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Instrumental response variation is inversely related to reward probability. Gharib, Derby, and Roberts (2001) theorized that individuals behave more variably when their expectation of reward is low. They postulate that this behavioral rule assists the discovery of alternative actions when a target response is unlikely to be reinforced. This suggests that response variability may be unaffected in a situation in which an animal's behavior is inconsequential to outcome delivery. We trained 6 pigeons in a within-subjects Pavlovian autoshaping procedure. On any given trial, the pigeons were presented with one of six colored discs on a touchscreen; each stimulus was associated with a particular probability of food, ranging from 100% to 0.6%. Pecking was more variable with low probabilities of food delivery, thus extending the rule relating variability and expectation to a Pavlovian situation. doi: 10.3758/LB.38.2.111

Pigeons were trained in a forced choice task with four alternatives to categorize arrays consisting of 1, 3, 5, or 8 dots. Before the pigeons chose a comparison stimulus, they were required to peck each dot sequentially. A single peck to a dot, which was defined as an indicating response, changed the color of the dot so that it was differentiated from those that remained to be counted. The pigeons successfully learned to categorize the numerical arrays and then displayed transfer to novel arrays consisting of two, four, six, or seven dots, in a manner according to the order of 1 < 2 < 3 < 4 < 5 < 6 < 7 < 8. Subsequent tests revealed that the pigeons discriminated the stimuli by relying on the number of indicating responses. They also utilized multiple information (surface area, time, and other confounded events), but this was of minor significance, and after training, the pigeons were able to disregard these cues. doi: 10.3758/LB.37.4.365

Two experiments were conducted to test predictions of 2 models for acquisition of preference in concurrent chains. Pigeons were trained with pairs of fixed-interval and variable-interval terminal-link schedules in a successive-reversal design. Results showed that acquisition rate was faster when fixed-interval schedules preceded and when variable-interval schedules followed the reversal and was approximately constant when the same pair of schedules was used. These results were predicted by both linear-operator and memory-representational models. However, only the linear-operator model predicted effects of schedule duration. Overall, these results demonstrated that a simple linear-operator model can account for the major features of preference acquisition in concurrent chains and suggest that choice in transition and steady state may provide convergent validation of a single delay-discounting function.

In each of 3 experiments, different sets of 4 pigeons (Columba livia) were trained to discriminate between 2 visual symbols that covered wells containing food items that varied in number, mass, or both. In Experiment 1, the symbols were associated with 0, 1, 3, 5, 7, or 9 pieces of grain reward. The pigeons learned to choose the symbol corresponding to the larger reward, and on summation tests, they chose the pair of symbols that summed to the larger total reward. When number of food pellets was varied but mass of reward was held constant in Experiment 2, preference for the larger number symbols failed to appear. When number was held constant and mass was varied in Experiment 3, the pigeons showed a clear preference for the larger mass symbols on single-symbol and summation tests. These findings show that pigeons summate the value of symbols and are more likely to represent symbols by mass of food reward than by number of food items.

Pigeons previously trained to peck 1 button (same) after the successive presentation of 16 identical pictures and to peck a 2nd button (different) after the successive presentation of 16 nonidentical pictures were tested on lists involving different degrees of variability, different list lengths, and different temporal organizations of list items. The pigeons' performances on this successive same-different task revealed a strong sensitivity to list entropy; but, their discrimination was also affected by their memory for list items and by the accumulated evidence for a same versus a different response. Statistical models confirmed and quantified the importance of these additional factors.

Training associated pairs of perceptually dissimilar stimulus classes with a common delay or probability of food reinforcement in pigeons. Then, different choice responses were trained to 1 component class in each pair. In a choice test, the untrained class in each pair occasioned the same response as did the choice-trained class. In a 3rd experiment, 2 classes had reinforcement delays of 1 s and 15 s, respectively, and 2 other classes had reinforcement probabilities of 0.1 and 0.9. The, 1 choice response was reinforced to a class previously associated with a better condition of reinforcement (e.g., 1-s delay or 1.0 probability), and a different response was reinforced to a class previously associated with a worse condition of reinforcement (0.1 probability or 0-s delay). Testing with all classes suggested that categorization was based on the relative reinforcement or hedonic value and not on the parametric details of reinforcement.

Error tendencies in matching-to-duration tasks are controlled by analogical coding and subjective shortening. Pigeons trained with differing duration tasks showed a choose-short effect. Extending the duration through variable delays showed systematic error tendencies of choose-short and -long effects, but only in birds trained with 1 and 30 s samples.

