Signal and Noise II. Preprocessing

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					I.Signal and Noise

 II. Preprocessing


BIAC Graduate fMRI Course
     October 19, 2005
1. Signal and Noise in
         fMRI
 What is signal? What is noise?
• Signal, literally defined
   – Amount of current in receiver coil
• How can we control the amount of received
  signal?
   –   Scanner properties (e.g., field strength)
   –   Experimental task timing
   –   Subject compliance (through training)
   –   Head motion (to some degree)
• What can’t we control?
   – NOISE
I. Introduction to SNR
      Signal, noise, and the General
               Linear Model



          Y  M  
                Amplitude (solve for)
Measured Data                                      Noise
                           Design Model




                                          Cf. Boynton et al., 1996
        Signal Size in fMRI
A      45     B         50




                               E



C   50 - 45   D   (50-45)/45
Differences in SNR
Voxel 1




          790   830   870


Voxel 2




          690   730    770

Voxel 3




          770   810    850
Effects of SNR: Simulation Data
• Hemodynamic response
  – Unit amplitude
  – Flat prestimulus baseline
• Gaussian Noise
  – Temporally uncorrelated (white)
  – Noise assumed to be constant over epoch
• SNR varied across simulations
  – Max: 2.0, Min: 0.125
                             SNR = 2.0
                 2


            1.5


                 1


            0.5


                 0
-5 -4 -3 -2 -1       0   1   2   3   4   5   6   7   8   9 10 11 12 13 14 15

           -0.5


             -1
                             SNR = 1.0
                 2


            1.5


                 1


            0.5


                 0
-5 -4 -3 -2 -1       0   1   2   3   4   5   6   7   8   9 10 11 12 13 14 15

           -0.5


             -1
                             SNR = 0.5
                 2


            1.5


                 1


            0.5


                 0
-5 -4 -3 -2 -1       0   1   2   3   4   5   6   7   8   9 10 11 12 13 14 15

           -0.5


             -1
                             SNR = 0.25
                 4


                 3


                 2


                 1


                 0
-5 -4 -3 -2 -1       0   1   2   3   4   5   6   7   8   9 10 11 12 13 14 15
             -1


             -2


             -3
                   SNR = 0.125
             6

             5

             4

             3

             2

             1

             0
-5 -4 -3 -2 -1 0   1   2   3   4   5   6   7   8   9 10 11 12 13 14 15
             -1

            -2

            -3

            -4

            -5
SNR = 4.0   SNR = 2.0




SNR = 1.0    SNR = .5
  What are typical SNRs for fMRI
              data?
• Signal amplitude
  – MR units: 5-10 units (baseline: ~700)
  – Percent signal change: 0.5-2%
• Noise amplitude
  – MR units: 10-50
  – Percent signal change: 0.5-5%
• SNR range
  – Total range: 0.1 to 4.0
  – Typical: 0.2 – 0.5
 II. Properties of Noise in fMRI

Can we assume Gaussian noise?
                 Types of Noise
• Thermal noise
   – Responsible for variation in background
   – Eddy currents, scanner heating
• Power fluctuations
   – Typically caused by scanner problems
• Variation in subject cognition
   – Timing of processes
• Head motion effects
• Physiological changes
• Differences across brain regions
   – Functional differences
   – Large vessel effects
• Artifact-induced problems
    Why is noise assumed to be
            Gaussian?


• Central limit theorem
  – If X1 … Xn are a set of independent random
    variables, each with an arbitary probability
    distribution, then the sum of the set of
    variables (probability density function) will be
    distributed normally.
Is noise constant through time?
                                       0.40%             Young

                                       0.35%
Standard Deviation at Time Point (%)




                                                         Old

                                       0.30%

                                       0.25%

                                       0.20%

                                       0.15%

                                       0.10%

                                       0.05%

                                       0.00%
                                               -5 -4 -3 -2 -1 0    1   2   3   4   5    6   7     8   9 10 11 12 13
                                                                  Time since stimulus onset (s)
Is fMRI noise Gaussian (over space)?

