1. Auditory plasticity refers to the ability of the auditory system to change in response to experience and environmental conditions. 2. The auditory system develops both anatomically and functionally after birth, with many capabilities like sound detection, discrimination, and localization continuing to mature into childhood. 3. Early auditory experiences, particularly with speech and music, are important for the development and maturation of the auditory system. There appear to be sensitive periods where exposure can influence development.

1. Plasticity
Presenter: Nahid Shamsi

2. Major topics
 Development as auditory plasticity
 The development after birth
 Which auditory capabilities improve after birth?
 The importance of early experience; speech, music
 Maturation of auditory circuits in brain
 Plasticity in adult(Which hearing capabilities )
 Mechanisms of plasticity
 Learning/top-down processes in human
 . Plasticity with forms of altered auditory experience
1. Bilateral HL/deaf/after CI
2. Unilateral HL
3. Tinnitus
 Cross-modal plasticity:Whole-brain level
 Intra-modal plasticity: within the auditory system

3. Development
as an auditory
plasticity
Auditory development is a broadly defined term, which
refers to the fact that perception is influenced by a
combination of innate, genetically programmed changes
in anatomy and physiology, combined with auditory
experience
 the capacity to vary in developmental pattern, in phenotype, or in
behavior according to varying environmental conditions

4. 1.
Anatomic
plasticity
/Embryo and
pharyngeal
arches
Anatomy: Pharyngeal arches are paired structures that
grow on either side of the future head and neck of the
developing embryo and fuse at the centerline.
Pharyngeal arches produce the cartilage, bone, nerves,
muscles, glands, and connective tissue of the face and
neck.

5. Pharyngeal
arches and
neural tube

6. Otic placode
Formation of otic placode Thickening of ectoderm in hindbrain region

7. When does
hearing start?
 Start to invaginate and fold up into otocyst (which form cochlea
and otic ganglion cells and auditory nerve )

8. Otocyst
differentiation

9. neural tube
 Neurons of central auditory pathway : are produced in ventricular
zone of embryo’s neural tube
 Then they migrate to final destination to brain

10. Subcortical
and cortical
auditory
structures
• Subcortical auditory structures and cortical plate by 8 fetal week
• Temporal lobe become apparent in 27 th week of gestation

11. Neural
development

12. Making
specific
synaptic
connection
 After generating neurons
 During migration
 Sending out axons toward their targets
 Chemical guidance cues
 Role of receptors
 Exploring growth cones
 TrkB andTrkC receptor
(BDNF and NT-3)
Topographic map

13. Robust
synaptic
connection
 14th week of gestation:
Innervation of HC
 7 weeks later :
Starting connection
between thalamus and
cortical plate

14. Myelination
 For rapid and reliable connection of AP
 Begins at 26th week of gestation
 First synapses to sound can be measured( ERP)
 First reactions in the end of second trimester
 By the end of pregnancy : not only can sounds, but also can
discriminate between sounds

15. Gene
expression
 Forming the otocyst : expression of proneural bHLH
transcription factor Neurogenin 1 (Neurog1)
 Neuronal precursors: expression of another bHLH
transcription factor, Neurod1
 Specification and segregation of auditory and vestibular
neurons are not fully understood. Probably; Neurog1
expression
 Guidance the topographic map formation: two
neurotrophin receptors,TrkB andTrkC (BDNF and NT-3)

16. After birth
Has the development of auditory system’s anatomy
completed at birth time?

17. “External and
middle ears”
 External canal is shorter and straighter than it is in adults
 the increase in ear canal diameter and length during the first 2
years of life
 the development of the middle-ear cavity volume, which extends
into the late teenage years, which is likely to influence the
mechanics involved in middle-ear function
substantial effects on how sounds are absorbed, processed, filtered,
and transmitted to the auditory system

18. “Basilar
membrane
and cochlea”
 Little is known about maturation of the basilar membrane
 OAEs have become a highly utilized tool for investigating
maturation of cochlear function, and thus also for identifying
immature and abnormal peripheral auditory function.
 At birth, in contrast with the immature outer and middle ear, the
inner ear seems to be more mature, as characterized by OAEs .

