Elsevier

Appetite

Volume 81, 1 October 2014, Pages 242-252
Appetite

Research report
How partial reinforcement of food cues affects the extinction and reacquisition of appetitive responses. A new model for dieting success?

https://doi.org/10.1016/j.appet.2014.06.024Get rights and content

Highlights

  • Partial reinforcement of food cues slowed down extinction of appetitive responses.

  • Partial reinforcement slowed down the reacquisition of US expectancies.

  • Impulsivity may be related to a slower extinction of US expectancies.

  • Dieting success might be influenced by differences in prior learning schedules.

Abstract

Animals and humans can easily learn to associate an initially neutral cue with food intake through classical conditioning, but extinction of learned appetitive responses can be more difficult. Intermittent or partial reinforcement of food cues causes especially persistent behaviour in animals: after exposure to such learning schedules, the decline in responding that occurs during extinction is slow. After extinction, increases in responding with renewed reinforcement of food cues (reacquisition) might be less rapid after acquisition with partial reinforcement. In humans, it may be that the eating behaviour of some individuals resembles partial reinforcement schedules to a greater extent, possibly affecting dieting success by interacting with extinction and reacquisition. Furthermore, impulsivity has been associated with less successful dieting, and this association might be explained by impulsivity affecting the learning and extinction of appetitive responses. In the present two studies, the effects of different reinforcement schedules and impulsivity on the acquisition, extinction, and reacquisition of appetitive responses were investigated in a conditioning paradigm involving food rewards in healthy humans. Overall, the results indicate both partial reinforcement schedules and, possibly, impulsivity to be associated with worse extinction performance. A new model of dieting success is proposed: learning histories and, perhaps, certain personality traits (impulsivity) can interfere with the extinction and reacquisition of appetitive responses to food cues and they may be causally related to unsuccessful dieting.

Introduction

As obesity prevalence continues to rise, so do attempts to lose weight by restricting dietary intake (Hill, 2002). In a US sample, as many as 60% of overweight and 70% of obese women reported to be currently trying to lose weight (Bish et al., 2005). However, successful long-term weight loss is rare, and weight loss practises have even been linked to binge eating, weight gain and the development of obesity (e.g., Field et al, 2003, Jeffery et al, 2000, Neumark-Sztainer et al, 2006, Stice et al, 2005). Learning models can help explain why successful long-term weight loss is so difficult (Bouton, 2011, Jansen, 1998). After conditioned stimuli (CSs) have become associated with food intake (unconditioned stimulus; US), they will elicit conditioned responses (CRs). Exposure to a CS can induce (strong) desires for food, and may promote food consumption (e.g., Boggiano et al, 2009, Jansen, 1998).

Conditioning phenomena could also explain why some dieters are able to adhere to their diets more successfully than others. Intermittent reinforcement of conditioned food cues (i.e., the alternation of intake and restriction in response to food cues) has long been known to result in persistent conditioning effects in animals. For instance, rats that do not always receive a food US when presented with a CS (e.g., on 50% of the trials) are known to perform worse in an extinction training in which the CS is never followed anymore by the US, compared with rats that had received a 100% contingency conditioning procedure (e.g., Bouton et al, 2014, Haselgrove et al, 2004). Thus, paradoxically, although the reinforcement schedule is leaner in rats receiving partial (e.g., 50% contingency) instead of continuous (100% contingency) reinforcement, extinction is more difficult. This difficulty to extinguish conditioned responses after partial reinforcement is known as the partial reinforcement extinction effect (PREE). It is thought that the PREE occurs because, in case of partial reinforcement, the animal has received reward under conditions of nonreinforcement: extinction is harder to achieve because a nonreinforced CS signals possible US availability on the next trial (e.g., Amsel, 1962, Capaldi, 1994). Consequently, reward is expected after nonreinforced CS trials during extinction and appetitive responses are more persistent. Further, it can be argued that original learning with either continuous or partial reinforcement might result in differential response patterns when the CS−US contingency is again reinforced after extinction, i.e. during a reacquisition phase. Reacquisition after extinguished responses to a continuous schedule is often rapid (e.g., Ricker & Bouton, 1996); however, one might expect that the return of appetitive responses during reacquisition after extinction is less pronounced after partial reinforcement because a reinforced CS does not predict (as strongly) that the subsequent trial will be reinforced.

