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Behav Brain Res. Author manuscript; available in PMC 2017 January 01. Published in final edited form as: Behav Brain Res. 2016 January 1; 296: 418–430. doi:10.1016/j.bbr.2015.07.055.
Rats that sign-track are resistant to Pavlovian but not instrumental extinction Allison M. Ahrensa,*, Bryan F. Singera, Christopher J. Fitzpatrickb, Jonathan D. Morrowb,c, and Terry E. Robinsona,b aDepartment
of Psychology, The University of Michigan, Ann Arbor, MI 48109, USA
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bNeuroscience cDepartment
Graduate Program, The University of Michigan, Ann Arbor, MI 48109, USA
of Psychiatry, The University of Michigan, Ann Arbor, MI 48109, USA
Abstract
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Individuals vary in the extent to which they attribute incentive salience to a discrete cue (conditioned stimulus; CS) that predicts reward delivery (unconditioned stimulus; US), which results in some individuals approaching and interacting with the CS (sign-trackers; STs) more than others (goal-trackers; GTs). Here we asked how periods of non-reinforcement influence conditioned responding in STs vs. GTs, in both Pavlovian and instrumental tasks. After classifying rats as STs or GTs by pairing a retractable lever (the CS) with the delivery of a food pellet (US), we introduced periods of non-reinforcement, first by simply withholding the US (i.e., extinction training; experiment 1), then by signaling alternating periods of reward (R) and non-reward (NR) within the same session (experiments 2 and 3). We also examined how alternating R and NR periods influenced instrumental responding for food (experiment 4). STs and GTs did not differ in their ability to discriminate between R and NR periods in the instrumental task. However, in Pavlovian settings STs and GTs responded to periods of non-reward very differently. Relative to STs, GTs very rapidly modified their behavior in response to periods of non-reward, showing much faster extinction and better and faster discrimination between R and NR conditions. These results highlight differences between Pavlovian and instrumental extinction learning, and suggest that if a Pavlovian CS is strongly attributed with incentive salience, as in STs, it may continue to bias attention toward it, and to facilitate persistent and relatively inflexible responding, even when it is no longer followed by reward.
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Keywords Extinction; Pavlovian conditioning; Sign tracking; Goal tracking; Incentive motivation; Occasion setting; Discriminative stimuli
*
Corresponding author: Allison M. Ahrens, Ph.D., Department of Psychology, University of Michigan, 530 Church St., Ann Arbor, MI 48109, Tel.: +1 512 7731315, [email protected]. Publisher's Disclaimer: This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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1. INTRODUCTION
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In appetitive Pavlovian conditioning an initially neutral cue is paired with the delivery of a reward (the unconditioned stimulus; US), and as they become associated, the cue (conditioned stimulus; CS) may come to elicit a variety of conditioned responses (CRs) that anticipate reward delivery. CRs can take many forms, depending not only on the nature of the US, but also the properties of the CS (Holland 1977), including whether the CS is attributed with incentive salience (Anselme et al. 2013; Flagel et al. 2009; Meyer et al. 2014; Robinson et al. 2014a; Robinson and Flagel 2009; Saunders and Robinson 2013). For example, if presentation of a discrete, localizable CS is consistently followed by food reward in a response-independent manner some animals learn to approach the CS itself (termed "sign-trackers"; STs) (Hearst 1974), whilst others approach the site of food delivery (termed "goal trackers"; GTs) (Boakes 1977; Zener 1937). Although both STs and GTs learn the predictive value of the CS – it comes to evoke an anticipatory CR in both – it has been suggested that the form of the CR differs in part because STs attribute greater incentive salience to the CS than GTs (Flagel et al. 2009; Meyer et al. 2012a; Robinson and Flagel 2009; Saunders and Robinson 2013; Tomie et al. 2000; Beckmann et al. 2015; Anselme 2014). Not only are STs more attracted to the CS, but it is a more effective conditioned reinforcer and more effective in renewing reward-seeking behavior in STs than GTs (Robinson and Flagel 2009; Saunders et al. 2013; Yager and Robinson 2010). Furthermore, performance on the Pavlovian conditioned approach (PCA) task using food reward predicts the extent to which discrete drug cues acquire control over motivated behavior (Meyer et al. 2012b; Robinson et al. 2014b; Saunders and Robinson 2013). Thus, it appears that the cue is an equally effective CS in STs and GTs, but it acts as a more powerful, motivating incentive stimulus in STs.
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Both food and drug cues engage so-called brain ‘motive circuits’ to a greater extent in STs than GTs (Flagel et al. 2011a, 2011b; Yager et al. 2015), which may contribute to STs being more susceptible to develop impulse control disorders, such as addiction (Flagel et al. 2011b; Saunders and Robinson 2013; Tunstall and Kearns 2015; Beckmann et al. 2011). Addiction-like behaviors tend to persist despite non-reinforcement (Deroche-Gamonet et al. 2004), therefore, one might expect sign-tracking behavior to be more resistant to extinction than goal-tracking. However, in previous studies involving the extinction of instrumental responding for food (Yager and Robinson 2010) or cocaine (Saunders et al. 2014; Saunders and Robinson 2011; Yager and Robinson 2013), there was no difference between STs and GTs in the rate of instrumental extinction. However, in the studies above extinction was conducted in the absence of a reward cue (CS), or an explicit reward cue was not used in training. In another study presentation of an increasingly aversive outcome progressively decreased instrumental responding for cocaine at the same rate in STs and GTs, despite continued presence of a cue associated with cocaine delivery (Saunders et al. 2013). On the other hand, in one study in which extinction of cocaine self-administration behavior was conducted in the presence of the reward cue, STs were somewhat more resistant to extinction than GTs, but the effect was very small and was only evident on the first day (out of 28 days) of extinction training (Saunders and Robinson 2010). Thus, there is no
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compelling evidence that STs and GTs differ in extinction learning in an instrumental task, but we are not aware of any study comparing Pavlovian extinction in STs and GTs. It is important to examine Pavlovian extinction because, although there are many similarities in the processes involved in instrumental and Pavlovian extinction, there are also differences (e.g., Todd et al. 2014). Beckmann and Chow (2015) did examine Pavlovian extinction using a 2-CS procedure, in which rats developed a ST CR to a lever-CS and a GT CR to a tone-CS, when both CSs were presented within the same session. They reported that when the lever and tone were no longer reinforced the GT CR extinguished faster than the ST CR (Beckmann and Chow 2015). However, in this study all rats showed a ST CR during leverCS presentation; i.e., no rats showed a GT CR in response to the lever-CS. Therefore, it is still not known whether STs and GTs differ in Pavlovian extinction, when the same CS evokes two different CRs in different individuals.
