A recent study published in the journal Nature Communications sought to investigate whether post-encoding ripples improve emotional memory by restoring the amygdala-hippocampus memory or by maintaining stimuli in working memory. The study also explored whether either of these methods enhances stimuli similarities through post-encoding waves.
Amygdala-hippocampus connections during memory consolidation influence emotional memory through various neuromodulatory effects. Restoration of memory during the post-encoding phase predicts memory performance later in life. Waves are transient oscillations of the hippocampus associated with synchronous neural activity in the hippocampus and amygdala, crucial for memory trace binding and reactivation of emotional memory.
About the Study
The study aimed to determine if wave-regulated dynamic changes in the hippocampus and amygdala enhance emotional memory. Seven human subjects participated in the study, which included intracranial electroencephalography (iEEG) recordings from the hippocampus and amygdala alongside tests for encoding and discrimination of emotional memory. The participants were presented with stimuli and asked to evaluate their valence as positive, negative, or neutral.
During the retrieval phase, the participants were shown stimuli from one of three categories: repetitions (same), lure (slightly different), and novel (stimuli not observed during the encoding period) and were asked to classify each as old or new. The authors defined memory discrimination as the correct categorization of repeat stimuli as old, novel stimuli as new, and lure stimuli as new.
The researchers examined the relationship between post-encoding wave rates (number of occurrences of waves per second), emotional content of stimuli (arousal and valence induced by stimuli), and accurate lure discrimination after retrieval.
They also investigated whether post-encoding wave-induced stimulus similarities can improve memory discrimination later. The researchers computed stimulus similarities based on Spearman correlations between high-frequency activity power spectral vectors (PSVs) from the same trial for all combinations of encoding and retrieval time intervals.
The researchers evaluated the similarities of stimuli after encoding for each location relative to the wave peak to analyze the unique contributions of the hippocampus and amygdala to the phenomenon. They then examined the relationship between stimulus similarities after encoding, stimulus-induced arousal, and lure discrimination. The researchers hypothesized that the intervals of stimulus similarities would occur simultaneously in both structures and follow consistent temporal dynamics.
The team analyzed intracranial recordings from the human amygdala and hippocampus to assess their influence on encoding and discrimination of emotional memory. They confirmed behavioral findings of superior memory discrimination for arousing stimuli using intracranial electroencephalography (iEEG) recordings in epilepsy patients. The number of ripple events immediately after learning was associated with stimulus-induced arousal and subsequent discrimination accuracy.
The synchronized post-encoding stimulus similarities in the hippocampus and amygdala for post-encoding ripples predicted subsequent memory discrimination performance, with the amygdala’s stimuli similarity influencing hippocampal stimulus similarity.
Memory discrimination improved for emotional stimuli as individuals correctly identified repeated and novel stimuli. Discrimination accuracy was poorer for lure stimuli, suggesting image similarity led to memory confusions.
Stimulus-induced arousal was associated with accurate lure discrimination, consistent with previous findings. Response time showed no significant association with the emotional valence of stimuli. The lure discrimination index (LDI) showed significantly higher values for high arousal stimuli, indicating a higher probability of high arousal stimuli being classified as new.
Precise lure discriminations were closely associated with stimulus arousal and similarity, but not with valence. The team observed a strong connection between similarity and arousal, with low similarity and high arousal stimuli providing the most accurate lure discrimination.
Post-encoding type ripples were associated with enhanced discrimination of emotional stimuli, with higher post-encoding ripple rates linked to stimulus-induced arousal and predicting better lure discrimination after retrieval, but not with stimulus valence.
The study found that stimulus-induced arousal and subsequent accurate lure discrimination were the primary effects without significant interaction. Wave probability was much higher during low theta power periods, consistent with studies on wave suppression during robust theta oscillations.
Reactivation of different elements of stored stimuli was associated with post-encoding ripples and stimulus-locked stimulus similarities in the hippocampus and amygdala, suggesting these areas play a crucial role in memory discrimination. The study revealed that stimulus similarities in trials without post-encoding ripples did not significantly differ based on stimulus arousal levels or subsequent precise lure discrimination.
Overall, the study’s results indicated that post-encoding ripples improve emotional memory, with higher ripple rates associated with stimulus-induced arousal and accurate stimulus identification. The findings suggest waves can selectively enhance salient events.
There was no significant connection between encoding waves and lure discrimination accuracy, indicating facilitation of retrieval and strengthening of stored representations, thereby contributing to memory consolidation. Sensory similarity after encoding supports memory consolidation, peaking during the occurrence of waves.