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EUROPEAN JOURNAL OF BEHAVIOR ANALYSIS 2008, 9, xx - xx NUMBER 2 (WINTER 2008) 1 Humphreys (1939b) revisited: Is there a “verbal” PREE? Frode Svartdal and Kjetil Heggelund University of Tromsø, Tromsø, Norway Humphreys (1939b) reported the PREE (partial reinforcement extinction effect) of a “verbal” classically conditioned response. His interpretation in terms of expectation corresponds well to the modern cognitive view of conditioning. To date, however, Humphreys (1939b) is the only study we are aware of that attempts to demonstrate the involvement of a verbalized expectation in the PREE. We performed two replications of Humphreys’ experiment, and offer an alternative interpretation to the original conclusions. Specifically, we suggest that in Humphreys’ experiment, verbalized expectations were not classically conditioned responses but rather responses established by confirmatory consequences and strong task demands. In a third experiment we demonstrate a PREE of the conditioned verbal expectation response, but the effect was much weaker than that of the original experiment. Our results suggest that the role of cognition in the generation of the PREE is weaker than assumed by Humphreys and cognitive theory. Keywords: Persistence, verbal response, reward, extinction, PREE In 1939, Humphreys (1939a) published one of the first papers on the partial reinforcement extinction effect (PREE). In it, Humphreys reported data from a classical eyelid conditioning procedure with students as participants. The results demonstrated that partial, as compared to continuous, reinforcement during acquisition causes increased resistance to subsequent extinction. This effect, often coined the Humphreys’ paradox (e.g., Mazur, 2004), is now well established as one of the basic findings of the psychology of learning. Later the same year, Humphreys (1939b) published another paper that purportedly demonstrated the same effect, but now with verbalized expectations as the conditioned response. Using a classical conditioning procedure1 with 24 trials, student participants wrote down, for each trial, “yes” or “no” as to whether the US would occur following the CS. The verbalized anticipation of the US thus served as the CR. Two reinforcement conditions, 100% or 50%, were arranged. After acquisition, 12 extinction trials with no reinforcement followed. The results indicated that extinction performance following the 50% reinforcement phase was slower compared to performance following the 100% contingency. These results thus seemed to conform well to the initial PREE findings, and implied that behavioral (eyelid responses to the CS) and cognitive (verbalized expectations of the US) measures of the CR demonstrate a close correspondence. Humphreys (1939, p. 298) concluded that “extinction of verbal expectations of a stimulus duplicates rather closely the extinction of conditioned eyelid reactions”, and added: “This indicates that expectancy is, at least, an important variable in conditioning” (ibid.). 1 Humphreys noted (1939b, p. 295) that his procedure is “analogous” to a classical conditioning procedure, implying that alternative descriptions are possible. However, the procedure fulfills the basic requirements for a classical conditioning procedure, i.e., the presentation of two stimuli in a specified temporal order independently of the behavior of the participant (e.g., Catania, 1992). We therefore use the term “classical conditioning” in the description of the procedure throughout this paper. We thank Reidar Kvadsheim, Oslo University College, for helpful assistance in recruiting participants. Correspondence concerning this paper should be addressed to F. Svartdal, Department of Psychology, University of Tromsø, 9037 Tromsø, Norway. E-mail: [email protected] 1 2 Frode Svartdal and Kjetil Heggelund Humphreys’ (1939b) report on the role of expectancy in extinction was probably the first experimental study that attempted to assess the role of cognition in acquisition and extinction of the conditioned response. His data were correlational: Because of the similarity between the nonverbal and verbal PREE, the results were taken to support the view that acquisition as well as extinction involve explicit verbalization of contingencies. Interestingly, this view corresponds well to a current and very popular view of human conditioning that assumes conditioning and extinction to be dependent on explicit cognitive processes (e.g., Lovibond, 2004; Shanks & Lovibond, 2002; Shanks, 2005). Cognitive theory holds that learning and extinction depend on consciously available contingency knowledge, require attentional resources, and are influenced by language (Brewer, 1974; Lovibond, 2004; Shanks, 2005; Shanks & Lovibond, 2002). Overall, this view is supported by substantial experimental evidence, both within classical and operant conditioning (see, e.g., Lovibond, 2004; Shanks, 2005, for reviews). However, the cognitive interpretation of extinction – i.e., that behavioral extinction depends on changes in contingency beliefs (Lovibond, 2004) – is largely theoretical and has little empirical support. For the PREE specifically, Humphreys’ (1939b) experiment appears to be the only study that has attempted to empirically document the involvement of explicit cognition in extinction of a conditioned response. In