19. “Excitatory
and inhibitory
input”
Cortical plate
In animal
ganglionic eminence
(GE)

20. “Thalamocortic
al connection”
SP neuron

21. “cortex layers”

22. Hearing capabilities
improve after birth

23. Quite
sophisticated
capacity to
make sense of
auditory world
in infantsVs
animal species
 Distinguish between different phonemes
 Sensitive to pitch and rhythm of their mother’s voice
 Various aspects of music perception
 Able to distinguish different scales and chords
 Preference for consonant over dissonant
 Sensitive to the beat of rhythmic sound pattern

24. Human infant
vs adult infant
 Threshold detection ( specially in low frequency) : 10 years
 Frequency selectivity (over several years)
 Frequency resolution ( mature earlier)
 Sound localization ( 5 years)
 Binaural masking level ( 5years)
 Precedence effect ( couple of years)
 Backward masking (15 years)

25. Detection of
sound
(Threshold
detection)
 Infants can have thresholds up to 25 dB worse than adults
 a 10–15 dB gap by the time that children reach 5 years of age
 depends on the frequency
Mechanisms:
 Growth of the external ear and increases in the efficiency of
middle ear transmission
 OAE; frequency tuning at the level of the cochlea
 Primary neural maturation is probably also involved in threshold
maturation
 Improvements in synaptic transmission efficiency within the
brainstem

26. Frequency
discrimination
 Definition: the ability to perceive a change in the frequency of
tonal stimuli
 Test : train of tones, and switching them to a tone of difference
frequency misstream, then checking the behavioral changes
 Adult: ~1% change in the frequency
 Results:
1. At 3 month: discrimination ability is poorer for 4000-Hz tones
than 500-Hz tones
2. At 6 months : the pattern is reversed and infants are better at
discriminating changes imposed on 4000-Hz tones than 500-Hz
tones
 Adult-like performance: is reached between 6 and 12 months of
age
 Point: considerable maturation between 3 and 6 months of age/
The importance of memory and attention load

27. Intensity
discrimination
 Definition: the ability to detect a change in the level (in dB sound
pressure level) at which a sound source is presented
 Test : measuring the ability of a listener to detect a change from a
background stimulus
 Adult: can hear differences in intensity as small as 1–2 dB
 Results:
1. Infants between the ages of 5 and 7 months need approximately 6 dB
difference
2. Infants are particularly worse than adults for low-frequency stimuli
(~400 Hz) than high-frequency stimuli (4000 Hz)
 Adult-like performance: continues into childhood
 Point: intensity discrimination is more immature than frequency
discrimination early in life

28. Auditory scene
analysis(ASA)
 The basic process through which the neural
mechanisms involved in perception are able to parse
out various sound sources and assign meaning to
appropriate sound sources
 A more specific example of ASA that occurs for speech
sounds is the cocktail-party effect

29. Energetic
masking
 Definition: In energetic masking, one sound interferes with our
ability to detect or otherwise hear another sound because the two
sounds excite similar auditory neurons in the periphery, thereby
limiting the extent to which information about the target can be
perceived in the presence of the masker
 Results:
 1. thresholds of infants aged 6–24 months are higher than those
of adults by 15–25 dB
 2. A more rapid maturation for hearing signals in noise, with high-
frequency thresholds

30. Auditory
streaming
 Definition: the notion that when listeners are presented with sounds
that share some dimensions and vary along other dimensions, they
perceive them either as one coherent sound, or as two distinct sounds
Results:
 Older children( ages 9–11 years ) like adults, required small
differences between the frequencies of the two tone
 younger children : 8 years old or younger required larger frequency
differences
2. infants by the age of ~4 months infants can use similar acoustic cues
to those used by adults
 Adult-like performance: starts from school-age years
 Point: ASA, as measured with auditory streaming, develops well into
school-age years

31. SPATIALAND
BINAURAL
HEARING

32. Sound
localization
 Definition: a perceptual representation of where sounds are
located relative to the head.
 Test :minimum audible angle (MAA), the smallest change in the
angular position of a sound source that can be reliably
discriminated
 Adult: 10.2° ± 10.72° SD
 Results:
1. Newborn infants orient towards the direction of auditory stimuli
within hours after birth/ unconditioned, or reflexive, integrate
auditory and visual information
2. largest decrease in MAA occurs between 2 months of age and 2
years of age, with continued improvement through 5 years of age
 Adult-like performance: 5 years of age
 Point: auditory cortex plays a key role in determining the ability
of an animal to localize, and to learn to localize or to relearn novel
maps of space

33. Sound
localization
 First is that lesions of the auditory cortex (the primary
auditory cortex (A1) in particular) lead to a reduction in
the ability of animals to relearn spatial hearing maps
 Second, their behavior improves most dramatically
with training and feedback
 non-sensory factors are likely to be involved in the
emergence and preservation of spatial hearing maps.

