Transcript Document

Week 6:
(March 15, 2011)
Auditory Maps and
orienting: need for
Coordinate Transformations
The Barnowl (tyto alba):
Ears are placed asymmetrically
View on left side
View on right side
Convergence of many ICc neurons onto a single ICx neuron
creates ITD sensitivity
(Takahashi)
Neurons in the owl’s
ventral lemniscus
are sensitive to
the interaural level
difference (ILD)!
(equivalent to the
LSO in mammals)
(Takahashi)
Formation of a space-specific neuron in the owl’s ICx:
Convergence of ITD and ILD sensitive neurons
(Knudsen/Konishi)
The neural map of auditory space in the owl’s ICx
The formation of this
computational map
depends critically on
the quality of the
owl’s visual system
through feedback
connections from the
SC (later.....).
General Organization of the Mammalian Auditory System
subcortical
pathways
ACOUSTIC
I
The Direct Sound-Localization and Orienting Pathway:
Brainstem:
acoustic
cue
processing
Inferior
Colliculus:
Superior
Colliculus:
sound
direction
eye-head
orienting
Orienting of eyes, head, body (and pinnae)
involves the midbrain Superior Colliculus (SC):
motor
‘sensory’
EYE
VISION
SC
AUDITION
HEAD
SC:
- Multisensory
- Sensorimotor ‘interface’
- Topographic map of saccadic gaze shifts
SC:
Topographic map of gaze - shifts: saccade vectors
Vert. Eye Pos
Hor.
Eye
Hor.
Eye
PosPos
Firing
rate
time
Independent of eye position
Making an eye movement towards a sound
requires a coordinate transformation:
This transformation necessiates a signal
about eye position re. head, E
A
C
Eye and Head Aligned
Eye and Head Not Aligned
AV
EH
E
-20
20
B
AV’
V
H
20
D
rostral
rostral
up
2
up
2
5
5
10
V
A
10
20
30
down
0
40
-60
-30
caudal
A
V
A’
20
V’
down
40
-60
-30
30
0
caudal
Jay and Sparks
(1984/1987):
Auditory responses in SC are
in eye-centered
coordinates.
Question:
How do these cells get their
information?
Hypothesis:
The midbrain IC could convey this
signal.
Tuning
of an IC neuron
to eye position.
1. FR increases
for rightward
eye fixations.
2. FR increases
only during
the acoustic
response:
“GAIN FIELD”
Neural Network Model of IC-SC mapping
sound at
(AZ,EL)
HRTF/ILD
Eye position
IC
SC
Up
Hor
freq
freq
Down
freq
Brainstem
pathways
Tonotopic code
of sounds
Topographic code
of eye-motor error
Activity of model IC neurons:
-
R = F0 × (1 + a × I P + h × H + e × E ) × T ( f )
Peak
Sound level and
sound position
modulation
Eye position
modulation
Gaussian
Tuning
Curve
randomly distributed across the IC population
e , h , a are
(240 IC neurons, 12 freq bands; 100 SC neurons in map).
Example of a typical IC model neuron: ‘gain field’
-
R = F0 × (1 + a × I P + h × H + e × E ) × T ( f )
Simulation result for TH =(+20,+30)deg
and EH =(-30,+30) => ME = (50,0)
M
F
T
H
E
O
M = H-E
Reference frames: including the head and body
OMR
Eye
+
Vision:
Target re. Eye
Eye
re. Head
Head +
Vision is Eye-Centered
Target
Audition:
Target re. Head
Head
re. Body
Audition is Head-Centered
Somatosensation:
Target re. Body
+
Body
Eye movements require oculocentric error signals
Head movements require craniocentric error signals
In head-free
orienting(human):
Eyes(Go)and
head(Ho)indeed
move both toward a
visual or auditory
target.
Goossens & Van Opstal,
Exp. Brain Research, 114
(1997)
Studying coordinate transformations - I:
Does the auditory system keep sounds
in head-centered coordinates?
First, the rationale behind the underlying
idea:
“the double-step paradigm and
pure-tone localization”
Goossens and Van Opstal,
J Neurophysiol 1999
Studying coordinate transformations - I
Double-Step
Paradigm
Pure-Tone
Localization
Paradigm
Goossens & Van Opstal,
J Neurophysiol 1999
The sound-localization system should be able
to account for intervening movements of the
eyes and head:
Elevation
(S)
S
TH
S
GH
GS
∆G
F
Gaze shift
GH=TH
(V)
V
GS=TH - ∆G
Azimuth
Sound localization responses are spatially accurate
(Goossens and Van Opstal, 1999)
Pure-Tone Localization Behaviour:
do head movements help?
Pure-Tone Localization:
(in)dependent of head
orientation?
Pure-Tone
Localization:
depends on
head orientation
AND
on tone
frequency!
Sounds appear to be represented in a spatial
(body-centered) reference frame (TSPACE = THEAD+HSPACE):
Computation WITHIN the (tonotopic) auditory system
Dynamic coordinate transformations for multisensory orienting: