Enrique T. Segura

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.

Reward downshift in the toad

The effect of reward downshift on runway performance using water as reward is studied in toads. Toads do not show negative contrast effect but a decline in performance to the same level as the small reward unshifted controls when downshifted after 50 and 80 preshift trials. These results suggest that the role of reward in toads differs from that in rats.

GEOGRAPHIC, ECOLOGICAL AND SUBSPECIFIC VARIATION IN THE SONG OF THE RUFOUS-BROWED PEPPERSHRIKE (CYCLARHIS GUJANENSIS)'

We describe the patterns of song variation in the Rufous-browed Peppershrike (Cyclarhis gujanensis). Individual variation was estimated using a sample of 659 songs belonging to 2 1 individuals. This analysis showed that this species uses a repertoire of up to seven song types. Geographic, ecological, and subspecific patterns of variation were estimated using four songs from each of 93 birds recorded from Mexico to Argentina. On each sonogram we measured 10 temporal, frequency, and structural features of the song. Principal component analysis based on the correlation matrix of these data showed that subspecies sing similar songs. However, there were significant relationships between principal component values and latitude, indicating that songs from equatorial areas are shorter in duration, have higher maximum frequency and number of syllables, broader bandwidth and are less repetitive than those from more temperate areas. In addition, we classified the recording sites into three categories according to the actual vegetation: open, mixed, and closed. A multivariate analysis of variance after removing the effect of latitude and altitude, showed that habitat types differ in song structure. Songs from open and mixed areas have a narrow bandwidth, and a lower maximum, minimum and emphasized frequencies than those from closed sites. We analyzed these patterns according to the current ideas about sound propagation in natural environments and the geographical pattern of body size vari- ation in the Rufous-browed Peppershrike.

The tale of the screaming hairy armadillo, the guinea pig and the marginal value theorem

Abstract Foraging by screaming hairy armadillos, Chaetophractus vellerosus, and guinea pigs, Cavia porcellus, was studied in the laboratory. The main question was whether patch exploitation varies with overall capture rate as predicted by the marginal value theorem (MVT). Armadillos in experiment I and guinea pigs in experiment II experienced a single travel time between depleting patches of two kinds: good and poor. There were two treatments, which differed in the quality of poor patches. MVT predicts that within a treatment, more prey should be taken from good than from poor patches and between treatments, good patches should be exploited in inverse relation to the quality of poor patches and poor patches should be exploited in direct relation to their own quality. In experiment III, guinea pigs experienced three treatments which differed in the travel requirement, while the two patch types remained the same. MVT predicts that within a treatment more prey should be taken from good than from poor patches and that between treatments more prey should be taken from both patch types as travel requirement increases. The qualitative predictions were supported in the three experiments. The quantitative fit was good but there was a bias towards more severe patch exploitation than predicted. The results indicate that in these species patch exploitation depends on overall food availability as predicted by the MVT when overall food availability differs either because of patch type composition or because of differences in travel requirement between patches.

Neural set point for the control of arterial pressure: role of the nucleus tractus solitarius

BackgroundPhysiological experiments have shown that the mean arterial blood pressure (MAP) can not be regulated after chemo and cardiopulmonary receptor denervation. Neuro-physiological information suggests that the nucleus tractus solitarius (NTS) is the only structure that receives information from its rostral neural nuclei and from the cardiovascular receptors and projects to nuclei that regulate the circulatory variables.MethodsFrom a control theory perspective, to answer if the cardiovascular regulation has a set point, we should find out whether in the cardiovascular control there is something equivalent to a comparator evaluating the error signal (between the rostral projections to the NTS and the feedback inputs). The NTS would function as a comparator if: a) its lesion suppresses cardiovascular regulation; b) the negative feedback loop still responds normally to perturbations (such as mechanical or electrical) after cutting the rostral afferent fibers to the NTS; c) perturbation of rostral neural structures (RNS) to the NTS modifies the set point without changing the dynamics of the elicited response; and d) cardiovascular responses to perturbations on neural structures within the negative feedback loop compensate for much faster than perturbations on the NTS rostral structures.ResultsFrom the control theory framework, experimental evidence found currently in the literature plus experimental results from our group was put together showing that the above-mentioned conditions (to show that the NTS functions as a comparator) are satisfied.ConclusionsPhysiological experiments suggest that long-term blood pressure is regulated by the nervous system. The NTS functions as a comparator (evaluating the error signal) between its RNS and the cardiovascular receptor afferents and projects to nuclei that regulate the circulatory variables. The mean arterial pressure (MAP) is regulated by the feedback of chemo and cardiopulmonary receptors and the baroreflex would stabilize the short term pressure value to the prevailing carotid MAP. The discharge rates of rostral neural projections to the NTS would function as the set point of the closed and open loops of cardiovascular control. No doubt, then, the RNS play a functional role not only under steady-state conditions, but also in different behaviors and pathologies.