In 2 experiments, pigeons were trained on, and then transferred to, delayed simple discriminations in which the initial stimuli signalled reinforcement versus extinction following a retention interval. Experiment 1 showed that discriminative responding on the retention test transferred to novel test stimuli that had appeared in another delayed simple discrimination but not to stimuli having the same reinforcement history off-baseline. By contrast, Experiment 2 showed that performances transferred to novel initial stimuli whether they had been trained on-baseline or off-baseline. These results suggest that the test stimuli in delayed simple discriminations acquire control over responding only in the memory task itself. On the other hand, control by the initial stimuli, if coded as outcome expectancies, does not require such task-specific training.

Pigeons searched for any of several targets in experiments designed to explore attentional priming or search-image formation. Three experiments assessed the runs advantage, seen in improved search efficiency during single-target trial sequences compared with search efficiency during mixed-target sequences. Experiment I varied the number, of items in the search display and the target set. Well-practiced pigeons showed the runs advantage only for a display size of 36 and a target set of 8. Experiments 2 and 3 followed the runs effect over training. For a target set of 4, the runs advantage was present initially but diminished with practice. For a target set of 12, the runs advantage persisted. The data suggest that learning permits a shift from controlled to automatic processing of memory, but constrains the number of items that can be addressed in parallel.

Results of an experiment with pigeons tend to support memory retrieval theories in which unreinforced trials serve as a retrieval cue for not responding. Eliminating autoshaped responding by random or negatively-contingent response-reductive methods may not eliminate a previously acquired association. In two experiments, the autoshaped responding of pigeons was gotten rid of by a backward or negative contingency.

Behavioral contrast was produced using a percentage reinforcement procedure with pigeons. However, changes in probability of reinforcement could not be distinguished from alterations in the rate of reinforcement. Behavioral contrast occurs when there is an inverse relation between response rate in one segment of a multiple schedule and the reinforcement conditions of the other component. Behavioral contrast did not vary by whether nonreinforcement in the changed component of the schedule was signaled or unsignaled.

The option to make a choice appears to determine the asymmetrical, 'respond-short' effects in pigeons' retention functions for duration. The respond-short effect occurs in choice tasks, but not in delayed-matching-to-sample tasks. Experiments with hybrid tasks eliminated several possible explanations for the difference in retention. Temporal parameters of the stimulus schedule, timing of the fixed interval, relative amount of time exposed to the sample and test stimuli, or simultaneous versus successive exposure to test stimuli did not appear to be critical features.

A series of experiments with pigeons demonstrated that their behavior was affected by interval duration. The fixed intervals apparently functioned as both retention intervals and stimuli for discrimination. The pigeons were able to alter their temporal discrimination in response to the stimulus preceding the fixed interval. Accuracy was greater after short intervals than after long ones, but accuracy was still greater than chance after long intervals.

Pigeons trained through block clock procedures pecked more often both throughout and at the end of a cycle of four colors than did those trained with standard autoshaping. Total pecking in the former experiment correlated positively with the number of stimuli, contrary to response-dependent, or operant, procedures. The difference in the final clock segment was most striking, as conditions then were identical. The presence of clock stimuli apparently has opposite effects for the two conditioning methods.

Pigeons trained to peck at a spot on a computer screen showing a grassy field and several landmarks responded jointly to global cues and the landmark closest to the spot. Some pigeons depended to an extent on two landmarks, if both were close to the spot, and color appeared to be much more important than shape in landmark identification. Overall screen location affected control but the configuration of landmarks did not. The experiment generalized several earlier findings using simpler computer figures.

Two experiments with pigeons examined retention and mediated transfer in many-to-one matching with hue, response and food samples. Transfer tests with hue samples provided the strongest evidence for common coding, whereas alternative coding may have occurred during interim training for response and food samples. In retention tests, greater accuracy was associated with presentation of different samples rather than with repeated presentations of the same sample. In addition, asymmetrical results were obtained in each dimension, with shallower retention gradients for no-food, green and one response than for food, red and twenty response.

Two experiments were conducted with pigeons to study the relation between delay-cue pecking and accuracy on a delayed-matching-to-sample task. The cues signaled remembering, forgetting or ambiguous conditions. The pigeons were separated into omission and substitution groups. The substitution group was given a simple discrimination task when no memory test was given, whereas in the omission group the trial ended without testing. In the omission group, no relation was found between delay-cue pecking and performance on the delayed-matching-to-sample task. Absence of a memory test did not affect accuracy.

