Working memory is active short-term memory storage that is easily accessible and underlies various activities, such as maintaining phone numbers in mind for a short period [1,2]. There is accumulating theoretical and physiological evidence that memorized items are represented rhythmically by neural oscillation in the theta range (4-7 Hz) [3,4]. However, the impact of this process on human behavior is yet to be examined. Here we show that simply memorizing sequential information affects a behavioral index (i.e., reaction time, RT) in a rhythmic manner. In the main experiment (Experiment 1), we measured RTs to a visual probe that appeared at one of two sequentially memorized locations after a random interval. Consequently, RTs to the first and second probes each fluctuated in the theta range as a function of the random interval, and the phases of the two theta fluctuations were not in phase or anti-phase, but shifted by approximately 270 degree. Interestingly, the 270 degree phase difference corresponded to the rhythm of "phase coding", where sequential information is represented on the specific phase of theta oscillation [5-7]. These relationships were not observed in tasks simply requiring attention (Experiment 2) or memorization (Experiment 3) of spatial locations without sequential order. In conclusion, the current results demonstrate that our behavior fluctuates when recalling memorized sequential items in the theta-range, suggesting that accessibility to sequential working memory is rhythmic rather than stable, possibly reflecting theta-phase coding.
Typical Behaviour of Random Interval Homeomorphisms
