How habitual caffeine consumption and dose influence flavour preference conditioning with caffeine

Physiology & Behavior 82 (2004) 317 – 324 How habitual caffeine consumption and dose influence flavour preference conditioning with caffeine Elizabeth M. Tinleya, Paula J. Durlachb, Martin R. Yeomansa,* a Department of Psychology, School of Life Sciences, University of Sussex, Falmer, Brighton BN1 9QG, UK b Army Research Institute, Orlando, FL, USA Received 14 April 2003; accepted 25 March 2004 Abstract This study investigated the effects of both habitual caffeine use and dose administered in determining the ability of caffeine to reinforce conditioned changes in flavour preference. Thirty overnight-withdrawn moderate caffeine consumers and 30 non or low-dose caffeine (non/ low) consumers evaluated five novel-flavoured fruit teas. Subsequently, their median-rated tea was used in four ensuing conditioning sessions. Either placebo, 1 or 2 mg/kg of caffeine (n=10 consumers, 10 non/low consumers in each condition), was added to the target tea, and all five teas were reevaluated at a final tasting. Pleasantness ratings over the four conditioning sessions indicated that non/low consumers’ liking increased for the noncaffeinated fruit tea with no change for the tea containing either 1 or 2 mg/kg of caffeine. Among consumers, pleasantness ratings tended to decrease for the noncaffeinated fruit tea but increased significantly at the 1-mg dose and showed a tendency to increase at the 2-mg dose. Similar effects were shown in the evaluations made before and after conditioning, with no change in the nonexposed drinks. These results show that 1.0 mg/kg of caffeine reinforces changes in flavour pleasantness in acutely withdrawn habitual consumers but not in nonconsumers or nondependent low-caffeine consumers, further endorsing the negative-reinforcement theory of conditioning with caffeine. D 2004 Elsevier Inc. All rights reserved. Keywords: Caffeine; Flavour preference; Conditioning; Reinforcement; Liking 1. Introduction Recent research has demonstrated that caffeine can reinforce changes in preference and liking for novel flavours; however, such conditioning with caffeine depends critically on certain factors. The two most important factors appear to be the habitual consumption level of the population under test [1] and the caffeine-deprivation state of consumers when tested (e.g., [2 – 5]). Evidence of the importance of habitual consumption levels is limited to one published study to date, yet these data are critical to our understanding of the nature of caffeine reinforcement. It is therefore important to establish how robust these findings are, and the present study was designed to evaluate further the importance of habitual caffeine consumption levels as a predictor of changes in liking for novel flavours paired with caffeine as a function of caffeine dose. * Corresponding author. Tel.: +44-1273-678617; fax: +44-1273678058. E-mail address: martin@sussex.ac.uk (M.R. Yeomans). 0031-9384/$ – see front matter D 2004 Elsevier Inc. All rights reserved. doi:10.1016/j.physbeh.2004.03.018 Abstention from caffeine by habitual consumers results in well-characterised withdrawal symptoms which occur after a relatively short period of deprivation (approximately 12 h; for review see Ref. [6]). Symptoms include both physical and affective changes, with headaches being the most frequently reported symptom (e.g., Ref. [7 –10]) and tiredness/lethargy (e.g., [11]) also commonly reported. These withdrawal symptoms are not restricted to high and moderate caffeine consumers, they have also been reported by those ingesting as little as 100 mg of caffeine per day (e.g., Ref. [12]). Recent findings that implicate the importance of habitual caffeine use in the expression of conditioning with caffeine suggest that caffeine acts as a negative reinforcer by relieving withdrawal symptoms encountered after acute cessation of caffeine use. In the earliest study [1], low caffeine consumers (mean caffeine consumption<120 mg/day) expressed an overall increased preference for a target drink irrespective of caffeine content. However, moderate consumers (mean caffeine>120 mg/day) only showed a (nonsignificant) increase in preference for a caffeinated drink, and a decreased preference if the drink 318 E.M. Tinley et al. / Physiology & Behavior 82 (2004) 317–324 was noncaffeinated. Inasmuch as low consumers would not encounter the dysphoric effects of withdrawal, one explanation for these findings is that caffeine had no reinforcing effects on drink preference in the absence