Exploring the temporal dynamics of the spatial working memory n-back task using steady state visual evoked potentials (SSVEP)

The neural networks associated with spatial working memory (SWM) are well established. However, the temporal dynamics of SWM-related brain activity are less clear. This study examined changes in temporal neurophysiology during the spatial n-back task using steady state probe topography (SSPT) to record cortical steady state visual evoked potentials (SSVEPs) at 64 scalp locations. Twenty healthy male volunteers participated in the study. The findings identified three different time periods of significance during the spatial n-back task--an early perceptual/encoding period (approximately 0-500 ms), an early delay period just following the stimulus disappearing from view (approximately 850-1400 ms), and a late period lasting the final second of the delay and anticipation of the new stimulus (approximately 2500-3500 ms). The delay period was associated with increases in frontal and occipital region amplitude, consistent with previous findings in more basic working memory tasks. The two different SSVEP components during the delay appear reflective of the additional "executive" demands associated with the n-back and may suggest variable roles for the PFC during different stages of the delay. All three n-back levels demonstrated a relative consistent electrophysiological profile, indicating that this pattern is specific to the spatial n-back task. Nevertheless, these findings supported the hypothesis that memory load modulates activity within the networks identified, consistent with previous neuroimaging studies. The current findings may offer a framework in which to further investigate the temporal aspects of SWM