The role of emotion and meaning-based congruence in memory formation
Abstract
Since Ebbinghaus, researchers have tried to isolate human memory phenomena in order to determine more precisely their nature. In an attempt to approach perfect analytical objectivity, they often have used the simplest kind of stimuli possible and created neutral situations with the minimum amount of emotional content, also avoiding any meaningful links with previous knowledge the participants might possess. Notwithstanding the merits of this approach, it has unfortunately made it difficult to appreciate and understand how these critically important factors interact with memory.
Classically the encoding of memories was explained as a straightforward process through which new information is stored into a blank empty space. However, it has become increasingly clear that encoding is not a simple affair. A variety of factors and subprocesses influence how memories will be encoded and stored. These are decisive in determining whether a memory will be successfully formed and also the qualitative nature of the resulting memory traces. This dissertation will focus on two of the main psychological factors that influence the outcome of encoding operations: emotion and meaning-based congruence.
The work contained in this thesis attempted, by using electrophysiological measures such as electroencephalography (EEG) recording and skin conductance recording (SCR), to investigate these main questions: i) how does fear influence learning and ii) how does meaning-based congruence between and event and prior information influence learning? In addition, we developed a naturalistic episodic sequence paradigm with greater ecological validity to investigate how meaning-based congruence between and event and prior information influences learning.
In study 1 we found that the implicit, but not the explicit, memory traces of the fearful context of an episode can be detected at long-term through SCR. Such automatic bodily responses measured with SCR were found associated only to verbatim and not gist-based memories, thus highlighting the dissociation between explicit and implicit memory systems. This suggests that contextual fear differentially affects both memory systems in a way that possibly interferes with their processes over time. We found that even moderate levels of fear produce memory disturbances hypothesized to cause and maintain post-traumatic–stress disorder (PTSD).
The results from study 2 reveal encoding-dependent brain mechanisms related to the rapid integration of congruent semantic information and its consequences for long-term memory. The results suggest that congruent events can trigger an accelerated onset of neural encoding mechanisms supporting the integration of semantic information with the event input, and shed light on how humans can efficiently encode into memory rapid streams of events. Thus, our results suggest congruent events are integrated together with semantic associations, after initial semantic access reflected by the N400 effect, and before encoding into long-term memory, resulting in memory traces enhanced with a greater amount of associated semantic information. This may explain the associated increase in semantically-related false memories.
In Study 3, we found that oscillation dynamics dissociate between successful encoding of congruent and incongruent events. Our results indicate that semantic offline encoding processes of both congruent and incongruent events have neural correlates in the alpha frequency range. We found that neural correlates in the alpha frequency range are more closely tied to the observed inter-individual variations in successful encoding rates of congruent events. This is in line with the idea of alpha desynchronization reflecting processing of semantic features, an increase in the amount of total information represented in the brain, and the controlled access to previous knowledge stores for the orientation in time, space and context necessary for the successful encoding of a coherent and meaningful memory trace.
Increases in theta power during encoding predicted successful encoding of incongruent events, supporting the idea that theta synchronization plays an important role in the encoding of new information and non-semantic encoding processes. This increase in theta synchronization may be triggered by an automatic mismatch detection mechanism that increases medial temporal lobe (MTL) dependent encoding processes. The behavioral results confirm that congruent events are more easily integrated into real-life episodic memories, through a facilitated encoding of the congruent related semantic information.
The results presented here provide novel insights into how meaning-based congruence and emotion influence the neural processes that support the initial encoding stage which is critical in the successful formation of memories. I hope that the thoughts, findings and suggestions contained in this thesis may aid in the efforts towards a richer and more comprehensive understanding of memory and possibly provide ideas useful in starting novel research initiatives that aim to improve our understanding of the paper of emotion and meaning-based congruence in learning.