(A) simulation study on memory formation and maintenance using spiking neural network model

Abstract

Learning new information and storing memories are two of the most important functions of the brain. The purpose of the thesis is to develop a computational model that accounts for the mechanism of memory formation and maintenance. To approach this question, we designed three simulations covering the circuit, molecular, and system levels, respectively. First, we discuss the roles of diverse spike-timing-dependent plasticity (STDP) profiles in flexible and stable memory formation. Through computer simulation, we found that an asymmetric learning rate generates flexible memory that is volatile and easily overwritten by newly appended information. Moreover, a symmetric learning rate generates stable memory that can coexist with newly appended information. Our results demonstrate that various attributes of memory functions may originate from differences in synaptic stability. Second, we suggested the roles of intrinsic excitability in memory allocation. Using the model network, we successfully generated experimental results that showed that a neuron excited just before learning has a higher probability of being allocated in a memory ensemble than control group neurons. This result suggests that temporal excitability fluctuation not only can link two memories encoded close in time, but also boost the capacity of the network. Finally, we present a novel systematic model in which the brain regions responsible for learning and coding are separated. We found that our model can account for the observed dynamics of an ensemble in the basolateral amygdala during fear conditioning. The simulated ensemble dynamics well described the observed data such as bi-directional plasticity, which never can be achieved by a single-layered network simulation. Overall, our thesis presents an integrated approach to memory formation and maintenance through computer simulation. We hope this study can provide a unified description of how and why our memories are formed and can be maintained.