It has been proposed that hippocampal neurogenesis degrades existing memories. This may be due to increased competition between the new and existing neurons, followed by the replacement of synapses in preexisting memory circuits. This theory has been supported in mouse models in which increasing neurogenesis levels also increased forgetting.[57][58] Additionally, decreasing neurogenesis after new memory formation resulted in decreased forgetting.[58] Are 'lost' infant memories permanently erased (i.e., storage failure) or do they become progressively inaccessible with time (i.e., retrieval failure)?