Symbolic neural dynamics abstracting the functionalities of synaptic plasticity has been proposed as a new approach to modeling brain cognitive capabilities and used to define the basic mechanisms of an associative memory. This formalism is extended here to reproduce optogenetic manipulations, thus defining a computational model of memory engrams. It is illustrated through simulations of reversible retrograde amnesia and false memories of contextual fear conditioning that reproduce the behavioral schedules of actual experiments. These results support the hypothesis that separate processes are involved in long term memory i.e., the retention of specific patterns of connectivity between engram cells required for the storage of information, on one hand, and the synaptic strengthening needed for its consolidation and retrieval, on the other. Defined by a logic program, this simulation platform could be used to design and predict the results of experiments involving inhibitory/excitatory loops formed between various brain regions