Probing the relevance of the hippocampus for conflict-induced memory improvement

Abstract

The hippocampus plays a key role for episodic memory. In addition, a small but growing number of studies has shown that it also contributes to the resolution of response conflicts. It is less clear how these two functions are related, and how they are affected by hippocampal lesions in patients with mesial temporal lobe epilepsy (MTLE). Previous studies suggested that conflict stimuli might be better remembered, but whether the hippocampus is critical for supporting this interaction between conflict processing and memory formation is unknown. Here, we tested 19 patients with MTLE due to hippocampal sclerosis and 19 matched healthy controls. Participants performed a face-word Stroop task during functional magnetic resonance imaging (fMRI) followed by a recognition task for the faces. We tested whether memory performance and activity in brain regions implicated in long-term memory were modulated by conflict during encoding, and whether this differed between MTLE patients and controls. In controls, we largely replicated previous findings of improved memory for conflict stimuli. While MTLE patients showed response time slowing during conflict trials as well, they did not exhibit a memory benefit. In controls, neural activity of conflict resolution and memory encoding interacted within a hippocampal region of interest. Here, left hippocampal recruitment was less efficient for memory performance in incongruent trials than in congruent trials, suggesting an intrahippocampal competition for limited resources. They also showed an involvement of precuneus and posterior cingulate cortex during conflict resolution. Both effects were not observed in MTLE patients, where activation of the precuneus and posterior cingulate cortex instead predicted later memory. Further research is needed to find out whether our findings reflect widespread functional reorganization of the episodic memory network due to hippocampal dysfunction.

Proportion of conflict, contingency learning, and recency effects in a Stroop task

“…The absence of contingency learning observed in this study, together with the persistent effects of the most recent previous episode involving a distracter, can be seen as informative on the current debate on how learning and memory modulate conflict effects, and vice versa. It is important to note that the argument about the role of conflict on both learning and memory was raised to justify both observed improvements in these processes as a result of conflict (Krebs et al, 2015; Ramm et al, 2021; Rosner et al, 2015; Verguts & Notebaert, 2008) and a decrease in learning in conditions of higher conflict (Whitehead et al, 2018). Other studies have failed to substantiate either of these two opposing claims (Ortiz-Tudela et al, 2017, 2018), even under conditions in which the trained items were repeatedly presented in either congruent or incongruent contexts (Jiménez et al, 2020).…”