A temporal gap between feature and target did not affect pigeons' learning of serial feature-positive discrimination tasks, but did affect learning of serial feature-negative discrimination tasks. For the serial feature-negative discrimination tasks, the discrimination without the temporal gap was learned, but the discrimination with the gap was not. Results are compared with other experimental results for pigeons and for rats.

Timing processes were similar, but memory processes differed for pigeons in successive versus choice delayed matching-to-sample tasks. The pigeons were trained in either successive or choice tasks and then administered tests involving delay manipulation, double or multiple samples, intertrial interval manipulations, no sample and 40-second samples. Only manipulation of delay produced qualitatively different results for the choice and successive tasks. The results suggest that the difference between the two tasks specifically involves the memory code used to retain temporal information during delay.

Pecking and food were equally effective as single coded events for pigeons in conditional discrimination learning experiments, suggesting that their memory processes are flexible. Pigeons using pecking, food or both as a basis for the single code all exhibited nonparallel retention functions for food-sample versus no-food-sample trials. Parallel retention functions were obtained only for the group of pigeons that could use neither pecking nor food for the single code.

Evidence that pigeons use prospective coding was provided by an experiment using successive delayed conditional discrimination with variation in the number of sample and test stimuli and the interstimulus interval. The group which had two test stimuli performed better than that with four test stimuli during delay testing but not during acquisition. Evidence suggested that shape stimuli were more difficult to discriminate than line stimuli.

Two experiments with pigeons investigated potentiation and overshadowing using a variable interval schedule with delayed reinforcement. The potentiation effect found by Schachtman and associates was replicated by obtaining a higher response rate with presentation of a houselight signal during a three-second delay. An overshadowing effect, or lowered response, occurred when the houselight was not associated with reinforcement. The overshadowing effect did not occur when the signal was located on the key to eliminate competition between response to the signal and the keypecking response.

Delayed matching-to-sample experiments were used to test aspects of pigeons' memory for duration involving length of delay and intertrial intervals. As predicted by the relative duration hypothesis, a shorter intertrial interval was associated with more systematic errors in response to manipulations of delay intervals. Varying the intertrial and delay intervals during training did not significantly affect the acquisition of a temporal reference memory.

Two experiments with pigeons investigated the effects of differential outcome expectancies on memory for temporal and nontemporal stimuli. The choose-short effect was enhanced for the differential outcome group with high probability of reinforcement for the short sample and eliminated for the group with high probability of reinforcement for the long sample. With nontemporal stimuli, greater accuracy was associated with higher probability of reinforcement. Results indicate that temporal codes, outcome expectancy codes and response bias all affect performance.

Common coding of stimuli was examined in pigeons in 3 experiments involving many-to-one mapping of lines and hues onto common events. The common events were shapes in Experiment 1 (involving delayed symbolic matching-to-sample) and food-no-food outcomes in Experiments 2 and 3 (involving simple discriminations). In Phase 2 of Experiments 1 and 2, the hue discriminations were reversed for Group Hue, the line discriminations were reversed for Group Line, and both discriminations were reversed for Group Hue-Line. Line reversals were learned faster by Group Hue-Line than by Group Line, but differences in reversal learning were not found with hues. In Experiment 3, both hue and line discriminations were repeatedly reversed until reversal transfer was stable. Relative to this baseline, significantly poorer performance was found on a line-only reversal. Overall, the results suggest that when a hue and a line are associated with a common event, both may be centrally represented as the hue.

In 3 experiments, extinction was conducted with a compound conditioned stimulus. For Experiment 1, the compound was composed of elements that had separately taken part in conditioning. The loss of the conditioned properties of the elements in these circumstances was slower than if they were presented alone during extinction. In Stage 1 of Experiments 2 and 3, reinforced trials with a compound AB were intermixed with nonreinforced presentations of A (Experiment 2) or A and B (Experiment 3). This training was followed by conditioning trials with A and then extinction with AB. During extinction, AB elicited a higher rate of responding than a compound composed of 2 elements that had separately taken part in conditioning. Overall, the results are more consistent with a configural than an elemental account of conditioning.

