Transcript Functional and Structural MRI of the Human Auditory System

Behavioral Neuroscience & Neurology
• Lesion studies
– Animals
– Patient case studies
• Phineas Gage
• H.M.
• Single cell recordings
• Behavioral studies
• TMS (Transcranial Magnetic Stimulation)
– Creating artificial & temporary lesions in the brain
Transcranial Magnetic Stimulation (TMS)
Cognitive Neuroscience
STRUCTURAL Neuroimaging
– CT (Computerized Tomography)
– MRI (Magnetic resonance imaging)
• 3-D combine x-ray or MRI slices
FUNCTIONAL Neuroimaging
– EEG (Topography) 1929
• MEG (Magnetoencephalogram)
– PET (Positron Emission Tomography –
1970s)
• cerebral glucose levels
• high spatial resolution but low
temporal and invasive
– fMRI (functional MRI) – 1990s
functional Magnetic Resonance
Imaging (fMRI)
• Measures cerebral hemodynamic response
(BOLD signal)
– Blood flow in brain – levels of oxygen in
(deoxy)hemoglobin recruited by firing neurons
• Structure AND function in single scan
– High spatial resolution with modest temporal resolution
• Problem: loud scanner noises, machine artifacts
MRI physics
(1) Static magnetic field B0 is on
B0
Water molecules align with B0
(equilibrium state)
B0
(2) Magnetic field B1 is applied
(orthogonal to B0 and
at proton’s resonance frequency)
Water molecules are “flipped”
into excited state
(not equilibrium)
B1 (RF pulse)
B0
(3) Relaxation (T1 and T2)
Water molecules return to
equilibrium state
(this creates changing magnetic field)
T1 (white matter)
MR signal
T1
time
T2
Changing magnetic field creates
current in the wire loop which
can be measured
Fourier Transform
MR signal
B0
(4) NMR signal detection
T1 (gray matter)
T2 (gray matter)
T2 (white matter)
Conventional MRI
white
matter
gray
matter
• MRI is sensitive to myelin
HIGH Spatial Resolution of MRI
MRI
Histology
Layer IV
MRI vs. fMRI
MRI
- shows difference between different types of tissues
(“difference in space”, e.g. white vs. gray matter)
fMRI
-shows difference between stimulated and non-stimulated tissue
(“difference in time”)
What is functional MRI (fMRI)?
Stimulus Off (Condition 1):
Stimulus On (Condition 2):
increased metabolism
requires more oxygen
MR signal
T2 (stimulated tissue)
more oxygen changes magnetic
properties of the tissue
T2 (tissue at rest)
MR signal increases (S)
S
time
• fMRI detects subtractive difference in signal (S = Cond. 2-Cond.1)
• fMRI is indirect (hemodynamic) method of cellular communication
Structural MRI &
Anatomical MRI
(T1-weighted)
Functional MRI
Structural MRI
(gray matter thickness map)
Functional MRI
(activation to music)
Diffusion Tensor MRI
(white matter tracts)
Spatio - temporal scales
Spatial Resolution (mm)
10
8
6
MEG / EEG
PET
4
fMRI
2
Single / Multi
Unit Recording
0
1 msec
1 sec
1 min
10 min 1 hour
Temporal Resolution (sec)
Why fMRI?
• non-invasive => can be used in humans
• measures population neural activity
• can study whole brain at once
MRI scanner
Magnetic field:
• is VERY STRONG ( 3T ~ 30,000 times the earth’s magnetic field)
• is ALWAYS ON
Image Artifacts and Subject Safety
With hair band
Without hair band
Typical fMRI experiment
Stimulus
Stimulus
Stimulus
Stimulus
(30 s)
(30 s)
(30 s)
(30 s)
Off
Stimulus (e.g. music) is presented to subject
repeatedly every 30 sec.
•
Images are acquired ~2 sec.
•
MR signal is estimated for every voxel as a
function of time
Off
On
...
0
4
TIME
(minutes)
image acquired
at each tick mark
Image Intensity
•
On
...
time
•
Change in MR signal between “On” and “Off”
conditions is calculated for every voxel.
•
Voxels with significant change of the MR signal
are displayed on the anatomical image.
Scanner Noise
• Scanner noise sources:
– scanner cooling system (ongoing)
– gradient coils (every image acquisition)
• Noise is a PROBLEM:
– want response to stimulus, get response to stimulus+scanner noise
– scanner noise reduces dynamic range of response
– scanner noise masks stimulus
• Solutions:
– blocking noise transmission to the subject (earplugs, earmuffs)
– modifying scanner
– modifying experiment
Subcortical Activation
MGB
IC
SOC
CN
Auditory Pathway
Inferior Colliculus
(sound intensity varied)
35 dB SL
55 dB SL
75 dB SL
p = 0.01
p = 2 x 10-9
Inferior Colliculi
Image
Signal
Change
Sound
On
1
0
0
30
0
30
0
30
Time (seconds)
1
Auditory Cortex
music
noise
Percent Signal Change
auditory
cortex
Sound
On
0
30 Time (sec)
0
30
Time (sec)
Millisecond changes in
electrophysiology (ERPs)
Functional Neuroimaging Resolution
Spatial
Temporal
EEG/MEG
1-3 cm2
1 ms
PET
3 mm3
20 s
(+ 3-8 s lag)
fMRI
1 mm3
2s
(+ lag of 3-8 s)
Dis/Advantages of each technique
Mind vs Brain
• VHS 1518.25 Dr Phil- Cognitive
Neuroscience