Spatial correspondence based asymmetry analysis in FMRI

Asymmetry analysis of brain activation in functional magnetic resonance imaging (fMRI) experiments is commonly used to determine hemispheric specialization for cognitive function. Asymmetry is typically expressed in terms of differences in the number of suprathreshold voxels activated, normalized to total activation, within specific regions known to subserve the function of interest or over the entire hemisphere. We introduce methodologies for carrying out asymmetry analysis both for region of interest (ROI) and whole brain based studies that take into account information about spatial correspondence of voxels on two sides of the brain. We apply this methodology to make determination of activation asymmetry during a memory encoding task in patients with refractory temporal lobe epilepsy (TLE). Memory lateralization is an important step in the presurgical evaluation of such patients for temporal lobectomy. Using structure specific analysis, our asymmetry scores in hippocampus are found to have a strong correspondence with hemispheric dominance given by Intracortical Amobarbital Testing (IAT), which is the widely accepted gold standard for determining laterality. Whole brain analysis, on the other hand, is used to locate regions in the brain with asymmetric activation in an exploratory analysis. These methods of quantifying asymmetry using spatial correspondence have the potential for generating more robust asymmetry measurements.