Localizing Sites of Activation in Primary Visual Cortex Using Visual-Evoked Potentials and Functional Magnetic Resonance Imaging

Abstract

This study compared retinotopic map identification in primary visual cortex (V1) using: (i) functional magnetic resonance imaging (fMRI) and (ii) visual evoked potentials (VEPs) coupled with dipole source localization (DSL). A multielectrode array was used to record VEPs while subjects viewed a flickering dartboard pattern modulated by a 16-bit m-sequence. The stimulus preferentially activates V1. Using a common time function DSL algorithm, the primary source of each stimulus patch was found independent of the fMRI. The VEP/DSL and fMRI localization data for each subject were aligned by a rigid translation and rotation. The average distance between VEP and corresponding fMRI sources was 10.8 mm +/- 3.8 mm. To assess the significance of the results, fMRI and DSL solutions were scrambled so the comparisons were no longer for corresponding patches. The average distance between the noncorresponding data sets was 17.2 mm for 50 million scrambles. The probability of the scrambled data yielding a better fit than the real data was p < 10(-7). The combination of multielectrode recording, multiinput visual stimulation and common time function DSL analysis can provide a detailed retinotopic map of visual cortex that has high correspondence with independent fMRI localization analysis on the same subject.

Electrical source dynamics in three functional localizer paradigms

“…Using EEG alone, a resolution of about 3 degrees of visual angle can be obtained when individual MRIs are used to define the source space, and with a prior on where electrical signals are likely to originate (Im et al, 2007). Furthermore, by accurately mapping the likely current direction in a restricted source space around primary visual areas, it is possible to differentiate the activations in V1 and V2 (Ales et al, 2009;Baker et al, 2006). This way, it could be shown that V1 and V2 have comparable response latencies.…”

“…This assumes that the EEG scalp potentials result from one, or a few, cortical generators. With dipole source localization, striate cortex can be localized from the EEG scalp data (Di Russo et al, 2002;Ikeda et al, 1998) in good agreement with fMRI data (Baker et al, 2006). But even though the scalp potentials of the first evoked EEG component, the C1 at around 80-90 ms, are consistent with a single cortical dipole (Jeffreys and Axford, 1972), the underlying source activity is known to be more complex.…”

Combining MRI and VEP imaging to isolate the temporal response of visual cortical areas

“…5 More recently Sharon and colleagues compared source identification using EEG and MEG separately as well as combined. 6 Localization perfomance based on EEG data was similar to our earlier findings.…”

“…We have previously shown that by assuming a common time function for all cortical sources that correspond to a ring of stimulus patch within a visual area, an accurate estimation of the time function is possible (see Appendix 3 for details.). 5,13 Here we apply additional cortical constraints given by the MRI/fMRI results. The voltages (V) recorded on electrodes (e) for a ring of stimulus patches (p), or hemi ring of patches for a single hemisphere is:…”

“…Weighted norm methods can produce more localized sources. 2,3,4 Dipole source modeling methods have recently started to incorporate cortical constraints to reduce the problem of nearby sources 5,6 to improve isolation of the dominant dipole, the likely V1…”