34. Early experience in
speech , music

35. The
importance of
early
experience;
speech
 Importance of sensory experience
 Reduced auditory input has profound impact on development od
central auditory system
 During the first year : can perceive phonetic contrast in their
mother tongue/ lose sensitivity to sounds in foreign languages
 This process occurs : as early as 6 month ages for vowels and by
10 months for consonant

36. The
importance of
early
experience;
speech
 Over the same period , learn to get sensitive to the correspondence
between speech and talker’s face in their own language
 The importance of social interaction in maturation
 Sensitive period: lasts for about 7 years/other cognitive abilities will
improve
 Example: vocal learning in songbirds
1. Sensitive period
2. Highly variable vocal attempts
3. Auditory feedback during a sensorimotor
4. Crystalized
Speech
Babbling

37. ERP responses
in infancy
 Language functions are lateralized
 But less specialized
 Recorded responses are much slower compared to adult
 ERPs: by 7.5 months of age, the brain is more sensitive to phonetic
contrast in child’s native language than in non-native language
 Neural correlate of word learning: the first year of life
 Violation of syntactic word result in ERP differences : at around 30
months

38. The
importance of
early
experience;
music
 Another related area: Music
 Remarkable advance sensitivity to to different aspect of music
 At first: infant respond similar way to music of any culture
 6 month: sensitive to rhythmic variation in music of different cultures
 12 month: show a culture –specific bias
After that: brief exposure would improve/ adult: it is not the case
Existence of sensitive period in music perception
 Passive exposure : leads to change in neural sensitivity
 Training effect

39. The
importance of
early
experience;
music
 Example: absolute pitch
 musicians with Absolute Pitch have greater volume in their
auditory cortex than musicians without the ability, or non-
musician controls
 Sensitive period
 The difference: left superior temporal sulcus (STS)
 STS :involved in categorization tasks, its activation might
suggest that AP musicians involve categorization region in tonal
tasks.
 Functional difference in auditory BS in musician

40. The
importance of
early
experience;
music
 Musical disorders:
 Functional difference in auditory BS in musician
 Tone deaf:
 unable to perceive differences of musical pitch accurately.
 Occurs in 10 percent of people
 Reduced links between parts of brain which are involved in sound
processing and those responsible for vocal production
 reduction in the size off the “arcuate fasciculus” (a fiber that connects
temporal lobe to frontal lobe of cortex)
 Plasticity??

41. arcuate
fasciculus” in
tone deaf
people

42. Plasticity in adult

43. Maturation of
auditory
circuits in brain
Plasticity of the frequency map:
1. manipulations of the environment
2. changes to the auditory system itself
including associative learning
3. release of neuromodulatory transmitters
4. Aging
5. extended exposure to sounds
6. lesioning of the peripheral receptor surface

44. 1.Spectral
integration
 Point :A critical aspect of any functional map is the
parameter resolution that the map can provide
 Research results:
 1. frequency discrimination training : increased cortical
representation of the trained frequency range in the
tonotopic map/ sharpness of tuning in the range of the
frequency trained

45. Spectral
integration
 2. Spectral integration bandwidth ----- spatial variability and
modulation rate
 Modulated stimuli repeatedly delivered to one site on the
receptor surface: increase spectral bandwidth
 unmodulated stimuli delivered to different locations:
decrease RF size
 Long-term exposure of adult animals to broad-band noise:
increase the spectral bandwidth of neurons
 Training: decrease the spectral bandwidth

46. Response
Magnitude
 Point: associative plasticity can create or refine an intensity-
specific maximal firing rate
 In sound intensity discrimination task: in AI following paired
stimulus reinforcement and instrumental conditioning
paradigms, became more strongly nonlinear
 code for sound intensity within AI can be derived from
intensity-tuned neurons

47. Response
Magnitude
 The primary sensory cortex is positioned at a confluence of:
1. bottom-up dedicated sensory inputs
2. top-down inputs related to higher-order sensory features
3. attentional state
4. behavioral reinforcement
 Enduring receptive field plasticity in the adult auditory cortex
may be shaped by :
1. task-specific top-down inputs that interact with bottom-up
sensory inputs
2. reinforcement-based neuromodulator release