                        Outside Brain
                        Edge of Brain
                        Boundary of Brain
                        Middle of Brain
                                   Is Signal Gaussian (over voxels)?
                                  18%

                                  16%                     Young
                                  14%
Percentage of All Active Voxels




                                  12%
                                                          Old

                                  10%

                                  8%

                                  6%

                                  4%

                                  2%

                                  0%
                                        0   0.5    1                  1.5                   2   2.5
                                                  Peak Amplitude (% change over baseline)
Variability
 Variability in Subject Behavior: Issues

• Cognitive processes are not static
  – May take time to engage
  – Often variable across trials
  – Subjects’ attention/arousal wax and wane
• Subjects adopt different strategies
  – Feedback- or sequence-based
  – Problem-solving methods
• Subjects engage in non-task cognition
  – Non-task periods do not have the absence of thinking

        What can we do about these problems?
    Response Time Variability

A                  B
    Intersubject Variability
A       B


                A & B: Responses across
                  subjects for 2 sessions
                C & D: Responses within
                  single subjects across
C       D         days
                E & F: Responses within
                  single subjects within a
                  session

E        F
                      - Aguirre et al., 1998
                                                                 Young Adults
                                                                620
                                               1.50%            617
                                                                595
Signal Change over Pre-stimulus Baseline (%)




                                               1.30%            566
                                                                560
                                               1.10%            559
                                                                555
                                               0.90%            551
                                                                549
                                               0.70%            520
                                                                383
                                               0.50%

                                               0.30%

                                               0.10%

                                               -0.10% -5 -4 -3 -2 -1   0    1   2   3   4   5    6   7     8   9 10 11 12 13

                                               -0.30%
                                                                           Time since stimulus onset (s)
 Implications of Inter-Subject Variability
• Use of individual subject’s hemodynamic responses
   – Corrects for differences in latency/shape

• Suggests iterative HDR analysis
   – Initial analyses use canonical HDR
   – Functional ROIs drawn, interrogated for new HDR
   – Repeat until convergence

• Requires appropriate statistical measures
   – Random effects analyses
   – Use statistical tests across subjects as dependent measure
     (rather than averaged data)
    Spatial Variability?
A              B




                    McGonigle et al., 2000
   Standard
Deviation Image
Spatial Distribution of Noise
               A: Anatomical Image
               B: Noise image
               C: Physiological noise
               D: Motion-related noise
               E: Phantom (all noise)
               F: Phantom (Physiological)

                 - Kruger & Glover (2001)
 Low and High Frequency Noise
750

740

730

720

710

700
                       2


690                  1.5

                       1


680                  0.5

                       0


670                  -0.5

                      -1


660                  -1.5

                      -2


650
      1   51   101          151   201
III. Methods for Improving
           SNR
Increasing Field Strength
     Theoretical Effects of Field
              Strength
• SNR = signal / noise
• SNR increases linearly with field strength
  – Signal increases with square of field strength
  – Noise increases linearly with field strength
  – A 4.0T scanner should have 2.7x SNR of 1.5T
    scanner
• T1 and T2* both change with field strength
  – T1 increases, reducing signal recovery
  – T2* decreases, increasing BOLD contrast
Adapted from Turner, et al. (1993)
Measured Effects of Field Strength
• SNR usually increases by less than theoretical
  prediction
   – Sub-linear increases in SNR; large vessel effects may
     be independent of field strength
• Where tested, clear advantages of higher field
  have been demonstrated
   – But, physiological noise may counteract gains at high
     field ( > ~4.0T)
• Spatial extent increases with field strength
• Increased susceptibility artifacts
                Trial Averaging
• Static signal, variable noise
   – Assumes that the MR data recorded on each trial are composed
     of a signal + (random) noise


