“…Beyond this general trend, however, we have found that the PREE appears much earlier in ontogeny than do other paradoxical effects. The PREE first appears at 12-14 days (Chen & Amsel, 1980;Letz, Burdette, Gregg, Kittrell, & Amsel, 1978), whereas the MREE fails to appears clearly until about 21 days (Burdette, Brake, Chen, & Amsel, 1976;Chen, Gross, & Amsel, 1981;; SNC first appears at 25-26 days (Chen et al, 1981;; and there is no evidence of the OEEprior to 25-26 days . The sequence of appearance of these extinction effects remains to be explained by theories that attribute these effects to a common set of learning mechanisms.…”