2016 Volume 5 Pages 43-48
Alpha-band (8–13 Hz) rhythm during encoding of sequentially presenting memory items was recorded using a 76-channel magnetoencephalograph. In a memory task, seven images each with an arrow pointing up, down, left or right were presented sequentially. Thereafter, a recall number was presented. Each participant memorized the arrow directions in the order of presentation and recalled the direction of the arrow for the recall number, and responded by pressing a button. Each participant also took part in a control task, in which the participant looked at the same images as those in the memory task but pressed the button previously instructed. Ten right-handed male volunteers performed 280 epochs of the memory task and 120 epochs of control task. The amplitudes of alpha-band rhythm during presentation of midterm (3rd–5th) items were compared with those of beginning (1st and 2nd) items. Fifteen sensors showed significant differences (midterm > beginning) in the memory task (0.01 < p < 0.05 at 11 sensors, 0.001 < p < 0.01 at 4 sensors), while no sensors showed a significant increase in the control task. Using the statistical parametric mapping (SPM) method, the region of increase in alpha-band rhythm from beginning to midterm was estimated to be located in the visual cortex and around bilateral temporal gyri. Additionally, the increase in amplitude from beginning to midterm was significantly greater before a correct answer than before a wrong answer (0.01 < p < 0.05 at 7 sensors and p < 0.001 at 1 sensor). Previous works demonstrated that alpha-band rhythm is increased by active inhibition of visual inputs that are irrelevant to memory. The region of increase estimated to be in the visual cortex agrees with previous works. An interpretation of our results is that since irrelevant visual inputs are successfully inhibited before a correct answer, the amplitude of alpha-band rhythm increases more than that before a wrong answer. These results indicate the possibility that alpha-band amplitude during memory encoding can be used as an index of memory performance.