Is the center of mass (COM) a reliable parameter for the localization of brain function in fMRI?

Abstract

The center of mass (COM) in functional MRI studies is defined as the center of a cerebral activation cluster. Although the COM is a well-accepted parameter for exactly localizing brain function, the reliability of COMs has not received much attention until now. Our goal was to investigate COM reliability as a function of the thresholding technique, the threshold level, and the type of COM calculation. Therefore 15 subjects were examined repeatedly using simple hand and tongue movement paradigms. Postprocessing was performed with uncorrected, corrected, and proportional thresholding as well as different threshold levels. Geometric and T-weighted COMs of left-hemispheric primary hand and tongue motor clusters were calculated. The COM variation was evaluated within and between repeated sessions depending on the different postprocessing setups. Mean COM variations over three repeated sessions varied between 1.6 mm and 9.8 mm for the hand paradigm and between 7.0 mm and 14.4 mm for the tongue task. Stringent thresholding techniques and high threshold levels were required to assess reliable results, whereas the kind of COM calculation was of lesser relevance. Thus, COM reliability cannot be presupposed; it depends strongly on the individual postprocessing techniques. This should be considered when using COMs for localizing brain function.

Bad Mood—Bad Activation?

“…This is clearly supported by several reliability studies, where considerable differences between the results of repeated measurements are reported [4][5][6][7][8][9][10][11][12][13][14][15][16][17][18]. Many factors such as magnetic field inhomogeneities, involuntary movements, respiration, cyclic heart function, and so on do have substantial influence on fMRI results [19][20][21][22][23][24][25][26].…”

“…Shown are selected axial slices (z = 55 mm and 60 mm) z = 55 mm z = 60 mm F-value effects, but also very easy and overlearned tasks such as finger tapping may be affected by a second task which is concurrently performed, and (2) even discrete changes like an alternated emotional state are sufficient to influence fMRI results. Intrinsic factors like emotion or attention may explain unsatisfactory results in reliability studies that were assessed under strictly controlled conditions to exclude as many extrinsic influencing factors as possible [4,6,9].…”

Motor map reliability and aging: a TMS/fMRI study

“…SigniWcance of resultant F statistics were held to a whole-brain derived false discovery rate (FDR) corrected value set to q = 0.001, and a minimum cluster volume of 50 L. This threshold was selected to minimize variability of center of mass (CoM) reported to be associated with BOLD measurement Fesl et al 2008). CoM shift was calculated as the average coordinate distance oVset, measured by the three dimensional Euclidean distances between CoM locations from mapping sessions 1-2 and from mapping sessions 2-3 (analogous to calculations of CoG shift from TMS).…”

“…In motor mapping, this could be considered as the within-subject stability of motor map size and location across testing sessions. A number of studies have approached the topic of upper extremity motor map reliability with fMRI (Bennett and Miller 2010;Gountouna et al 2010;Havel et al 2006;Fesl et al 2008;Maitra 2009) and with TMS (Mortifee et al 1994;Uy et al 2002;Wolf et al 2004;Malcolm et al 2006;Christie et al 2007) having varied Wndings. TMS intersession reliability measures have generally been reported as being high (Mortifee et al 1994;Uy et al 2002;Wolf et al 2004) in terms of motor map size and center of gravity (CoG) even across diVering muscles (Malcolm et al 2006).…”

Reproducibility of functional MRI localization within the human somatosensory cortex

“…Studies using presurgical paradigms (3,4) have been used to explore the level of reproducibility of fMRI localization; however, findings were often constrained by the paradigm design or the use of patients. Studies in healthy subjects have determined reproducibility within visual (5), motor (6), and somatosensory (7) cortices and have explored aspects such as voxel size and spatial smoothing (7) and thresholding (6). There is, however, limited evidence in the existing literature describing the effect on reproducibility due to (i) region size, (ii) method of identifying the location of functional activation, and (iii) registration errors.…”