fact, we are aware of only three additional studies that actually have investigated the role of beliefs, expectations and verbalizations related to the PREE (Svartdal, 2000, 2003; Svartdal & Silvera, 2008). Whereas Humphreys’ results are consistent with a cognitive interpretation of PREE, the three other studies provide no evidence for the involvement of explicit cognitive processes in the PREE. For example, in one set of experiments (Svartdal, 2003), student participants were exposed to CRF vs. PRF operant contingencies, then to extinction contingencies. As expected, the conventional PREE appeared in behavioral measures. We also obtained information about explicit and implicit verbalizations of the learning contingencies. Verbalizations closely matched acquisition contingencies. In contrast, beliefs about extinction persistence did not reflect the PREE and instead demonstrated an undifferentiated and gradual decrease in persistence over extinction trial probes. In a later study (Svartdal & Silvera, 2008, Exp. 2), we actually demonstrated a behavior-cognition dissociation under extinction conditions. Following exposure to CRF vs. PRF contingencies, participants had a small number of extinction trials. The conventional PREE appeared in the behavioral measures, but no difference was observed in probes for explicit or implicit contingency beliefs; instead, these probes indicated a gradual but non-differentiated extinction response. In contrast to Humphreys’ conclusion, our findings indicate that contingency beliefs under extinction conditions are not related to the immediately preceding reinforcement history and therefore cannot explain the behavioral PREE. Thus, whereas probes are sensitive to extinction conditions in the sense that verbalized probes indicate gradual extinction, thus supporting cognitive theory in a general sense, such probes are not sensitive to the CRF vs. PRF manipulation. Because Humphreys’ experiment may be the only study to directly support the notion that verbalization is closely correlated with the PREE, it is of importance to verify its validity. To our knowledge, no replication of this experiment has been performed. Furthermore, the 1939b paper has been cited more than 200 times, and its interpretation of conditioning and extinction is almost identical to a modern cognitive view of conditioning and extinction. The present paper presents data from replications and extensions of the Humphreys’ investigation, and argues that Humphreys may not have demonstrated the PREE in his explicit cognition (verbalization) study. We will argue that the response he observed most probably was shaped by consequences of responding and strong task demands. If correct, this will question the validity of Humphreys’ conclusion and hence its support of a cognitive account of the PREE. We start by presenting a critical evaluation Humphreys Revisited of the Humphreys’ 1939b investigation (see also Williams & Hartley, 1982). The procedure was very simple. First, over 24 acquisition trials, participants guessed whether the US would occur, given the CS. Two mutually excluding response alternatives, “yes” or “no”, were available. In the 100% condition, “yes” responses were consistently confirmed by the occurrence of the US, and any “no” response would be disconfirmed by the occurrence of the US. Then, under the subsequent extinction phase, “no” answers would now be continuously confirmed by the absence of the US, and any “yes” response would be disconfirmed. Given such powerful consequences of the emission of two response alternatives under acquisition and extinction, it is highly likely that the prediction task was greatly facilitated. Accordingly, Humphreys reported CRF “learning” curves that quickly approached asymptotic level during acquisition trials, and also quickly diminished to zero during extinction trials. Humphreys noted that these results differed from typical animal data; in fact, extinction was complete by the third extinction trial (see Humphreys, 1939b, Figure 2). We hypothesize, therefore, that the verbal “yes” response observed during acquisition trials in the 100% condition was not a classically conditioned response; if anything, it was most likely a response strongly controlled by immediate consequences and hence could more correctly be seen as an operant response.2 However, the task demands in the guessing task generated a guessing pattern in the 100% condition that may render this interpretation doubtful as well. In fact, if the procedure is seen as a simple discrimination task (“predict if a stimulus will occur, given a signal”), most participants will easily provide the correct response within few acquisition trials relying on confirmatory feedback. Then, under extinction, the correct response is easily identified applying the same confirmatory strategy. 2 It should also be noted that Humphreys’ procedure, although formally a classical conditioning procedure, did not involve an unconditioned response (UR) that could elicit an US. The lack of the usual automatic US-UR relation (this was present in his 1939a experiment in the form of blink responses to airpuffs) further indicates that the role of classical conditioning in the 1939b experiment was marginal. 