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The Role and Dynamic of Strengthening in the Reconsolidation Process in a Human Declarative Memory: What Decides the Fate of Recent and Older Memories?
Congreso; 45th meeting of the European Brain and Behaviour Society (EBBS); 2013
Aims: Several reports have shown that after specific reminders are presented, consolidated memories pass from a stable state to one in which the memory is reactivated. This reactivation implies that memories are labile and susceptible to amnesic agents. This susceptibility decreases over time and leads to a re-stabilization phase usually known as reconsolidation. With respect to the biological role of reconsolidation, one of the proposed functions considers that the reconsolidation process strengthens the original memory. We have previously demonstrated that at least two successive triggering of the process strengths a declarative memory in humans.  Demonstrating strengthening creates a new scenario for the reconsolidation process, being a new factor that may transform the dynamics of memory. The aim of this study was to go into detail about the role of strengthening in the reconsolidation process, using a declarative memory paradigm in humans under various conditions. 1) We analyzed whether strengthening of the original memory by repeated labilization-reconsolidations maintained the memory available for longer periods of time. 2) We evaluated how strengthening via repeated labilization-reconsolidation processes modified the effect of an amnesic agent presented immediately after subsequent labilizations. 3) We examined whether the effect of strengthening could depend on the age of the memory. Methods: Our paradigm consisted of learning verbal material (five pairs of nonsense cue-response syllables in rioplatense Spanish) acquired in the Training Session on Day 1 (List 1-training). After this declarative memory had been consolidated, it could be made labile by presenting a specific reminder (cue-reminder) during the Treatment Session. After this, the memory passes through a subsequent stabilization process. Then the memory could be evaluated in the Testing session: The subjects were required to write down the corresponding response-syllable for each cue-syllable presented. 1) We conducted a seven-day experiment using three groups. On Day 1, subjects learned the List 1). On Day 2, two groups received a treatment. Groups differed in the number or presence or absence of reminders (without any reminder presentation or received one cue reminder or, two cue reminders).  All subjects underwent testing on Day 7. 2) A second learning task served as an interfering agent to impair re-stabilization of a reactivated memory. We designed a three-day experiment which included five groups. On Day 1, the subjects of four of these groups received a training session (List 1). On Day 2, they received a treatment (one or two presentations of the cue reminder); some groups received the second learning task (List 2), which served as an amnesic agent. The remaining group learned List 2 only. All subjects underwent testing on Day 7. 3) We conducted an eight-day experiment using four groups. On Day 1, subjects learned a list of paired syllables (List 1). On Day 7, three groups received a treatment. One group received one cue reminder; other group received two-cue reminders; the third group received 4 cue reminders. Finally, the no-reminder group did not receive any treatment on Day 7. All subjects were tested on Day 8. Results: 1) We found that just one labilization-reconsolidation process was enough to strengthen a memory that was evaluated 5 days following its re-stabilization. We also demonstrated that this effect was not triggered by retrieval only. 2) Our results indicated that repeated labilization-reconsolidations rendered memories more resistant to interference during the re-stabilization phase. 3) Strengthening appeared to depend on the age of the memory. In this case, forgetting could be considered a process that weakens the effect of this function. Conclusions: Considering that this study examined strengthening in various experimental scenarios, this research may shed light onto the role of reconsolidation in the fate of declarative memories in humans.