48. Response
Timing
 The relative and absolute timing of cortical responses is
a highly relevant aspect of cortical processing
 Plasticity effects on the timing of cortical responses
 Behavioral training and nucleus basalis stimulation
can enhance the ability of cortical neurons to phase-
lock to faster amplitude modulation signals
 Training and enrichment:
1.faster and briefer responses to sound onsets
2. enhanced phase-locking to modulated sounds
 Long-term exposure to broad-band noise resulted, by
contrast, in longer peak latencies and longer
response durations

49. Response
Timing

50. Sound
Location
 At first : neural sensitivity to ILD and ITD are
observed in MSO and LSO
 After experience : inhibitory projection from MNTB
to LSO…. Neural sensitivity to ILD
 Many of connections die off and those remain
……switch to inhibitory
 After experience: given to the size of head,
precisely timed inhibitory input to MSO …. Neural
sensitivity to ITD

51. MSO after
auditory
experience

52. Importance of
central
auditory
factors /
Spatial tuning
of neurons in
the inferior
colliculus
 Larger amount after
birth

53. Conflicting
auditory inputs
Optic
tectum
SC

54. Manipulating/
conflicting
visual input

55. optic tectum
an IC

56. Manipulating/
conflicting
visual input
 Videos
 Barn owl

57. Mechanisms
of Map
Plasticity
 representational maps of auditory features remain
plastic throughout the lifespan
 How about “rules”?
 Age with sensitivity to passive experience:
onset of hearing and ending at some time before sexual
maturity
 repeated presentation of artificial meaningless sounds
in adult animals: have no long-term effect on map
organization (aversive)

58. Mechanisms of
Map Plasticity
 tonotopic remapping in subcortical auditory nuclei :
longer exposure periods and are more transient than
tonotopic map plasticity in AI
cortex may be the primary site of plasticity

59. 1.Stimulus-
specific
adaptation
(SSA)
 Stimulus-specific adaptation is a reduction in the
response of a neuron to a repeated stimulus
1.auditory cortex (AI)
2.midbrain and thalamus (IC) and (MGB)
 Which parts:
(the external and dorsal cortices of the IC and the medial and dorsal divisions
of the MGB)

60. 1.Stimulus-
specific
adaptation
(SSA)
Example of SSA: “oddball” paradigm( (“deviants”) (“standard”).
 mis-match negativity (MMN)
 mechanisms of auditory : adaptation of narrowly tuned modules
(ANTM) model

61. 1.Stimulus-
specific
adaptation
(SSA)
 SSA has many properties in common with behavioural
habituation
 auditory cortical habituation:
1. little attention
2. a decrease in the responses of layer 2/3 pyramidal
neurons………. activity of somatostatin-expressing
inhibitory neurons

62. 2.
Neuromodulator
y and synaptic
mechanisms of
plasticity
 most forms of auditory cortical plasticity are changes in synaptic
efficacy within existing patterns of connectivity

63. 2.
Neuromodulator
y and synaptic
mechanisms of
plasticity/ACh
 The neocortex receives diffuse extra thalamic projections from five
different subcortical cell groups in learning-related cortical
plasticity
 cholinergic and noradrenergic systems arising in the nucleus basalis
(NB) and locus coeruleus, respectively
 A particular frequency shifted the tuning of AI neurons towards the
stimulation frequency, such that there was an expanded
representation of that frequency
 produces a stimulus-specific rapid reduction in inhibition
 followed by an increase in excitation at the paired frequency

64. 2.
Neuromodulator
y and synaptic
mechanisms of
plasticity/GABA
 In contrast to the changes in frequency selectivity associated with
learning and attention, those produced by cochlear lesions do
not involve cholinergic modulation

65. 2.
Neuromodulat
ory and
synaptic
mechanisms of
plasticity/
GABA
 Shortly after the onset of hearing:
1. GABA-mediated two-tone suppression is weak
2. GABAA receptor subunit composition is immature
3. inhibitory synaptic currents are sluggish with
frequency tuning that is not yet precisely co-
registered with excitation
 As sound-evoked inhibition becomes sharper and more
robust, repeated exposure to pure tones is no longer
able to induce a long-term remodeling of frequency
tuning

66. 2.
Neuromodulator
y and synaptic
mechanisms of
plasticity/GABA
 STD: short-term depression
 STF: short-term facilitation
 PTP: posttetanic potentiation
 LTD: Long-term depression
 LTF : Long-term potentiation