of withdrawal. In contrast, the acutely withdrawn moderate consumers would have experienced relief of withdrawal after consuming the caffeinated drink, and the association of the beneficial effects of caffeine could explain the slight increase in preference for the caffeinated drink. In addition, the noncaffeinated version could have been associated with the (possibly increasing) withdrawal effects leading to the subsequent decrease in preference for that flavour. Yeomans et al. [3] provided further evidence of this and highlighted the negative reinforcing effect of caffeine. Acutely withdrawn moderate caffeine consumers reported an increased liking for a caffeinated drink, yet liking for the flavour decreased if it did not contain caffeine. No such changes in flavour liking were seen in the absence of caffeine deprivation. Subsequent research further confirmed the sensitivity of such conditioning to deprivation state [5]. Moderate consumers who had been trained following overnight abstinence and thus in a caffeine-deprived state reported an increase in liking for a novel-flavoured caffeinated drink throughout training, but when subsequently tested undeprived (and thus no longer experiencing withdrawal symptoms), liking for that flavour fell significantly. The importance of the relief of withdrawal symptoms in self-administration of caffeine was suggested by Hughes et al. [13] who commented that effects such as headaches ‘prospectively predicted subsequent self-administration of caffeine’. Further evidence of the importance of withdrawal relief in reinforcement with caffeine was the finding that participants were willing to forfeit more money to avoid placebo than they would pay to receive caffeine [14]. Although the case that caffeine reinforces changes in flavour preference and liking through reversal of the negative consequences of caffeine abstinence in dependent consumers appears well supported, this conclusion relies heavily on the findings from one study [1] stating that nondependent and moderate caffeine consumers responded differently in a flavour – caffeine conditioning paradigm. However, whereas subsequent caffeine-conditioning studies have measured flavour pleasantness, the original study used a measure of relative preference to assess conditioning. Although changes in liking and preference covary, a change in flavour pleasantness need not always reflect a change in preference and vice versa (see Ref. [15]). A preference reflects a choice between two or more options, whereas liking is a pure hedonic evaluation. Thus, for example, although a person may have a higher hedonic rating for the flavour of a hot drink than for the flavour of a cold drink, their preference may be to consume the cold drink depending on ambient temperature. Thus, the finding that the lowand moderate caffeine consumers differed in the way that they altered their preference for flavours paired with the absence or presence of caffeine need not necessarily imply that changes in flavour liking showed the same changes. It could be, for example, that low consumers increased their liking for the caffeine-paired drink, but inasmuch as they prefer not to consume caffeine, this liking was not translated into a flavour preference. Thus, the present study aimed to replicate and extend the earlier study by Rogers et al. [1], by including both a measure of liking (the rated pleasantness of the conditioned flavour) and preference for that flavour. Another relevant factor to reinforcement with caffeine is dose. Among habitual consumers, low doses of caffeine in the range of 25– 100 mg are liked, whereas higher doses of caffeine are not. This was demonstrated in studies revealing increased liking for coffee containing caffeine at doses of 25 and 50 mg expressed by both high [16] and moderate consumers [17], with both groups having reported a monotonic decrease in liking with doses of 100 – 600 mg of caffeine. These ‘liked’ doses closely resemble the amount of caffeine contained in commonly consumed caffeinated beverages (30 – 125 mg of caffeine). The present study set out to reexamine the relevance of habitual caffeine consumption level (non/low consumers, defined as less than 65 mg of caffeine/day, vs. moderate consumers, defined as those consuming between 75 and 500 mg/day) on the ability of 0, 1, and 2 mg/kg to reinforce both changes in liking and preference for the caffeine-paired flavour. The lower dose (1 mg/kg bodyweight) provided the equivalent amount of caffeine to that consumed when drinking tea or instant coffee, and the higher dose (2 mg/kg bodyweight) approximately equated to that found in filter coffee. 