Where was the Toaster? Interplay of Episodic Memory Traces and Semantic Knowledge during Scenario Construction

Experiences that are retrieved from memory are often not accurate, but prone to biases. The interplay of already existing semantic knowledge and recently generated episodic memory traces might explain some of the underlying mechanisms. The scenario construction model postulates that during encoding, only the gist of an episode is stored in the episodic memory trace and during retrieval, any needed information that is missing from that trace is constructed from semantic knowledge. The current study aimed to investigate semantic construction in a realistic, yet controlled setting. Using a desktop virtual reality (VR) participants navigated through a flat in which some household items appeared in unexpected rooms, creating conflicts between the experienced episode and semantic expectations. During recall after two separate retrieval delays (one day vs one week) we were able to identify influences from semantic knowledge. To manipulate salience, some objects were task-relevant, and some were irrelevant to the sequence of actions. We used spatial and temporal recall measures. Both congruency and task-relevance, but not time, predicted correct episodic memory retrieval. In the spatial memory retrieval, semantic construction was more likely than guessing in cases of episodic memory failure and occurred more frequently for task-irrelevant objects. In the temporal recall at the second retrieval delay we could show that object-pairs belonging to the same semantic room-category were temporally clustered together compared to object-pairs from different semantic room-categories. Taken together, our findings support the predictions of the scenario construction model, as we found influences of semantic knowledge on both spatial and temporal memory recall. The new VR-paradigm appears to be a promising tool for investigating semantic construction.

Recovery from memory failure when recalling a memorized performance: The role of musical structure and performance cues

To perform reliably and confidently from memory, musicians must able to recover from mistakes and memory failures. We describe how an experienced singer (the second author) recovered from mistakes and gaps in recall as she periodically recalled the score of a piece of vocal music that she had memorized for public performance, writing out the music six times over a five-year period following the performance. Five years after the performance, the singer was still able to recall two-thirds of the piece. When she made mistakes, she recovered and went on, leaving gaps in her written recall that lengthened over time. We determined where in the piece gaps started ( losses) and ended ( gains), and compared them with the locations of structural beats (starts of sections and phrases) and performance cues ( PCs) that the singer reported using as mental landmarks to keep track of her progress through the piece during the sung, public performance. Gains occurred on structural beats where there was a PC; losses occurred on structural beats without a PC. As the singer’s memory faded over time, she increasingly forgot phrases that did not start with a PC and recovered at the starts of phrases that did. Our study shows how PCs enable musicians to recover from memory failures.

Oscillations support co-firing of neurons in the service of human memory formation

Brain oscillations have been demonstrated to support information transfer between neurons in animal models of memory. However, direct evidence for a similar role of oscillations in humans has so far remained unclear. Here we show that theta and gamma oscillations in the medial-temporal-lobe synchronize neural firing during a memory task. We observe that faster oscillations at theta- and gamma frequencies correlate with co-firing of neurons at short latencies (~20-30 ms) and occur during successful memory formation. Slower oscillations in these same frequency bands, by contrast, correlate with longer co-firing latencies and occur during memory failure. A computational model supports the present effects and links these findings to synaptic plasticity. Together, the results support the long-standing assumption that correlated neural firing supports human episodic memory formation.One Sentence Summaryheta and gamma oscillations induce co-firing of neurons in the human medial temporal lobe during successful memory formation.

Relating Lurie's problem, Hopfield's network and Alzheimer's disease

Alzheimer's disease is a degenerative brain disorder that affects millions of people around the world and still without cure. A very common application of Hopfield neural networks is to simulate a human memory as well as to evaluate problems of degeneration and memory loss. On the other hand, from the control area, one has Lurie's problem, which emerged in the 1940s and which still does not have a general solution. However many works and results came in an attempt to solve it. In this paper, the Hopfield's network is shown as a particular case of Lurie's problem, then one of the consequences of Alzheimer's disease, memory failure, is modeled using Hopfield's networks and nally a recent result of Lurie's problem is applied to the computationally modeled disease to correct the problem of memory loss. The correction is made using a controller via DK-iteration. Simulations are performed to validate the computational model of the disease and to demonstrate the effectiveness of the application of the recent Lurie's problem theorem. Therefore, in addition to the results presented, this work aims at encouraging the researches in the area, so that in the future, better diagnostic and treatment conditions will be achieved.

1. The challenges of dementia

While cognitive problems have long been associated with ageing, dementia is a relatively new concept. ‘The challenge of dementia’ shows how our understanding of it was shaped by research in the late 19th and early 20th centuries. Dementia often first appears as short-term memory failure, but symptoms vary depending on the type and the regions of the brain affected. The hippocampus is often implicated, as are synaptic failures that lead to cell death. Despite a rapidly ageing population, dementia research is underfunded. Research presents challenges for scientists, including obtaining brain samples and addressing the complex processes of neurodegeneration. Dementia does not just affect the elderly.

Memory destabilization and reconsolidation dynamically regulate the PKMζ maintenance mechanism

Useful memory must balance between stability and malleability. This puts effective memory storage at odds with plasticity processes like reconsolidation. What becomes of memory maintenance processes during synaptic plasticity is unknown. Here we examined the fate of the memory maintenance protein PKMζ during memory destabilization and reconsolidation. We found that NMDA receptor activation and proteasome activity induced a transient reduction in PKMζ protein following retrieval. During reconsolidation, new PKMζ was synthesized to re-store the memory. Failure to synthesize new PKMζ during reconsolidation impaired memory but uninterrupted PKMζ translation was not necessary for maintenance itself. Finally, NMDA receptor activation was necessary to render memories vulnerable to the amnesic effect of PKMζ-antisense. These findings outline a transient collapse and renewal of the PKMζ memory maintenance mechanism during plasticity. We argue that dynamic changes in PKMζ protein levels can serve as an exemplary model of the molecular changes underlying memory destabilization and reconsolidation.