Figure 4. Generation of an “addiction index” for individual animals.

(A–B) Exploratory factor analysis on multiple behavioral endpoints reduced multi-dimensional behavioral data to 8 “factors.” A composite score, or “addiction index (AI),” of those factors most strongly associated with behaviors reflective of an addicted-like phenotype was generated using the individual transformed data for Factors 1, 3, & 4. (C–K) Data for individual animals for each behavior and each factor are presented. Each animal is represented by the same unique shape and color. (C, F, I) Factor loading, or associations, of Factors 1, 3, & 4 with SA behaviors (yellow = positive; blue = negative) are presented. (D, G, J) Individual data presented for the behaviors associated with each factor. (D) Factor 1 associated with intake and infusions; (G) Factor 3 is positively associated with active lever and negatively associated with inactive lever under an FR2 schedule; (J) Factor 4 is positively associated with FR2 lever presses and negatively associated with lever pressing on an FR1 schedule. (E, H, K) Individual transformed data for Factors 1 (E), 3 (H) and 4 (K). The product of these values was calculated to generate an AI for each individual. An animal must display high performance on all three factors (▲) to have a high AI. By contrast, if an animal performs poorly on one of the behaviors (× or ■) their AI is lower.