2. Method 2.1. Design Using a mixed design, changes in pleasantness across time (within-subject) were contrasted between six treatment groups based on consumer status (‘consumer’ or ‘non/low consumer’) and administered caffeine dose (0 [placebo], 1, or 2 mg/kg). 2.2. Participants Sixty participants were recruited from Sussex University, using e-mail, personal communication, and poster adverts. All applicants had previously completed a questionnaire devised at Sussex University, combining the Three Factor Eating Questionnaire [18] with questions on general food and drink use. Questions regarding caffeine consumption were embedded within the section on drinks. Thus, potential participants were not aware that recruitment was specifically based on their daily caffeine consumption. Caffeine consumption levels were estimated from self-reported daily drink consumption using the following approximations of normal caffeine content: cola=30 mg/can, tea=60 mg of 319 E.M. Tinley et al. / Physiology & Behavior 82 (2004) 317–324 caffeine/cup, instant coffee=70 mg/cup and filter coffee=125 mg/cup (adapted from Ref. [19]). Respondents reporting medical contraindications (diabetes, use of medication [excluding the contraceptive pill], allergies to any of the breakfast ingredients, or diagnoses of an eating disorder) were excluded from the study. The first 30 respondents consuming less than 65 mg of caffeine/day were assigned to the ‘non/low consumer’ groups (mean=27F5 mg/day), and 30 respondents consuming between 75 – 500 mg of caffeine/ day to the ‘consumer’ groups (mean=221F19 mg/day). The two groups did not differ in BMI, weight, or age (see Table 1). However, further analysis of habitual caffeine consumption by the consumers, divided into the three treatment conditions, revealed an unexpected difference between groups [ F(2,27)=2.99, P=.06], with the group who were assigned to the 2.0-mg/kg-caffeine condition habitually consuming significantly less than the group assigned to placebo ( P<.05), with the 1.0 mg/kg group intermediate. The protocol was approved by the University of Sussex Ethics Committee, and the study was conducted according to the ethical standards laid down in the Declaration of Helsinki 1964. 2.3. Test drinks The teas used were Lemon and Ginger (Twinings); Camomile and Spiced Apple (Twinings); Ginger, Orange, and Honey (Twinings); Sweet Berry Swirl (London Herb and Spice); and Camomile and Honey (London Herb and Spice). These were chosen from a pilot study of 10 fruit teas, and were those rated most novel (to try to ensure minimal prior exposure to the flavours) and mid-range for pleasantness (to allow for both an increase and decrease in ratings). Drinks were made by steeping a tea bag in 120 mg of boiling water for 3 min. 2.4. Procedure Participants attended the laboratory at the Psychopharmacology Unit of the University of Sussex on six nonconsecutive days between 8.30 and 9.30 a.m., having refrained from eating or drinking anything other than water from 11.00 p.m. on the evening prior to each session. At least one ‘nonexperimental’ day between sessions ensured that participants returned to their normal caffeine consumption patterns and were not fasting on consecutive nights. At the first visit (pretest session), participants read and signed an information sheet and a standard volunteer consent form. The information sheet described the purpose of the experiment as determining the effect of common drink ingredients on mood. It included sucrose, glucose, fructose, aspartame, acesulfame, saccharin, caffeine, natural food flavourings, and natural colourings as possible additives to the drinks. The purpose of this deception was to prevent responses biased by participants’ preconceptions of caffeine effects. To try to ensure compliance with the overnight food and drink restrictions, each participant provided a saliva sample, which they were told would be analysed, but, however, was not. They were then presented with a sample of all five test teas, each of which was tasted in an order chosen by the participant. Each tea was evaluated for ‘pleasant’, ‘novel’, ‘sweet’, ‘bitter’, and ‘sour’ qualities using 100-mm line scales end-anchored with ‘‘Not at all’’ and ‘‘Extremely’’, and each participant recorded their most preferred tea. A standard breakfast of 60 g of Crunchy Nut Cornflakes (Kellogg’s brand) and 160 g of semiskimmed milk (total 310 kcal) was then given. Once these were eaten, the participants’ height and weight were measured. Rated pleasantness for the teas was assessed, and the tea which received the median score on the pleasantness scale was designated as the target tea for that participant. Sessions 2 –5 were designed as conditioning trials (subsequently referred to as Conditioning Days 1– 4) during which participants consumed their target tea containing either placebo (2 mg/kg of maltodextrin, Cerestar UK), which has been previously used in this and other laboratories to maintain the double-blind, or 1 or 2 mg/kg of caffeine (Courtin and Warner), according to the allocated dosage group. All drug administration was double-blind. As before, participants reported to the Psychopharmacology Unit between 8.30 and 9.30 a.m., and saliva samples were taken at each visit. On each conditioning day, a mood questionnaire indicating current moods was completed, after which the target tea was consumed and a tea-rating questionnaire was completed. The mood questionnaire has been successfully used to assess subjective moods in relation to caffeine in previous research [20]. Sixteen adjectives (friendly, angry, cheerful, dejected, confident, uncertain, clearheaded, muddled, calm, placid, jittery, tense, energetic, lively, drowsy, and tired) were rated on a 100-mm visual analogue scale from 0 mm (not at all) to 100 mm (extremely). Also included were ‘headache’, a recognised symptom of caffeine Table 1 Mean (FS.E.) habitual caffeine usage, body size and weight, and age of the six groups of participants Consumer group Caffeine intake BMI Weight Age Non/low consumer group 0 mg/kg 1 mg/kg 2 mg/kg 0 mg/kg 1 mg/kg 2 mg/kg 275.5F37.8 21.1F0.5 60.1F2.1 22.6F1.3 223.0F32.5 24.0F1.2 71.0F2.9 21.5F0.8 167.0F21.5 21.1F0.9 60.7F3.4 23.5F2.2 36.0F9.8 21.5F0.5 67.1F2.9 23.6F1.6 21.5F9.3 21.2F0.4 58.7F1.7 20.1F0.7 24.0F8.7 22.12F1.0 65.4F2.5 20.1F1.4 320 E.M. Tinley et al. / Physiology & Behavior 82 (2004) 317–324 withdrawal, ‘hunger’, and ‘thirst’. Once the entire fruit tea was consumed, subjects received breakfast and ate as much as they wanted. Thirty minutes later, a second mood questionnaire was completed, again according to current mood. On Day 6 (posttest session), subjects again provided a saliva sample and completed a mood questionnaire. As were in the pretest, all five tea flavours were rated, this time with the relevant caffeine or placebo dose added to the target tea. Again, participants indicated their most preferred tea. Following breakfast, a structured debriefing was conducted with participants asked what they thought the experiment was about, whether they thought it involved caffeine, and whether they thought there was caffeine in the fruit tea. Participants were then given a full explanation of the purpose of the experiment, thanked, and paid for participating. 2.5. Data analysis The rated pleasantness of the fruit teas was assessed across the four conditioning days using a mixed model ANOVA, with group (non/low consumers vs. consumers) and dose (0, 1, and 2 mg/kg) as between-subject factors, and conditioning day [1 –4] as a within-subject factor. Inasmuch as we were specifically predicting an increase in rated pleasantness for the caffeine-paired flavour with exposure in the moderate consumer group, we also calculated the difference in rated pleasantness on Conditioning Day 4 (when change should be maximal) relative to Conditioning Day 1 and contrasted these change scores between groups using two-way ANOVA. The same approach was used to assess change in pleasantness for the target tea from the pretest and posttest sessions. Change in ranked preference was assessed from the pleasantness ratings made on all five teas at the pretest and posttest sessions. The fruit tea given the lowest pleasantness rating was assigned Rank 1, and the fruit tea with the highest pleasantness rating assigned as Rank 5. Change in ranking was assessed by subtracting the rank of the tea at posttest from the ranking at pretest. To assess changes in pleasantness for the nontarget teas, paired t tests were conducted on the pleasantness ratings for each of the nontarget teas at the Pre- and posttest sessions. ANOVA was not possible for these data inasmuch as different participants were exposed to different teas, depending on their initial pleasantness evaluation, and so different teas were available for analysis for different consumers. Using separate protected t tests therefore allowed us to include all data for nonexposed teas before and after conditioning. To assess whether caffeine had any effect on mood, ratings were taken prior to, and 30 min after ingestion of the tea. Those mood ratings previously shown to be sensitive to caffeine (jittery, energetic, lively, tired, clearheaded, and headache) as well as hunger and thirst were analysed by a 2 (group)3 (dose)4 (day) repeated-measure ANOVA over the four conditioning days. Inasmuch as the number of possible tests was large, significance was adjusted using the Bonferoni correction to avoid Type 2 errors. For brevity, we only discuss significant findings. 