Acquisition of barpressing in rats following experience with response-independent food

In a three-group experiment, one group of rats (response-contingent) learned to contact a food cup for Noyes food pellets delivered according to a fixed-ratio schedule of reinforcement. Subjects in another group received a response-independent pellet each time its yoked counterpart earned one. A third (control) group received the same amount of food as the other two groups each day, but the pellets were delivered in mass. Following this training, the rats were placed in a novel experimental chamber in which all responses on a bar were reinforced with the presentation of food pellets. The results showed the response-independent animals to be slower in acquiring the barpress response than the naive control subjects, and the response-contingent subjects to be the fastest.     

Transfer of persistence across reinforced behaviors

Rats barpressed for food on a variable-interval schedule and then received food in a runway for one of three degrees of effort. Finally, all animals again barpressed for food. Requiring five runway shuttles per food pellet produced a greater subsequent rate of barpressing than reward for each shuttle, which, in turn, yielded more barpressing than free food. A follow-up study showed the five-shuttle treatment to produce more subsequent barpressing than a control condition which omitted any runway treatment. Another experiment indicated that the higher rate of barpressing following the five-shuttle treatment was not due to greater conditioned general activity, since the five-shuttle treatment failed to increase the number of grid crossings to the cue of food presentation and produced no more unconditioned barpressing than following free food in the runway. Two possible interpretations of the results were compared: (1) The degree of accustomed effort per reinforcer becomes a generalized component of instrumental behavior, (2) high effort increases the habituation of frustration-produced disruptive responses.     

Motivational control after extended instrumental training

Hungry rats were trained to press a lever for food pellets prior to an assessment of the effect of a shift in their motivational state on instrumental performance in extinction. The first study replicated the finding that a reduction in the level of food deprivation has no detectable effect on extinction performance unless the animals receive prior experience with the food pellets in the nondeprived state (Balleine, 1992; Balleine & Dickinson, 1994). When tested in the nondeprived state, only animals that were reexposed to the food pellets in this state between training and testing showed a reduction in the level of pressing during the extinction test relative to animals tested in the deprived state. The magnitude of this reexposure effect depended, however, on the amount of instrumental training. Following more extended instrumental training, extinction performance was unaffected by reexposure to the food pellets in the nondeprived state whether or not the animals were food deprived at the time of testing. A second study demonstrated that the resistance to the reexposure treatment engendered by overtraining was due to the animals’ increased experience of the food pellets in the deprived state during training rather than to the more extensive exposure to the instrumental contingency. In contrast to the results of the first two experiments, however, a reliable reexposure effect was detected after overtraining in a final study, in which the animals were given greater reexposure to the food pellets in the nondeprived state.     

Prior fixed ratio training and durable persistence in rats

Following barpress training with different terminal fixed ratios (FR), rats were given the interpolated experiences of runway acquisition and extinction (as part of another experiment) followed by 2 months of vacation. Then they were tested in FR 10 barpress reacquisition, FR 10 barpress extinction, consistently reinforced runway reacquisition, and a second runway extinction. In a start (response initiation) measure, resistance to extinction during the FR 10 extinction and in the second runway extinction was positively related to the terminal FR values of the initial barpress training, an indication of highly durable differential persistence effects attributable to the initial training to different terminal fixed ratios of barpress responding.     

Transfer between Pavlovian facilitators and instrumental discriminative stimuli

Two experiments assessed the degree to which Pavlovian facilitators were interchangeable with instrumental discriminative stimuli (S^ds). In Experiment 1, rats were trained in a Pavlovian paradigm in which one stimulus (i.e., a facilitator) signaled the reinforcement of another stimulus (i.e., a target). Next, the rats were given instrumental discrimination training in which an S^d signaled the reinforcement of barpressing. A transfer test then assessed the capacity of the Pavlovian facilitator to promote barpressing. The results showed that the facilitator promoted significant barpressing, both when it was presented alone and when it was presented in compound with the S^d. Reliable transfer was not obtained with a “pseudofacilitator” control stimulus that, during training, was uninformative about the reinforcement of its target. Experiment 2 showed that a stimulus trained as an instrumental S^d reliably augmented responding to a stimulus previously trained as a target in a Pavlovian facilitation paradigm. A “pseudo-S^d” that, during training, was uninformative about the reinforcement of barpressing failed to promote such transfer. These results show that Pavlovian facilitators and instrumental S^ds are interchangeable to a significant degree, and suggest that facilitators and S^ds may act via similar mechanisms.     