67. Mechanisms
and unsolved
mysteries
underlying
auditory
cortical map
reorganization

68. 3.Learning/top
-down
processes
 The degree of plasticity may determine the strength of learning
 Classical conditioning with a tonal conditioned stimulus (CS):
1. an increase in the response at the CS frequency
2. Increase in contrast sensitivity
3. decrease in response at the pre-training best frequency (BF)
 Can be very quick
 Short-live or long lasting(based on task)
 First stage: reduction in auditory cortex inhibition
 Role of the cholinergic input originating in the nucleus basalis
(NB)
 Modulated by top-down influences from “higher-order” cortical
areas mediating attention

69. 3.Learning/top
-down
processes
Activation of FC neurons
sources of these neuromodulatory systems
changes in AI auditory responses(sometimes belt)

70. 3.Learning/top
-down
processes
 connections between the auditory system and the lateral
amygdala (LA)
changes in AI auditory responses(sometimes belt)

71. 3.Learning/top
-down
processes
 Only auditory cortical ?
 similar effects of learning and attention have been
reported in the medial geniculate body (MGB) and
inferior colliculus (IC) as centrifugal influences from
auditory cortex

72. 3.Learning/top
-down
processes in
human

73. 4. Plasticity
with forms of
altered
auditory
experience/
disorders
 Compensatory plasticity
 Cross-modal plasticity
 Intra-modal plasticity
 Cross-modal plasticity and intra-modal plasticity

74. Cross-modal
plasticity:
Whole-brain
level
1.Visuo-auditory plasticity in deafness:
 attempts to compensate: contextual cues, including speech-
reading
 activation of superior temporal lobe ,or primary part by
speechreading
2.Visuo-auditory plasticity after cochlear implantation
 Audiovisual integration(35%)
 Cortical plasticity and audiovisual integration

75. Cortical
plasticity and
audiovisual
integration

76. Intra-modal
plasticity:
within the
auditory
system
Brain plasticity after
unilateral hearing loss
 5weeks
 physiological and
cytoarchitectonic
mechanisms described :
1. a loss of contralateral
inhibition
2. 2.reinforcement of
the number of fiber
connections along the
healthy ear auditory
pathway

77. Brain plasticity
after unilateral
hearing loss
 The lateralization in NH
subjects : contralateral
 In unilateral deafness:
ipsilateral dominance
 a deficit in sound
localization
performances
 weaker involvement of
auditory dorsal stream
in UHL patients
compared with controls

78. Tinnitus as
plasticity
 a certain degree of
hearing loss and other
peripheral nerve input
reduction:
1.Reduction in input
2.Compensation of
nervous system
3. Reduction in inhibitory
effect on efferent nerves
4. Increased spontaneous
discharge activity of the
auditory cortex

79. Tinnitus as
plasticity/
Changes of
central
neurotransmitt
ers
 The main neurotransmitters in the auditory pathway are GABA, 5-
hydroxytryptamine (5-HT), glutamic acid, dopamine, etc.
 A GABAergic neuron is an inhibitory neurotransmitter in auditory
cortex.
 A decrease of GABA receptors may also be an important
mechanism of tinnitus

80. Tinnitus as
plasticity/
Changes of
central
neurotransmitt
ers

82. References
1.Auditory map plasticity: Diversity in causes and consequences
2. Plasticity in the Auditory System,Dexter R. F. Irvine
3. Development of Auditory Cortex Circuits ,Minzi Chang1 , and Patrick O.
Kanold1
4. Brain plasticityandhearingdisorders, Malzaher
,NVannson,ODeguine,MMarx,PascalBarone,Kstrelnikov
5. Molecular Aspects of the Development and Function of Auditory Neurons,
Gabriela Pavlinkova
6.Myelin Development, Plasticity, and Pathology in the Auditory System,
Patrick Long1,§, GuoqiangWan2,§, MichaelT. Roberts1, and Gabriel Corfas1
7.Development of the auditory system,Ruth Litovsky
8. Infant auditory capabilities, Lynne A.Werner, PhD
9. Analysis on the neurological mechanism of acupuncture treatment in
Idiopathic tinnitus based on the theory of “central plasticity”, Peng-Xi Zhang1,
Tong-Sheng Su2*

83. Bibliography
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2. Role of attention in the generation and modulation of tinnitus, Larry E.
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