Learning in honeybees as a function of amount of reward: Tests of the equal-asymptote assumption

In Experiments 1 and 2, honeybee foragers visiting the laboratory were fed on targets of two different colors, one containing 5 μl and the other containing 20 μl of 50% sucrose solution. The targets were presented singly in quasi-random sequences on the training visits, after which preference was measured in an unrewarded choice test. In Experiment 1, 16 differentially rewarded training trials with each color were followed by the same number of trials with the color-amount relation reversed; no preference for either color was found in the subsequent choice test. In Experiment 2, 20 differentially rewarded training trials with each color—enough to produce a clear preference for the 20-μl color when given directly after pretraining—were given after 10 feedings to repletion on each color that were calculated to generate near-asymptotic associative strength; no preference for either color was found in the subsequent choice test. In Experiment 3, there were 12 feedings to repletion on one color and, on the other, 12 feedings to repletion followed by 15 trials with a small (5 μl) reward; no preference was found in a subsequent choice test. The results of all three experiments support a nonrepresentational interpretation of the role of amount of reward in the learning of honeybees.     

Reward and learning in honeybees: analysis of an overshadowing effect

Previous experiments have shown that honeybees trained with colored targets baited with 5- versus 20-µl drops of sucrose solution fail to develop a preference for the 20-µl color when the location of the drop on each target is marked by a white dot (dot-color overshadowing) but that discrimination is not impaired by dots when the targets differ in odor rather than in color. In Experiments 1–3, dot-color overshadowing failed to appear with differences in concentration rather than amount of sucrose (50% vs. 20% or 0%), but it did appear in Experiments 4 and 5 with a difference in probability of reward (consistent vs. partial). Experiment 6 showed no dot-odor overshadowing with a difference in probability of reward. The results are not generally predictable from the Rescorla-Wagner principle of shared associative strength, but point instead (in conjunction with those of earlier experiments) to competition for visual attention.     

Short-term spatial memory in honeybees

Foraging honeybees were trained individually in two-choice spatial problems. Differentially rewarded for spatial alternation in Experiment 1 (“win-shift” training), they showed instead a clear tendency to perseverate—that is, to prefer on each trial the location of reward on the immediately preceding trial. On the basis of the results of Experiments 2 and 3, in which one location was rewarded over shorter or longer series of consecutive trials, an associative interpretation of the perseveration found in the first experiment was rejected in favor of an interpretation in terms of short-term spatial memory. Experiment 4, in which the animals were rewarded on each trial for choosing either location, also showed perseveration. Honeybees, like rats, seem to remember a rewarded location recently visited, but tend to return to it rather than, like rats, to avoid it.     

Learning in honeybees as a function of amount of reward: Further experiments with color

Foraging honeybees were trained individually with successively presented targets differing in color, one containing 5 µl and the other 20 µl of 50% sucrose solution, after which preferences were measured in unrewarded choice tests. The targets were conical, designed to control for the possibility of differential delay of reward stemming from the greater detectability of the larger as compared with the smaller drops of sucrose when the drops were presented on the conventional flat targets. The new results for color, like recent results for odor, can be understood on the assumption that the attractiveness of a stimulus increases as a function of the strength of its association with reward and that the effect of amount of reward is on asymptotic strength.     

Effects of forced-choice runway variations on rats’ T-maze serial pattern learning

Rats learned an ordered RNR/RNN serial pattern task in a T-maze where they were shifted to a different runway on Trial 3 only in the RNR series (shift-win/stay-lose group) or only in the RNN series (stay-win/shift-lose group). The shift-win/stay-lose group developed faster speeds on Trial 3 of the RNR than on Trial 3 of the RNN series more easily than the stay-win/shift-lose group. This difference occurred whether all rats were forced onto the same runway on the first two trials (Experiment 1) or onto a different runway on Trial 2 from that on Trial 1 in each series (Experiment 2). Posttraining probe tests revealed that the shift-win/stay-lose group in each experiment relied on the runway shift event in Trial 3 or on the series position to anticipate the second reward within a series. Such reward expectancies were greater when the runway shift occurred in the same series position as during training. These probe tests revealed that the stay-win/shift-lose group relied only on the series position in Experiment 2. Our findings do not support predictions based on an associative predictive validity model. Rather, they reflect rats’ predisposition to spontaneously alternate choices in the T-maze, a tendency corresponding to their inherent win-shift foraging strategy. Rats in each group also reduced their speeds less on the nonrewarded Trial 2 when it preceded a rewarded rather than a nonrewarded Trial 3. This effect suggests that rats were able to determine which series contained a second rewarded trial. We discuss the theoretical implications of this Trial 2 speed effect in terms of rats’ uncertainty about where this second rewarded trial might occur in the RNR series.     