3. Results 3.1. Pleasantness and novelty ratings for the target tea Analysis of the pleasantness ratings over the four conditioning days (Fig. 1) revealed a significant GroupDose Day interaction, [ F(6,162)=2.47, P<.05]. To simplify analysis of this complex interaction, these changes in pleasantness were examined further by calculating overall changes in pleasantness across the four conditioning trials, at which time any changes through association should be maximal (Fig. 2). Here, a GroupDose interaction [ F(2,54)=5.99, P<.005] confirmed that non/low consumers rated pleasantness of the noncaffeinated fruit tea increased significantly [t(9)=2.41, P<.05], whereas consumers rated pleasantness tended (NS) to decrease for this drink. Conversely, rated pleasantness of the tea containing 1 mg/kg of caffeine increased significantly over this time in the consumer group as predicted [t(9)=2.04, P<.05, one-tailed], but no significant changes in pleasantness were observed in the non/low consumer group. With 2 mg/kg, there was a tendency for pleasantness to increase in the consumers and to decrease in the non/low consumers. As expected, novelty ratings decreased over the 4 days [ F(3,162)=7.512, P<.001]. The changes in pleasantness during the conditioning period were also reflected in the evaluations made on Days 1 and 6, before and after conditioning. A GroupDose interaction [ F(2,54)=3.284, P=.045] of the change in pleasantness for the target tea reflected distinct patterns in the consumer and non/low consumer groups. In the latter, pleasantness increased significantly for the noncaffeinated fruit tea [t(9)=3.47, P<.01] and tended to decrease for the fruit tea containing 2 mg/kg of caffeine, whereas consumers rated the fruit tea containing 1 mg/kg of caffeine as significantly more pleasant after than before conditioning [t(9)=1.84, P<.05, one-tailed], but rated pleasantness for the drinks with 0 or 2 mg/kg did not change significantly. The present design relies on all five potential target teas showing similar changes in liking when paired with the presence or absence of caffeine. If this was not so and if choice of target had differed between the six test groups, this would question the validity of any conclusions we might make about conditioned changes in flavour evaluation. To test this assumption, we examined the frequency with which the five teas were selected as target for the six test groups. All five teas were used, and the frequency with which the teas were used did not differ form chance [chi-squared (4)=8.5, NS]. The frequency with which each drink was used as target by the consumer and nonconsumer groups 321 E.M. Tinley et al. / Physiology & Behavior 82 (2004) 317–324 Fig. 1. Mean (FS.E.) rated pleasantness of the target fruit tea on the four conditioning trials for the non/low consumers (left hand panel) and moderate consumers (right hand panel). The target drink contained either 0 ( ), 1 ( ), or 2 ( ) mg/kg of caffeine. was the following: Lemon and Ginger, consumers=6, non/ low consumers=4; Camomille and Spiced Apple, consumers=10, non/low consumers=9; Ginger, Orange, and Honey, consumers=7, non/low consumers=8; Sweet Berry Swirl, consumers=2, non/low consumers=4; Camomille and Honey, consumers=5, non/low consumers=5. Thus, there were no differences between consumers and nonconsumers, and although numbers in each category were too small to allow statistical analysis, inspection of target choice across the six groups did not suggest any systematic differences in the frequency with which each tea was used as target across groups. 3.2. Pleasantness of the nontarget teas Table 2 shows mean pleasantness ratings for each of the nontarget teas on Day 1 (pretest) and Day 6 (posttest). There were no significant differences in liking for these teas between Day 1 and Day 6 [ F(1,59)=3.61, NS]. Although, there was a nonsignificant trend for an overall increase in pleasantness (from 40.1F2.7 to 43.8F2.1). Inasmuch as the only group which increased liking for the target drink when paired with caffeine was the consumers who received 1 mg/ kg of caffeine, it was possible that liking for the nontarget teas also increased in that group, but it was also possible that this change was masked by the lack of change in the remaining five groups. However, analysis of pleasantness ratings for the unexposed teas in the consumer/1.0-mg/kg caffeine group alone did not support this [ F(1,9)=1.26, NS], with a nonsignificant increase in liking for the unexposed teas (from 37.4F4.5 to 41.2F2.7) in line with the overall nonsignificant change in pleasantness. 