Binge eating is a core clinical feature of bulimia nervosa (BN). Enhanced reinforcing value of food may play a role in this behavioral disturbance, but a systematic behavioral assessment of objective measures of the rewarding value of binge eating is lacking. The purpose of this study was to quantify the reinforcing value of food in BN patients as compared with normal controls. A progressive ratio (PR) computerized work task was completed under binge and non-binge instruction. The task consisted of 12 trials. The first trial required 50 keyboard taps to earn one portion of yogurt shake, and subsequent trials required progressive work increments of 200 taps for each additional portion. Completion of all 12 trials required 13,800 taps to earn 2100 ml of shake. The breakpoint, defined as the largest ratio completed before a participant stopped working, was the measure of reinforcing efficacy. Ten patients and 10 controls completed the experiment. Under binge instruction, patients completed more trials and taps, and had a higher breakpoint than controls. The non-binge instruction yielded opposite findings; compared to controls, patients completed fewer trials and taps, and had a lower breakpoint. These results support the feasibility and potential utility of a PR task to quantify the reinforcing value of food in patients with BN.
Binge eating, accompanied by inappropriate compensatory behavior to avoid weight gain, is a core clinical feature of bulimia nervosa (BN). Enhanced reinforcing value of food may play a role in this behavioral disturbance (Bohon & Stice, 2011), but an objective behavioral assessment of measures of reward is lacking in this patient population.
The field of behavioral economics provides methods by which to measure the reinforcing efficacy of motivated behaviors, like smoking (Bulik and Brinded, 1994 and Epstein et al., 1991), drug use (Comer et al., 1998 and Stafford et al., 1998), physical activity (Saelens & Epstein, 1999), and eating (Epstein and Leddy, 2006, Epstein et al., 2007, Lappalainen and Epstein, 1990, Raynor and Epstein, 2003 and Saelens and Epstein, 1996) in a laboratory setting. In general, these laboratory paradigms quantify the reinforcing efficacy of a target behavior in terms of the amount of “work” an individual is willing to expend to access it (Hodos, 1961). Specifically, the progressive ratio (PR) task measures the reinforcing value of a motivated behavior by requiring the participant to expend progressively increasing amounts of work to gain access to it (Roane, 2008). In humans, effort or “work” is often based on the number of taps on a computer keyboard. The PR breakpoint is defined as the number of responses completed for a reinforcer before the participant stops working for it; the more reinforcing a stimulus is, the higher the breakpoint (Hodos, 1961).
Human eating behavior can be measured during laboratory test meals and is related to two food attributes, hedonics and liking (Mitchell et al., 1998 and Walsh and Boudreau, 2003). It has, however, been suggested that the reinforcing value of food may be a more powerful determinant of food intake than either food hedonics or liking (Epstein & Leddy, 2006).
Laboratory studies demonstrate that when asked to binge eat, patients with BN consume binge quantities of food at single-item and multi-item meals (Kaye et al., 1992, Kissileff et al., 1986, LaChaussee et al., 1992, Walsh et al., 1989a and Walsh et al., 1989b). The purpose of the current study was to determine if patients with BN were willing to work at a PR task in order to obtain a binge quantity of food during a single-item laboratory test meal. We hypothesized that: (1) Both BN and NC subjects would perform more total work and achieve a higher breakpoint under binge vs. non-binge instruction; and (2) under binge instruction, BN subjects would work harder (more total work and a higher breakpoint) than NC subjects to obtain a larger quantity of food.