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Thus, we conducted a series of studies on extinction in STs and GTs. In the first study, after initial training, the US was simply withheld over 4–8 days of extinction training. Next, we examined within-session extinction and recovery of ST and GT CRs during alternating blocks of rewarded (R) and non-rewarded (NR) trials. The R and NR periods were signaled by either a diffuse and continuous change in illumination (experiment 2) or by spatially and temporally discrete light cues (experiment 3). These signals were intended to facilitate discrimination between R and NR phases by acting as occasion setters, indicating that the CS would be rewarded when accompanied by one signal but would not be rewarded when accompanied by the other (Bouton 2004; Bouton et al. 2014; Crombag et al. 2008; Holland 1992). Finally, in a fourth experiment changes in illumination were used as discriminative stimuli (Bouton et al. 2014; Cardinal et al. 2002) signaling whether an instrumental response would be reinforced or not. STs were highly resistant to extinction, relative to GTs, but only in the Pavlovian setting, when the CS was present.
2. METHODS 2.1. Subjects
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A total of 133 male Sprague-Dawley rats purchased from Charles River or Harlan were used (Fitzpatrick et al. 2013). All rats weighed 250–275g on arrival, and were singly housed in standard polycarbonate cages with food and water available ad libitum throughout experiments (i.e., the rats were not food deprived at any time). Rats were housed on a reverse 12-h light/12-hr dark cycle, with all testing taking place during the dark phase of the cycle. Testing began after one week of acclimation and handling. All procedures were approved by the University of Michigan Committee on the Use and Care of Animals (UCUCA). 2.2. Apparatus All experiments took place in standard test chambers (30.5 × 24.1 × 21 cm) located inside sound-attenuating cabinets with ventilating fans, which masked outside noise (Med Associates, Inc.; St Albans, VT, USA). Each chamber contained a pellet dispenser connected to a recessed pellet magazine, which was mounted in the center of the front wall
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and equipped with an infrared sensor to record magazine head entries. During Pavlovian conditioned approach (PCA) training a single retractable lever was mounted to the right or left of the magazine (the position of the lever was counterbalanced across rats). An LED behind the lever was lit any time the lever was inserted into the cage and illuminated the slot through which the lever protruded. During instrumental sessions, the lever was removed and active and inactive nose-poke ports were installed on either side of the magazine, with the left or right position of the active port counterbalanced across rats. Chambers also contained a red house light mounted at the top of the back wall, and two white stimulus lights, one mounted on the back wall of the chamber, and one mounted on the front wall, with the left or right position relative to the magazine counterbalanced. Behavioral responses (lever deflections, magazine entries, and nose pokes) were recorded with MED-PC® software (Med Associates, Inc.).
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2.3. Pavlovian conditioned approach (PCA) training
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All experiments began with PCA training, during which the insertion of the lever (the conditional stimulus; CS) into the cage was paired with the delivery of a single food pellet (unconditional stimulus, US; a 45 mg banana-flavored pellet; #F0059; Bio-Serv; Frenchtown, NJ, USA), using methods described previously (Flagel et al. 2007). For two days prior to the start of training, rats were given 25 pellets in their home cages in order to familiarize them with this food. On a single pre-training day, 25 pellets were delivered on a variable time (VT)-30 (0–60 s) schedule without a CS, in order to train rats to retrieve pellets from the magazine. One rat failed to eat the banana pellets, and was excluded from the study. PCA sessions began the following day. There were 25 trials per session separated by VT-90 (30–150s) intervals. In each trial, the lever was inserted into the cage for 8 s, then retracted and immediately followed by the response-independent delivery of a banana pellet. The chamber was illuminated by the red house light during PCA training. Lever presses and magazine entries were recorded during the 8-s CS period when the lever was present. 2.4. Quantification of PCA behavior
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As in previous studies the lever-CS evoked different CRs in different rats. The relative occurrence of a sign-tracking CR (ST; approach and engagement with the lever) vs. a goaltracking CR (GT; approach and engagement with the food cup) was quantified by measuring: 1) the number of contacts with the lever or magazine, 2) the latency to first contact the lever or magazine, and 3) the probability of contacting the lever or magazine on any given trial. To classify rats as STs or GTs, we used a composite of these three measures termed the PCA Index Score (Meyer et al. 2012a). For each of the five training sessions we determined a PCA Score, which was the average of three measures: response bias [(lever presses - magazine entries / total contacts)], latency score [(magazine latency - lever latency / 8s)], and probability difference (lever probability - magazine probability). PCA Scores range from 1 (indicating an animal exclusively made ST CRs in every trial) to −1 (indicating exclusively GT CRs on every trial). PCA Scores from the last two training sessions were averaged to produce a PCA Index Score that was used to classify rats as STs (+0.5 to 1) or GTs (−0.5 to −1). Rats that fell in the middle range (−0.5 to 0.5) were classified as intermediates and were not included in these studies.
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2.5. Experiment 1 - Extinction of ST vs. GT CRs
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Rats were initially trained for 7–9 sessions using the PCA procedures described in section 2.3. The day after the last day of training all rats were then subjected to extinction conditions, in which the lever-CS was presented as before (25 trials per session, VT-90 s interval) but was not followed by a food pellet. There was no signal provided to indicate extinction conditions were in effect, other than reward was not provided. One group of 15 rats had four extinction sessions, and another group of 34 rats had eight extinction sessions. In both groups data from the first 4 sessions were very similar, so the two groups were pooled in the analyses below. 2.6. Experiment 2–Alternating rewarded (R) and non-rewarded (NR) periods signaled by illumination of the chamber
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In experiment 1 there was no explicit signal indicating whether the CS would be followed by reward. In this, and in the following experiment, we asked whether STs would show similar resistance to extinction as seen in experiment 1 if alternating blocks of reward and nonreward periods were explicitly signaled. In this experiment a ‘diffuse’ signal (illumination of the entire chamber) indicated whether presentation of the CS would or would not be followed by reward. All rats (N=21) were first trained using the PCA procedure described above for 5 daily sessions. On each of the following 5 days sessions consisted of 60 trials (i.e., 8-s CS presentations) separated by VT-90s intervals. However, the sessions were divided into two blocks in which the CS was followed by presentation of the food reward (‘Rewarded’; R) and two blocks in which presentation of the CS was not followed by reward (‘Non-Rewarded’; NR). The two R and NR periods lasted for 15 trials each. Thus, during R blocks, the red house light was constantly illuminated (as during the PCA training phase), and thus the chamber would appear dimly lit, and a banana pellet followed each presentation of the lever-CS. NR blocks were signaled by the continuous illumination of a white stimulus light, which illuminated the entire chamber and was brighter than the red house light, and under this condition the 8-s lever-CS presentations were not followed by a food pellet. The four periods alternated in two different patterns (R, NR, R, NR or NR, R, NR, R), which were counterbalanced across rats and across days. 2.7. Experiment 3- Alternating rewarded (R) and non-rewarded (NR) periods signaled by a discrete signal
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To extend experiment 2 we repeated the experiment, but further asked whether similar results would be obtained when R and NR periods were signaled by a spatially and temporally discrete cue, rather than illumination of the entire chamber. Illumination of the entire chamber may be perceived as a change in ‘context’, which could be processed differently than a discrete cue (e.g., (Bouton 2004; Burns and Domjan 2001; Domjan 2003; Holland and Bouton 1999). A separate group of 39 rats was used in this experiment. All rats first received 5 sessions of PCA training, as above, followed by 5 sessions of discrimination training, as in experiment 2. However, in this experiment R periods were signaled by illumination of a white stimulus light mounted at the front of the cage, and NR periods by illumination of an identical white stimulus light at the back of the cage. Unlike in experiment 2, these lights were illuminated for only 6 s immediately prior to each lever-CS
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presentation, and the red house light stayed on throughout the session. When the lever-CS was preceded by the R signal, the trial ended with delivery of a pellet. When the lever-CS was preceded by the NR signal, no pellet was delivered. During discrimination training there were 60 trials per session separated by VT-90-s intervals. There were two R and two NR blocks that consisted of 15 consecutive trials each, alternating in two counterbalanced patterns (R, NR, R, NR or NR, R, NR, R). 2.8. Experiment 4–Effects of alternating rewarded (R) and non-rewarded (NR) periods signaled by illumination of the chamber on instrumental responding for food reward In our previous studies on instrumental extinction, in which STs and GTs did not differ, the extinction period was not indicated by any explicit signal. The purpose of this experiment was to again examine instrumental extinction in STs and GTs but using a discrimination procedure that better matched the procedures used in experiments 2 and 3 above.
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2.8.1. Acquisition—Following PCA training, a separate group of 24 rats were trained to nose poke in the active port to receive a food pellet (responses in the inactive port had no consequences). In sessions 1–3 rats were trained on a fixed ratio (FR1) schedule with a 10-s time-out period after each pellet delivery. For the next three sessions rats were trained on variable interval (VI) schedules that increased the time between rewarded responses: day 4 VI-20 (10–30s), day 5 VI-40 (20–60s), day 6 VI-60 (30–90s). These first six sessions lasted until rats self-administered 30 pellets, to ensure that all animals experienced the same number of reinforced responses. On days 7 and 8, rats were trained on the VI-60 schedule, but sessions had a fixed duration of 30 min to facilitate comparison with discrimination sessions. A red house light was constantly illuminated throughout all acquisition sessions.
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2.8.2. Discrimination—For the next 10 days rats were trained on discrimination sessions that consisted of four alternating 15-min blocks, much like in experiment 2. During the R periods, the red house light was constantly illuminated and responses in the active port were rewarded on a VI-60 schedule of reinforcement. During NR periods, the red light was extinguished and a white stimulus light mounted above the inactive port (the same as used in experiment 2) was constantly illuminated, and responses in the active port were never reinforced. The order of R and NR phases (R, NR, R, NR or NR, R, NR, R) was counterbalanced across rats and across days. 2.9. Statistics
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Between-group comparisons were analyzed with repeated-measures (RM) ANOVAs comparing ST and GT groups across sessions. Significant interaction results were followed by post hoc independent t tests (Bonferroni-corrected) comparing STs and GTs in each of the sessions included in the ANOVA. In experiments 2 and 3, within-group comparisons were analyzed with RM ANOVAs comparing R and NR blocks across the five sessions of discrimination training, with both session and block as within-subjects variables. Significant interaction results were followed by post hoc paired t tests (Bonferroni-corrected) comparing R and NR results across all sessions included in the ANOVA. Corrected p values are reported if data did not pass Levene’s test of equality of variance or Mauchly’s test of sphericity. All statistical procedures were performed with SPSS for Windows (Version 21).
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3. RESULTS 3.1. PCA training: ST and GT groups
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As described previously, individual rats differed in the form of conditioned approach behavior they developed during the PCA training phase, with some rats primarily approaching the lever and others the food cup during the 8-s CS periods (Flagel et al. 2009; Meyer et al. 2012a; Robinson and Flagel 2009). Across the four experiments, 72 rats were classified as STs, and 61 were classified as GTs. Additional rats were classified as intermediates (INs), but these rats were not used further because we wanted to directly compare groups that varied markedly in their propensity to attribute incentive salience to the lever-CS (Meyer et al. 2012a). The number of intermediates that were excluded from each experiment are: experiment 1 – 22 INs, experiment 2 – 11 INs, experiment 3 – 25 INs, experiment 4 - 1 IN. Measures of probability, contacts, and latency are shown for leverdirected behavior and food cup-directed behavior during the first five sessions of PCA training for all rats used in subsequent experiments (Fig. 1). For all measures the behavior of STs and GTs diverged with repeated days of training, and for each of the six comparisons there were significant interaction effects [Fs(4, 328) = 48.51 – 75.47, ps < .001].
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During discrimination training in experiments 2 and 3, the majority of rats maintained the ST or GT classification that was established during PCA training. However, in some rats discrimination training disrupted their initial preference for the lever or food cup, and these rats switched from sign-tracking to goal-tracking or from goal-tracking to sign-tracking by the end of the discrimination training phase. We calculated PCA Index Scores from days 4– 5 of initial PCA training and from days 4–5 of discrimination (R/NR) training. There were 8 STs that switched from having positive scores during PCA training to having negative scores during discrimination (3 from experiment 2 and 5 from experiment 3). There was also 1 GT (from experiment 3) that switched from having a negative score during PCA training to a positive score during discrimination. These rats were excluded from subsequent analyses; therefore, all results shown below include only the rats that maintained scores in the direction of their original classification (STs > 0 and GTs < 0). 3.2. Experiment 1 - Extinction of ST vs. GT CRs
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In this experiment, non-reinforced exposure to the lever-CS (without any signal) resulted in faster extinction of PCA behavior in GTs (n = 21) than STs (n = 28). Within-session changes in probability of approach are shown for the last session of PCA training (baseline), and for extinction sessions 1 and 4. GTs showed a rapid decrease in approach toward the food cup within the first extinction session, as well as a further decrease by the fourth extinction session (Fig. 2A). In STs, approach toward the lever did not change from baseline within the first extinction session; however, by the fourth extinction session the STs did show a significant decrease in sign-tracking behavior (Fig. 2B). Fig. 2C shows probability of approach averaged across all 25 trials per session, analyzed with a 2 × 5 RM ANOVA (including baseline plus extinction sessions 1–4). Responding during extinction was significantly lower in GTs compared to STs [group F(1, 47) = 14.57, p< .001]. Repeated extinction sessions decreased approach in both groups [session F(4, 188)
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= 171.79, p< .001]; however, the rate of decrease differed in GTs and STs, with a significant group × session interaction effect [F(4, 188) = 13.97, p< .001]. Paired t tests comparing extinction sessions to baseline (Bonferroni-corrected) showed that changes from baseline were significant for GTs in all four sessions, but were only significant for STs in the last three sessions (Fig. 2C).
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In Fig. 2D the first 3 trials of each session were examined separately to evaluate the recall of extinction from one session to the next. GTs showed more immediate recall of extinction than STs, with a significant main effect of group [F(1, 47) = 32.97, p< .001]. Though recall improved in both groups with repeated days of extinction training [session F(4, 188) = 32.32, p< .001], this improvement was better in GTs than STs, with a significant group × session interaction effect [F(4, 188) = 7.25, p< .001]. Paired t tests comparing extinction sessions to baseline (Bonferroni-corrected), indicated that GTs showed significant recall of extinction in sessions 2–4, whereas STs showed significant, but weaker, recall in sessions 3 and 4 (Fig. 2D). We also examined the first 3 trials of each session in the subset of rats that received 8 extinction sessions instead of just 4 (9 GTs and 25 STs), and found that the probability of approach continued to be significantly lower in GTs than STs, with significant differences persisting throughout the additional sessions [session F(8, 256) =24.28, p< .001; group F(1, 32) = 30.17, p< .001; interaction F(8, 256) = 2.77, p< .01]. 3.3. Experiment 2 - Alternating rewarded and non-rewarded periods signaled by illumination of the chamber
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Fig. 3 shows the effect of alternating rewarded (R) and non-rewarded (NR) periods, signaled by chamber illumination, on the probability of sign-tracking and goal-tracking behavior, in STs and GTs, on the first and fifth session of R/NR training. It is clear that even in the very first session the behavior of GTs (n = 11) changed markedly between R and NR periods, but this was not the case for STs (n = 7). GTs rapidly decreased responding during NR phases, and rapidly resumed making GT CRs during R phases (Fig. 3A–B). In contrast, in the first session the behaviors of STs did not differ between the R and NR periods (Fig. 3C–D). By the fifth session GTs continued to show excellent discrimination between the R and NR periods, and by this time STs did so as well, based on probability of making a ST CR (see below). In addition, Fig. 3 shows that the introduction of discrimination training had essentially no effect on the very low levels of food cup-directed behavior in STs or on lever directed behavior in GTs.
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For lever-directed behavior in STs and food cup-directed behavior in GTs, measures of probability, contacts, and latency were averaged across the 30 R trials and 30 NR trials per session, and analyzed for each group separately with 2 × 5 (block × session) RM ANOVAs. STs initially showed no discrimination between R and NR periods, but with repeated training sign-tracking responses began to diverge between the two blocks. For all three measures, there were significant effects of block [Fs(1, 6) = 7.78 – 30.24; ps < .05 – .01] and significant interaction effects [Fs(4, 24) = 4.46 – 9.21; ps < .01 – .001; Fig. 4A–C]. In STs, post hoc tests comparing R and NR blocks were significant in session 5 for measures of probability and latency, but not contacts (Fig. 4A–C). In addition, during the discrimination sessions sign-tracking behavior was somewhat disrupted during R periods relative to the last
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session of PCA training. When lever-directed behavior during PCA training session 5 was compared to the reinforced blocks of R/NR session 5 (paired t tests), significant reductions were seen for probability [t(6) = 2.60, p< .05] and contacts [t(6) = 2.51, p< .05], but not for latency [t(6) = 1.55, n.s.] (Fig. 4A–C).
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In contrast to STs, GTs showed excellent discrimination between R and NR blocks in all five sessions. For probability, latency, and contacts there were significant differences between R and NR blocks [Fs(1, 10) = 29.93 – 298.21, ps < .001]. The GTs also showed some improvement in discrimination across the five discrimination sessions, with significant interaction effects for all three measures [Fs(4, 40) = 3.51 – 4.50, ps< .05 – .01]. Post hoc tests showed that the differences between R and NR periods were significant in all of the five sessions (Fig. 4D–F). In addition, and unlike sign-tracking (above), goal-tracking behavior was maintained in the R periods at the same level as during the last session of PCA training, with no significant differences when PCA training session 5 was compared to R/NR session 5 [ts(10) = 1.07 – 1.53, n.s.](Fig. 4D–F).
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To more directly compare the performance of STs and GTs, we calculated difference scores for each measure (R responses minus NR responses), and performed 2 × 5 (group × session) RM ANOVAs. For probability, the difference scores of GTs were significantly higher than STs [group F(1, 16) = 11.01, p< .01]. Although the scores of both groups increased across sessions [session F(4, 64) = 15.07, p< .001], the STs improved to the point where they matched the GTs on the last session, with a significant interaction effect [interaction F(4, 64) = 3.63, p< .01]. Post hoc tests showed significant differences between STs and GTs only in the first two sessions (Fig. 4G). For contacts, difference scores were significantly higher in GTs than STs, and for both groups improved across sessions at similar rates, with significant main effects but no interaction [group F(1, 16) = 9.32, p< .01; session F(4, 64) = 7.01, p< .001; interaction F(4, 64) = 0.76, n.s.] (Fig. 4H). For latency, there was a significant difference between STs and GTs and an improvement in discrimination across sessions, but no interaction effect [group F(1, 16) = 33.74, p< .001; session F(4, 64) = 10.04, p< .001; interaction F(4, 64) = 2.27, n.s.] (Fig. 4I). Thus, for these latter two measures STs never achieved the same level of discrimination between R and NR periods, even after 5 days of discrimination training. 3.4. Experiment 3 - Alternating rewarded and non-rewarded periods signaled by a discrete signal
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When discrete cues were used to signal whether or not the lever-CS would be followed by reward the results looked very much as in experiment 2 above. GTs (n = 18) showed much better discrimination between the R and NR periods than STs (n = 15). Fig. 5 shows changes in lever- and food cup-directed behavior during blocks of R and NR trials. In the very first session, as well as the fifth session, GTs rapidly decreased responding during NR periods, and rapidly resumed making GT CRs during R periods (Fig. 5A–B). In contrast, the STs showed no difference in responding between R and NR periods in the first session, though discrimination was evident by the fifth session (Fig. 5C–D). In Fig. 6, measures of lever-directed behavior for STs, and food cup-directed behavior for GTs, were averaged across the 30 R trials and 30 NR trials per session, and analyzed with 2 Behav Brain Res. Author manuscript; available in PMC 2017 January 01.
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× 5 (phase × session) RM ANOVAs. Similar to experiment 2, STs showed no difference between R and NR periods in the first session, but they showed improvement in discrimination in subsequent sessions. For all three measures (probability, contacts, and latency) there were significant effects of block [Fs(1, 14) = 9.21 – 23.59; ps < .01 – .001] and significant block × session interactions [Fs(4, 56) = 5.12 – 5.43; ps < .01 – .001]. Post hoc tests showed significant differences between R and NR periods emerging in the third session (Fig. 6A–C). Although in later trials STs showed more sign tracking during R trials compared to NR trials, sign-tracking behavior during the R trials was reduced compared to baseline levels on PCA session 5. For all three measures, lever-directed behavior was diminished during R trials in session 5 compared to PCA 5 [paired t tests: probability, t(14) = 3.44, p
Behav Brain Res. Author manuscript; available in PMC 2017 January 01. Published in final edited form as: Behav Brain Res. 2016 January 1; 296: 418–430. doi:10.1016/j.bbr.2015.07.055.
