Localization capabilities of VEP source localization and functional magnetic resonance imaging in human visual cortex

The ideal brain imaging technique would have high spatial and temporal resolution to answer where and when cortical activation occurs in response to visual stimulation. As to where, functional magnetic resonance imaging (fMRI) offers high spatial resolution but has very poor temporal resolution. On the other hand, visual evoked potentials (VEPs) coupled with dipole source localization (DSL) methods offer high temporal resolution, in the 1 msec range, but, historically, has rather poor spatial resolution.; In order to explore the spatial localization capabilities of the VEP/DSL method, sources in the primary visual cortex were compared to equivalent sources found using fMRI for two subjects. 48 sources in the primary visual cortex (left and right hemisphere) for each subject were aligned using a 4-parameter fit (1 rotation and 3 translations). The RMS distance between fMRI and VEP/DSL sources was 10.8 mm.; Due to the assumption of fMRI as the gold standard in spatial localization, it was necessary to analyze spatial localization error introduced by inaccurately estimating the hemodynamic delay and the threshold. By varying the hemodynamic delay and threshold in data analysis, it was shown that small changes in variables result in shifting of the fMRI source. An improved fMRI retinotopic map localization method was developed. This new method optimized the amount of data used and allowed the calculation of error bars of the fMRI spatial accuracy on the fMRI flatmap.