• Effects of averaging
   – Signal is present on every trial,
      so it remains constant when averaged
   – Noise randomly varies across trials,
      so it decreases with averaging
   – Thus, SNR increases with averaging
  Increasing Power increases Spatial
               Extent
                                 Trials    Subject 1   Subject 2
                                Averaged




                                    4
500 ms             500 ms
                            …      16

                                   36
         16-20 s
                                   64

                                  100

                                  144
A   B
    0.00   0.00   0.00   0.00   0.00   0.00   0.00   0.00   0.00   0.00   3.85

    0.00   0.00   0.00   0.00   0.00   0.00   0.00   0.00   0.00   2.44   2.41

    0.00   0.00   0.00   0.00   0.00   0.00   3.36   3.68   2.79   1.78   1.84

    0.00   0.00   0.00   0.00   5.88   6.79   8.36   2.09 -0.50 -3.08 -0.96

    0.00   0.00   3.20   5.46   2.00   6.50   6.13   5.67 -0.06 -3.41 -1.56

    2.66   2.42   0.01   5.81   5.88   5.86   6.84   5.63   3.71 -1.76 -2.25

    3.74   3.42   1.43   0.68   2.13   6.47   8.05   8.96 10.27 2.45      0.29

    4.60   2.27   0.77   1.41   0.80   1.71   9.65   9.91 12.19 3.17      1.75

    0.94   1.38   1.22   2.96   0.30 -1.58 2.19      4.10   5.84   3.06   0.53

    -0.46 -1.11 -0.31 1.27 -0.94 -4.97 -3.26 -1.93 -1.07 0.28 -1.21

    -4.05 -2.33 -2.67 -2.17 -1.64 -7.44 -7.22 -4.83 -3.93 0.00            0.55
                                     Effects of Signal-Noise Ratio on extent of
                                             activation: Empirical Data
                               100
                                                                                                 Subject 1
                                90
Number of Significant Voxels



                                80

                                70
                                                                                                         Subject 2
                                60

                                50

                                40
                                                                           VN = Vmax[1 - e(-0.016 * N)]
                                30
                                                                                  Subject1                       Subject 2
                                                     Peak latency of
                                                                          4 sec    5 sec         6 sec   4 sec    5 sec      6 sec
                                20                   reference HDR

                                                          Vmax             89       96            72      25        80        98
                                10                  Correlation of data
                                                                          0.997    0.995         0.993   0.960    0.994      0.998
                                                     with prediction

                                 0
                                     0    25   50         75              100              125           150         175             200
                                               Number of Trials Averaged
                  Active Voxel Simulation
                                     Signal + Noise (SNR = 1.0)
                            2
                          1.5
                            1
                          0.5
                            0                                                         2




                                 1

                                     3

                                         5

                                             7

                                                 9

                                                     11

                                                          13

                                                               15

                                                                    17

                                                                         19
                          -0.5                                                       1.5
                            -1                                                        1
                          -1.5                                                       0.5
                            -2                                                        0




                                                                                           1

                                                                                               3

                                                                                                   5

                                                                                                       7

                                                                                                           9

                                                                                                               11

                                                                                                                    13

                                                                                                                         15

                                                                                                                              17

                                                                                                                                   19
                                                                                    -0.5
                            2
                                                                                      -1
                          1.5
                                                                                    -1.5
                            1
                                                                                      -2
                          0.5
                            0
                                 1

                                     3

                                         5

                                             7

                                                 9

                                                     11

                                                          13

                                                               15

                                                                    17

                                                                         19
                          -0.5
                           -1
                          -1.5
                           -2




1000 Voxels, 100 Active                                                       • Signal waveform taken from
                                     Noise                                    observed data.
                            2
                          1.5                                                 • Signal amplitude distribution:
                            1
                          0.5                                                 Gamma (observed).
                            0
                                 1

                                     3

                                         5

                                             7

                                                 9

                                                     11

                                                          13

                                                               15

                                                                    17

                                                                         19




                                                                              • Assumed Gaussian white noise.
                          -0.5
                            -1
                          -1.5
                            -2
Effects of Signal-Noise Ratio on extent of
                activation:
             Simulation Data
 Number of Activated Voxels   120