Rats that sign-track are resistant to Pavlovian but not instrumental extinction Allison M. Ahrensa,*, Bryan F. Singera, Christopher J. Fitzpatrickb, Jonathan D. Morrowb,c, and Terry E. Robinsona,b aDepartment
of Psychology, The University of Michigan, Ann Arbor, MI 48109, USA
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bNeuroscience cDepartment
Graduate Program, The University of Michigan, Ann Arbor, MI 48109, USA
of Psychiatry, The University of Michigan, Ann Arbor, MI 48109, USA
Abstract
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Individuals vary in the extent to which they attribute incentive salience to a discrete cue (conditioned stimulus; CS) that predicts reward delivery (unconditioned stimulus; US), which results in some individuals approaching and interacting with the CS (sign-trackers; STs) more than others (goal-trackers; GTs). Here we asked how periods of non-reinforcement influence conditioned responding in STs vs. GTs, in both Pavlovian and instrumental tasks. After classifying rats as STs or GTs by pairing a retractable lever (the CS) with the delivery of a food pellet (US), we introduced periods of non-reinforcement, first by simply withholding the US (i.e., extinction training; experiment 1), then by signaling alternating periods of reward (R) and non-reward (NR) within the same session (experiments 2 and 3). We also examined how alternating R and NR periods influenced instrumental responding for food (experiment 4). STs and GTs did not differ in their ability to discriminate between R and NR periods in the instrumental task. However, in Pavlovian settings STs and GTs responded to periods of non-reward very differently. Relative to STs, GTs very rapidly modified their behavior in response to periods of non-reward, showing much faster extinction and better and faster discrimination between R and NR conditions. These results highlight differences between Pavlovian and instrumental extinction learning, and suggest that if a Pavlovian CS is strongly attributed with incentive salience, as in STs, it may continue to bias attention toward it, and to facilitate persistent and relatively inflexible responding, even when it is no longer followed by reward.
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Keywords Extinction; Pavlovian conditioning; Sign tracking; Goal tracking; Incentive motivation; Occasion setting; Discriminative stimuli
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Corresponding author: Allison M. Ahrens, Ph.D., Department of Psychology, University of Michigan, 530 Church St., Ann Arbor, MI 48109, Tel.: +1 512 7731315, [email protected]. Publisher's Disclaimer: This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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1. INTRODUCTION
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In appetitive Pavlovian conditioning an initially neutral cue is paired with the delivery of a reward (the unconditioned stimulus; US), and as they become associated, the cue (conditioned stimulus; CS) may come to elicit a variety of conditioned responses (CRs) that anticipate reward delivery. CRs can take many forms, depending not only on the nature of the US, but also the properties of the CS (Holland 1977), including whether the CS is attributed with incentive salience (Anselme et al. 2013; Flagel et al. 2009; Meyer et al. 2014; Robinson et al. 2014a; Robinson and Flagel 2009; Saunders and Robinson 2013). For example, if presentation of a discrete, localizable CS is consistently followed by food reward in a response-independent manner some animals learn to approach the CS itself (termed "sign-trackers"; STs) (Hearst 1974), whilst others approach the site of food delivery (termed "goal trackers"; GTs) (Boakes 1977; Zener 1937). Although both STs and GTs learn the predictive value of the CS – it comes to evoke an anticipatory CR in both – it has been suggested that the form of the CR differs in part because STs attribute greater incentive salience to the CS than GTs (Flagel et al. 2009; Meyer et al. 2012a; Robinson and Flagel 2009; Saunders and Robinson 2013; Tomie et al. 2000; Beckmann et al. 2015; Anselme 2014). Not only are STs more attracted to the CS, but it is a more effective conditioned reinforcer and more effective in renewing reward-seeking behavior in STs than GTs (Robinson and Flagel 2009; Saunders et al. 2013; Yager and Robinson 2010). Furthermore, performance on the Pavlovian conditioned approach (PCA) task using food reward predicts the extent to which discrete drug cues acquire control over motivated behavior (Meyer et al. 2012b; Robinson et al. 2014b; Saunders and Robinson 2013). Thus, it appears that the cue is an equally effective CS in STs and GTs, but it acts as a more powerful, motivating incentive stimulus in STs.
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Both food and drug cues engage so-called brain ‘motive circuits’ to a greater extent in STs than GTs (Flagel et al. 2011a, 2011b; Yager et al. 2015), which may contribute to STs being more susceptible to develop impulse control disorders, such as addiction (Flagel et al. 2011b; Saunders and Robinson 2013; Tunstall and Kearns 2015; Beckmann et al. 2011). Addiction-like behaviors tend to persist despite non-reinforcement (Deroche-Gamonet et al. 2004), therefore, one might expect sign-tracking behavior to be more resistant to extinction than goal-tracking. However, in previous studies involving the extinction of instrumental responding for food (Yager and Robinson 2010) or cocaine (Saunders et al. 2014; Saunders and Robinson 2011; Yager and Robinson 2013), there was no difference between STs and GTs in the rate of instrumental extinction. However, in the studies above extinction was conducted in the absence of a reward cue (CS), or an explicit reward cue was not used in training. In another study presentation of an increasingly aversive outcome progressively decreased instrumental responding for cocaine at the same rate in STs and GTs, despite continued presence of a cue associated with cocaine delivery (Saunders et al. 2013). On the other hand, in one study in which extinction of cocaine self-administration behavior was conducted in the presence of the reward cue, STs were somewhat more resistant to extinction than GTs, but the effect was very small and was only evident on the first day (out of 28 days) of extinction training (Saunders and Robinson 2010). Thus, there is no
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compelling evidence that STs and GTs differ in extinction learning in an instrumental task, but we are not aware of any study comparing Pavlovian extinction in STs and GTs. It is important to examine Pavlovian extinction because, although there are many similarities in the processes involved in instrumental and Pavlovian extinction, there are also differences (e.g., Todd et al. 2014). Beckmann and Chow (2015) did examine Pavlovian extinction using a 2-CS procedure, in which rats developed a ST CR to a lever-CS and a GT CR to a tone-CS, when both CSs were presented within the same session. They reported that when the lever and tone were no longer reinforced the GT CR extinguished faster than the ST CR (Beckmann and Chow 2015). However, in this study all rats showed a ST CR during leverCS presentation; i.e., no rats showed a GT CR in response to the lever-CS. Therefore, it is still not known whether STs and GTs differ in Pavlovian extinction, when the same CS evokes two different CRs in different individuals.
