Stairway to memory: Left-hemispheric alpha dynamics index the progressive loading of items into a short-term store

The encoding, maintenance, and subsequent retrieval of memories over short time intervals is an essential cognitive function. Load effects on the neural dynamics supporting the maintenance of short-term memories have been well studied, but experimental design limitations have hindered the study of similar effects during the encoding of information into online memory stores. Theoretically, the active encoding of complex visual stimuli into memory must also recruit neural resources in a manner that scales with memory load. Understanding the neural systems supporting this encoding load effect is of particular importance, as some patient populations exhibit difficulties specifically with the encoding, and not the maintenance, of short-term memories. Using magnetoencephalography, a visual sequence memory paradigm, and a novel encoding slope analysis, we provide evidence for a left-lateralized network of regions, oscillating in the alpha frequency range, that exhibit a progressive loading effect of complex visual stimulus information during memory encoding. This progressive encoding load effect significantly tracked the eventual retrieval of item-order memories at the single trial level, and neural activity in these regions was functionally dissociated from that of earlier visual networks. These findings suggest that the active encoding of stimulus information into short-term stores recruits a left-lateralized network of frontal, parietal, and temporal regions, and might be susceptible to modulation (e.g., using non-invasive stimulation) in the alpha band.