3.3. Ranked preference for the target tea The target tea was selected as the median-preferred tea at the pretest session, and so it was possible to test whether Table 2 Mean (FS.E.) pleasantness ratings of the unexposed (nontarget) teas on Days 1 (pretest) and 6 (posttest) Tea Fig. 2. Mean (FS.E.) change in rated pleasantness between Conditioning Days 1 and 4 trials for the non/low consumers ( ) and moderate consumers ( ) consuming the target drink with either 0, 1, or 2 mg/kg of added caffeine. Lemon and ginger Camomile and spice apple Ginger, orange, and honey Sweet berry swirl Camomile and honey n 50 41 45 54 50 Pleasantness rating Day 1 Day 6 39.4F3.8 43.4F4.3 32.8F3.9 47.6F4.3 31.6F4.1 43.7F3.4 50.5F3.4 39.2F4.2 46.2F4.4 35.2F4.2 322 E.M. Tinley et al. / Physiology & Behavior 82 (2004) 317–324 exposure to these teas with or without added caffeine during conditioning resulted in a change in ranked preference at the posttest session (Day 6). Although the ranked preference changes were in the same direction as actual changes in rated flavour pleasantness (Table 3), analysis found no significant effects of group or caffeine dose on the ranking measure. 3.4. Mood ratings Consumers reported higher headache ratings than non/ low consumers [ F(1,54)=10.296, P<.005], indicating acute caffeine withdrawal effects among the consumer group. Headaches were relieved by consumption of the fruit tea [ F(1,54)=12.18, P=.001], however, this was irrespective of group or caffeine content. Non/low consumers reported feeling more clearheaded than consumers [ F(1,52)=7.22, P<.01], and although clearheadedness increased after fruit tea consumption [ F(1,52)=7.703, P<.01], it was unaffected by dose [ F(2,52)=0.593, NS]. As expected, there was a main effect of time on hunger and thirst ratings with participants less hungry and thirsty after consumption [ F(1,52)=152.78, P<.001 and F(1,52)=80.29, P<.0001, respectively], but this was unaffected by group or caffeine dose. 4. Discussion This experiment aimed to verify the reinforcing effect of caffeine between non/low and moderate consumer populations and at different doses. The caffeine doses used in this study provided ecologically valid amounts, as mean bodyweight of participants was 63.8 kg, thus 1 mg/kg provided caffeine similar to that contained in tea (60 mg/cup), while 2 mg/kg yielded an amount similar to that found in filter coffee (125 mg/cup). A difference in the reinforcing effects of caffeine between non/low consumer and consumer groups was evident. Increased liking for caffeine-paired flavours only occurred among regular moderate caffeine consumers, with liking for the noncaffeinated version reduced among this group. Conversely, for non/low consumers, pleasantness ratings tended to decrease for both caffeinated drinks yet increased significantly for a noncaffeinated drink. This supports and extends the view that caffeine is primarily a negative Table 3 Mean (FS.E.) change in rank preference for the target fruit tea between pretest and posttest sessions as a function of normal caffeine consumption and level of caffeine during exposure Group Non/low consumers Consumers Caffeine dose in drink during conditioning (mg/kg) 0 1 2 0.50F0.55 0.20F0.41 0.00F0.44 0.00F0.38 0.80F0.47 0.15F0.44 reinforcer in this context, as only moderate consumers would be responsive to the withdrawal alleviation of the caffeine, while non/low consumer groups may experience negative effects from ingestion of caffeine resulting in decreased liking. In the earlier study [1], low consumers (<120 mg/day) reported an increased preference for the target drink regardless of caffeine content, whereas the present study only found an increase in pleasantness for the drink without caffeine. Inspection of the earlier data [1], however, suggests that in that study, the change in preferences was also greater for the noncaffeinated than the caffeinated drink, but this difference was not significant. Moreover, the different daily caffeine intake levels of the populations in the two studies may also partly explain the slight difference between the two studies. Maximal habitual caffeine use of low consumers in Rogers et