Download The world of sounds

Plasticity in sensory systems
Jan Schnupp on the monocycle
Activity and
size of auditory
cortex…
Schneider et al. Nat.
Neurosci. 2003
…Are correlated…
…and correlated with musical
abilities
Is musical practice increasing the
size of auditory cortex, or do
people with large auditory cortex
become musicians?
What do we learn when we
learn a new skill?
Nat. Neurosci. 2006
Human psychoacoustical
performance
Frequency differences
Pressure ratio between
softest and loudest
sounds…
Hair motion at absolute threshold…
Learning protocol
Perceptual learning
• Partially non-specific
– Playing tetris improves frequency
discrimination
• Partially due to passive exposure
• But also to some extent requires active
task performance
Animal models of auditory plasticity
• Classical conditioning
– Fear conditioning: associating a sound with a
foot shock
• Environmental enrichment and relatives
– Manipulating the environment can have both
beneficial and disruptive effects on the
auditory system
• Spatial hearing
Nat. Rev. Neurosci. 2004
Fear conditioning…
…changes cortical neurons
Brain Research 2007
Environmental enrichment…
Environmental enrichment…
Environmental enrichment…
Plasticity in auditory enriched
environments
Auditory plasticity requires stimuli
but not interaction
Just noticeable differences in azimuth
at the center, tone stimuli
Binaural Cues for Localising
Sounds in Space
time
Interaural Time Differences (ITDs)
Interaural Level Differences (ILDs)
Interaural Time Difference (ITD)
Cues
ITD
ITDs are powerful cues to sound source direction,
but they are ambiguous (“cones of confusion”)
Binaural disparities in humans
ITD
ILD
Disambiguating the cone of confusion
• Sounds on the median
plane (azimuth 0,
different elevations) have
zero binaural disparities
• This is a special case of
the cone of confusion
• Nevertheless, humans
and other animals can
determine the elevation of
a sound source
Spectral information about space
The barn owl…
Binaural Cues in the Barn Owl
Barn owls have highly
asymmetric outer
ears, with one ear
pointing up, the other
down. Consequently,
at high frequencies,
barn owl ILDs vary
with elevation, rather
than with azimuth (D).
Consequently ITD and
ILD cues together
form a grid specifying
azimuth and elevation
respectively.
Phase locking at high
frequencies in the barn owl
C. Köppl, 1997
Processing of Interaural Time Differences
To the Inferior Colliculus
Sound on the
ipsilateral side
Contralateral side
Medial superior olive
Interaural time difference
Preservation of Time Cues in
AVCN
spherical
bushy
cell
endbulb
of Held
VIII nerve
fiber
• Auditory Nerve Fibers connect
to spherical and globular
bushy cells in the anteroventral cochlear nucleus
(AVCN) via large, fast and
secure synapses known as
“endbulbs of Held”.
• Phase locking in bushy cells is
even more precise than in the
afferent nerve fibers.
• Bushy cells project to the
superior olivary complex.
The coincidence detection model of Jeffress (1948) is the
widely accepted model for low-frequency sound
localisation
Response
0
Interaural Time Difference
Response
0
Interaural Time Difference
0 s Time Delay
0 s
Cochlear
Nucleus
Left Ear
Cochlear
Nucleus
Right Ear
Se mi cir cul ar
Can als
Win dow
MSO
Auditory Nerve Activity
Large calyx synaptic ending
0 s Time Delay
Arrives at left ear 300 s
later than at the right
300 s
Cochlear
Nucleus
Left Ear
Cochlear
Nucleus
Right Ear
Se mi cir cul ar
Can als
Win dow
MSO
Auditory Nerve Activity
Large calyx synaptic ending
300 s Time Delay
Coincident spikes
Arrives at left ear 300 s
later than at the right
0 s Time Delay
300 s
0 s
Cochlear
Nucleus
Left Ear
Cochlear
Nucleus
Right Ear
Se mi cir cul ar
Can als
Win dow
MSO
Auditory Nerve Activity
Large calyx synaptic ending
300 s Time Delay
Coincident spikes
0 s Time Delay
Interaural Phase Sensitivity in the MSO to 1000 Hz
1 ms
1 ms
Yin and Chan (1988)
Processing of Interaural Level Differences
To the Inferior Colliculus
Sound on the
ipsilateral side
Lateral superior olive
I>C
Contralateral
side
C>I
Interaural intensity difference
The Calyx of Held
• MNTB relay neurons receive
their input via very large calyx of Held
synapses.
• These secure synapses would not be
needed if the MNTB only fed into “ILD
pathway” in the LSO.
• MNTB also provides precisely timed
inhibition to MSO.
Ipsilateral
Contralateral
100
20
20
Sound level (dB SPL)
100
0.125
32
0.125
32
Frequency (kHz)
Caird and Klinke 1983
The Superior Olivary Nuclei – a Summary
• Most neurons in the
MSO respond best to
sounds that occur
earlier in the
contralateral ear.
• Most neurons in the
LSO respond best to
sounds that are louder
in the ipsilateral ear.
• Space representation is
crossed, and therefore
LSO projects mostly
contralaterally and
MSO ipsilaterally.
Excitatory Connection
Inhibitory Connection
Midline
IC
LSO
IC
MNTB
MSO
CN
CN
Spatial hearing is plastic
Plasticity in adults
Nat. Neurosci. 1998
New ears…
Sound localization by humans
-30
0
30
Sound localization by humans
Effect of modifying the ear
Learning the
new ears
Knowing
both ears
Plasticity of the space map
Knudsen, Nature 2002
Orientation responses to auditory
and visual stimuli are congruent…
Auditory orientation
response
Visual orientation response
Prisms that shift the visual scene
Auditory responses adapt to the
visual shift
The brain of the barn owl
The ICC, ICX and the Superior
Collicullus (Optic Tectum)
Point-to-point correspondence
between ICX and OT
Neural correlate of the shift of
auditory responses
Shift in ITD sensitivity occurs first in
ICX
Axonal sprouting cause shift of ITD
sensitivity in ICX
Axonal sprouting cause shift of ITD
sensitivity in ICX
Time course of ITD shift
Cellular mechanisms of ITD shift
Anatomy of the instructive signal
Visual activity in ICX uncovered by
removing inhibition in OT
Cellular mechanisms of ITD shift
NMDA receptors are present at the
transition stage…
…but not when the shift is
complete
Cellular mechanisms of ITD shift
GABA participates in the suppression of
the normal responses
Bicuculline
Control
Plasticity and age
Old animals cannot change
A sensitive period…
During the sensitive period,
plasticity potential is very large
The normal map is robust and can
be recovered at any age
Recovery of the normal map
requires rich environment
Adult plasticity is possible after
juvenile experience
Adult plasticity is possible after
juvenile experience
Time course of adult adjustment