Abstract
Many theoretical models suggest that neural oscillations play a role in learning or retrieval of temporal sequences, but the extent to which oscillations support sequence representation remains unclear. To address this question, we used scalp electroencephalography (EEG) to examine oscillatory activity over learning of different object sequences. Participants made semantic decisions on each object as they were presented in a continuous stream. For three "Consistent" sequences, the order of the objects was always fixed. Activity during Consistent sequences was compared to "Random" sequences that consisted of the same objects presented in a different order on each repetition. Over the course of learning, participants made faster semantic decisions to objects in Consistent, as compared to objects in Random sequences. Thus, participants were able to use sequence knowledge to predict upcoming items in Consistent sequences. EEG analyses revealed decreased oscillatory power in the theta (4-7 Hz) band at frontal sites following decisions about objects in Consistent sequences, as compared with objects in Random sequences. The theta power difference between Consistent and Random only emerged in the second half of the task, as participants were more effectively able to predict items in Consistent sequences. Moreover, we found increases in parieto-occipital alpha (10-13 Hz) and beta (14-28 Hz) power during the pre-response period for objects in Consistent sequences, relative to objects in Random sequences. Linear mixed effects modeling revealed that single trial theta oscillations were related to reaction time for future objects in a sequence, whereas beta and alpha oscillations were only predictive of reaction time on the current trial. These results indicate that theta and alpha/beta activity preferentially relate to future and current events, respectively. More generally our findings highlight the importance of band-specific neural oscillations in the learning of temporal order information.
Highlights
Episodic memories can be conceptualized as temporally-organized sequences of events (Allen & Fortin, 2013)
We found a significant Study Block (Early vs. Late) by Serial Position interaction (F2.95, 56.07 = 5.43, p < 0.003) and a significant Sequence Type by Serial Position interaction (F4,40, 83.60 = 2.95, p < 0.05), all of which pointed to the fact that reaction times for objects in Consistent and Random sequences diverged from each other over the course of learning2
This analysis revealed no main effects or interactions, most likely due to subjects’ near-ceiling performance. These results demonstrate that participants were able use their sequence knowledge to predict upcoming objects in the Consistent sequences to facilitate their reaction times
Summary
Episodic memories can be conceptualized as temporally-organized sequences of events (Allen & Fortin, 2013). Available evidence suggests that theta activity might contribute to sequence processing (e.g., Heusser, Poeppel, Ezzyat, & Davachi, 2016; Hsieh et al, 2011; Roberts et al, 2013). EEG studies of working memory (Hsieh et al, 2011; Roberts et al, 2013) have shown that theta power is higher during active maintenance of temporal order information than during maintenance of detailed object information. These findings indicate that theta power could reflect online maintenance of temporal order relationships.
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