Beauchamp:Retinotopy

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Phase-Mapped Retinotopy

Visual areas are retinotopic. An efficient way to map visual areas is to present stimuli that traverse the visual field. Here are three snapshots of an eccentricity mapping stimulus:

Over the course of a 5-minute scan series, the ring expands from central to peripheral five times.

Here are three snapshots of a polar angle mapping stimulus:

Over the course of a 5-minute scan series, the wedge rotates around the fixation point five times.

Processing Phase-Mapped Retinotopy Data in AFNI

Here are some processing steps to analyse this data in AFNI:

1. First step: measure motion in each run separately, perform the registration, save motion parameters
2. create average EPI image to register to

 3dTcat -prefix {$ec}rall {$ec}r1+orig.HEAD {$ec}r2+orig.HEAD {$ec}r3+orig.HEAD {$ec}r4+orig.HEAD {$ec}r5+orig.HEAD \
 {$ec}r6+orig.HEAD {$ec}r7+orig.HEAD {$ec}r8+orig.HEAD
 3dTstat -mean -prefix {$ec}EPImean {$ec}rall+orig.HEAD

1. perform registration

 foreach r (1 2 3 4 5 6 7 8)
  3dvolreg -prefix {$ec}r{$r}vr -base {$ec}EPImean+orig  -input {$ec}r{$r}+orig  -1Dfile {$ec}r{$r}vr.1D -cubic
 end

1. remove quadratic trend from each run separately

 foreach r (1 2 3 4 5 6 7 8)
  3dDetrend -prefix {$ec}r{$r}vrdt -polort 2  {$ec}r{$r}vr+orig
 end

1. blur slightly

 foreach r (1 2 3 4 5 6 7 8)
  3dmerge -doall -1blur_rms 3 -prefix {$ec}r{$r}vrdtbl {$ec}r{$r}vrdt+orig 
 end
 rm junk*
 foreach r (1 2 3 4 5 6 7 8)
 # Convert to float as required by DFT
  3dcalc -prefix junkr{$r} -datum float -a  {$ec}r{$r}vrdtbl+orig -expr "a"
   # perform FFT on each run separately
   3dDFT -prefix {$ec}r{$r}vrdtblFFT -detrend junkr{$r}+orig
 end

 foreach r (1 2 3 4 5 6 7 8)
 # Retinotopic stimulus has periodicity at frequency 5, calculate phase at only that frequency
  3dcalc -datum short -prefix {$ec}r{$r}vrdtblFFTf5 -cx2r PHASE -a {$ec}r{$r}vrdtblFFT+orig'[5]'  -expr "a"
  3drefit -sublabel 0 FFTr{$r}f5 -subrepkey 0 FFTr{$r}f5 {$ec}r{$r}vrdtblFFTf5+orig
 # Calculate magnitude squared at all frequencies
  3dcalc -datum short -prefix {$ec}r{$r}vrdtblFFTMAG -cx2r ABS -a {$ec}r{$r}vrdtblFFT+orig  -expr "a*a"
 end

1. Average magnitude squared across runs

 3dMean -prefix {$ec}rallvrdtblFFTMAG {$ec}r?vrdtblFFTMAG+orig.HEAD

1. Calculate Fratio between frequency of interest and neighboring frequencies

 rm junk*
 3dcalc -prefix junkMAGSTAT -a3 {$ec}rallvrdtblFFTMAG+orig -b4 {$ec}rallvrdtblFFTMAG+orig -c5 {$ec}rallvrdtblFFTMAG+orig \
 -d6 {$ec}rallvrdtblFFTMAG+orig -e7 {$ec}rallvrdtblFFTMAG+orig -expr "c^2 / ( (a^2 + b^2 + d^2 + e^2)/4 )"

1. Convert to fbuc

 3dbucket -prefix {$ec}rallvrdtblFFTMAGSTAT  -fbuc junkMAGSTAT+orig

1. Give it correct stats--numerator degrees of freedom is 2*number of runs, denominator degrees of freedom is 4*number of runs

 3drefit  -sublabel 0 Significance -substatpar 0 fift 16 64 -subrepkey 0 Significance {$ec}rallvrdtblFFTMAGSTAT+orig

1. Calculate appropriate phases
2. flip CW and CON runs

 foreach r (3 4 7 8)
  3dcalc -datum short -prefix {$ec}r{$r}vrdtblFFTf5flip -a {$ec}r{$r}vrdtblFFTf5+orig  -expr "-a"
  3drefit -sublabel 0 FFTr{$r}f5flip -subrepkey 0 FFTr{$r}f5flip {$ec}r{$r}vrdtblFFTf5flip+orig
  end

1. Average together

  rm junk*
  3dcalc -prefix junkeccavg -datum short -a {$ec}r2vrdtblFFTf5+orig -b {$ec}r4vrdtblFFTf5flip+orig \
  -c {$ec}r6vrdtblFFTf5+orig -d {$ec}r8vrdtblFFTf5flip+orig -expr "mean(a,b,c,d)"
  
  3dcalc -prefix junkpolavg -datum short -a {$ec}r1vrdtblFFTf5+orig -b {$ec}r3vrdtblFFTf5flip+orig \
  -c {$ec}r5vrdtblFFTf5+orig -d {$ec}r7vrdtblFFTf5flip+orig -expr "mean(a,b,c,d)"

1. Combine stats, individual runs, and averages in one file for ease of viewing

  3dbucket -prefix {$ec}Ret -fbuc  {$ec}rallvrdtblFFTMAGSTAT+orig junkeccavg+orig junkpolavg+orig \
 {$ec}r{$r}vrdtblFFTf5+orig.HEAD   {$ec}r?vrdtblFFTf5flip+orig.HEAD

Here is the resulting flat map showing the threshold eccentricity and polar angle maps: