Rubén N. Muzio

Instrumental learning in toads (Bufo arenarum): reinforcer magnitude and the medial pallium.

Two experiments studied the effects of shifts in the amount of water reinforcement on the instrumental performance of toads (Bufo arenarum) using one trial per day. In the first experiment, a shift from a large to a small reward magnitude led to a gradual change in performance without evidence of negative contrast. A shift from large or small reward magnitude to extinction led to similar extinction rates and provided no indication of the magnitude of reinforcement extinction effect. In the second experiment, the gradual change in performance (with no contrast) after a shift from a large to a small reward magnitude was completely eliminated by ablation of the medial pallium. These results provide support for the hypothesis that the medial pallium in amphibians is homologous to the hippocampal formation in mammals and also indicate that species differences in these learning phenomena may be related to the differential development and differentiation of the hippocampal formation.

Administration of corticosterone after the first downshift trial enhances consummatory successive negative contrast

Rats given access to a 32% sucrose solution and then downshifted to a 4% solution exhibit less contact with the sipper tube than unshifted controls always given access to 4% solution. This phenomenon, called consummatory successive negative contrast, was facilitated in Experiment 1 by a post-trial injection of corticosterone (3 mg/kg) administered immediately after the first downshift trial. Experiment 2 demonstrated that this facilitatory effect of post-trial corticosterone does not occur when administered 3 hr after the first downshift trial. These results support the hypothesis that corticosterone strengthens an aversive emotional component elicited by the surprising downshift in reward magnitude during the initial downshift trial.

Effect of schedule and magnitude of reinforcement on instrumental learning in the toad, Bufo arenarum ☆

Abstract Extinction after training with continuous (CR) or 50% partial (PR) reinforcement, and with different magnitudes of reward, was studied in the amphibian Bufo arenarum , in a runway situation. In Experiment 1, a group of toads received massed-trial, CR training with access to water as the reward. Performance improved during acquisition, including an improvement on the first trial of each session. Extinction was rapid and there was evidence for spontaneous recovery of the running response. In Experiment 2, groups of toads received PR or CR training at a rate of one trial per day. PR impaired acquisition and resulted in poor responding during extinction, compared to CR. Experiment 3 factorially studied the effects of schedule (PR vs CR) and distribution of practice (15 s vs 300 s intertrial interval). Acquisition was impaired by PR training but had little effect on extinction performance. Different magnitudes of water reinforcement were used in Experiment 4 in a one-trial-per-day situation. Terminal acquisition performance was a monotonic function of reward magnitude, but there were no differences in extinction performance across groups. The results are discussed in relation to comparative and developmental data on the paradoxical effects of reward.

Learning under partial reinforcement in the toad (Bufo arenarum): effects of lesions in the medial pallium.

Two experiments studied the adjustment of toads (Bufo arenarum) to partial reinforcement in a runway. In Experiment 1, two groups received 24 daily trials of either continuous reinforcement (CR) or 50% partial reinforcement (PR). Training parameters that facilitate the PR extinction effect (greater resistance to extinction after PR than CR training) in rats were selected. PR impaired performance during acquisition but had no effect on performance during extinction relative to CR. In Experiment 2, four groups were trained in a factorial design involving CR and PR, and a lesion of the medial pallium and a sham operation. Performance during acquisition was again impaired by PR, but the medial pallium lesions had no effect. The lesion, however, increased resistance to extinction after both CR and PR training. The results are discussed in relation to comparative research on learning and to the hypothesized homology of the amphibian medial pallium and the mammalian hippocampal formation.

Effects of lesions in the medial pallium on instrumental learning in the toad (Bufo arenarum)

Lesions of the medial pallium in toads significantly retarded extinction of a water-reinforced instrumental response in a runway training situation, relative to sham-operated and intact controls. The lesion had no effect on acquisition or in the amount of water uptake during acquisition. The results suggest that the medial pallium of toads plays a role in response inhibition. This possibility is discussed in relation to the results of analogous experiments with hippocampectomized rats.

Common Toads (Bufo arenarum) Learn to Anticipate and Avoid Hypertonic Saline Solutions

Toads (Bufo arenarum) were exposed to pairings between immersion in a neutral saline solution (i.e., one that caused no significant variation in fluid balance), followed by immersion in a highly hypertonic saline solution (i.e., one that caused water loss). In Experiment 1, solutions were presented in a Pavlovian conditioning arrangement. A group receiving a single neutral-highly hypertonic pairing per day exhibited a greater conditioned increase in heart rate than groups receiving either the same solutions in an explicitly unpaired fashion, or just the neutral solution. Paired toads also showed a greater ability to compensate for water loss across trials than that of the explicitly unpaired group. Using the same reinforcers and a similar apparatus, Experiment 2 demonstrated that toads learn a one-way avoidance response motivated by immersion in the highly hypertonic solution. Cardiac and avoidance conditioning are elements of an adaptive system for confronting aversive situations involving loss of water balance.

Goal orientation by geometric and feature cues: spatial learning in the terrestrial toad Rhinella arenarum

Although of crucial importance in vertebrate evolution, amphibians are rarely considered in studies of comparative cognition. Using water as reward, we studied whether the terrestrial toad, Rhinella arenarum, is also capable of encoding geometric and feature information to navigate to a goal location. Experimental toads, partially dehydrated, were trained in either a white rectangular box (Geometry-only, Experiment 1) or in the same box with a removable colored panel (Geometry–Feature, Experiment 2) covering one wall. Four water containers were used, but only one (Geometry–Feature), or two in geometrically equivalent corners (Geometry-only), had water accessible to the trained animals. After learning to successfully locate the water reward, probe trials were carried out by changing the shape of the arena or the location of the feature cue. Probe tests revealed that, under the experimental conditions used, toads can use both geometry and feature to locate a goal location, but geometry is more potent as a navigational cue. The results generally agree with findings from other vertebrates and support the idea that at the behavioral-level geometric orientation is a conserved feature shared by all vertebrates.

Aftereffects of the surprising presentation and omission of appetitive reinforcers on key-pecking performance in pigeons

The reinforcement-omission effect (ROE), also known as frustration effect, refers to greater response strength immediately after nonreinforcement (N) than reinforcement (R). The ROE was traditionally interpreted as transient invigoration after N induced by primary frustration. Pigeons demonstrate similar ROEs whether outcomes are surprising (partial R) or expected (discrimination training) in runway (Experiment 1) and Skinner box situations (Experiments 2-3). Variations in the interval between N and the opportunity to respond indicate that the ROE results from an aftereffect of food consumption (Experiment 4). Increasing reinforcer magnitude increased the after-R effect, without modifying the after-N function (Experiment 5). These results are reviewed in the context of comparative research on spaced-trial successive negative contrast and related phenomena that have failed to appear in experiments involving nonmammalian vertebrates.

β-adrenergic control of the water permeability of the skin during rehydration in the toad Bufo arenarum

1. Toads dehydrated to 80% of their standard weight (% SW) were rehydrated during 3 hr in distilled water. 2. Water permeability of the skin was positively correlated with the degree of dehydration in the range 80-100% SW. 3. Systemic administration of the beta-adrenergic agonist isoproterenol (5 mg/kg) 90 min after rehydration started (animals fully hydrated) increased skin permeability to the values observed in 80% SW dehydrated animals. 4. The administration of the beta-adrenergic blocker propranolol (5 mg/kg) 15 min before rehydration started produced a long-lasting decrease in water permeability during the 3 hr of rehydration. 5. The results are consistent with the hypothesis of a beta-adrenergic control of the water permeability of the skin during rehydration.

Incentive or Habit Learning in Amphibians

Toads (Rhinella arenarum) received training with a novel incentive procedure involving access to solutions of different NaCl concentrations. In Experiment 1, instrumental behavior and weight variation data confirmed that such solutions yield incentive values ranging from appetitive (deionized water, DW, leading to weight gain), to neutral (300 mM slightly hypertonic solution, leading to no net weight gain or loss), and aversive (800 mM highly hypertonic solution leading to weight loss). In Experiment 2, a downshift from DW to a 300 mM solution or an upshift from a 300 mM solution to DW led to a gradual adjustment in instrumental behavior. In Experiment 3, extinction was similar after acquisition with access to only DW or with a random mixture of DW and 300 mM. In Experiment 4, a downshift from DW to 225, 212, or 200 mM solutions led again to gradual adjustments. These findings add to a growing body of comparative evidence suggesting that amphibians adjust to incentive shifts on the basis of habit formation and reorganization.