Logarithmic transformation for high-field BOLD Functional Magnetic Resonance Imaging data
permalinkExperimental Brain Research - 2005-09-01Lewis S, Jerde T, Tzagarakis C, Gourtzelidis P, Georgopoulos MA, Tsekos N, Amirikian B, Kim SG, Ugurbil K, Georgopoulos AP10.1007/s00221-005-2336-4Parametric statistical analyses of BOLD
Functional Magnetic Resonance Imagingdata 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
fMRIdata 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
fMRIdata need to be log-transformed before parametric statistical analyses are applied.