Fig. 2.

(Top) Layout of working memory paradigm and (bottom) time-frequency spectrograms. (Top) Each trial consisted of four phases: (1) a fixation phase lasting 1,000 ms that included the baseline (−400 to 0 ms), (2) an encoding phase lasting 2,000 ms and consisting of six letters presented simultaneously within a grid, (3) a maintenance phase lasting 3,000 ms during which the six letters disappeared from the grid, and (4) a retrieval phase lasting 900 ms that required participants to identify whether the probe letter appeared in the original encoding set. Each participant completed 128 trials during each MEG session. (Bottom) Group mean time-frequency spectrograms averaged across all participants, with a sensor from the left parieto-occipital region shown on the left and the same for the right. In each spectrogram, time (sec) is shown on the x-axis and frequency (Hz) on the y-axis. Percentage of power change was computed by dividing the mean power of each time-frequency bin by the respective bin's baseline power and multiplying this value by 100. The color legend is displayed to the right. Strong theta synchronization and alpha desynchronization was observed shortly after the encoding grid was presented, and this evolved into a narrower alpha synchronization during maintenance (with theta oscillations dissipating)