Different roles for theta- and alpha-band brain rhythms during sequential memory

Numerous studies have demonstrated that brain rhythms are modulated according to memory performance or memory processing. In sequential memory tasks, memory performance can be reduced by shortening the intervals between memory item presentations. To clarify the neurophysiological mechanism underlying this, we recorded magnetoencephalograms in 33 healthy volunteers performing two sequential memory tasks with either short or long intervals between memory items (hereafter, fast and slow conditions, respectively). Memory accuracy, and theta- and alpha-band activities originating from occipital and frontal brain areas were analyzed. Memory performance was significantly lower for the fast condition than the slow condition. Meanwhile, occipital and frontal theta activities were significantly lower for the fast condition than the slow condition. Increased occipital-alpha, a sign of active inhibition of task-irrelevant visual input, occurred regardless of condition. However, memory processing related to occipital- and frontal-theta activities had some temporal limitations. Namely, the shorter intervals of the fast condition attenuated theta activity, likely disrupting working memory processing, thereby leading to the observed decline in memory performance.