Concurrent observations of barpress suppression and freezing: Effects of CS modality and on-line vs. off-line training upon posttrial behavior

In a conditioned suppression study with rats, CS modality (light vs. noise) and type of conditioning (on-line vs. off-line training) were manipulated. All rats were then tested on-line with only half the test trials reinforced. Some results and conclusions were as follows: (1) During initial training, suppression following reinforced noise trials was moderately strong at first but weakened over days; for the light, it was weak from the start. It was suggested that this strong influence of CS modality might complicate interpretations of posttrial suppression as a measure of US effectiveness. (2) During testing, posttrial suppression and freezing were greater following non-reinforced trials than following reinforced trials (US-omission effect), and this effect was stronger for noise than for light. Since noise also produced more freezing than light, this result favors the hypothesis that the US-omission effect is due to persistent CS-elicited freezing that is un-disrupted by a shock US. (3) Although noise produced more freezing, both noise and light produced similar barpress suppression. This result is consistent with the suggestion that noise and light acquire equal associative strength but elicit different defensive behaviors.     

Frustration-mediated learning of a barpress response

Following 72 reinforced placements in a box, rats received 30 additional nonreinforced placements during which time a bar was initially available. Under one condition a barpress response resulted in the rat’s being immediately removed from the box. Under the other condition, a barpress resulted in removal of the bar from the box while the rat remained in the box for a fixed period of time. Barpress speeds of both groups, but especially of the latter, improved with trials, whereas the slower speeds of control subjects, which received nonreinforced placements prior to barpress testing, showed no evidence of learning. The results were interpreted as supporting a prediction from elicitation theory that frustration can mediate learning through consistent elicitation of a response that is characteristic of frustration conditions, even when reinforcement in the form of frustration reduction is absent.     

Poison-avoidance learning to food-related tactile stimuli: Avoidance of texture cues by rats

Rats injected with lithium chloride after ingesting familiar food pellets presented in textured metal sleeves learned aversions to the sleeved food. In a choice between sleeved and unsleeved food, the aversions were evident following conditioning with toxicosis delayed as long as 120 min after exposure to the sleeved food (Experiment 1). Texture-specific aversions resulted from procedures in which rats were exposed to food in both rough- and smooth-textured sleeves but were injected with lithium only in conjunction with one of the textures (Experiments 2–4). This differential aversion learning occurred when lithium treatment was delayed 30 min after exposure to the sleeved food (Experiments 3 and 4) and was equally evident in rats conditioned and tested in total darkness or in normal room-level illumination (Experiment 4). However, differential texture aversion learning was not observed with 90- or 300-min delayed toxicosis (Experiment 3). The present experiments highlight the importance of tactile cues in the poison-avoidance learning of species that handle their food during the course of ingestion.     

Facilitating stimulus effects of reward and punishment in discrimination learning

To demonstrate a facilitating stimulus effect, as opposed to an incentive effect, of food reward, rats were trained on an easy, light-dark discrimination with different amounts of reward for correct and incorrect responses (1-0, 2-0, 3-1, and 5-1 pellets, respectively), and with shock or no shock administered in the correct goalbox. Both errors and trials to criterion were fewer with a large reward differential (LRD: 2-0 and 5-1), as compared with a small reward differential (SRD: 1-0 and 3-1), but were not affected by the “base” reinforcement condition of either 1 or 0 pellets for the incorrect response. In addition, choice and arm speeds during early training were positively related to the combined, or average, number of pellets contingent upon both correct and incorrect responses, indicating a generalization of reward expectancies. Although shock uniformly suppressed arm speeds under all reward conditions, it facilitated discrimination learning in the SRD conditions. That such facilitation occurred only when the conditions of reward for correct and incorrect responses were relatively similar indicates that not only shock, but also food can function as a distinctive cue: As a stimulus selectively applied to one response, it can decrease the similarity of the alternatives, and, in this manner, it can faciltate performance.     

Integrating control generated by positive and negative reinforcement on an operant baseline: Appetitive-aversive interactions

The interaction of opposing motivational states was measured within a design in which rats barpressed for food in one stimulus and to avoid shock in another. Tone and light discriminative stimuli (Sds) were counterbalanced over incentive conditions. Extinction eliminated responding when neither Sd was present. To minimize the influence of competing peripheral operants or reinforcer-elicited behaviors during appetitive-aversive interactions, contingency parameters were manipulated to generate similar rates and patterns of barpressing in both Sds and stimulus-compounding tests were administered in extinction. On these tests, rates in tone, light, and tone plus light (T+L) were equivalent. In contrast, when the same reinforcer (i.e., food or shock avoidance) maintained comparable training rates in tone and in light, in testing, T+L controlled double the rates of the single stimuli—strong additive summation. These results were strikingly similar to those of single-incentive experiments concerned with the contribution of excitatory and inhibitory incentive states to the results of stimulus compounding. Simultaneously presenting two Sds whose implicit stimulus-reinforcer (S-S^r) contingencies were arranged to make them, respectively, conditioned appetitive and aversive exciters (present experiment) produced test results comparable to those of two Sds whose implicit S-S^r contingencies were arranged to make them both conditioned inhibitors. Reciprocal antagonism between these two motive states more than neutralizes them. It appears to produce a negative (i.e., an inhibitory) motive state.     

