Fig. 6. Hypothetical relations between a phenotypic trait and the amount of running exhibited during the final week of a multi-week exposure to wheels. In both A and B, it is assumed that mice housed without wheels (not shown) would have values of the phenotype lower than or equal to (about 24) those exhibited by Control mice housed with wheels. (A) The greater phenotypic values for the Selected lines (S; black circles) as compared with Control lines (C; gray squares) are explainable statistically by their greater amount of running (`more pain, more gain'). A real example of this pattern involves the level of brain-derived neurotrophic factor (BDNF) in the hippocampus of S and C mice after 1 week of access to running wheels [see fig. 2 (Johnson et al., 2003)]. (B) There is no relation between the amount of running and phenotype within either group and for a given amount of running the increase in phenotype (relative to the values when mice do not have wheel access) is greater for selected lines than for C lines. Hence, S lines exhibit a greater plastic response. See text for further explanation.