Two experiments examined memory for event duration in pigeons using choice and successive matching tasks. In the choice task, two stimuli were presented following the sample, and the correct choice depended upon the duration of the preceding sample. In the successive task, only one of two stimuli was presented following the sample, and whether responding was or was not reinforced depended upon the duration of the preceding sample. In the first experiment, a successive matching task was employed and accuracy decreased at an equivalent rate as a function of delay on short-sample and long-sample trials, regardless of whether the events which differed in duration were samples of houselight or food. The second experiment, which employed as samples different durations of houselight, replicated this result both in naive subjects and in subjects previously trained in a choice matching task. In addition, it was found that naive subjects which were trained and tested in a choice matching task demonstrated a more rapid decrease in accuracy on long-sample trials than on short-sample trials as a function of delay; a result which has been obtained in several previous studies and has been referred to as the 'choose-short' effect. It was also found, however, that the choose-short effect did not occur in the choice matching task if subjects had been trained and tested previously in the successive matching task. It was concluded that pigeons employ different coding strategies in matching to sample duration in the choice and successive tasks.

In each of two phases, pigeons were trained on a 9-s fixed interval schedule with one signal, and a 30-s fixed interval schedule with another signal. Across two phases, the intervals associated with the signals were reversed. On occasional empty trials, one of the signals alone, or both signals simultaneously were presented for a long time with no reward dispensed. The distribution of responses in time for these three types of tests showed the following three properties: 1) Distributions were nearly symmetric about the time of peak responding, implying that the internal clock runs linearly in time. 2) The width of the distribution relative to the height on tests with single signals was a constant proportion of the fixed interval the birds were timing, supporting Weber's Law. 3) When both signals were presented simultaneously, single-peaked distributions were found, with the time of peak responding in the majority of cases some time between 9 s and 30 s. This implies that the birds were using both signals in timing. The criterion durations of the arrival of reward associated with each signal (around 9 s and 30 s, respectively) were averaged in a weighted fashion to arrive at one criterion for timing. The same three phenomena in the psychophysics of two domains of spatial search are discussed.

What are the minimal conditions for the formation of chuncks by a pigeon learning an arbitrary list? Experiment 1 compared the acquisition of two types of chunkable list (each composed of colors and achromatic geometric forms): A B C D' E' (OR A' B' C' D E) and A B C' D' E' (r A' B' C D E). The first type of list was acquired more rapidly than the second. On both lists, however, evidence of chunking did not emerge until the four-item phase of training (e.g., pauses at the end of one category of list item). In Experiment 2, chunking was shown to occur on four-item lists in which colors and forms were segregated (A B C' D' and A' B' C D), but not on lists in which the two types of items were interspersed (A B' C' D and A' B C D'). As in Experiment 1, evidence of chunking (pauses at chunk boundaries) did not appear until the fourth item was added.

Can a group of items that a pigeon chunks on one list function as such on a second list? In Experiment 1, the ordinal position of the chunk was held constant across both lists. Following training on a list of colors and achromatic geometric forms (A B C D' E'), the integrity of the color chunk [A B C] was maintained on a list of five colors (A B C F G) even though the basis for establishing that chunk was eliminated. The integrity of the chunk [A B C] was also maintained on a new list of colors and forms (A B C D* E*). In Experiment 2, the ordinal position of a chunk established on [list. sub. 1 and 2], the integrity of the chunks [A B] and [C' D'] was maintained on lists X' A B Y' and X' C' D' Y', respectively. Conversely, the heterogeneous list X' B C' Y' took longer to learn than the original list.

Chunking by pigeons was demonstrated by comparing performance on different types of lists. Experiment 1 showed that Groups II and IV (who leanred lists in which colors and achromatic geometric forms were segregated: A B C D' E' and A B C D E', respectively) executed lists more rapidly than did Group I (who learned a homogeneous list of colors: A B C D E) OR Groups II and III (who learned lists consisting of unsegregated colors and forms: A B' C D' E and A B C' D E, respectively). Experiment 2 showed that Groups II and IV tolerated interruptions of the list better than did Groups I, III, and V. The accuracy of responding of Groups I, III, and V decreased as a function of the duration of the interruption and the point in the sequence at which it occurred. The performance of Group II was unaffected by interruptions; Group IV was minimally affected. These results indicate that Groups II and Iv organized their lists as ordered chunks.

Three groups of pigeons were trained to discriminate between a target temporal sequence, consisting of red, green, and blue, and distractor sequences consisting of all other combinations of these three colors presented on a pecking key. Response alternatives were provided during the course of the trial (on-line decisions) as well as at the end of the time (postsequence decisions). Different temporal phrasing patterns emphasized the first stimulus in the sequence, the final stimulus, or all stimuli equally. The phrasing pattern did not affect the speed of acquisition, but groups receiving emphasized stimuli relied more heavily on those stimuli than on the other stimuli for their discriminations. The patterns of both on-line choices during the sequence and terminal choices following the sequence were consistent with the use of hierarchical representations and inconsistent with a simple item-by-item prospective discrimination scheme.

