Measurements and source estimations of extremely low frequency brain magnetic fields in a short-term memory task by a whole-head neurogradiometer

To investigate the characteristics of visual short-term memory in humans, brain magnetic fields evoked during a delayed paired comparison task were recorded using a whole-head neuromagnetometer. The visual stimulus consisted of a circle with different colors in each quadrant. In the memory condition, subjects reacted with the index finger, when the first stimulus (Sample) was identical in color configuration to the second stimulus (Test), and with the middle finger when they differed. For the control condition, the subjects ignored the Sample, and moved the index or middle finger alternately in response to the Test. Extremely low frequency components of brain magnetic fields were observed 500 ms after the Sample onset in the temporal and/or the occipital region in the memory condition, but not in the control condition. Sources for the low frequency components were localized in the inferior part of the occipital lobe, in the vicinity of the supramarginal gyrus and the angular gyrus, and the inferior frontal gyrus. The results suggest that the activities in the inferior part of the occipital lobe controls the storage process of short-term visual memory.