A broader phenotype of persistence emerges from individual differences in response to extinction.

The typical practice of averaging group performance during extinction gives the impression that responding declines gradually and homogeneously. However, previous studies of extinction in human infants have shown that some individuals persist in responding, whereas others abruptly cease responding. As predicted by theories of control, the infants who quickly resign typically display signs of sadness and despair when the expected reward is omitted. Using genetically diverse mice, here we observed a similar pattern of individual differences and the associated phenotypes. After learning to approach a food reward, upon extinction, some animals rapidly abandoned approach to the goal box, whereas other animals persisted in entering and searching the goal box. Interestingly, the persistent mice were slower to "give up" when confined to an inescapable pool of water (a test asserted to be indicative of susceptibility to depression) and exhibited a more extensive pattern of search for omitted rewards. Thus, extinction reveals a continuum in persistence, in which low values might reflect a susceptibility to the negative effects of stress and might predispose individuals to depression.