                                        SNR = 1.00
                              100


                               80                    SNR = 0.52
                                                     (Young)


                               60                                 SNR = 0.35
                                                                  (Old)

                               40                                              SNR = 0.25


                                                                                             SNR = 0.15
                               20
                                                                                                    SNR = 0.10
                                0
                                    0                50                100                  150                  200
                                                 Number of Trials Averaged

                                            Old (66 trials)           Young (70 trials)            Ratio (Y/O)
                              Observed            26                        53                         2.0
                              Predicted          57%                       97%                         1.7
                     Caveats
• Signal averaging is based on assumptions
  – Data = signal + temporally invariant noise
  – Noise is uncorrelated over time
• If assumptions are violated, then averaging
  ignores potentially valuable information
  – Amount of noise varies over time
  – Some noise is temporally correlated (physiology)
• Nevertheless, averaging provides robust,
  reliable method for determining brain activity
II. Preprocessing of
     FMRI Data
      What is preprocessing?
• Correcting for non-task-related variability
  in experimental data
  – Usually done without consideration of
    experimental design; thus, pre-analysis
  – Occasionally called post-processing, in
    reference to being after acquisition

• Attempts to remove, rather than model,
  data variability
Quality Assurance
       Tools for Preprocessing
•   SPM
•   Brain Voyager
•   VoxBo
•   AFNI
•   Custom BIAC scripts
Slice Timing Correction
Motion Correction
Head Motion: Good, Bad,…
… and catastrophically bad
Why does head motion introduce
         problems?

     A       B                     C




                 507   89    154       663   507   89

                 119   171   83        520   119   171

                 179   117   53        137   179   117
Simulated Head Motion
 Severe Head Motion: Simulation




Two 4s movements of
8mm in -Y direction         Motion
(during task epochs)
  Severe Head Motion: Real Data




Two 4s movements of
8mm in -Y direction         Motion
(during task epochs)
       Correcting Head Motion
• Rigid body transformation
  – 6 parameters: 3 translation, 3 rotation
• Minimization of some cost function
  – E.g., sum of squared differences
  – Mutual information
Effects of Head Motion Correction
   Limitations of Motion Correction
• Artifact-related limitations
   – Loss of data at edges of imaging volume
   – Ghosts in image do not change in same manner as
     real data

• Distortions in fMRI images
   – Distortions may be dependent on position in field, not
     position in head

• Intrinsic problems with correction of both slice
  timing and head motion
What is the best approach for minimizing the
 influence of head motion on your data?
Coregistration
        Should you Coregister?
• Advantages
  –   Aids in normalization
  –   Allows display of activation on anatomical images
  –   Allows comparison across modalities
  –   Necessary if no coplanar anatomical images

• Disadvantages
  – May severely distort functional data
  – May reduce correspondence between functional and
    anatomical images
Normalization
            Standardized Spaces
• Talairach space (proportional grid system)
   –   From atlas of Talairach and Tournoux (1988)
   –   Based on single subject (60y, Female, Cadaver)
   –   Single hemisphere
   –   Related to Brodmann coordinates

• Montreal Neurological Institute (MNI) space
   – Combination of many MRI scans on normal controls
        • All right-handed subjects
   – Approximated to Talaraich space
        • Slightly larger
        • Taller from AC to top by 5mm; deeper from AC to bottom by 10mm
   – Used by SPM, fMRI Data Center, International Consortium for
     Brain Mapping
  Normalization to Template
Normalization Template   Normalized Data
Anterior and Posterior Commissures




