STEP 4: Scaling the Data - as percent of the mean

• STEP 4: Scaling the Data - as percent of the mean

• • For each run,
  • • for each voxel:
    • • compute the mean value of the time series
      • scale the time series so that the new mean is 100
  • Scaling becomes an important issue when comparing data across subjects:
  • • using only one scanner, shimming affects the magnetization differently for each subject (and therefore affects the data differently for each subject)
    • different scanners might produce vastly different EPI signal values
  • Without scaling, the magnitude of the beta weights may have meaning only when compared with other beta weights in the dataset
  • • Example, what does a beta weight of 4.7 mean? Basically nothing, by itself.
    • • It is a small response, if many voxels have responses in the hundreds.
      • It is a large response, if it is a percentage of the mean.
  • By converting to percent change, we can compare the activation calibrated with the relative change of signal, instead of the arbitrary baseline of FMRI signal