Publications

Abstract

When a novel word is learned, its memory representation is thought to undergo a process of consolidation and integration. In this study, we tested whether the neural representations of novel words change as a function of consolidation by observing brain activation patterns just after learning and again after a delay of one week. Words learned with meanings were remembered better than those learned without meanings. Both episodic (hippocampus-dependent) and semantic (dependent on distributed neocortical areas) memory systems were utilised during recognition of the novel words. The extent to which the two systems were involved changed as a function of time and the amount of associated information, with more involvement of both systems for the meaningful words than for the form-only words after the one-week delay. These results suggest that the reason the meaningful words were remembered better is that their retrieval can benefit more from these two complementary memory systems

Abstract

Navigating through space is fundamental to human nature and requires the ability to retrieve relevant information from the remote past. With the passage of time, some memories become generic, capturing only a sense of familiarity. Yet, others maintain precision, even when acquired decades ago. Understanding the dynamics of memory consolidation is a major challenge to neuroscientists. Using functional magnetic resonance imaging, we systematically examined the effects of time and spatial context on the neural representation of landmark recognition memory. An equal number of male and female subjects (males N = 10, total N = 20) watched a route through a large-scale virtual environment. Landmarks occurred at navigationally relevant and irrelevant locations along the route. Recognition memory for landmarks was tested directly following encoding, 24 h later and 30 days later. Surprisingly, changes over time in the neural representation of navigationally relevant landmarks differed between males and females. In males, relevant landmarks selectively engaged the parahippocampal gyrus (PHG) regardless of the age of the memory. In females, the response to relevant landmarks gradually diminished with time in the PHG but strengthened progressively in the inferior frontal gyrus (IFG). Based on what is known about the functioning of the PHG and IFG, the findings of this study suggest that males maintain access to the initially formed spatial representation of landmarks whereas females become strongly dependent on a verbal representation of landmarks with time. Our findings yield a clear objective for future studies

Abstract

So far, studies that investigated interference effects of post-learning processes on episodic memory consolidation in humans have used tasks involving only complex and meaningful information. Such tasks require reallocation of general or encoding-specific resources away from consolidation-relevant activities. The possibility that interference can be elicited using a task that heavily taxes our limited brain resources, but has low semantic and hippocampal related long-term memory processing demands, has never been tested. We address this question by investigating whether consolidation could persist in parallel with an active, encoding-irrelevant, minimally semantic task, regardless of its high resource demands for cognitive processing. We distinguish the impact of such a task on consolidation based on whether it engages resources that are: (1) general/executive, or (2) specific/overlapping with the encoding modality. Our experiments compared subsequent memory performance across two post-encoding consolidation periods: quiet wakeful rest and a cognitively demanding n-Back task. Across six different experiments (total N = 176), we carefully manipulated the design of the n-Back task to target general or specific resources engaged in the ongoing consolidation process. In contrast to previous studies that employed interference tasks involving conceptual stimuli and complex processing demands, we did not find any differences between n-Back and rest conditions on memory performance at delayed test, using both recall and recognition tests. Our results indicate that: (1) quiet, wakeful rest is not a necessary prerequisite for episodic memory consolidation; and (2) post-encoding cognitive engagement does not interfere with memory consolidation when task-performance has minimal semantic and hippocampally-based episodic memory processing demands. We discuss our findings with reference to resource and reactivation-led interference theories

Abstract

The complementary learning systems account of word learning states that novel words, like other types of memories, undergo an offline consolidation process during which they are gradually integrated into the neocortical memory network. A fundamental change in the neural representation of a novel word should therefore occur in the hours after learning. The present EEG study tested this hypothesis by investigating whether novel words learned before a 24-hr consolidation period elicited more word-like oscillatory responses than novel words learned immediately before testing. In line with previous studies indicating that theta synchronization reflects lexical access, unfamiliar novel words elicited lower power in the theta band (4–8 Hz) than existing words. Recently learned words still showed a marginally lower theta increase than existing words, but theta responses to novel words that had been acquired 24 hr earlier were indistinguishable from responses to existing words. Consistent with evidence that beta desynchronization (16–21 Hz) is related to lexical-semantic processing, we found that both unfamiliar and recently learned novel words elicited less beta desynchronization than existing words. In contrast, no difference was found between novel words learned 24 hr earlier and existing words. These data therefore suggest that an offline consolidation period enables novel words to acquire lexically integrated, word-like neural representations.

Abstract

General knowledge acquisition entails the extraction of statistical regularities from the environment. At high levels of complexity, this may involve the extraction, and consolidation, of associative regularities across event memories. The underlying neural mechanisms would likely involve a hippocampo-neocortical dialog, as proposed previously for system-level consolidation. To test these hypotheses, we assessed possible differences in consolidation between associative memories containing cross-episodic regularities and unique associative memories. Subjects learned face–location associations, half of which responded to complex regularities regarding the combination of facial features and locations, whereas the other half did not. Importantly, regularities could only be extracted over hippocampus-encoded, associative aspects of the items. Memory was assessed both immediately after encoding and 48 h later, under fMRI acquisition. Our results suggest that processes related to system-level reorganization occur preferentially for regular associations across episodes. Moreover, the build-up of general knowledge regarding regular associations appears to involve the coordinated activity of the hippocampus and mediofrontal regions. The putative cross-talk between these two regions might support a mechanism for regularity extraction. These findings suggest that the consolidation of cross-episodic regularities may be a key mechanism underlying general knowledge acquisition.

Abstract

To test the hypothesis that thalamic midline nuclei play a transient role in memory consolidation, we reanalyzed a prospective functional MRI study, contrasting recent and progressively more remote memory retrieval. We revealed a transient thalamic connectivity increase with the hippocampus, the medial prefrontal cortex (mPFC), and a parahippocampal area, which decreased with time. In turn, mPFC-parahippocampal connectivity increased progressively. These findings support a model in which thalamic midline nuclei serve as a hub linking hippocampus, mPFC, and posterior representational areas during memory retrieval at an early (2 h) stage of consolidation, extending classical systems consolidation models by attributing a transient role to midline thalamic nuclei.

Abstract

According to the complementary learning systems (CLS) account of word learning, novel words are rapidly acquired (learning system 1), but slowly integrated into the mental lexicon (learning system 2). This two-step learning process has been shown to apply to novel word forms. In this study, we investigated whether novel word meanings are also gradually integrated after acquisition by measuring the extent to which newly learned words were able to prime semantically related words at two different time points. In addition, we investigated whether modality at study modulates this integration process. Sixty-four adult participants studied novel words together with written or spoken definitions. These words did not prime semantically related words directly following study, but did so after a 24-hour delay. This significant increase in the magnitude of the priming effect suggests that semantic integration occurs over time. Overall, words that were studied with a written definition showed larger priming effects, suggesting greater integration for the written study modality. Although the process of integration, reflected as an increase in the priming effect over time, did not significantly differ between study modalities, words studied with a written definition showed the most prominent positive effect after a 24-hour delay. Our data suggest that semantic integration requires time, and that studying in written format benefits semantic integration more than studying in spoken format. These findings are discussed in light of the CLS theory of word learning.