Learning in honeybees as a function of amount of reward: Control of delay

Six experiments on learning in honeybees were prompted by the possibility that results previously attributed to a difference in amount of reward (20- versus 5-μl drops of sucrose solution presented on colored targets) might be due at least in part to a difference in delay of reward attendant on greater difficulty in locating the 5-μ1 drops. Substantial reduction in the diameter of the targets, which was designed to facilitate location of the drops, impaired discrimination of the colors, perhaps because their salience was reduced in the process (Experiment 3). White dots used to mark the location of the drops on larger targets also impaired discrimination of the colors, which presumably were overshadowed by the dots (Experiments 1, 2, and 4). That the dots did not serve merely to equate delay but were themselves discriminated was demonstrated in Experiment 5, which produced as well the first indication of an effect of amount of reward uncontaminated by the possibility of differential delay: Animals trained with a 5-μl drop on a dotted target of one color and a drop of the same size on an undotted target of a second color preferred the dotted target, but animals trained with a 5-μl drop on a dotted target of one color and a 20-μl drop on an undotted target of a second color preferred the undotted target. In Experiment 6, with odors substituted for the colors on the assumption that they were less likely to be overshadowed by the dots, what could be interpreted as a pure amount effect was found again. Aside from their relevance to questions about the role of amount of reward, the results have some interesting implications for the theory of discriminative learning in honeybees.     

Discrimination of color-odor compounds by honeybees: Tests of a continuity model

In experiments previously reported, individual honeybees were trained in a variety of problems to choose between two visually identical but differently scented targets, one or the other of which contained sucrose solution. The results could be simulated accurately with simple equations for computing changes in associative strength produced by reinforcement or nonreinforcement and for predicting choice on the basis of relative strength. In the present experiments, the targets used differed in color as well as in odor, and the animals were trained in a variety of problems with color-odor compounds. Contrary to expectation, the new results could be simulated accurately with the same equations as before on the further assumption that the components of a compound gain and lose associative strength independently (independence rule) and that the associative strength of a compound is equal to the sum of the strengths of its components (summation rule).     

Acquisition and extinction in the presence of the same intertrial stimuli: The effects of partial reward

Two groups of rats (N = 11) were trained at two trials a day for 16 days in Phase I and 13 days in Phase II. Responses on Trial 1 were always rewarded in both phases. Percentage of reward (50% vs 100%) was varied on Trial 2 of each day of Phase I. Trial 2 on each day of Phase II was never rewarded. A partial reward effect (PRE) was observed on Trial 2 of Phase II. The implications of the results for intertrial explanations of the PRE were discussed.     

Learning in honeybees as a function of amount and frequency of reward

In a series of four experiments with free-flying honeybees, individual foragers were trained with targets of two different colors that contained 5 or 20 μl of 50% sucrose solution. The two targets were singly presented in quasi-random sequences on each visit, with the amount of reward to be found on each target perfectly predictable from its color. The number of training visits (4–32) was varied both within and between experiments, and so also was the relative frequency of trials with the 5- and 20-μl targets (1:1, 2:1, 3:1, and 9:1). At the conclusion of training under each condition, unrewarded responses to the targets were measured in a 10-min extinction test, with the targets presented either separately to two different groups of animals (Experiment 1) or as a pair (Experiments 2–4). When the number of training trials with each target was the same (Experiments 1 and 2), the animals responded more in extinction to the 20-μl target than to the 5-μl target, although there was a decline in the overall level of responding to both targets (an overlearning-extinction effect) as the number of training trials increased. After nine times as many, or only three times as many, training trials with the5- μl target as with the 20-μl target, the animals responded more in extinction to the 5-μl target (Experiment 3); after twice as many training trials with the 5-μl target as with the 20-μl target, there was equal responding to both (Experiment 4). The preferences shown in the choice tests of Experiments 2–4 could be simulated rather accurately on the assumptions of a model previously developed to deal with the discrete-trials choice behavior of honeybees and the further assumption that associative strength grows at a rate increasing with amount of reward to an asymptote independent of amount of reward.     

