title: From evoked potentials to cortical currents: Resolving V1 and V2 components using retinotopy constrained source estimation without fMRI.
abstract: Despite evoked potentials' (EP) ubiquity in research and clinical medicine, insights are limited to gross brain dynamics as it remains challenging to map surface potentials to their sources in specific cortical regions. Multiple sources cancellation due to cortical folding and cross-talk obscures close sources, e.g. between visual areas V1 and V2. Recently retinotopic functional magnetic resonance imaging (fMRI) responses were used to constrain source locations to assist separating close sources and to determine cortical current generators. However, an fMRI is largely infeasible for routine EP investigation. We developed a novel method that replaces the fMRI derived retinotopic layout (RL) by an approach where the retinotopy and current estimates are generated from EEG or MEG signals and a standard clinical T1-weighted anatomical MRI. Using the EEG-RL, sources were localized to within 2 mm of the fMRI-RL constrained localized sources. The EEG-RL also produced V1 and V2 current waveforms that closely matched the fMRI-RL's (n = 2) r(1,198)  = 0.99, P < 0.0001. Applying the method to subjects without fMRI (n = 4) demonstrates it generates waveforms that agree closely with the literature. Our advance allows investigators with their current EEG or MEG systems to create a library of brain models tuned to individual subjects' cortical folding in retinotopic maps, and should be applicable to auditory and somatosensory maps. The novel method developed expands EP's ability to study specific brain areas, revitalizing this well-worn technique. Hum Brain Mapp 37:1696-1709, 2016. © 2016 Wiley Periodicals, Inc.
authors: Eccles Institute of Neuroscience, John Curtin School of Medical Research, Australian National University, Canberra, ACT, Australia.|Australian Research Council Centre of Excellence in Vision Science and Research School of Biology, Australian National University, Canberra, ACT, Australia.|Wyss Institute, Harvard University, Boston, Massachusetts.
authors: Samuel A
authors: SA
authors: Inverso
authors: Eccles Institute of Neuroscience, John Curtin School of Medical Research, Australian National University, Canberra, ACT, Australia.|Australian Research Council Centre of Excellence in Vision Science and Research School of Biology, Australian National University, Canberra, ACT, Australia.
authors: Xin-Lin
authors: XL
authors: Goh
authors: Department of Neuroscience and Biomedical Engineering, Aalto University, Espoo, Finland.|AMI Centre, Aalto Neuroimaging, Aalto University, Finland.
authors: Linda
authors: L
authors: Henriksson
authors: AMI Centre, Aalto Neuroimaging, Aalto University, Finland.|Clinical Neurosciences, Neurology, University of Helsinki and Helsinki University Hospital, Helsinki, Finland.
authors: Simo
authors: S
authors: Vanni
authors: Eccles Institute of Neuroscience, John Curtin School of Medical Research, Australian National University, Canberra, ACT, Australia.|Australian Research Council Centre of Excellence in Vision Science and Research School of Biology, Australian National University, Canberra, ACT, Australia.
authors: Andrew C
authors: AC
authors: James
mesh_terms: D000328:Adult; D001931:Brain Mapping; D004569:Electroencephalography; D005074:Evoked Potentials, Visual / Q000502:physiology*; D007839:Functional Laterality; D006801:Humans; D007091:Image Processing, Computer-Assisted; D008279:Magnetic Resonance Imaging; D015225:Magnetoencephalography; D008875:Middle Aged; D010775:Photic Stimulation; D012160:Retina / Q000000981:diagnostic imaging / Q000502:physiology*; D014793:Visual Cortex / Q000000981:diagnostic imaging* / Q000502:physiology*; D014795:Visual Pathways / Q000000981:diagnostic imaging / Q000502:physiology*
keywords: EEG; dipole model; fMRI; retinotopy constrained source estimation; visual evoked current; visual evoked potential