3 Performance in the 50% group of the Humphreys (1939b) experiment was more complex. During acquisition, responding seemed to random, i.e., the “yes” and “no” answers had no relation to the occurrence of the US. Then, under extinction conditions, participants first demonstrated an increased probability of “yes” responses, then they shifted to “no”. Thus, increased extinction “persistence” seemed to occur. As there was no way for participants to reliably predict the occurrence of the US during acquisition, “yes” and “no” guesses were confirmed and disconfirmed randomly. “Adaptation” to the PRF contingency would therefore imply “yes” responses in about half the trials. This is what Humphreys actually observed. In consequence, the slower reduction of the “yes” response under extinction was probably caused by that prior chance relation between the CS and the US (learned irrelevance). Further, the PRF contingency implied the presentation of the US on average every second or third trial. In light of this, the burst of “yes” responses early in extinction might reflect the expectation that the US was due or even overdue (i.e., a gambler’s fallacy; Tversky & Kahneman, 1974). In summary, we argue that the verbal “yes” and “no” responses observed in 100 and 50% conditions the Humphrey’s 1939b experiment probably had little to do with conditioning and hence with the PREE. For the PREE to occur, a history of conditioning must precede extinction trials. This premise is not well satisfied in the Humphreys’ experiment. In the 50% condition there is little evidence for any specific conditioned response, and in the 100% condition consequences and strong task demands greatly facilitated adaptation. Of course, as the PREE is observed in classical as well as in operant conditioning (e.g., Mazur, 2004), it is not critical whether the verbal conditioning response is respondent or operant. It is critical, however, that the response involved in the demonstration of the PREE is in fact a conditioned response. If this premise is not satisfied in the Humphreys 1939b study, this will question the validity of his study and hence its support of a cognitive interpretation of the PREE. In the remainder of this paper we present data to support and Frode Svartdal and Kjetil Heggelund 4 extend this contention, and to demonstrate that cognition is not strongly involved in generation of the PREE. Experiment 1 The first experiment was designed as a replication of Humphreys (1939b) original study with a few minor modifications, and its purpose was to replicate the original experiment and to confirm our interpretation of the Humphreys’ results, i.e., that responding was strongly determined by confirmatory consequences. Whereas Humphreys used a within-subjects design with repeated measures, Experiment 1 applied a between-groups design in which separate groups of participants were exposed to the task under CRF or PRF conditions. The rationale for this modification was that PREE will occur more reliably when comparing separate groups of subjects (e.g., Amsel, 1992, 1994; Capaldi, 1994; Nevin, 1988). Also, in contrast to Humphreys’ original experiment we did not include the chance series control condition. Method Participants. Participants were 27 students of both sexes (mean age = 24.0 yrs.) from the University of Tromsø. Apparatus. We implemented Humphreys’ procedure in software via a web interface (Inquisit Web Edition; www.inquisit.com). The experiment could then be performed on any PC with a web browser. Procedure. Participants first received general instructions and were then were guided to an introductory page explaining the procedure of the experiment and requirements for its completion. The experimental session started with random selection of experimental condition, CRF (100% reinforcement) or PRF (50% reinforcement). All participants then received brief instructions on the computer screen. Each trial started with the CS (a green square, 40 x 40 mm) being presented on the computer screen. The participant was then, by a written message presented on the screen, requested to guess whether another stimulus, a red square (same size and position as the CS), would follow. In the CRF condition, the US always followed; in the PRF condition, the US followed the CS in 50% of all experimental trials. The PRF schedule was identical for all participants in that group, and implied a maximum run of 2 of extinction or US trials (the actual sequence was us, us, ext, us, ext, ext, us, ext, ext, us, us, ext, us, ext, ext, us, ext, ext, us, ext, us, us, ext, us). Participants responded in terms of “yes” or “no”, indicated by the keyboard keys “Y” and “N”. These guesses (anticipation of the red square) thus served as “conditioned responses”. The acquisition phase consisted of 24 trials. Then, 12 extinction trials followed. In this phase, no US was presented; the CS was presented, and participants guessed whether or not the US would occur. Results and discussion There were no effects of sex or age, so these factors were disregarded in the analyses. The data are presented in the same manner as Humphreys’ (1939b) investigation to facilitate direct comparison. Figure 1, left panel, shows the acquisition data. “Probability of response YES” was computed as the probability of “yes” answers over blocks of four trials and subjected to repeated measures ANOVA. The CRF group gradually established a functional response with a high asymptotic level, whereas the PRF group seemed to answer “yes” or “no” in