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Thus, we conducted a series of studies on extinction in STs and GTs. In the first study, after initial training, the US was simply withheld over 4–8 days of extinction training. Next, we examined within-session extinction and recovery of ST and GT CRs during alternating blocks of rewarded (R) and non-rewarded (NR) trials. The R and NR periods were signaled by either a diffuse and continuous change in illumination (experiment 2) or by spatially and temporally discrete light cues (experiment 3). These signals were intended to facilitate discrimination between R and NR phases by acting as occasion setters, indicating that the CS would be rewarded when accompanied by one signal but would not be rewarded when accompanied by the other (Bouton 2004; Bouton et al. 2014; Crombag et al. 2008; Holland 1992). Finally, in a fourth experiment changes in illumination were used as discriminative stimuli (Bouton et al. 2014; Cardinal et al. 2002) signaling whether an instrumental response would be reinforced or not. STs were highly resistant to extinction, relative to GTs, but only in the Pavlovian setting, when the CS was present.
2. METHODS 2.1. Subjects
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A total of 133 male Sprague-Dawley rats purchased from Charles River or Harlan were used (Fitzpatrick et al. 2013). All rats weighed 250–275g on arrival, and were singly housed in standard polycarbonate cages with food and water available ad libitum throughout experiments (i.e., the rats were not food deprived at any time). Rats were housed on a reverse 12-h light/12-hr dark cycle, with all testing taking place during the dark phase of the cycle. Testing began after one week of acclimation and handling. All procedures were approved by the University of Michigan Committee on the Use and Care of Animals (UCUCA). 2.2. Apparatus All experiments took place in standard test chambers (30.5 × 24.1 × 21 cm) located inside sound-attenuating cabinets with ventilating fans, which masked outside noise (Med Associates, Inc.; St Albans, VT, USA). Each chamber contained a pellet dispenser connected to a recessed pellet magazine, which was mounted in the center of the front wall
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and equipped with an infrared sensor to record magazine head entries. During Pavlovian conditioned approach (PCA) training a single retractable lever was mounted to the right or left of the magazine (the position of the lever was counterbalanced across rats). An LED behind the lever was lit any time the lever was inserted into the cage and illuminated the slot through which the lever protruded. During instrumental sessions, the lever was removed and active and inactive nose-poke ports were installed on either side of the magazine, with the left or right position of the active port counterbalanced across rats. Chambers also contained a red house light mounted at the top of the back wall, and two white stimulus lights, one mounted on the back wall of the chamber, and one mounted on the front wall, with the left or right position relative to the magazine counterbalanced. Behavioral responses (lever deflections, magazine entries, and nose pokes) were recorded with MED-PC® software (Med Associates, Inc.).
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2.3. Pavlovian conditioned approach (PCA) training
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All experiments began with PCA training, during which the insertion of the lever (the conditional stimulus; CS) into the cage was paired with the delivery of a single food pellet (unconditional stimulus, US; a 45 mg banana-flavored pellet; #F0059; Bio-Serv; Frenchtown, NJ, USA), using methods described previously (Flagel et al. 2007). For two days prior to the start of training, rats were given 25 pellets in their home cages in order to familiarize them with this food. On a single pre-training day, 25 pellets were delivered on a variable time (VT)-30 (0–60 s) schedule without a CS, in order to train rats to retrieve pellets from the magazine. One rat failed to eat the banana pellets, and was excluded from the study. PCA sessions began the following day. There were 25 trials per session separated by VT-90 (30–150s) intervals. In each trial, the lever was inserted into the cage for 8 s, then retracted and immediately followed by the response-independent delivery of a banana pellet. The chamber was illuminated by the red house light during PCA training. Lever presses and magazine entries were recorded during the 8-s CS period when the lever was present. 2.4. Quantification of PCA behavior
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As in previous studies the lever-CS evoked different CRs in different rats. The relative occurrence of a sign-tracking CR (ST; approach and engagement with the lever) vs. a goaltracking CR (GT; approach and engagement with the food cup) was quantified by measuring: 1) the number of contacts with the lever or magazine, 2) the latency to first contact the lever or magazine, and 3) the probability of contacting the lever or magazine on any given trial. To classify rats as STs or GTs, we used a composite of these three measures termed the PCA Index Score (Meyer et al. 2012a). For each of the five training sessions we determined a PCA Score, which was the average of three measures: response bias [(lever presses - magazine entries / total contacts)], latency score [(magazine latency - lever latency / 8s)], and probability difference (lever probability - magazine probability). PCA Scores range from 1 (indicating an animal exclusively made ST CRs in every trial) to −1 (indicating exclusively GT CRs on every trial). PCA Scores from the last two training sessions were averaged to produce a PCA Index Score that was used to classify rats as STs (+0.5 to 1) or GTs (−0.5 to −1). Rats that fell in the middle range (−0.5 to 0.5) were classified as intermediates and were not included in these studies.
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2.5. Experiment 1 - Extinction of ST vs. GT CRs
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Rats were initially trained for 7–9 sessions using the PCA procedures described in section 2.3. The day after the last day of training all rats were then subjected to extinction conditions, in which the lever-CS was presented as before (25 trials per session, VT-90 s interval) but was not followed by a food pellet. There was no signal provided to indicate extinction conditions were in effect, other than reward was not provided. One group of 15 rats had four extinction sessions, and another group of 34 rats had eight extinction sessions. In both groups data from the first 4 sessions were very similar, so the two groups were pooled in the analyses below. 2.6. Experiment 2–Alternating rewarded (R) and non-rewarded (NR) periods signaled by illumination of the chamber
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In experiment 1 there was no explicit signal indicating whether the CS would be followed by reward. In this, and in the following experiment, we asked whether STs would show similar resistance to extinction as seen in experiment 1 if alternating blocks of reward and nonreward periods were explicitly signaled. In this experiment a ‘diffuse’ signal (illumination of the entire chamber) indicated whether presentation of the CS would or would not be followed by reward. All rats (N=21) were first trained using the PCA procedure described above for 5 daily sessions. On each of the following 5 days sessions consisted of 60 trials (i.e., 8-s CS presentations) separated by VT-90s intervals. However, the sessions were divided into two blocks in which the CS was followed by presentation of the food reward (‘Rewarded’; R) and two blocks in which presentation of the CS was not followed by reward (‘Non-Rewarded’; NR). The two R and NR periods lasted for 15 trials each. Thus, during R blocks, the red house light was constantly illuminated (as during the PCA training phase), and thus the chamber would appear dimly lit, and a banana pellet followed each presentation of the lever-CS. NR blocks were signaled by the continuous illumination of a white stimulus light, which illuminated the entire chamber and was brighter than the red house light, and under this condition the 8-s lever-CS presentations were not followed by a food pellet. The four periods alternated in two different patterns (R, NR, R, NR or NR, R, NR, R), which were counterbalanced across rats and across days. 2.7. Experiment 3- Alternating rewarded (R) and non-rewarded (NR) periods signaled by a discrete signal
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To extend experiment 2 we repeated the experiment, but further asked whether similar results would be obtained when R and NR periods were signaled by a spatially and temporally discrete cue, rather than illumination of the entire chamber. Illumination of the entire chamber may be perceived as a change in ‘context’, which could be processed differently than a discrete cue (e.g., (Bouton 2004; Burns and Domjan 2001; Domjan 2003; Holland and Bouton 1999). A separate group of 39 rats was used in this experiment. All rats first received 5 sessions of PCA training, as above, followed by 5 sessions of discrimination training, as in experiment 2. However, in this experiment R periods were signaled by illumination of a white stimulus light mounted at the front of the cage, and NR periods by illumination of an identical white stimulus light at the back of the cage. Unlike in experiment 2, these lights were illuminated for only 6 s immediately prior to each lever-CS
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presentation, and the red house light stayed on throughout the session. When the lever-CS was preceded by the R signal, the trial ended with delivery of a pellet. When the lever-CS was preceded by the NR signal, no pellet was delivered. During discrimination training there were 60 trials per session separated by VT-90-s intervals. There were two R and two NR blocks that consisted of 15 consecutive trials each, alternating in two counterbalanced patterns (R, NR, R, NR or NR, R, NR, R). 2.8. Experiment 4–Effects of alternating rewarded (R) and non-rewarded (NR) periods signaled by illumination of the chamber on instrumental responding for food reward In our previous studies on instrumental extinction, in which STs and GTs did not differ, the extinction period was not indicated by any explicit signal. The purpose of this experiment was to again examine instrumental extinction in STs and GTs but using a discrimination procedure that better matched the procedures used in experiments 2 and 3 above.