al. study [1] was almost twice that of the non/low consumers in the present study (<70 mg/day). In the present study, the daily caffeine intake of the non/low consumer group was below the minimal level at which withdrawal effects have been reported (100 mg; [12]), but withdrawal reversal may still have been an element of preference change in the low consumer group from the study by Rogers et al. [1]. Inasmuch as it has also been reported that consumption of two evenly spaced caffeinated drinks per day was enough to sustain optimal mood and psychomotor performance among caffeine consumers [21], the low consumer group in the earlier study [1] may also have incorporated consumers who regularly experienced beneficial effects from the ingestion of caffeine. Furthermore, although no detrimental effects of caffeine consumption were evident from the mood data in the present study, it is possible that the non/low consumers experienced some aversive effects from the caffeinated drink, particularly at the higher caffeine dose, and this aversive effect was the basis for the tendency for pleasantness to decrease in that group. Thus, the slight differences in responses between the non/low consumer group in the present study and that reported by Rogers et al. [1] could be explained by differences in study power along with inclusion of some mildly caffeine-dependent consumers in the low consumer group in the earlier study. Overall, the present results show a trend towards an inverse relationship between the reinforcing effect of caffeine and dose. Among the moderate consumers, rated flavour pleasantness increased, and preference tended to increase, at the 1-mg/kg dose, but these effects were marginal at the higher (2-mg/kg) dose, supporting previous research of similar dose effects. For example, voluntary intake of coffee was significantly greater at lower (25- and 50-mg) doses than at a higher (100-mg) dose [22]. Higher caffeine doses have also been reported as less reinforcing than lower doses (100 and 200 mg; [23]), and coffee with very high caffeine content is aversive (400 and 600 mg; [23]). Together, these findings suggest that there is an optimal dose range at which caffeine reinforces flavour preferences among moderate caffeine consumers, which E.M. Tinley et al. / Physiology & Behavior 82 (2004) 317–324 falls within levels contained in regular caffeinated drinks. In the present study, any such negative effects of the 2.0-mg/kg caffeine dose in the consumer group may have been exacerbated by them having (by chance) a lower habitual caffeine consumption than the placebo-treated consumer group inasmuch as tolerance would be predicted to be related to habitual caffeine usage. The current experiment measured change in preference and change in rated pleasantness for the target drink. The results are similar to those reported in previous flavour conditioning studies in that change in rated pleasantness (e.g., Ref. [2 – 5,24]) provided evidence of conditioned changes in liking. However, relative preference for the target drink, the method used in the earliest investigation of flavour preference conditioning with caffeine [1], resulted in nonsignificant trends only although these trends were broadly in line with changes in rated pleasantness. The difference between changes in pleasantness and changes in ranked preference could be explained by the sensitivity of the measurement methods used. Firstly, change in rank produces ordinal scores ranging from 2 to +2, whereas pleasantness ratings were measured on a 100-point scale. Thus, the greater variability in scores permitted by the rating method allowed for a more sensitive evaluation of change than with ranked scores. Secondly, in the current experiment, the fruit teas were assigned ranks calculated from pleasantness ratings on Days 1 and 6, rather than a separate evaluation of preference. Thus, for a drink to alter in ranked preference using this method, the increase in rated pleasantness had to exceed the difference in pleasantness between itself and the next highest ranked tea at pretest. Thus, if the teas that were ranked one higher than the target tea had pleasantness ratings that were at least 15 points higher on the 100-mm rating scale, a change in ranked preference could only be seen if the rated pleasantness of the tea at posttest was at least 15 units. In practice, the ranking procedure used in the present study may not have been optimal, and an independent ranked preference measure may have been more appropriate. However, other studies in this laboratory suggest that where choice is