Effects of sequences of sucrose reward magnitudes with short ITIs in rats

Two experiments assessed the role of aftereffect learning in rats rewarded with sucrose solutions. In Experiment 1, rats were trained in a single straight runway for two trials on each of 18 days, each trial terminating with either large (20% scurose) or small (3% sucrose) reward. The ITI was 3–5 min. The sequence of daily rewards for each of four groups was small-small (SS), small-large, (SL), large-small (LS), or large-large (LL). Response patterning and a simultaneous negative contrast effect were observed in LS and SL relative to the consistently rewarded controls. During 10 massed extinction trials, resistance to extinction was greatest for Group SL, followed in order by Groups SS, LL, and LS. Experiment 2 examined single alternation of large and small rewards administered for 10 trials on each of 31 days with an ITI of 60 sec. Reward for one group was 20% or 3% sucrose while another received 1 or 10 45-mg Noyes pellets. Appropriate patterning developed only in the food-pellet rewarded animals. The overall results suggest that sucrose rewards may produce high-amplitude and long-duration aftereffects which interfere with learning in designs employing several massed daily trials, but which may facilitate learning—relative to food-pellet rewards—with longer intertrial intervals and fewer daily trials.     

Effects of sucrose concentrations on single-alternation performance in rats

In Experiment 1, rats received single-alternation training with 32% or 4% sucrose reward (Phase 1) followed by a shift in reward from 32% to 4%, and vice versa (Phase 2). In Phase 1, high reward facilitated alternation performance over low reward. In Phase 2, performance on rewarded trials increased as reward increased but was unchanged as reward decreased. Performance on nonrewarded trials showed negligible effects of shifts in reward. In Experiment 2, rats received goalbox placements with 32% or 4% sucrose alternated with nonreward in Phase 1; and in Phase 2, they received alternation runway training with the same or the opposite reward from that of placements. Performance on rewarded trials was faster, the higher the reward in runway training; performance on nonrewarded trials was slower, the higher the reward in placements. In Experiment 3, Phase 1 provided placements with 64%, 32%, 16%, or 4% sucrose or dry mash alternated with nonreward; Phase 2 provided alternation runway training with dry mash reward. Alternation prerformance developed more rapidly, the higher the sucrose concentration in placements. Only 64% sucrose produced performance superior to that for dry-mash placements.     

Ordinal position in the serial learning of rats

Rats were runway trained on each of two, three-trial series consisting of different varieties of reward (X, Y, and Z) and nonreward (N) serving as trial outcomes. The two series are represented as XNY and ZNN. Distinguishing the two series were different brightness and texture cues on the runway floor. Transfer tests, conducted after the rats had developed faster running for rewarded trials than for nonrewarded trials and slower running on Trial 2 of ZNN than on Trial 2 of XNY, provided evidence that trial position, rather than item memories, was controlling the discriminations. In Experiment 1, reversing the floor cues completely reversed the discriminations. In Experiment 2, transfer to NNN did not change the routine patterns of approach that had been established.     

Analysis of choice in honeybees

Free-flying honeybees were trained in a set of four problems to choose between two differently scented targets, one or the other of which contained sucrose solution. The training was simulated quantitatively, always with the same simple linear equations for computing changes produced by reinforcement and nonreinforcement in the strength of association between each target and the sucrose, but with a diverse array of functions for predicting choice on the basis of relative strength. Accuracy of prediction was indexed by the root-mean-square (RMS) deviation of simulated data from real data. The results provide some good approximations of what is tentatively assumed to be the true choice function, setting the stage for further development of the associative features of the model to encompass more complex phenomena of honeybee learning in choice situations.     

Blocking in landmark-based search in honeybees

Two experiments tested blocking in landmark-based search in honeybees. Honeybees in the experimental group were trained in Phase 1 with a single landmark in a constant spatial relation to the target (sugar water). In the compound training second phase, the landmark used in Phase 1 (blocking landmark) and a new landmark (blocked landmark) were presented at constant spatial relations to the target. The blocking and blocked landmarks differed from each other in color and position, and the blocking landmark retained the same spatial relationship to the target as in Phase 1. In Experiment 1, the control group experienced only Phase 2 training with two landmarks. In Experiment 2, the control group was trained with a different landmark in a different position in Phase 1. Blocking was found in both cases.     

