Long-term memory storage by the interaction of structural and synaptic plasticity in recurrent neuronal networks

• Applicant: Dr. Michael Fauth
• Subject Area: Cognitive, Systems and Behavioural Neurobiology
• Term: from 2017 to 2018
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 336760888

In order to survive in complex and changing environments, humans and animals learn from past experiences by forming memories. Hereby, an experience, which triggers memory formation, causes neuronal activation in the brains networks leading to changes of the connections between neurons, which represent the memory of this experience. The retention of this memory item is believed to depend on the persistence of these changes. However, recent experiments show that synapses are created and removed on a daily basis and large fractions of synapses are exchanged on a timescale of weeks. This poses the question, how long term-memories, which can persist for years and decades, can be stored and maintained on such a variable substrate. Specifically, it has to be clarified how, in spite of synaptic turnover, long-term memory can emerge from the interaction between neuronal activities and different plasticity processes involved in changing the neuronal connectivity namely structural plasticity, describing the creation and removal of synapses, and synaptic plasticity, describing changes of the synaptic transmission efficacies. To investigate this complex interaction on the network level, we propose a theoretical study using mathematical models of the involved processes. We hypothesize that the interaction between activity and the described plasticity mechanisms leads to a collective dynamics in populations of cells, which can be controlled by the incoming stimulation: At high stimulation levels, a positive feedback between neuronal activities and connectivity leads to the formation of highly interconnected clusters of cells - so-called cell assemblies - which are considered as memory items. At low stimulation levels, the connections between these cells will be pruned, such that the memory is forgotten. At intermediate stimulation levels, we expect that already formed assemblies can be maintained for long period of time. More precisely, we hypothesize that the positive feedback between activity and connectivity in these assemblies stabilizes them against synaptic turnover, such that they can be retained on the timescale of long-term memory. To test these hypotheses, we will use both network simulations and analytical mathematical methods, which will allow us to identify the determinants of assembly stability. The outcome of this study will be a dynamic memory model which can form or remove memories in an input-dependent fashion and maintain them on the timescale of long-term memory despite synaptic turnover.

• DFG Programme: Research Fellowships
• International Connection: United Kingdom
• Host: Professor Dr. Mark van Rossum