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2.8.1. Acquisition—Following PCA training, a separate group of 24 rats were trained to nose poke in the active port to receive a food pellet (responses in the inactive port had no consequences). In sessions 1–3 rats were trained on a fixed ratio (FR1) schedule with a 10-s time-out period after each pellet delivery. For the next three sessions rats were trained on variable interval (VI) schedules that increased the time between rewarded responses: day 4 VI-20 (10–30s), day 5 VI-40 (20–60s), day 6 VI-60 (30–90s). These first six sessions lasted until rats self-administered 30 pellets, to ensure that all animals experienced the same number of reinforced responses. On days 7 and 8, rats were trained on the VI-60 schedule, but sessions had a fixed duration of 30 min to facilitate comparison with discrimination sessions. A red house light was constantly illuminated throughout all acquisition sessions.
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2.8.2. Discrimination—For the next 10 days rats were trained on discrimination sessions that consisted of four alternating 15-min blocks, much like in experiment 2. During the R periods, the red house light was constantly illuminated and responses in the active port were rewarded on a VI-60 schedule of reinforcement. During NR periods, the red light was extinguished and a white stimulus light mounted above the inactive port (the same as used in experiment 2) was constantly illuminated, and responses in the active port were never reinforced. The order of R and NR phases (R, NR, R, NR or NR, R, NR, R) was counterbalanced across rats and across days. 2.9. Statistics
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Between-group comparisons were analyzed with repeated-measures (RM) ANOVAs comparing ST and GT groups across sessions. Significant interaction results were followed by post hoc independent t tests (Bonferroni-corrected) comparing STs and GTs in each of the sessions included in the ANOVA. In experiments 2 and 3, within-group comparisons were analyzed with RM ANOVAs comparing R and NR blocks across the five sessions of discrimination training, with both session and block as within-subjects variables. Significant interaction results were followed by post hoc paired t tests (Bonferroni-corrected) comparing R and NR results across all sessions included in the ANOVA. Corrected p values are reported if data did not pass Levene’s test of equality of variance or Mauchly’s test of sphericity. All statistical procedures were performed with SPSS for Windows (Version 21).
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3. RESULTS 3.1. PCA training: ST and GT groups
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As described previously, individual rats differed in the form of conditioned approach behavior they developed during the PCA training phase, with some rats primarily approaching the lever and others the food cup during the 8-s CS periods (Flagel et al. 2009; Meyer et al. 2012a; Robinson and Flagel 2009). Across the four experiments, 72 rats were classified as STs, and 61 were classified as GTs. Additional rats were classified as intermediates (INs), but these rats were not used further because we wanted to directly compare groups that varied markedly in their propensity to attribute incentive salience to the lever-CS (Meyer et al. 2012a). The number of intermediates that were excluded from each experiment are: experiment 1 – 22 INs, experiment 2 – 11 INs, experiment 3 – 25 INs, experiment 4 - 1 IN. Measures of probability, contacts, and latency are shown for leverdirected behavior and food cup-directed behavior during the first five sessions of PCA training for all rats used in subsequent experiments (Fig. 1). For all measures the behavior of STs and GTs diverged with repeated days of training, and for each of the six comparisons there were significant interaction effects [Fs(4, 328) = 48.51 – 75.47, ps < .001].
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During discrimination training in experiments 2 and 3, the majority of rats maintained the ST or GT classification that was established during PCA training. However, in some rats discrimination training disrupted their initial preference for the lever or food cup, and these rats switched from sign-tracking to goal-tracking or from goal-tracking to sign-tracking by the end of the discrimination training phase. We calculated PCA Index Scores from days 4– 5 of initial PCA training and from days 4–5 of discrimination (R/NR) training. There were 8 STs that switched from having positive scores during PCA training to having negative scores during discrimination (3 from experiment 2 and 5 from experiment 3). There was also 1 GT (from experiment 3) that switched from having a negative score during PCA training to a positive score during discrimination. These rats were excluded from subsequent analyses; therefore, all results shown below include only the rats that maintained scores in the direction of their original classification (STs > 0 and GTs < 0). 3.2. Experiment 1 - Extinction of ST vs. GT CRs
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In this experiment, non-reinforced exposure to the lever-CS (without any signal) resulted in faster extinction of PCA behavior in GTs (n = 21) than STs (n = 28). Within-session changes in probability of approach are shown for the last session of PCA training (baseline), and for extinction sessions 1 and 4. GTs showed a rapid decrease in approach toward the food cup within the first extinction session, as well as a further decrease by the fourth extinction session (Fig. 2A). In STs, approach toward the lever did not change from baseline within the first extinction session; however, by the fourth extinction session the STs did show a significant decrease in sign-tracking behavior (Fig. 2B). Fig. 2C shows probability of approach averaged across all 25 trials per session, analyzed with a 2 × 5 RM ANOVA (including baseline plus extinction sessions 1–4). Responding during extinction was significantly lower in GTs compared to STs [group F(1, 47) = 14.57, p< .001]. Repeated extinction sessions decreased approach in both groups [session F(4, 188)
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= 171.79, p< .001]; however, the rate of decrease differed in GTs and STs, with a significant group × session interaction effect [F(4, 188) = 13.97, p< .001]. Paired t tests comparing extinction sessions to baseline (Bonferroni-corrected) showed that changes from baseline were significant for GTs in all four sessions, but were only significant for STs in the last three sessions (Fig. 2C).
