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The Auditory System

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					The Auditory System



    Sound is created by pressure waves in air; these waves
    are induced by vibrating membranes such as vocal
    cords. Because the membranes usually vibrate in a
    regular manner, the pressure waves have a fixed
    spacing.




    Sound has two main characteristics: A. the spacing
    between waves or period. This can also be thought of in
    terms of how many wave cycles pass by in one second-
    that is, the sound’s frequency in Hz=cycles/second.
    B. The intensity of amplitude of the sound measured in
    decibels.

      Bear et al.
                               The Auditory Periphery




Bear et al.                                           Kandel et al.


The outer ear and canal guide and filter sound. The tympanic membrane and ossicles transmit
the vibrations to the cochlea itself; the vibrations enter the cochlea via the round window and
exit via the round window. As they pass through the endolymph of the scala vestibuli and
tympani, sound waves cause the basilar membrane to vibrate. This is the key to auditory
function.
              The Cochlea


                            Sound waves cause the basilar
                            membrane to vibrate.




                        The basilar membrane is stiff at its base
                        and loose at its apex. Just like a guitar
                        string, this causes it to resonate to high
                        frequencies at its base and low
                        frequencies at its apex. There is
                        therefore a tonotopic map- a location
                        code- formed on the cochlea.This is a
                        fundamental feature of auditory coding
                        and is preserved right up to cortex.
                        Note that this is a separate code from
                        synchronization.



Bear et al.
Bear et al.
                   Hair Cells

                                       Hair cells transduce vibrations into
                                       depolarization. This in turn leads
                                       to vesicular release that excites
                                       auditory afferent fibers and
                                       causes them to discharge.




              Hair cells are firmly attached to the basilar membrane
              and therefore move up and down with it as it vibrates.
              The “hairs” or cilia of these cells are attached to a
              tectorial membrane; this membrane is fixed- it does not
              vibrate in response to sound. So, as you can imagine,
              when the basilar membrane moves upward, the cilia will
              be bent. This is the first step in the transduction process.

              A scanning electron microscopic view of the beautiful
              organization of hair cell cilia in the cochlea.



              Kandel et al.
                                          Hair Cells 2




When the cilia bend in one direction it       This effect is due to the mechanical coupling of the
causes the hair cell to hyperpolarize;        cilia to K+ channels at their tips. The depolarization
bending in the opposite direction             causes Ca2+ entry and the fusion of vesicles and
causes depolarization.                        release of glutamate from hair cells. This cause
                                              excitation and spiking of the auditory afferent
                                              fibers.
Bear et al.
                             Auditory Afferent fibers




Each auditory afferent fiber is tuned
to a specific frequency.                      The tuning is simply due to the location of its
                                              hair cell along the cochlea. This tonotopic
                                              mapping is preserved in the projection of
                                              cochlear afferents to the cochlear nuclei in
                                              the medulla.


                              As I mentioned earlier, cochlear afferents are also phase locked
                              to sound (especially low frequency sounds). So there are two
                              ways to code for sound. A firing rate “place” code (tonotopy)
                              and a temporal code. The central auditory system uses both
                              codes for various purposes.
                              This is a general principle. Sensory systems are flexible and
                              can use multiple coding strategies.
   Bear et al.
        Central Auditory Pathways




                                Auditory reach the cochlear nuclei of the
                                medulla (DCN, VCN). From these nuclei
                                a direct pathway goes to the inferior
                                colliculus, then the thalamic medial
                                geniculate nucleus (MGN) and then onto
                                the auditory primary cortex.
                                Note that this pathway is bilateral unlike
                                the contralateral somatosensory
                                pathway. This makes sense since sound
                                always reaches both ears.


                On the way up to cortex axons from VCN also terminate
                in nuclei of the superior olivary complex. Neurons in this
                cell group use relative sound timing and intensity in the
                two ears to estimate the spatial location of a sound
                source.

Bear et al.
                                    Sound Localization




                                             A cell in the superior olive responds with an increase
                                             in firing rate to a time difference in the arrival of
                                             sound to the ears. This cue to the sounds location is
                                             conveyed up to the inferior colliculus and onto cortex.
                                             Relative sound intensity is also used as a cue in
Sounds coming from the right arrive          different neurons in the superior olive region.
at the right ear a little earlier than the   The neural mechanisms involved are becoming
at the left ear. This small time             understood but are beyond the scope of an
difference is used by the superior           introductory course.
olive.
Bear et al.
                         Cortical Processing of Sounds
Bear et al.



                                                          The MGN projects up to the primary
                                                          auditory cortex (there are secondary
                                                          auditory areas as welll). Notice that
                                                          the cortex still has tonotopy.
                                                          However auditory cortex neurons
                                                          also respond to complex features of
                                                          sound such as modulations of
                                                          amplitude or frequency. If you pay
                                                          attention to speech or music you will
                                                          hear many examples of both kinds of
                                                          modulation.


   Auditory cortex projects to numerous secondary cortical areas including multisensory areas
   (allow us to recognize animals or other humans by both sound and sight) and to regions
   specifically involved in communication. Communication and environmental sounds are
   separated after the AC.
   It is also noteworthy that the AC and MGN project to the amygdala; as we’ll see later this
   permits sounds to be linked with dangerous stimuli (fear conditioning for conditioned
   avoidance).
               Processing of Natural Sounds
Nelken, 2004



                         Acoustic input typically comes from many different
                         sources; they have different combinations of
                         frequencies, their amplitudes change
                         independently and they have different locations.
                         But the sound waves coming into the ears are just
                         the sum of all these different acoustic signals.

                         The auditory system then separates these
                         acoustic inputs to generate the “sounds” we hear-
                         the “auditory scene”. This takes extensive learning
                         early in life but the mechanisms are not
                         understood.

                         One major point is that the both the signal fine
                         structure (the frequencies present) and the
                         envelope (amplitude modulation of the individual
                         frequencies) must be extracted and connected
                         with their location.

                         This process occurs in cortex.

				
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posted:7/9/2011
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