OPTIMAL REFERENCE ELECTRODE SELECTION FOR ELECTRIC SOURCE IMAGING

One goal of recording voltages on the scalp is to form images of elect rical sources across the cerebral cortex (electric source imaging). In this study, an objective criterion is introduced for selecting the op timal location for the reference electrode to attain the maximum spati al resolution of the source image, for example as provided here by the truncated singular value decomposition pseudoinverse solution. The he ad model features a realistic cortex within a 3-shell conductive spher e, and pyramidal cell activity is represented by 9104 normal current e lements distributed across the cortical area. On the scalp, 234 electr odes provide the measurements with respect to a chosen reference elect rode. The effects of the reference electrode when located at the masto id, occipital pole, vertex or center of the head are analyzed by a sin gular value decomposition of the lead field matrices. Sensitivity to n oise, and hence the spatial resolution, is found to depend on characte ristics of the lead field matrix that are determined by the choice of the image source surface, electrode array and location of the referenc e electrode. Using a reference close to a source surface increases the sensitivity of the measurement system in identifying the nearby activ ity of low spatial frequency content. However, this feature is comprom ised by a reduction in spatial resolution for distant cortical areas d ue to noise in the measurements. A new performance measure, the image sensitivity map, is introduced to identify the cortical regions that p rovide peak image sensitivity. This measure may be exploited in design ing the geometry of an electrode array and selecting the location of t he reference electrode to follow the activity on a specific area of th e cortical surface.