a random manner. An overall ANOVA demonstrated a significant main effect of condition (CRF vs. PRF), F(1, 25) = 120.64 p < .01, a marginal effect of trials, F(5, 125) = 2.08, p = .07, and a significant effect of the condition x trial interaction, F(5, 125) = 10.46, p < .001. The significant interaction effect indicates that responding developed differentially over trials (see Figure 1, left panel). Note that whereas learning was rapid in the CRF group, no learning seemed to occur in the PRF group. Performance in this group was random, and if anything the probability of “yes” answers diminished over acquisition trials. The extinction data were analyzed over the 12 extinction trials. Figure 1, right panel, shows that the occurrence of “yes” responses diminished quite rapidly in the CRF condition, Humphreys Revisited 5 Figure 1. Acquisition (left panel) and extinction (right panel) of the verbal anticipation of the US in the CRF and PRF groups, Experiment 1. The acquisition data are means of 4 trials, the extinction data are shown trial by trial. Error bars have been omitted to facilitate direct comparison to Humphreys’ (1939b) original. whereas the “yes” responses in the PRF condition first increased and then diminished over the 12 extinction trials. The ANOVA demonstrated a significant main effect of condition, F(1, 25) = 9.12, p < .001, a significant effect of trials, F(11, 275) = 7.19, p < .001, and a significant effect of their interaction, F(11, 275) = 4.79, p < .001. Again, the significant interaction effect indicates a differential development of responding over trials between the two groups. Figure 2 shows the data from Humphrey’s 1939b investigation. These (Figure 2, right panel, Series I and II) appear to be very similar to our data. In particular, the CRF data in both experiments demonstrated a rapid reduction in the probability of “yes” responses, indicating complete extinction by the 4th or 5th extinction trial. Performance in the PRF groups was also similar in the two experiments, with much more “persistent” responding compared to the CRF groups. Two features of the PRF extinction curves must be emphasized. First, note the peak after 3-5 extinction trials. This burst of “yes” responses was studied by Williams and Hartley (1982). They suggested an analysis in terms of the run lengths of the US. Because there was no specific learning in the PRF group, we propose that the peak can be understood in terms of an increased expectation that the US will occur, given the no-occurrence of the US during initial extinction trials (i.e., a gambler’s fallacy). Second, note the relatively steep extinction curve in the PRF group once the “yes” responses were discounted. Following 3-4 extinction trials, the power of the confirmatory feedback for “no” answers seemed to produce an “extinction” curve that resembles that observed in the CRF condition. On the face of it, these data are compatible with the PREE, but as argued it is highly likely that performance under the arranged classical conditioning procedure was facilitated by instrumental contingencies. Such operantclassical interactions are well known from 6 Frode Svartdal and Kjetil Heggelund Figure 2. The original data from Humphreys’ (1939b) experiment. The left panel shows acquisition data, the right panel extinction data. The percentages refer to reported expectation among 78 participants as to whether a green light would be followed by a red light. Series I is the CRF condition (100% reinforcement), Series II is the PRF condition (50% reinforcement). The chance series is not relevant for the present paper. animal research (e.g., Brown & Jenkins, 1966; Williams & Williams, 1969). In the present case we suggest that the strong confirmatory 100% acquisition contingency established a response strategy that could be applied also under extinction conditions, albeit in reversed form, creating almost perfect adaptation to the extinction contingency in 3-4 trials. This confirmatory strategy probably also was applied in the PRF condition following the initial extinction peak. Also note that this peak produced a number of disconfirmatory outcomes (“yes” followed by the non-occurrence of the US), further suggesting the involvement of implicit instrumental contingencies. If it is recognized that the conditioning procedure used in Humphreys (1939b) and our Experiment 1 involves powerful implicit instrumental contingencies in the CRF conditions, it would be of great interest to replicate Humphreys’ experiment without an explicit response that can be shaped by consequences. A way of eliminating or reducing the role of an instrumental contingency in this context would be simply to omit the emission of explicit verbalized responses during acquisition trials. In this way, there would at least be no explicit response that could be subject to differential shaping, and a more unbiased performance during extinction trials must be expected. Experiment 2 Experiment 2 was a modified replication of the Humphreys experiment. The effect of implicit instrumental contingencies was reduced by instructing participants not to respond during acquisition trials; responding only occurred in extinction trials. Participants had 24 acquisition trials in which the CS was followed by the US in all (CRF) or 50% of all trials (PRF). In contrast to Experiment 1 of this paper, participants now