Adult Fears Why are fear memories almost impossible to get rid of—even with extensive extinction training? Animal studies have shown that the efficacy of extinction learning depends on age. Fear memories in young animals can be permanently erased, but in adults they can be easily recovered after extinction training. Perineuronal nets, the highly organized form of extracellular matrix around inhibitory neurons, mediate the shift from juvenile to adult forms of learning in sensory systems. Gogolla et al. (p. 1258 ; see the Perspective by Pizzorusso ) have discovered that the formation of perineuronal nets in the amygdala coincides with the developmental shift in the ability to erase fear memories by extinction. Removal of perineuronal nets in adult animals re-enabled the erasure of fear memories. Thus, in adults it appears that fear memories are actively protected from erasure by the perineuronal nets.