not driven by factors other than the pleasantness dimension (i.e., where factors, such as belief, drink temperature, etc, are controlled for), pleasantness and preference closely match. An interesting question for future research would be to contrast changes in pleasantness and preference where preference may be driven by a nonhedonic factor. Change in pleasantness was specific to the target flavour, as pleasantness ratings for the four other flavours sampled at the initial and final sessions did not change significantly between the pretest and posttest sessions, and similar findings were reported in a related recent study [25]. This was the case overall and, independently, for the only group who increased liking for a caffeine-paired flavour (the consumer group receiving 1.0 mg/kg of caffeine). Together, these results imply that changes in pleasantness were due to the postingestive effects of caffeine associated with the specific 323 flavour encountered during conditioning and provides further evidence of Pavlovian conditioning in flavour preference conditioning. Analysis of the mood data in the present study showed significant differences between the non/low consumer and consumer populations for headache (consumers reporting higher ratings at both time points) and clearheadedness, with non/low consumers more clearheaded at both time points. During debriefing, one of the participants from the non/low consumer group receiving 1 mg/kg commented on feeling jittery between 1 and 2 h after the conditioning sessions, and another nonconsumer receiving 2 mg/kg mentioned feeling jittery half an hour after the sessions finished. Therefore, to better understand the time-course of these effects and whether they differ between consumers and non/low consumers, future studies should examine changes in mood over a more prolonged period postingestion. Although the data overall clearly demonstrated an increase in liking for a flavour paired with 1.0 mg/kg of caffeine consumed by overnight-deprived moderate caffeine consumers, the study suffered form a number of methodological shortcomings. Firstly, the contrast in evaluations of the drinks pre- and postconditioning was complicated by the addition of the relevant additives from the training phase into the drink at posttest. The reasons for this were pragmatic: inasmuch as pretest determined which drink was to be the target, we could not include the relevant additives at that stage. However, once participants had become familiar with the drink with the relevant additive during training, subsequent omission of that small flavour component could have affected their response at posttest. In practice, the effects of having caffeine or placebo present are likely to have been small, and inasmuch as caffeine is bitter and therefore mildly aversive in flavour, any effect of the added caffeine would have been counter to the increase in pleasantness predicted for the caffeine-paired flavour. Indeed, our past studies have consistently failed to find effects of does of caffeine up to 100 mg on liking for these types of drinks [3– 5,24]. Where the presence of caffeine may have been important is in the 2.0 mg/kg group, where the bitter flavour may have acted to counter any increase in liking through association and so masked learning in these groups. In conclusion, the present experiment supported and extended earlier findings [1,3,5] that flavour preference conditioning with caffeine is dependent on habitual caffeine consumption levels and is not seen in participants who consume very little or no caffeine. This supports the theory that flavour preference conditioned by caffeine is primarily the consequence of negative reinforcement in that it negates the effects of short-term caffeine withdrawal. Interpretation of the present data as evidence of negative reinforcement is further supported by the more recent finding that habitual caffeine consumers who have been fully withdrawn from caffeine for 2 weeks prior to flavour –caffeine conditioning developed an aversion to the caffeine-paired flavour, while consumers who were maintained on caffeine developed a 324 E.M. Tinley et al. / Physiology & Behavior 82 (2004) 317–324 conditioned preference [2]. The present results also suggest that the reinforcing effects of caffeine on flavour preferences are stronger at 1 mg/kg (a dose similar to that in most popular caffeinated beverages) than at 2 mg/kg, where any tendency to increase preference may be confounded by mildly aversive effects of higher dose caffeine. 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