                       Posterior
          Anterior     Commissure
          Commissure
         Should you normalize?
• Advantages
  –   Allows generalization of results to larger population
  –   Improves comparison with other studies
  –   Provides coordinate space for reporting results
  –   Enables averaging across subjects
• Disadvantages
  – Reduces spatial resolution
  – May reduce activation strength by subject averaging
  – Time consuming, potentially problematic
       • Doing bad normalization is much worse than not normalizing
         (and using another approach)
 Slice-Based Normalization
Before Adjustment (15 Subjects)




After Adjustment to Reference Image




                                      Registration courtesy Dr. Martin McKeown (BIAC)
Spatial Smoothing
    Techniques for Smoothing
• Application of
  Gaussian kernel
  – Usually expressed in
    #mm FWHM
  – “Full Width – Half
    Maximum”
  – Typically ~2 times
    voxel size
      Effects of Smoothing on Activity
Unsmoothed Data




Smoothed Data (kernel width 5 voxels)
  Should you spatially smooth?
• Advantages
  – Increases Signal to Noise Ratio (SNR)
      • Matched Filter Theorem: Maximum increase in SNR by filter with
        same shape/size as signal
  – Reduces number of comparisons
      • Allows application of Gaussian Field Theory
  – May improve comparisons across subjects
      • Signal may be spread widely across cortex, due to intersubject
        variability


• Disadvantages
  – Reduces spatial resolution
  – Challenging to smooth accurately if size/shape of signal is not
    known
Temporal Filtering
        Filtering Approaches
• Identify unwanted frequency variation
  – Drift (low-frequency)
  – Physiology (high-frequency)
  – Task overlap (high-frequency)
• Reduce power around those frequencies
  through application of filters
• Potential problem: removal of frequencies
  composing response of interest
Power Spectra
                Segmentation
• Classifies voxels within an image into different
  anatomical divisions
   – Gray Matter
   – White Matter
   – Cerebro-spinal Fluid (CSF)




                                  Image courtesy J. Bizzell & A. Belger
  Histogram of Voxel Intensities
 0.04




0.035

                       Anatomical
 0.03
                       Functional
0.025




 0.02




0.015




 0.01




0.005




   0
Bias Field Correction
  Region-of-Interest (ROI) drawing
• Allows direct, unbiased measurement of activity
  in an anatomical region
  – Assumes functional divisions tend to follow
    anatomical divisions
• Improves ability to identify topographic changes
  – Motor mapping (central sulcus)
  – Social perception mapping (superior temporal sulcus)
• Complements voxel-based analyses
                Resources
• Drawing Tools
  – BIAC software (e.g., Overlay2)
  – Analyze
  – IRIS/SNAP (G. Gerig from UNC)
• Reference Works
  – Print atlases
  – Online atlases
• Analysis Tools
  – roi_analysis_script.m
ROI Examples
                                                                                                                                                                                            -2
                                                                                                                                                                                                 -1
                                                                                                                                                                                                      0
                                                                                                                                                                                                          1
                                                                                                                                                                                                              2
                                                                                                                                                                                                                  3
                                                                                                                                                                                                                      4
                                                                                                                                                                                       -3
                                                                                                                                                                                     -1.5
                                                                                                                                                                                        0
                                                                                                                                                                                      1.5
                                                                                                                                                                                        3
                                                                                                                                                                                      4.5
                                                                                                                                                                                        6




                                                                                                                                                                                80
                                                                                                                                                                                      7.5




                                                                                                                                                                                     60
                                                                                                                                                                                        9
                                                                                                                                                                                     10.5
                                                                                                                                                                                       12
                                                                                                                                                                                     13.5
                                                                                                                                                                                       15
                                                                                                                                                                                     16.5
                                                                                                                                                                                       -3
                                                                                                                                                                                     -1.5
                                                                                                                                                                                        0
                                                                                                                                                                                      1.5
                                                                                                                                                                                        3
                                                                                                                                                                                      4.5
                                                                                                                                                                                        6




                                                                                                                                                                                75
                                                                                                                                                                                      7.5