Partial reinforcement and latent inhibition effects on stimulus–outcome associations in flavor preference conditioning

Two experiments with thirsty rats explored the harmful effects of non-reinforced exposures to a flavor cue in the control by sensory-specific flavor–sucrose associations in a conditioned flavor preference paradigm. Experiment 1 demonstrated that rats learned to prefer a flavor cue that was consistently paired with sucrose over one that was paired with sucrose the same number of times but was also presented without sucrose on other occasions. However, rats for which sucrose was devalued following the conditioning phase preferred the partially reinforced flavor cue over the consistently reinforced flavor, suggesting that non-reinforcement weakened the ability of that flavor cue to evoke a specific representation of sucrose during the preference test. Experiment 2 demonstrated comparable effects of non-reinforcement in a latent inhibition procedure, although relatively more non-reinforced pre exposures to the flavor, in conjunction with fewer flavor–sucrose pairings, were required to see the effect. Together, the results suggest, as is often found with more traditional learning paradigms, that non-reinforcement of a flavor cue has deleterious effects on preference learning and/or performance.     

Long-delay,one-trial conditioned preference and retention in monkeys (Cebus apella)

Cebus monkeys explored a small T-maze for 5 min, and their preference for the striped or black arm of the maze was assessed. On the next day, the experimental animals were placed into the nonpreferred arm for a 1-min period (exposure to the CS), removed from the T-maze for a 30-min delay interval, and then returned to the startbox of the maze, where they received a food reward (UCS). One control group (CS only) received the placement experience but was not rewarded after the 30-min period. A second control group (noncontingent UCS) received the reward in the startbox but not the placement experience. A second preference test showed that the experimental, but not the control, animals reversed their original preference, now showing a preference for the arm associated with reward. A retention test given 4 months after three such training-test trials revealed considerable retention of the preferences exhibited by the experimental and CS-only control subjects.     

Learning in honeybees as a function of amount of reward: Rejection of the equal-asymptote assumption

Foraging honeybees were trained individually with successively presented targets differing in odor, one containing 5 µl and the other 20 µl of a 50% sucrose solution, after which preferences were measured in choice tests. In Experiment 1, there were either 8 training trials with each target, 16 trials with each, or 8 trials with the 20-µ1 target and 16 trials with the 5-µl target. In Experiments 2 and 3, the odor-amount relation was reversed after either 24 or 16 trials with each target. In Experiment 4, differential reward was introduced only after two, four, or six feedings-to-repletion on each target. All of the results could be simulated quantitatively and with considerable accuracy on the assumption that the attractiveness of an odor is given by the strength of its association with sucrose; that asymptotic associative strength is an increasing function of amount of reward; and that choice between two odors is determined by their relative associative strength.     

Compound-component and conditional discrimination of colors and odors by honeybees: Further tests of a continuity model

In experiments previously reported, individual honeybees were trained in a variety of problems to discriminate color-odor compounds. The results could be modeled accurately on the assumption that the associative strength of each component of a compound stimulus changes independently with reinforcement or nonreinforcement of the compound (independence rule) and that the associative strength of a compound is equal to the sum of the strengths of its components (summation rule). In the present experiments, which were designed to challenge the model, honeybees were trained in compound-component problems (choosing between compounds and their separate components) and in conditional problems (choosing between colors on the basis of a common odor or between odors on the basis of a common color). The new data, together with all of the previous data, could be modeled accurately on the further assumption that interaction among the components of a compound generates a new, compound-unique component that gains and loses associative strength in the same way as other components and contributes in the same way to the strength of the compound; the independence and summation rules continue to apply.     

Performance of honeybees in reversal and ambiguous-cue problems: Tests of a choice model

Individual honeybees visiting the laboratory regularly for food were trained in a variety of discrimination-reversal and ambiguous-cue problems to choose between two differently scented targets, of which one or the other contained sucrose solution. The training was simulated with a set of equations for predicting choice on the basis of associative strength, and the accuracy of prediction was indexed by the root-mean-square deviation of simulated data from obtained data. Developed earlier on the basis of performance in other problems (Couvillon & Bitterman, 1985), the model proved adequate for the new problems as well.     

Intramodal blocking in honeybees

Foraging honeybees were trained in a concurrent blocking design with a compound stimulus (AX) reinforced and one of its components (A) either reinforced for a blocking group or nonreinforced for a control group. In Experiment 1, a compound of two colors was used; in Experiment 2, a compound of two odors was used; in Experiment 3, a color-position compound, with position defined in terms of proximity to a distinctive visual landmark, was used; and, in Experiment 4, an odor-position compound was used. In each of the first three experiments, the blocking group responded less than did the control group in a subsequent test with X; in the fourth experiment, the two groups did not differ. The results are in accord with expectations based on those of previous experiments with honeybees in which the independence assumption was found to hold for intermodal compounds but not for intramodal compounds.