observed the CS-US presentations but did not respond. They were instructed to pay attention to the two stimuli and detect any pattern that might govern their presentation. Then, in 12 subsequent extinction trials, participants indicated, following the presentation of the CS, whether or not the US would occur. As in the previous experiment, the US was never presented during extinction trials. We decided to maintain an extinction session Humphreys Revisited of similar length to the original Humphreys experiment. By omitting the instructed response during acquisition trials, this procedure is less prone to the influence of implicit operant contingencies. It is of course possible that implicit responses and expectations during acquisition trials might be affected by the US presentations, but we reason that the effect of such implicit contingencies should be much less compared to the explicit shaping that probably occurred in Experiment 1. As the experiment involved two phases, an observation phase (acquisition trials), and then a response phase (extinction trials), instructions about this difference could be given at the outset of the experimental session or in two parts. To prevent an interruption of the flow of the experiment, we decided to provide all instructions at the outset of the experiment. However, the tradeoff with this solution is a possible loss of initial extinction responses. We considered this as less of a problem than the interruption with new instructions would represent. Method Participants and apparatus. A total of 40 students of both sexes (mean age = 22.8 yrs.) at the University of Tromsø participated. The apparatus was identical to that of Experiment 1 except that the procedure for acquisition trials now did not involve responding. Results and discussion There were no effects of sex or age, so these factors were disregarded in the analyses. An overall ANOVA indicated a nonsignificant effect of Condition, F(1, 38) = 0.01, a significant effect of Trials, F(11, 418) = 4.51, p < .001 and a significant effect of the Condition x Trials interaction, F(11, 418) = 3.72, p < .001. Figure 3 shows these data. It is apparent that the effect of extinction conditions in reducing the probability of “yes” responses was weaker compared to the results of Experiment 1. This was seen both in the CRF and PRF conditions. The significant Condition x Trials interaction indicates a conventional PREE, but extinction performance was not significantly different 7 between the groups when the two first extinction trials were excluded from the analysis in a planned comparison, F(1, 38) = 1.14, p = .29. The present data thus demonstrate a weak effect of the CRF vs. PRF conditions in extinction performance; in fact, the main difference between the groups could be traced to different asymptotic levels when entering extinction conditions. Figure 3. Extinction of verbal anticipation of an US in the CRF and PRF groups, Experiment 2. E1-E12 refers to individual extinction trials. It could be argued that the change in procedure from explicit responding to no responding under acquisition would imply less attention to the task, and that such reduced overall attention could explain weaker extinction effects. Note, however, that initial extinction performance was quite similar to that of Experiment 1, and the probability of “yes” response in the two first extinction trials exceeded .80. When compared to the results of Experiment 1, these results seem to indicate a more realistic extinction performance in that much of the effect of the implicit instrumental contingencies has been removed. However, we must assume that even the procedure of Experiment 2 might involve implicit instrumental contingencies and thus exaggerate effects in the direction of a conventional PREE. In reality, therefore, Experiment 2 provides very weak evidence for the existence of a PREE when a verbalized expectation is used as the CR. Frode Svartdal and Kjetil Heggelund 8 Experiment 3 The purpose of Experiment 3 was to repeat Experiment 2 with one potentially important modification. Humphreys (1939b), as well as our Experiments 1 and 2, applied dichotomous response alternatives, “yes” or “no”. It is possible that the implicit feedback contingencies could act in a very direct way on such responding, creating an exaggerated PREE in the Humphreys (1939b) experiment and in our Experiment 1. The weakened PREE seen in our Experiment 2 might then more correctly reflect the “net” PREE when most of the effect of implicit instrumental contingencies has been removed. It is possible, however, that the weak PREE might also be attributed to the operationalization of the response, i.e., that “yes” and “no” answers do not reflect the more nuanced expectations that may be generated during acquisition. If conditioning and extinction depend on beliefs (Lovibond, 2004), such beliefs must change gradually to match gradually increasing and decreasing response rates. Maybe, then, graded response alternatives could render a more sensitive cognitive measure. We therefore applied an expectation response that asked participants to indicate the likelihood for US to occur in five separate steps, from “very likely” to “very unlikely”. Because of the nonsignificant PREE seen in Experiment 2, we also could suspect that this null result could be attributable to relatively low statistical power. We