text: ||*ts*− ̂*t*|| 
  ̂*t*
  ε*j*= 100 × mean (  ) 
  
  Where*ts*is the morphometric measurement of a parcel for session*s*
  ̂*t*is the mean of*t*across sessions55,79. Thus, we defined variability 
  and 
  as the mean absolute percent difference between each individual and 
  the mean across sessions. Across-session variability estimates for all 
  400 regions were then averaged across eight participants, and a global 
  measure of cortical GMV variability was computed by averaging across 
  the 400 regions. This approach was repeated independently for the 
  T2 hippocampal scans, wherein we computed across-session variability 
  for each parcel of the ASHS parcellation scheme (*n*= 7 bilateral sub- 
  fields). However, it is important to note that raw subfield values (that 
  is, no manual retouching) were used for Day2Day variability assess- 
  ments and should be interpreted with caution. Finally, to better com- 
  pare against our own data, we repeated this approach using our 