"In what can be called the eraser theory of REM sleep, Crick and Mitchison have treated its reported absence in the echidna as evidence that it amounts to a mechanism for reverse learning, in which stimulation of the forebrain weakens the synaptic strength of undesirable “parasitic modes” of neuronal activity, thus fine-tuning the brain’s operation ... The echidna, it is said, gets by without REM sleep because its surprisingly large neocortex makes reverse learning unnecessary. If true, an inverse relationship between size of neocortex and REM sleep quotas is to be expected in other species, but supportive data are lacking."