Richards, J.E. (2006). Realistic Head Models for Cortical Source Analysis in Infant Participants. Society for Researh in Child Development, Atlanta GA. (PDF)
Cortical source analysis can identify cortical areas that are active during infant cognitive processing. This analysis uses high-density EEG recording and quantitative models that identify dipole sources inside the head to account for the EEG data. These sources can be related to the EEG activity in the time domain, to the experimental procedures, and to the cognitive processes occurring during the task. Cortical source analysis with infant participants has used adult models for the electrical and spatial characteristics of the head. The current poster will overview a method for using infant MRIs to develop realistic head models for infant participants for cortical source analysis.
The method consists of several steps: 1) the MRI of a representative infant or the infant in the psychophysiological experiment must be obtained; 2) the head must be “segmented” with computer programs that analyze the MRI recording; 3) values for impedance for skull, scalp, CSF, and brain for infant participants must be estimated; 4) sufficient number of electrodes must be recorded; 5) realistic “forward” models using the spatial topography of the MRI must be computed. These steps allow the use of “realistic models” in computer programs that do cortical source analysis (e.g., “equivalent current dipole” analysis in BESA, or Source-Signal EMSE programs).
This technique has been applied to cortical sources involve in infant spatial cueing, infant visual recognition memory, the response of infants to mother’s and stranger’s face, and EEG during object disappearance and appearance. The result of using realistic head models is illustrated in four findings. First, infant skull and scalp impedances (conductivity; resistance) are much lower than adults, so that using appropriate infant values results in cortical sources closer to the surface. Second, the number of recording channels affects cortical source analysis, with less than ~50 electrodes resulting in models that fit poorly and that mislocate the cortical source. Third, using realistic head models for areas of current leakage in the skull (sutures / seams between skull bones, partially closed fontenal) or for varying head characteristics (varying skull thickness, placement of brain with respect to skull landmarks) result in better fits and more appropriate locations of cortical sources. Finally, this is illustrated with a “representative MRI” approach in which a single MRI is used for all participants, and with an “individual MRI” approach in which the infant has both an anatomical MRI and participates in the psychophysiological experiment.