therefore increased the number of participants for Experiment 3 substantially. This increased n was made possible because the experimental procedure could be administered as a web application. Web-based implementations of experiments may be associated with a high drop-out rates, and they also lack the stringent control that laboratory experiments provide (e.g., Birnbaum, 2000). However, numerous web studies have consistently replicated classic experimental results (e.g., Eichstaedt, 2002; Hewson & Charlton, 2005), and we reasoned that the relatively simple procedure of the present experiments would be well suited for administration over the Web. Finally, for this experiment we included three conditions. In addition to the CRF and PRF conditions, we included a group receiving 60% reinforcement during acquisition. The rationale for this was that because the PRF condition did not indicate any specific conditioned response in Experiments 1 and 2, maybe some effect would be observed under conditions with a slightly increased rate of reinforcement. Method Participants and apparatus. A total of 395 students at the Oslo University College participated. The apparatus was identical to that of Experiment 2. Procedure. As in Experiment 2, the acquisition trials did not involve responding. Further, instead of giving dichotomous responses as to whether the US would occur, participants now answered by a number on the keyboard in five grades of certainty, from “very unlikely” (1) to “very unlikely” (5). Participants were randomly allocated to the conditions CRF (100% reinforcement), PRF (50%), and PRF60 (60%). Data scoring and analysis. Because the acquisition phase of the experiment did not require any action, transition to the extinction phase was not observed immediately by all participants. Thus, only 8 participants responded in the first extinction trial. We therefore decided to include extinction trials 2-12 in the statistical analyses. Further, because of a relatively large number of missing data over trials, we decided that participants that responded in less than 8 of the extinction trials were excluded from the analysis. Missing data in a given trial was replaced by the mean of that variable. Finally, participants that did not demonstrate any variability in their responses during extinction trials were excluded from the analysis. This left 225 participants, which indicates a dropout rate not far from 40%. This is a bit higher than the 30% rate common for web-based studies (Birnbaum, 2000), but it must be remembered that we used relatively strict inclusion criteria. Results and discussion The overall ANOVA of the extinction data Humphreys Revisited indicated a nonsignificant effect of Condition, F(2, 222) = 2.27, p > .10, a significant effect of Trials, F(10, 2220) = 20.92, p < .001 and a significant effect of the Condition x Trials, F(20, 2220) = 2,62, p < .001. Figure 4 shows these data. Inspection of Figure 4 indicates that no difference seemed to appear between the two PRF conditions. Further, the significant interaction effect indicates that the CRF and the PRF conditions developed differently over extinction trials, which is also apparent in the figure. As this interaction involved a crossing of curves early in extinction, the proper analysis of predicted CRF vs. PRF group difference is to compare late extinction trials. Thus, a contrast analysis demonstrated that the PRF and PRF60 groups differed significantly from the CRF group over the final 8 extinction trials, F(1, 222) = 10.71, p = .001. Figure 4. Extinction performance over extinction trials 2-12 in the CRF, PRF and PRF60 groups, Experiment 3. The relatively rapid decrease of the anticipation response in the CRF group compared to the slower decrease in the PRF groups demonstrates a conventional PREE. Also note that the anticipation response in the CRF group demonstrates a level well above the minimum 1 response option at the end of the extinction trials. This indicates an extinction rate quite similar to Humphreys’ original eyelid data (1939b), and very different from his verbal conditioning extinction data (1939b). Finally, note that even though the difference between the CRF and 9 PRF groups was significant, the magnitude of the PREE observed here is not striking. General discussion The present paper criticized the study of Humphreys (1939b), and concluded that the data originally presented by Humphreys probably did not demonstrate classical conditioning of a verbalized anticipation response. Further, Experiment 1 of this report replicated Humphreys’ procedure. The results were similar to the original data. Analysis of the results in both experiments supports the view that the “PREE” observed in these experiments had little to do with PREE; instead, the verbal anticipation responses were most probably shaped by immediate consequences (i.e., the presentation of the US following the verbalized expectations) and thus involved strong task demands that greatly facilitated the prediction task. To support this conclusion, Experiments 2 and 3 tested Humphreys’ hypothesis by means of a procedure with reduced possibility of the US acting as a reinforcer to shape an explicit anticipation response. In both experiments the PREE was observed, but in a much weaker form compared to Experiment 1. To our knowledge, the data from Experiments 2 and 3 are the first