Visual-evoked magnetic fields associated with geometric and phonetic discrimination

The endogenous negativity of the early negative component of event-related potential or event-related magnetic field might reveal some aspects of cognitive processes. In this paper, we identified the activated area for an early negative component (designated NA) which seems to be associated with pattern recognition processes by measuring the magnetic fields. The difference between the geometric-discriminative response and the phonetic-discriminative response was also obtained. The visual evoked magnetic field was measured with a 204-channel whole-scalp superconducting quantum interference device gradiometer and estimated the equivalent current dipoles (ECD) to clarify the source localization of NA. Localization of a single ECD was applied for the main peak of the NA component in the magnetoencephalogram response. In this paper, the ECD sources were localized in the parietal and temporal sections of the brain.