Choice responding following multiple schedule training

Pigeons’ choice responding on 10-sec interpolated probes was studied after baseline training on multiple variable-interval variable-interval schedules of food reinforcement. Unreinforced choice following training with three different relative reinforcement rates (Experiment 1), with a 3-ply multiple schedule (Experiment 2), and with three different relative reinforcement durations (Experiment 3) was examined. Least squares lines were fit to choice relative response rate and schedule relative response rate as functions of training relative reinforcement rate; choice slope was significantly greater than schedule slope in all three experiments. This result is counter to the prediction of Herrnstein’s (1970) theory that these slopes should not differ. Luce’s (1959) theory also failed to account for the data. It was concluded that choice responding was controlled by both approach to the stimulus associated with the smaller mean interreinforcer interval or the longer duration, and avoidance of the other stimulus.     

Time-place learning in the eight-arm radial maze

Rats (n=4) searched for food on an eight-arm radial maze. Daily 56-min sessions were divided into eight 7-min time zones, during each of which a different location provided food; locations were randomized across subjects before training. The rats obtained multiple pellets within each time zone by leaving and returning to the correct location. Evidence that the rats had knowledge about the temporal and spatial features of the task includes the following. The rats anticipated locations before they became active and anticipated the end of the currently active locations. The rats discriminated currently active locations from earlier and forthcoming active locations in the absence of food transition cues. After the rats had left the previously active location, they visited the next correct location more often than would be expected by chance in the absence of food transition cues. The rats used handling or opening doors as a cue to visit the first location and timed successive 7-min intervals to get to subsequent locations.     

Transfer of persistence from fixed-ratio barpress training to runway extinction

Two experiments were performed to investigate transfer of persistence across different situations and response topographies. Experiment I demonstrated that FR 100 barpress training increased resistance to runway extinction as compared with a control. In Experiment II, resistance to extinction in the runway was systematically and positively related to terminal ratio requirements of prior barpress training.     

Cross-stimulus transfer of timing in pigeons

Pigeons were trained in a delayed matching task in which the samples were short (2 sec) and long (10 sec) presentations of either a houselight or a keylight. Transfer trials involved short and long presentations of the nontrained signal as the sample. In the intermittent transfer test, infrequent transfer trials were intermixed with more frequent training trials; in the sustained transfer test, all trials were transfer trials. The intermittent test revealed only weak transfer. The sustained test revealed transfer in Session 1 only in birds that had received pairings of the transfer signal and food prior to testing. However, regardless of whether the transfer signal had been previously paired with food, birds exposed to consistent contingencies between duration and choice across training and testing learned the transfer task more rapidly than did birds exposed to inconsistent contingencies. It was concluded that some training in which the transfer signal serves as the sample is required before the durations of a transfer signal are related to the rules associating duration and responding     

Amount of training effects in representationmediated food aversion learning: No evidence of a role for associability changes

Rats acquired aversions to food pellets when a previously trained signal for that food was paired with a toxin, but only after minimal signal—food training. After extensive signal—food training, signal—toxin pairings had no effect on food consumption even after manipulations that enhanced the associability of the signal. By contrast, conditioned responding to the signal retained its sensitivity to devaluation of the food reinforcer by food—toxin pairings after extensive training. These results suggest that the nature of associatively activated event representations changes over the course of training.     

A comparison of the short-term memory performances of pigeons and jackdaws

Two experiments employed a delayed conditional discrimination procedure in which half the trials began with the presentation of food and half with no food; following a retention interval, subjects were presented with a choice between red and green keys, a response to one of which was reinforced according to whether the trial had started with food or no food. In Experiment 1, after 38 training sessions during which the retention interval was gradually increased, pigeons performed at a moderate level with intervals of 5 to 7.5 sec. A final test produced a steep forgetting function for food trials, but not for no-food trials; performance was unaffected by the duration of the intertriai interval (10 or 40 sec). Experiment 2 used the delayed conditional discrimination procedure to compare short-term memory in jackdaws (Corvus monedulus) with that in pigeons. Although the performance of the jackdaws was below that of the pigeons at the start of training, they showed more rapid learning over long delays, and, in the final test, a shallower forgetting function for food trials than that shown by pigeons. The results suggested superior short-term memory in jackdaws, which may help to explain the better performance of corvids in general when compared with that of pigeons in certain complex learning tasks.     

