Assessing the longevity of the Theta-induced Memory Effect (TIME)

Theta-phase synchronisation between visual and auditory rhythmically modulated sensory stimuli during encoding has been shown to enhance associative memory, an effect known as Theta-Induced Memory Effect (TIME). However, whether this advantage is short-lived or persists over time remains unknown. To address this, we examined the TIME effect in 52 participants randomly assigned to a short-delay group (immediate retrieval) or a long-delay group (retrieval after 24 hours). Electroencephalography (EEG) was recorded during incidental encoding of audio-video pairs. The luminance of the video clips and the amplitude of the sounds were modulated at theta frequency (4 Hz) and presented either in-synchrony (0 phase offset between audio and video) or out-of-synchrony (the sound 180 phase-shifted relative to the video). Source reconstruction was used to estimate the actual phase difference between visual and auditory cortices for every trial. At the behavioural level, no significant difference in memory accuracy was found between the synchronous and asynchronous conditions in either group. However, after reconstructing the actual phase difference at the source level, the TIME effect was replicated in the short-delay group, with significantly higher memory accuracy at the optimal reconstructed theta phase compared to out-of-phase. In contrast, the long-delay group showed a reversed pattern, with higher accuracy in the asynchronous condition, though this difference did not reach significance. These findings suggest that the TIME effect is time-limited and may not survive or even reverse after overnight consolidation.