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.

Influence of Age on Stress Responses to Metabolic Cage Housing in Rats

Abstract1.We studied the effect of isolation stress in 3- and 12-month-old rats individually housed in metabolic cages for 7 days. Urine (24 hr) was collected daily from one group of animals of each age. The other group was tested in an open field and on a hot plate on days 1 and 7.2.Total deambulation in the open-field test was lower in young than in older rats both on day 1 (54.7 ± 9.9 vs 80 ± 8.9 crossings/session; P < 0.04) and on day 7 (21 ± 9 vs 48 ± 7 crossings per session; P < 0.04) and decreased significantly in the two groups when tested on day 7 (P < 0.03). Latency to paw-licking in the hot-plate test was longer in young than in older animals on day 1 (14 ± 2 vs 8 ± 4 sec; P < 0.05) but was similar in the two groups on day 7.3.Urinary excretions of norepinephrine (NE) and epinephrine (E) were determined by HPLC with electrochemical detection. Urinary NE in day 1 was similar in young and older animals (2627 ± 828 vs 3069 ± 598 ng/24 hr). In young animals NE excretion decreased along the study and was significantly (P < 0.02) lower than on day 1 during the last 3 days of the study. Conversely, in older animals urinary excretion of NE remained similar throughout the study. On day 7 urinary excretion of NE in older animals was about two fold that in young rats. Urinary E was similar in young and older rats (341 ± 127 vs 532 ± 256 ng/24 hr) on day 1 and showed a tendency to increase throughout the study.4.Urinary monoamine oxidase inhibitory (IMAO) activity was determined by testing the ability of urine extracts to inhibit rat liver MAO activity in vitro and was higher in young than in older animals throughout the study (day 1, 54.8 ± 4.2 vs 25.1 ± 5.1%; P < 0.02). In young rats excretion of IMAO was significantly higher during the last 3 days of the study than on day 1 (P < 0.05). In older animals urinary IMAO showed a tendency to increase at the end of the study.5.Isolation stress caused by housing rats in metabolic cages results in different behavioral and metabolic responses in young and older animals. Young animals exhibit a lower locomotor and analgesic response and excrete lower amounts of NE and higher IMAO activity in the urine than older rats. The metabolic and behavioral responses to isolation stress are highly dependent on the age of the animals tested. These results should be taken into consideration when designing experiments requiring the use of metabolic cages.

Seasonal variations of plasma adrenaline and noradrenaline in toads

Abstract Adrenaline and noradrenaline concentrations in plasma of toads were measured by use of a fluorimetric method. Seasonal variations in the plasma concentration of these catecholamines were verified. Adrenaline decreased markedly during the mating season. Towards the end of summer a progressive increase took place, reaching high values during autumn and winter. Maximal concentrations were found to coincide with the premating period. Noradrenaline decreased during autumn. No changes were observed in the other seasons. The possible correlation of these results with the seasonal variation of adrenal catecholamines in the toad is discussed. The mean annual concentration of adrenaline in plasma was 0.59 ≠ 0.04 μg per 100 ml, and that of noradrenaline was 0.67 ≠ 0.05 μg per 100 ml.