Factors affecting rats’ location during conditioned suppression training

If freezing underlies barpress conditioned suppression, then it seems odd that auditory cues paired with shock evoke more freezing than do visual cues, yet evoke similar suppression. Bevins and Ayres (1992) found that auditory and visual cues also evoked similar withdrawal from the bar and dipper areas and suggested that such withdrawal could explain the similar suppression. Seeking to understand that withdrawal, we found evidence in the present study that it was due either to adventitious punishment or to place-aversion learning. The cue for shock seemed to set the occasion for such learning. For example, we found that, as training progressed, rats’ tendency to leave the bar area during the cue first increased, then decreased, then increased again, reflecting, presumably, shock occurrence first inside, then outside, then inside the bar area again. Despite these changes in the rats’ location, barpress suppression remained stable, implying that leaving the bar area, though sufficient for barpress suppression, is unnecessary.     

Errors in pigeons’ memory for number of events

In Experiment 1, pigeons were trained in a within-subjects design to discriminate sequences of light flashes (illumination of the feeder) that varied in number, but not in time (2f/4sec and 8f/4sec), and in time, but not in number (4f/2sec and 4f/8sec). Number samples required a response to one of two comparison dimensions (either color or line), whereas time samples required a response to the remaining comparison dimension. Delay testing revealed a significant choose-small bias following number samples and a significant choose-long bias following time samples. In Experiment 2, testing confirmed that in the absence of a sample, there was a bias to respond small to the number comparisons and long to the time comparisons. Additional tests indicated that the birds were discriminating time samples on the basis of the number of light flashes occurring during the last few seconds of the time samples, rather than on the basis of the total duration of the flash sequence. Consequently, the choose-long bias observed for time samples during delay testing was really a choose-small bias. In Experiment 3, the birds received baseline training with a 5-sec delay and were subsequently tested at shorter and longer delays. A choose-large bias occurred at delays shorter than the baseline training delay, whereas a choose-small bias was again observed at delays longer than the baseline delay. These findings provide additional empirical support for the conceptualizing of memory for number and time in terms of a common mechanism.     

Motivational cues as determiners of stimulus control in rats

Four rats were each trained to perform a light-intensity discrimination and a sound-intensity discrimination. For half of the subjects, light training sessions were preceded by food deprivation, and correct choices were reinforced with food. Sound training sessions, on the other hand, were preceded by water deprivation, and correct choices were reinforced with water. For the remaining subjects, light training sessions were associated with water deprivation, whereas sound sessions were associated with food deprivation. When the rats were tested in the presence of compounds of sound and light, choices tended to be controlled by light when testing was preceded by the deprivation condition associated with light discrimination task. Reliably fewer light-consistent choices were made under the other deprivation condition, though some preference for responding on the basis of light remained. Following extended training in the presence of all four combinations of light and sound stimuli, this preference was reduced somewhat. When additional testing sessions were preceded by combined food and water deprivation, the tendency to respond on the basis of either light or sound was shown to be related to both deprivation and reinforcement factors.     

Pavlovian aversive to instrumental appetitive transfer: Evidence for across-reinforcement blocking effects

Rats received Pavlovian aversive (shock) conditioning in which white noise was established for different groups as a CS+, CSO, or CS?. Then, in an appetitive T-maze discrimination, the CSs were presented contingent upon a designated correct response for which food reinforcement was factorially varied at 0, 1, 2, or 4 pellets. Although the CS+ suppressed and the CS? facilitated speed of running in the correct arm at the start of discrimination training, these effects extinguished rapidly and did not interact with reward magnitude. Furthermore, choice learning was faciltated by the CS+ and retarded by the CS?, with these effects being comparable for the 1- to 4-pellet reinforcement conditions, but absent for the 0-pellet condition. These findings are difficult to reconcile with a transfer interpretation positing a general signaling property of the CS and are better interpreted as across-reinforcement blocking effects: By predicting a preferred outcome (safety) comparable to the preferred outcome of food reinforcement, the CS? blocks (retards) the association of reinforcement and the S^D; conversely, by predicting a nonpreferred (shock) outcome discrepant from the preferred food outcome, the CS+ “counterblocks” (enhances) the association of reinforcement and the S^D.