Estimation of encoding of pictorial stimuli from visually evoked magnetic fields

Magnetic fields were recorded from human subjects during the performance of a 1-back recognition short-term memory task with pictorial stimuli using a whole-head neuromagnetometer. In the memory condition the subjects were required to respond to the same stimuli and memorize the presented pictures for a short period of time, in control condition to respond to all stimuli and not memorize the picture. Magnetic event-related fields (MEFs) were computed and analyzed to reveal information about the encoding process of pictorial stimuli in short-term memory. Based on comparative MEFs analysis between conditions and behavioral response times, the encoding process for pictorial stimuli in this particular memory task was estimated at an interval with peaking activity around 500–550 ms from the stimulus presentation onset. Equivalent current dipoles (ECDs) were fitted to localize the brain sources related to visual information processing and the encoding process. A dipole located in the left anterior medial temporal lobe was estimated as an important source underlying encoding activity.