                                                                                                                                                                                     55
                                                                                                                                                                                        9
                                                                                                                                                                                     10.5
                                                                                                                                                                                       12
                                                                                                                                                                                     13.5
                                                                                                                                                                                       15
                                                                                                                                                                                     16.5
                                                                                                                                                                                       -3
                                                                                                                                                                                     -1.5
                                                                                                                                                                                        0
                                                                                                                                                                                      1.5
                                                                                                                                                                                        3
                                                                                                                                                                                      4.5
                                                                                                                                                                                        6




                                                                                                                                                                                70
                                                                                                                                                                                      7.5




                                                                                                                                                                                     50
                                                                                                                                                                                        9
                                                                                                                                                                                     10.5
                                                                                                                                                                                       12
                                                                                                                                                                                     13.5
                                                                                                                                                                                       15
                                                                                                                                                                                     16.5
                                                                                                                                                                                       -3
                                                                                                                                                                                     -1.5
                                                                                                                                                                                        0
                                                                                                                                                                                      1.5
                                                                                                                                                                                        3
                                                                                                                                                                                      4.5
                                                                                                                                                                                        6




                                                                                                                                                                                65
                                                                                                                                                                                      7.5




                                                                                                                                                                                     45
                                                                                                                                                                                        9
                                                                                                                                                                                     10.5
                                                                                                                                                                                       12
                                                                                                                                                                                     13.5
                                                                                                                                                                                       15
                                                                                                                                                                                     16.5
                                                                                                                                                                                       -3
                                                                                                                                                                                     -1.5
                                                                                                                                                                                        0
                                                                                                                                                                                      1.5
                                                                                                                                                                                        3
                                                                                                                                                                                      4.5
                                                                                                                                                                                        6




                                                                                                                                                                                60
                                                                                                                                                                                      7.5




                                                                                                                                                                                     40
                                                                                                                                                                                        9
                                                                                                                                                                                     10.5
                                                                                                                                                                                       12
                                                                                                                                                                                     13.5
                                                                                                                                                                                       15
                                                                                                                                                                                     16.5
                                                                                                                                                                                       -3
                                                                                                                                                                                     -1.5
                                                                                                                                                                                        0
                                                                                                                                                                                      1.5
                                                                                                                                                                                        3
                                                                                                                                                                                      4.5
                                                                                                                                                                                        6




                                                                                                                                                                                55
                                                                                                                                                                                      7.5




                                                                                                                                                                                     35
                                                                                                                                                                                        9
                                                                                                                                                                                     10.5
                                                                                                                                                                                       12
                                                                                                                                                                                     13.5
                                                                                                                                                                                       15
                                                                                                                                                                                     16.5
                                                                                                                                                                                       -3
                                                                                                                                                                                     -1.5
                                                                                                                                                                                                                          Left Hemisphere - Gaze Shifts




                                                                                                                                                                                        0
                                                                                                                                                                                      1.5
                                                                                                                                                                                        3
                                                                                                                                                                                      4.5
                                                                                                                                                                                        6



                                                                                                                                                                                50
                                                                                                                                                                                      7.5

                                                                                                                                                                                     30
                                                                                                                                                                                        9
                                                                                                                                                                                     10.5
                                                                                                                                                                                       12
                                                                                                                                                                                     13.5
                                                                                                                                                                                       15
                                                                                                                                                                                     16.5
                                                                                                                                                                                       -3
                                                                                                                                                                                     -1.5
                                                                                                                                                                                        0
                                                                                                                                                                                      1.5
                                                                                                                                                                                        3
                                                                                                                                                                                      4.5
                                                                                                                                                                                45      6
                                                                                                                                                                                      7.5
                                                                                                                                                                                     25
                                                                                                                                                                                        9
                                                                                                                                                                                     10.5
                                                                                                                                                                                       12
                                                                                                                                                                                     13.5
                                                                                                                                                                                       15
                                                                                                                                                                                     16.5
                                                                                                                                                                                       -3
                                                                                                                                                                                     -1.5
                                                                                                                                                                                        0
                                                                                                                                                                                      1.5
                                                                                                                                                                                        3
                                                                                                                                                                                      4.5
                                                                                                                                                                                        6
                                                                                                                                                                                40



