Fig. 1. Strategies and results for testing the necessity and sufficiency of the neuronal population, which is activated by learning. (A) Activated neurons during conditioning are marked by ArchT through gene expression control techniques. If this population is necessary for memory, the expression of fear memory associated with a context will be reduced by optogenetic silencing of this population. A reduced fear response by yellow light shows the necessity of the activation of this activated neuronal population for memory expression. (B) Activated neurons during conditioning are marked by ChR2 through gene expression control techniques. If this population is sufficient for memory, an artificial activation of this neuronal population will elicit a fear response in the neutral context. An induced fear response by blue light in neutral context shows the sufficiency of this activated neuronal population for memory expression.