Exploring the effectiveness of targeted memory reactivation on emotional implicit memory

Introduction: Targeted Memory Reactivation (TMR) is a technique employed to enhance memory consolidation by reactivating specific memory traces during sleep. Negative attentional bias (AB), characterized by a tendency to preferentially focus on negative rather than positive stimuli, has been demonstrated to play a role in perpetuating negative emotions and memories. TMR could potentially affect AB by selectively reactivating positive memories, thus influencing emotional memory processing. This study aimed to investigate the effectiveness of TMR in modulating emotional implicit memories to attenuate negative AB. Materials and Methods: Eight female students (mean age ± standard deviation, 21.50 ± 2.45) participated in a between-subjects design involving the assessment of AB through a modified version of the Dot-Probe Task (DPT) before (T1) and after (T2) a nocturnal sleep with acoustic stimulation (ASTMR vs. ASCONTROL) during slow-wave sleep. During the DPT, participants were simultaneously presented with a positive and a negative emotional face, followed by a probe that equally replaced one of the emotional stimuli (positive condition/negative condition). The subjects' assignment was to indicate the position of the probe (left vs. right) using the keyboard. AB in DPT was measured by comparing reaction times (RTs) in positive and negative conditions, with faster responses obtained when the probe appeared in the previously attended spatial location. Concurrently with the keyboard response, participants were exposed to two different auditory cues for positive and negative conditions. In the ASTMR group, participants were exposed during sleep to the auditory stimulus linked to the positive condition during the DPT. In the ASCONTROL group, a novel sound was presented. To assess the effect of TMR, a mixed ANOVA was applied to the RTs of the DPT, with condition (negative/positive), session (T1/T2), and group (ASTMR/ASCONTROL) as within/between factors. To evaluate the EEG correlates of TMR during sleep, an event-related spectral power perturbation of nocturnal stimulations was computed, comparing the two experimental groups using independent samples t-Test. Results: Analysis of DPT showed a significant effect of TMR on RTs (group x session: p<0.001). Post hoc comparisons highlighted a reduction of both positive and negative RTs in the ASTMR group compared to the ASCONTROL group. The comparisons on sleep EEG correlates of TMR evidenced an increase in spindle activity around 500 and 1000 ms post-stimulation on the whole topography for ASTMR compared to the ASCONTROL group (p<0.05). The midline and parieto-occipital sites in the ASTMR group also showed increased delta/theta activity (p<0.05) around 1000 ms post-stimulation. Conclusions: The findings indicate an effect of TMR in modulating attentional processes associated with emotional stimuli with no specificity for the stimulus valence. The electrophysiological correlates of nocturnal stimulation involve rhythms typically engaged in sleep-dependent memory consolidation, supporting the effectiveness of memory reactivation. The reduction in RTs associated with both positive and negative stimuli seems to suggest that TMR effectiveness may be contingent upon previous associative learning at the explicit level, implying a nuanced interplay between different memory processes. Further research is warranted to unravel these intricate interactions and refine the TMR application also for therapeutic and cognitive enhancement purposes.