to demonstrate that predictions of the US in classical conditioning, a “verbalized” CR, are different following CRF vs. PRF reinforcement and conform to a conventional PREE. However, in spite of the prediction task being relatively simple, the group differences over extinction trials were quite small, and PRF persistence was marked even in the final extinction trials. This pattern of results is similar to the eyelid conditioning data of Humphreys (1939a), but different from his verbal conditioning data (1939b). We therefore conclude that Humphreys was correct in assuming that PREE is demonstrable in the verbal anticipation response, but that he in fact did not demonstrate this effect. In a related set of experiments on predictions of persistence in operant conditioning, Svartdal (2003) and Svartdal and Silvera (2008) have shown that predictions of extinction persistence 10 Frode Svartdal and Kjetil Heggelund in between-groups manipulations of CRF and PRF do not conform to a conventional PREE. Even when independent groups of participants demonstrated a behavioral PREE under CRF vs. PRF conditions (Svartdal & Silvera, 2008, Experiment 2), corresponding verbalized persistence predictions were insensitive to the immediately preceding reinforcement history. These results from operant conditioning contrast the results of Experiments 2 and 3 of this report. We suggest that two differences between the procedures may explain this apparent conflict. First, in the operant conditioning procedure, predictions of persistence responses address the person’s own behavior, whereas in classical conditioning persistence predictions address external and often salient events. This difference relates not only to the difference of focus during prediction measurement (own behavior vs. external stimuli), but more generally also to differences in the complexity of the immediately preceding reinforcement history involving these factors. Thus, the classical conditioning situation is defined in terms of repetitions of response-independent CS-US presentations (in the present case 24 acquisition trials). In contrast, the operant situation is defined in terms of a response-reinforcer relation in which the participant’s own behavior during the acquisition phase is part of the contingencies. Second, although the procedure of Experiment 3 did not instruct responding during acquisition trials, it is possible and even likely that participants established implicit anticipatory responses that were confirmed or disconfirmed by consequences. These anticipatory responses were relatively easily translated to explicit responses during extinction trials. In contrast, the operant extinction experiments discussed had no obvious relationship between the conditioned operant response (key pressing according to complex rules) and the prediction measure (answers using a questionnaire). In sum, these two procedural differences may at least in part explain the dissimilarity between these two set of results. We believe that the extinction performance seen in Experiments 2 and 3 of this report is not strongly controlled by explicit cognition. This follows from the acquisition procedure, which did not require an explicit verbalized response during each acquisition trial. The resulting extinction curves, and particularly the modest differential change as a consequence of CRF vs. PRF contingencies, speak for control mechanisms that are not well represented cognitively. If correct, this conclusion in fact corresponds quite well to the previous studies of operant extinction performance mentioned; the difference between them would then be one of degree rather than of principle. The conclusion that cognition is not strongly involved in the production of the behavioral PREE implies that other non-cognitive mechanisms are involved. Thus, we find it more likely that associationistic mechanisms (e.g., Amsel, 1994; Capaldi, 1994) and/or very simple heuristics (e.g., Wong, 1977) may be responsible for the behavioral PREE. For example, a simple “win stay, lose shift” strategy may produce rapid behavioral changes but not be reflected in explicit contingency beliefs. Clearly, as the present experiments provide support for the notion that explicit cognition is not strongly involved in the PREE, further research must examine the actual mechanisms that are involved. Finally, the experimental procedure of Experiment 3 of this report must be commented. In contrast to a traditional laboratory experiment, Experiment 3 was implemented in a web-based system. Such procedures may be associated with a high drop-out rates, and they generally lack the stringent control over participants that a laboratory experiment provides (e.g., Birnbaum, 2000). In Experiment 3, a relatively large drop-out rate was observed. In addition, we applied strict exclusion criteria. However, if this affected our conclusions, it did so by removing noise. Thus, corresponding analyses with more liberal inclusion criteria indicated no effect of the CRF vs. PRF manipulation. In general, use of the Internet as a research tool in the social and behavioral science is becoming well documented (Birnbaum, 2000; Hewson, Yule, Laurent & Vogel, 2003; Mann & Stewart, 2000), and numerous web studies have replicated classic results (Eichstaedt, 2002; Hewson & Charlton, 2005). 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