Logarithmic transformation for high-field BOLD fMRI data.
Parametric statistical analyses of BOLD fMRI data often assume that the data are normally distributed, the variance is independent of the mean, and the effects are additive. We evaluated the fulfilment of these conditions on BOLD fMRI data acquired at 4 T from the whole brain while 15 subjects fixated a spot, looked at a geometrical shape, and copied it using a joystick. We performed a detailed analysis of the data to assess (a) their frequency distribution (i.e. how close it was to a normal distribution), (b) the dependence of the standard deviation (SD) on the mean, and (c) the dependence of the response on the preceding baseline. The data showed a strong departure from normality (being skewed to the right and hyperkurtotic), a strong linear dependence of the SD on the mean, and a proportional response over the baseline. These results suggest the need for a logarithmic transformation. Indeed, the log transformation reduced the skewness and kurtosis of the distribution, stabilized the variance, and made the effect additive, i.e. independent of the baseline. We conclude that high-field BOLD fMRI data need to be log-transformed before parametric statistical analyses are applied.
Lewis
Scott M
SM
Veterans Affairs Medical Center, Brain Sciences Center, One Veterans Drive, Minneapolis, MN 55417, USA. lewis093@umn.edu
Jerde
Trenton A
TA
Tzagarakis
Charidimos
C
Gourtzelidis
Pavlos
P
Georgopoulos
Maria-Alexandra
MA
Tsekos
Nikolaos
N
Amirikian
Bagrat
B
Kim
Seong-Gi
SG
Uğurbil
Kâmil
K
Georgopoulos
Apostolos P
AP
D000328:Adult; D000465:Algorithms; D003627:Data Interpretation, Statistical; D005260:Female; D005556:Form Perception / Q000502:physiology; D006801:Humans; D007091:Image Processing, Computer-Assisted; D008279:Magnetic Resonance Imaging / Q000706:statistics & numerical data*; D008297:Male; D010100:Oxygen / Q000097:blood*; D011597:Psychomotor Performance / Q000502:physiology
D010100:Oxygen