                                                                                                                                                                                      7.5
                                                                                                                                                                                     20



                                                                                                                                                                                        9
                                                                                                                                                                                     10.5
                                                                                                                                                                                       12
                                                                                                                                                                                     13.5
                                                                                                                                                                                       15
                                                                                                                                                                                     16.5
                                                                                                                                                                                       -3
                                                                                                                                                                                     -1.5
                                                                                                                                                                                        0
                                                                                                                                                                                      1.5
                                                                                                                                                                                        3
                                                                                                                                                                                      4.5
                                                                                                                                                                                        6
                                                                                                                                                                                35




                                                                                                                                                                                      7.5
                                                                                                                                                                                     15




                                                                                                                                                                                        9
                                                                                                                                                                                     10.5
                                                                                                                                                                                       12
              in elicited when a subject watches a character shift gaze toward or away from a target.


                                                                                                                                                                                     13.5
                                                                                                                                                                                       15
                                                                                                                                                                                     16.5
                                                                                                                                                                                       -3
                                                                                                                                                                                     -1.5
                                                                                                                                                                                        0
                                                                                                                                                                                      1.5
                                                                                                                                                                                        3
                                                                                                                                                                                      4.5
                                                                                                                                                                                        6
                                                                                                                                                                                30




                                                                                                                                                                                      7.5
                                                                                                                      Distance Posterior from the Anterior Commissure (in mm)
                                                                                                                                                                                     10




                                                                                                                                                                                        9
                                                                                                                                                                                     10.5
                                                                                                                                                                                       12
                                                                                                                                                                                     13.5
                                                                                                                                                                                       15
                                                                                                                                                                                     16.5
                                                                                                                                                                                       -3
BIAC is studying biological motion and social perception – here by determining how context modulates brain activity




                                                                                                                                                                                     -1.5
                                                                                                                                                                                        0
                                                                                                                                                                                      1.5
                                                                                                                                                                                        3
                                                                                                                                                                                     5




                                                                                                                                                                                      4.5
                                                                                                                                                                                        6
                                                                                                                                                                                25




                                                                                                                                                                                      7.5
                                                                                                                                                                                        9
                                                                                                                                                                                     10.5
                                                                                                                                                                                       12
                                                                                                                                                                                     13.5
                                                                                                                                                                                       15
                                                                                                                                                                                     16.5
                                                                                                                                                                                       -3
                                                                                                                                                                                                                          Right Hemisphere - Gaze Shifts




                                                                                                                                                                                     -1.5
                                                                                                                                                                                        0
                                                                                                                                                                                      1.5
                                                                                                                                                                                        3
                                                                                                                                                                                     0




                                                                                                                                                                                      4.5
                                                                                                                                                                                        6
                                                                                                                                                                                20




                                                                                                                                                                                      7.5
                                                                                                                                                                                        9
                                                                                                                                                                                     10.5
                                                                                                                                                                                       12
                                                                                                                                                                                     13.5
                                                                                                                                                                                       15
                                                                                                                                                                                     16.5
        Additional Resources
• SPM website
  – http://www.fil.ion.ucl.ac.uk/spm/course/notes01.html
  – SPM Manual
• Brain viewers
  – http://www.bic.mni.mcgill.ca/cgi/icbm_view/
    Signal-Noise-Ratio (SNR)


 Task-Related
  Variability

Non-task-related
  Variability
    A
                  t = 16




B             C
        t=8                t=5
 BOLD may reflect predominantly
       excitatory activity


     TMS results had indicated
     that M1 is inhibited in no-go
     condition.




M1                             SMA      M1                SMA

          Solid = go ; dashed = no-go        Waldvogel, et al., 2000
1% change         2% change

            Parrish et al., 2000

				
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posted:11/23/2011
language:English
pages:102