It seems likely that there are intra- and inter-individual differences in the extent to which food cues are reinforced, and thus, the extent to which partial reinforcement is practised. For instance, an individual might consume breakfast at a relatively similar timepoint every day while less consistently eating a snack in the afternoon, alternating ‘eating’ and ‘not eating’ on a day-to-day basis for some cues. Over time, the predominant learning schedule for inconsistently reinforced food cues essentially reflects partial reinforcement schedules. Additionally, across individuals, it seems likely that overall patterns of reinforcement differ: some may practise partial reinforcement to a greater extent than others. For instance, some individuals may show a relatively inconsistent eating pattern, reinforcing and nonreinforcing different sets of cues each day (e.g., Kirk & Hill, 1997). When an individual starts a diet, extinction is presumably practised because he or she is attempting to refrain from eating (US) in response to previously reinforced cues (CS). The PREE would predict that this extinction is more difficult to achieve for those who previously practised a greater degree of partial reinforcement. A more difficult extinction of conditioned responses would theoretically result in a greater difficulty to refrain from eating in response to those cues: during extinction of previously partially reinforced cues, a dieter's body keeps expecting to receive food in response to such cues (i.e., they experience PREEs). Thus, the dieter could experience heightened conditioned cravings for foods even after a period in which he or she has suppressed responses to the cues. However, when a dieter has successfully extinguished these responses and thus has overcome the PREE, one could expect that a history of partial reinforcement could be beneficial for long-term weight maintenance. Since returns of appetitive responses after extinction (‘relapse’) are thought to considerably thwart dieting efforts (Bouton, 2011), a history of partial reinforcement could decrease chances for a full-blown relapse because they may slow down reacquisition. To gain insight into the underlying mechanisms behind individual differences in dieting success, examining causes of potential differences in the extinction and reacquisition of appetitive responses could prove valuable.

Apart from learning histories, personality characteristics could also affect dieting success. Impulsivity has been especially associated with increased cue reactivity, overeating, and obesity (Guerrieri et al, 2008, Tetley et al, 2010). Importantly, impulsivity has been found to distinguish successful from less successful dieters – impulsivity has been related to less weight loss during a weight loss treatment (e.g., Nederkoorn, Jansen, Mulkens, & Jansen, 2007), less successful self-reported dieting (e.g., van Koningsbruggen, Stroebe, & Aarts, 2013), and in the lab, restrained eaters (dieters) only overate when they were also impulsive (Jansen et al., 2009). Also, it seems that impulsiveness increases the risk for relapse in addicts (e.g., Doran, Spring, McChargue, Pergadia, & Richmond, 2004), and evidence suggests that treatment outcome and maintenance could be improved by targeting impulsivity (Houben, Jansen, 2011, Houben et al, 2011, Verbeken et al, 2013). It is possible that a relationship between impulsivity and unsuccessful dieting could be explained by differences in the learning and ‘unlearning’ of conditioned responses to rewarding cues between more and less impulsive individuals. For instance, different aspects of impulsivity have been theoretically associated with an increased speed of acquisition of appetitive responses, although empirical tests have been inconclusive (Corr, 2001, Corr, 2002, Corr et al, 1995, Dawe et al, 2004, Gorenstein, Newman, 1980, Papachristou et al, 2013, Patterson, Newman, 1993, van den Akker et al, 2013). Further, one specific aspect of impulsivity, rash impulsiveness or the inability to inhibit predominant approach responses (as measured by the BIS-11) (Dawe et al, 2004, Patton et al, 1995) could be associated with impaired extinction. The underlying neural structure of rash impulsiveness is the orbitofrontal cortex (OFC): worse functioning of the OFC has been related to higher rash impulsiveness in healthy individuals (see Dawe et al, 2004, Horn et al, 2003). Investigations in rodents, monkeys, and humans have resulted in the conclusion that the OFC is necessary for integrating changing information about a reward and providing new estimations about expected outcomes, that is, learning when reward contingencies change (e.g., Butter, 1969, Gallagher et al, 1999, McDannald et al, 2014, Rolls, 2000, Rolls, 2004, Schoenbaum, Esber, 2010, Schoenbaum et al, 2009). A relatively poor functioning OFC in rash impulsive individuals could therefore result in less correct estimates of expected outcomes when CS−US contingencies are altered during extinction, i.e. resulting in extinction deficits. The present studies investigate whether a measure of rash impulsiveness influences the acquisition and extinction of appetitive responses.

In sum, dieters who previously practised intermittent reinforcement of food cues might be less successful in restricting their food intake through difficulties in achieving extinction (i.e., they might experience a PREE), while simultaneously being at reduced risk for relapse once extinction has been achieved. However, relatively few human appetitive conditioning studies involving food rewards have been conducted (e.g., Van Gucht, Vansteenwegen, Beckers, & Van den Bergh, 2008), and none examined effects of partial reinforcement on conditioned responses to food cues. The present two studies were designed to test the hypothesis that an appetitive partial reinforcement schedule causes a PREE in healthy humans and slows down a return of appetitive responses when CSs are reinforced again after extinction. Since impulsivity has theoretically been associated with a possibly faster acquisition of conditioned appetitive responses and slower extinction, the influence of impulsivity on the different phases of conditioning was also investigated.

Section snippets

Participants

Thirty-two participants took part in the study. Three participants did not develop a differential US expectancy, suggesting they were not aware of the CS−US contingency. They were replaced by three other participants ensuring full counterbalancing. All participants were undergraduate female students who were proficient in Dutch and indicated a liking for chocolate. Participants were instructed to have a small meal two hours prior to participation but to refrain from calorie intake thereafter.

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    Acknowledgements: The authors would like to thank Jacco Ronner for technical assistance. This study is part of an ongoing project that is financed by the Netherlands Organisation for Scientific Research (NWO): Vici Grant 453.10.006, awarded to Anita Jansen.

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