In 5 experiments, pigeons learned to discriminate uniform arrays of colored elements from mixed arrays containing 2 colors in various proportions. When only the uniform arrays were positive, mixture-negative gradients were obtained. When only the arrays containing equal numbers of red and blue elements were positive, mixture-positive gradients were obtained. When elements of different colors from 2 independent mixture-negative discriminations were recombined, mixture-negative gradients were obtained with the transfer patterns. When elements from a mixture-negative discrimination were combined with a new set of positive elements, responding was not suppressed. Appropriate combinations of the elements in mixture discriminations reduce responding, but the elements themselves do not acquire inhibitory properties.

Four pigeons were trained on a 6-key modified symbolic matching-to-sample procedure. The stimuli were paired based on humanly defined stimulus classes, for example, faces, signs, playing cards, trees, birds, and butterflies. The second peck on the sample key lighted only one comparison key. Every two pecks on the sample lighted another comparison key, up to a maximum of five keys. Pecks on keys of symbolically matching pictures (e.g., if the sample was a face, the bird pecked the comparison key that presented another face) produced grain. Pecks on nonmatching keys (e.g., pictures of signs or playing cards) turned off all lights on the comparison keys and repeated the trial. The birds learned to peck each sample until the symbolically matching comparison stimulus appeared on one of the five comparison stimulus keys, and then to peck that key. Later, the birds showed symbolic matching transfer to novel samples, novel comparison stimuli, and some novel stimulus classes. The birds showed better transfer with visually similar (tree-tree) relative to visually dissimilar (cups-butterflies) stimulus pairs, and learned to avoid the sample stimulus when it appeared as an incorrect comparison stimulus choice.

Twenty pigeons, divided into four groups, were trained to discriminate picture categories of birds and mammals in a natural-category experiment. Two groups are trained with 5 items from each category and two groups were trained with 35 items from each category. For one 5-item group and one 35-item group, stimuli were assigned according to bird (S+ set) and mammal (S- set) categories and for the other groups (5 and 35 items) the same stimuli were pseudorandomly assigned to an arbitrary S+ set and S- set. Groups with fewer items acquired the discrimination faster than groups with more items irrespective of item assignment. Only the 35-item category group, upon transfer to novel items, showed any evidence of concept learning. The 5-item category group learned its expanded 35-item discrimination in about one half the number of sessions required by the 5-item pseudocategory group to learn its expanded 35-item discrimination. The 5-item category group acquired the 35-item discrimination in fewer sessions than the original 35-item category group, indicating a savings attributable to first learning with few items. The 5-item pseudocategory group showed no savings in acquiring the 35-item discrimination relative to the original 35-item pseudocategory group. A continuum model of concept learning is suggested in which categorization is controlled by rote memorization of a few items, by exemplar processing when more items are involved, and by prototype processing when even more items are used.

In Experiment 1, pigeons were autoshaped with two stimuli (a and b) in one context, X, and two further stimuli (c and d) in a second context, Y, that differed from the first in a number of visual characteristics. In the test phase subjects received presentations of stimuli a and d in context X and c and b in context Y. The response rate to the stimuli that were tested in the context in which they had been trained, a and c, was higher than that to the stimuli that were trained and tested in different contexts, b and d. This finding was replicated in Experiment 2 in which the contexts were differentiated by their odor. These results suggest that contextual cues can influence the retrieval of associative information.

The present study examined whether the dual-element effect occurs when temporal and visual stimuli appear simultaneously in a zero-delayed, symbolic matching-to-sample task. Two groups of pigeons were first exposed to either a red or green sample stimulus, for either 30 s or 5 s. The sample was followed by the presentation of yellow and blue comparisons. For pigeons in one group, the duration of the sample was the relevant cue. Responses to the yellow comparison were reinforced if the sample was 30 s, and responses to the blue comparison were reinforced if the sample was 5 s. For the other group, sample duration was irrelevant. Responses to the yellow comparison were reinforced if the sample had been green and responses to the blue comparison were reinforced if the sample had been red. Both groups then learned a second matching task in which the sample and comparison stimuli were vertical and horizontal lines. Finally, matching performance was examined when the lines appeared together with the temporal or color elements. The results showed that the line matching task was acquired more slowly for pigeons that were first trained to attend to duration. More importantly, matching was reduced when the temporal and line elements appeared simultaneously, and the effects were similar to those obtained when visual elements are combined.