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In Fig. 2D the first 3 trials of each session were examined separately to evaluate the recall of extinction from one session to the next. GTs showed more immediate recall of extinction than STs, with a significant main effect of group [F(1, 47) = 32.97, p< .001]. Though recall improved in both groups with repeated days of extinction training [session F(4, 188) = 32.32, p< .001], this improvement was better in GTs than STs, with a significant group × session interaction effect [F(4, 188) = 7.25, p< .001]. Paired t tests comparing extinction sessions to baseline (Bonferroni-corrected), indicated that GTs showed significant recall of extinction in sessions 2–4, whereas STs showed significant, but weaker, recall in sessions 3 and 4 (Fig. 2D). We also examined the first 3 trials of each session in the subset of rats that received 8 extinction sessions instead of just 4 (9 GTs and 25 STs), and found that the probability of approach continued to be significantly lower in GTs than STs, with significant differences persisting throughout the additional sessions [session F(8, 256) =24.28, p< .001; group F(1, 32) = 30.17, p< .001; interaction F(8, 256) = 2.77, p< .01]. 3.3. Experiment 2 - Alternating rewarded and non-rewarded periods signaled by illumination of the chamber
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Fig. 3 shows the effect of alternating rewarded (R) and non-rewarded (NR) periods, signaled by chamber illumination, on the probability of sign-tracking and goal-tracking behavior, in STs and GTs, on the first and fifth session of R/NR training. It is clear that even in the very first session the behavior of GTs (n = 11) changed markedly between R and NR periods, but this was not the case for STs (n = 7). GTs rapidly decreased responding during NR phases, and rapidly resumed making GT CRs during R phases (Fig. 3A–B). In contrast, in the first session the behaviors of STs did not differ between the R and NR periods (Fig. 3C–D). By the fifth session GTs continued to show excellent discrimination between the R and NR periods, and by this time STs did so as well, based on probability of making a ST CR (see below). In addition, Fig. 3 shows that the introduction of discrimination training had essentially no effect on the very low levels of food cup-directed behavior in STs or on lever directed behavior in GTs.
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For lever-directed behavior in STs and food cup-directed behavior in GTs, measures of probability, contacts, and latency were averaged across the 30 R trials and 30 NR trials per session, and analyzed for each group separately with 2 × 5 (block × session) RM ANOVAs. STs initially showed no discrimination between R and NR periods, but with repeated training sign-tracking responses began to diverge between the two blocks. For all three measures, there were significant effects of block [Fs(1, 6) = 7.78 – 30.24; ps < .05 – .01] and significant interaction effects [Fs(4, 24) = 4.46 – 9.21; ps < .01 – .001; Fig. 4A–C]. In STs, post hoc tests comparing R and NR blocks were significant in session 5 for measures of probability and latency, but not contacts (Fig. 4A–C). In addition, during the discrimination sessions sign-tracking behavior was somewhat disrupted during R periods relative to the last
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session of PCA training. When lever-directed behavior during PCA training session 5 was compared to the reinforced blocks of R/NR session 5 (paired t tests), significant reductions were seen for probability [t(6) = 2.60, p< .05] and contacts [t(6) = 2.51, p< .05], but not for latency [t(6) = 1.55, n.s.] (Fig. 4A–C).
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In contrast to STs, GTs showed excellent discrimination between R and NR blocks in all five sessions. For probability, latency, and contacts there were significant differences between R and NR blocks [Fs(1, 10) = 29.93 – 298.21, ps < .001]. The GTs also showed some improvement in discrimination across the five discrimination sessions, with significant interaction effects for all three measures [Fs(4, 40) = 3.51 – 4.50, ps< .05 – .01]. Post hoc tests showed that the differences between R and NR periods were significant in all of the five sessions (Fig. 4D–F). In addition, and unlike sign-tracking (above), goal-tracking behavior was maintained in the R periods at the same level as during the last session of PCA training, with no significant differences when PCA training session 5 was compared to R/NR session 5 [ts(10) = 1.07 – 1.53, n.s.](Fig. 4D–F).
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To more directly compare the performance of STs and GTs, we calculated difference scores for each measure (R responses minus NR responses), and performed 2 × 5 (group × session) RM ANOVAs. For probability, the difference scores of GTs were significantly higher than STs [group F(1, 16) = 11.01, p< .01]. Although the scores of both groups increased across sessions [session F(4, 64) = 15.07, p< .001], the STs improved to the point where they matched the GTs on the last session, with a significant interaction effect [interaction F(4, 64) = 3.63, p< .01]. Post hoc tests showed significant differences between STs and GTs only in the first two sessions (Fig. 4G). For contacts, difference scores were significantly higher in GTs than STs, and for both groups improved across sessions at similar rates, with significant main effects but no interaction [group F(1, 16) = 9.32, p< .01; session F(4, 64) = 7.01, p< .001; interaction F(4, 64) = 0.76, n.s.] (Fig. 4H). For latency, there was a significant difference between STs and GTs and an improvement in discrimination across sessions, but no interaction effect [group F(1, 16) = 33.74, p< .001; session F(4, 64) = 10.04, p< .001; interaction F(4, 64) = 2.27, n.s.] (Fig. 4I). Thus, for these latter two measures STs never achieved the same level of discrimination between R and NR periods, even after 5 days of discrimination training. 3.4. Experiment 3 - Alternating rewarded and non-rewarded periods signaled by a discrete signal
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When discrete cues were used to signal whether or not the lever-CS would be followed by reward the results looked very much as in experiment 2 above. GTs (n = 18) showed much better discrimination between the R and NR periods than STs (n = 15). Fig. 5 shows changes in lever- and food cup-directed behavior during blocks of R and NR trials. In the very first session, as well as the fifth session, GTs rapidly decreased responding during NR periods, and rapidly resumed making GT CRs during R periods (Fig. 5A–B). In contrast, the STs showed no difference in responding between R and NR periods in the first session, though discrimination was evident by the fifth session (Fig. 5C–D). In Fig. 6, measures of lever-directed behavior for STs, and food cup-directed behavior for GTs, were averaged across the 30 R trials and 30 NR trials per session, and analyzed with 2 Behav Brain Res. Author manuscript; available in PMC 2017 January 01.
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× 5 (phase × session) RM ANOVAs. Similar to experiment 2, STs showed no difference between R and NR periods in the first session, but they showed improvement in discrimination in subsequent sessions. For all three measures (probability, contacts, and latency) there were significant effects of block [Fs(1, 14) = 9.21 – 23.59; ps < .01 – .001] and significant block × session interactions [Fs(4, 56) = 5.12 – 5.43; ps < .01 – .001]. Post hoc tests showed significant differences between R and NR periods emerging in the third session (Fig. 6A–C). Although in later trials STs showed more sign tracking during R trials compared to NR trials, sign-tracking behavior during the R trials was reduced compared to baseline levels on PCA session 5. For all three measures, lever-directed behavior was diminished during R trials in session 5 compared to PCA 5 [paired t tests: probability, t(14) = 3.44, p
