FIGURE 3.

Mean (±SEM) number of operant responses for food in nonobese (n = 31) and obese (n = 27) participants in the 300-, 100-, and 0-kcal groups at baseline and after daily intake across different schedules of reinforcement. Changes in the pattern of operant responding for food as a function of phase (baseline compared with after daily intake) were compared by using mixed-effects regression models (MRMs) (23). The MRMs compared the pattern of operant responses for food after the daily intake phase to responses for food at baseline, with BMI and portion-size group as time-invariant predictors, the schedule of reinforcement as the time-variant predictor, and age, hunger, and scores on the Binge Eating Scale and the 3 subscales of the Three-Factor Eating Questionnaire (dietary restraint, disinhibition, and hunger) as covariates. Each of the covariates was also tested as a moderator of responding by interacting it with phase and schedule of reinforcement. Because none of the participants in any condition responded on the 1024 schedule, this schedule was excluded from the analysis. Interactions from the primary MRM analysis were probed using separate MRM analyses for each phase, weight-status group, and portion-size group by using the linear and quadratic trends where appropriate. There was a significant group × weight status × reinforcement schedule effect (P < 0.0001). Further probing of this analysis showed that obese individuals in the 300-kcal group responded significantly more for food after the daily intake phase than at baseline (P < 0.0001), and nonobese individuals in the 300-kcal group responded less for food after the daily intake phase than at baseline (P = 0.01). There were no differences in the 0- or 100-kcal groups (all P > 0.05).