Two experiments used a between-group proceduree with pigeons responding for signalled reinforcement on a variable interval schedule to examine behavioral contast as an explanation of previously demonstrated instrumental overshadowing effects obtained with pigeons using a within-subjects design. Experiment 1 obtained overshadowing of the reesponse when reinforcement was signalled relative to a control grouop that received unsignalled reinforceement. In Experiment 2, potentiation of responding by the signal was found when responding was prevented during the delay interval and was compared to a control group that received comparable conditions except reinforcement was unsignalled. These experiments demonstrate that the effects of signalling reinforcement during instrumental responding by pigeons are not due to behavioral contrast.

Whereas humans display base-rate neglect in a behavioral analogue of the base-rate problem, pigeons have been shown to behave optimally in a comparable task, appropriately weighting base-rate and case-cue information. Previous studies have shown that prior experience may interfere with optimal decisions for human subjects, a result consistent with the position that poor and illogical decisions often follow from the misapplication of learned rules. The present study shows that pigeons will also display base-rate neglect if given extensive pretraining with informative case cues. Two experiments with pigeons, in a matching-to-sample procedure designed to mimic the classic base-rate problem, show that pigeons display base-rate neglect after extensive pretraining with the matching task.

In a simultaneous discrimination involving a positive (S+) and a negative (S-) stimulus, positive value appears to transfer from the S+ to the S-. However, negative value does not appear to transfer from the S- to the S+. Instead, when sufficient experience with the contingencies associated with responding to the S- is provided, it appears that the presence of the S- enhances the value of the S+ (i.e., a contrast effect is found). The purpose of the present experiments was to further examine the influence of the S+ on the S- in a simultaneous discrimination (between subjects in Experiment 1 and within subjects in Experiment 2). In both experiments, we found that under typical training conditions, given little direct experience with the value of the S-, value transfers from the S+ to the S-. If sufficient experience with the value of the S- is provided, however, contrast between the S+ and the S- can be demonstrated. Thus, in a simultaneous discrimination, value transfer from the S+ to the S- depends on the animal's having responded relatively little to the S-.

Dorrance, Kaiser, and Zentall (1998) trained pigeons on two concurrent simultaneous discriminations (A + B-; C+D-), with interspersed single-stimulus trials in which responding to the two positive discriminanda was differentially reinforced (A+; C[+ or -]). In each of four separate experiments, the birds showed a preference for stimulus B over stimulus D. Dorrance et al. concluded that the results of these experiments were best described in terms of value transfer theory (VTT; Fersen, Wynne, Delius, & Staddon, 1991). They reject the possibility that the results of these experiments might have been caused by differential nonreinforced experience with the test stimuli (B and D) on the basis of correlational analyses within each experiment. However, differences between experiments in choice of B over D are well predicted by differences in their history of reinforcement. Previous findings that were thought to favor VTT are also entirely consistent with a simpler associative analysis.

We examine some implications of an attentional model designed to explain dimensional contrast. Pigeons were trained to discriminate rectangular forms under conditions that produced positive dimensional contrast. In two experiments, the spacing of training stimuli was manipulated in ways that should have changed the allocation of attention. Experiment 1 compared the effect of an increased spacing between negative and positive stimuli with the effect of a decreased overall range of positive stimuli. Both manipulations increased the quality of discrimination performance. Experiment 2 demonstrated that dimensional contrast continued to occur under conditions that minimized the difficulty of the task. Results from these experiments were consistent, with quantitative predictions of the attentional model concerning the interaction of discriminability factors and factors related to the allocation of attentional resources.

A study was conducted in which two birds (pigeons) were trained to respond on a fixed-ratio (FR) 100 baseline, and three birds were trained to respond on a FR 20 baseline. After stable baseline performance, a single FR 10 was interpolated into the FR 100 baselines while a single FR 200 (FR 250 for one bird) was interpolated into the FR 20 baselines. This was called the uncued condition. The same interpolations were replicated in another condition except that a different stimulus was experimentally associated with the interpolated ratio (cued condition). The results showed that the pause following the uncued interpolated ratio was generally the shortest in the session compared to the postreinforcement pauses for the remaining ratios. This effect occurred regardless of the relative size of the interpolated ratio, although it was absent when the interpolated ratio was cued. Moreover, pauses in those ratio components following the uncued interpolated ratio were slightly reduced, suggesting that the interpolation procedure also produced field effects. The results are discussed in terms of their theoretical relevance to variable-ratio schedule performance.