 Auditory perception
- 9 Psychophysics
- 10 The perception of intensity
- 11 The perception of frequency
- 12 Sound localisation
- 12.1 Localisation of sound in the horizontal plane
- 12.2 Interaural time delays: non-continuous sounds
- 12.3 Interaural time delays: continuous tones
- 12.4 Interaural intensity differences
- 12.5 Localisation of sound in the vertical plane
- 12.6 Distance cues
- 12.7 Summary of Section 12
- 12.8 More revision questions
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 medial geniculate nucleus (MGN, labelled meidal geniculate body in Figure 27) and the auditory cortex. Conversion from frequency to position that originates on the basilar membrane is maintained all the way up to the auditory cortex. One source of information about sound frequency is therefore derived from tonotopic maps; the location of active neurons in the auditory nuclei and in the cortex is an indication of the frequency of a sound. Phase locking as a means of frequency coding is also present in centres further along the pathway.
There are, in fact, two distinct pathways that occur in the CANS:
The ‘what’ pathway which is monaural and receives information from only one ear. This pathway is concerned with the spectral (frequency) and temporal (time) features of a sound and is hardly concerned with the spatial aspects. It focuses mainly on identifying and classifying different types of sound.
The ‘where’ pathway which is binaural and receives information from both ears. It is involved in the localisation of a sound stimulus.
Despite the apparent dichotomy of these two processing pathways, the same types of acoustic cues may be important for the analysis that occurs in each. For example, spectral information is used in the ‘where’ pathway for determining a sound's elevation; and temporal information, used for our perception of frequency in the ‘what’ pathway, is also used in the ‘where’ pathway for determining a sound's horizontal location.