Do brain ripples boost emotional memory encoding?

By Pooja Toshniwal PahariaReviewed by Sophia CoveneyJan 7 2024

In a recent study published in the journal Nature Communications, researchers investigated whether post-encoding ripples improve emotional memory through amygdala-hippocampal memory restoration or stimulus retention in working-type memory. They also investigated whether either method improves stimulus similarities through post-encoding ripples.

During memory consolidations, amygdala-hippocampal connections affect emotional memory through various processes, including neuromodulatory effects. Memory reinstatement during the post-encoding stage predicts memory performance later in life. Ripples are transient hippocampal oscillations linked with synchronous neuronal activity in the hippocampus and amygdala, crucial in binding memory traces and reactivating emotional memory.

About the study

In the present study, researchers intended to determine whether ripple-regulated dynamic changes in the hippocampus and amygdala enhanced emotional memory.

Seven human subjects participated in the study, which included intracranial electroencephalography (iEEG) recordings obtained from the hippocampus and amygdala while undertaking tests for encoding and discriminating emotional memory. The team provided the participants with stimuli and asked them to judge the valence as positive, negative, or neutral.

In the retrieval phase, participants were shown one of three stimuli categories: Repeats (same), Lure (marginally different), and Novel (stimuli not observed during encoding times) and asked to classify each one as old or new. The team defined memory discrimination as correctly categorizing repeat stimuli as old, novel stimuli as new, or lure stimuli as new.

The researchers investigated the relationship between post-encoding ripple rate (number of ripple occurrences per second), emotional content (stimulus-elicited arousal and valence), and accurate Lure discrimination after retrieval.

They also investigated whether stimulus similarities in post-encoding ripple periods may improve memory discrimination later. The researchers calculated stimulus similarities based on Spearman correlations between HFA power spectral vectors (PSVs) from the same trial for all encoding-response time bin combinations.

The researchers assessed post-encoding stimuli similarities for each location relative to the ripple peak to analyze the unique contributions of the hippocampus and amygdala to the phenomenon. They subsequently investigated the relationship between post-encoding stimulus similarities, stimulus-elicited arousal, and Lure discrimination. The researchers anticipated that the intervals of stimulus similarities in both structures occur concomitantly and follow a consistent temporal dynamic.

Results

The team investigated intracranial recordings from the human amygdala and hippocampus to evaluate their impact on emotional memory encoding and discriminating. They confirmed behavioral findings of superior discriminate memory for stimulating stimuli using intracranial electroencephalographic (iEEG) recordings in epileptic patients. Ripple event counts immediately following learning were related to stimuli-elicited arousal and subsequent discriminating accuracy.

The synchronized post-encoding stimuli similarities across the hippocampus and amygdala for post-encoding ripples predict subsequent memory discernment performance, with the stimulus similarities in the amygdala influencing the hippocampal stimulus similarity.

Memory discrimination improved for emotional-type stimuli, with individuals correctly identifying repeat and new stimuli. The accuracy of memory discrimination was poorer for the Lure stimuli, indicating that picture similarity generated memory interference.

Stimulus-elicited arousal was linked to accurate lure discrimination, correlating with prior findings. Response durations did not show significant associations with the emotional valence of the stimuli. The Lure discrimination index (LDI) showed considerably higher values for the high-arousal-type stimuli, demonstrating the likelihood of high-arousal-type stimuli being classified as novel.

Precise lure discriminations were strongly related to stimulus arousal and resemblance but not valence. The team observed a strong relationship between similarity and arousal, with low similarity and high arousal-type stimuli yielding the most accurate Lure discrimination.

Post-encoding-type ripples were associated with improved emotional stimulus discrimination, with higher post-encoding ripple rates associated with stimulus-elicited arousal, and they also predicted better Lure discrimination following retrieval but not with stimulus valence.

The study indicated that stimulus-elicited arousal and later accurate Lure discrimination were the primary impacts, with no significant interaction. Ripple likelihood was much higher during periods of low theta power, which is consistent with studies of ripple suppression during periods of robust theta oscillations.

The reactivation of various elements of stored stimuli was related to post-encoding and ripple-locked stimuli similarities in the hippocampus and amygdala, indicating that these areas play a crucial role in memory discrimination. The study discovered that stimulus similarity on trials without post-encoding ripples did not differ significantly according to stimulus arousal level or subsequent accurate Lure discrimination.

Conclusions

Overall, the study findings showed that post-encoding ripples improve emotional memory, with higher ripple rates related to stimulus-elicited arousal and accurate stimulus identification. The findings indicated that ripples may selectively enhance salient events.

There was no significant relationship between encoding ripples and Lure discrimination accuracy, indicative of the mediation of retrieval and strengthening of stored representations, thus contributing to memory consolidation. Post-encoding sensory similarity aids memory consolidation, which peaks during ripples.