Generalisation decrement and not overshadowing by associative competition among pairs of landmarks in a navigation task with humans.

In three experiments, a virtual preparation for humans of the Morris water task (VMWT) was used. Experiment 1 established that four landmarks were of similar salience. Then, in Experiments 2 and 3, participants were trained to locate a hidden platform in the presence or either two or four of the previous landmarks. In Experiment 2, one pair of groups was trained with four visual landmarks spaced at equal intervals around the edge of the pool, while a second pair was trained with two landmarks only, either relatively close to or far from the hidden platform. After training, a reciprocal overshadowing effect was found: on a test without the platform with two landmarks only (either close to or far from the platform position), the participants trained with four landmarks spent less time in the platform quadrant than those trained with only two. Finally, Experiment 3 showed that at least participants trained with two landmarks relatively close to the platform and then tested with four also performed worse on test than those trained and tested with two close landmarks only. This result suggests that generalisation decrement, rather than associative competition, could provide a sufficient explanation for the overshadowing observed in Experiment 2 in the proximal groups. The present set of experiments extend, although only partially, the generalisation decrement results documented in rats to human participants.

A diet enriched with plant sterols prevents the memory impairment induced by cholesterol loss in senescence-accelerated mice.

Cholesterol reduction at the neuronal plasma membrane has been related to age-dependent cognitive decline. We have used senescent-accelerated mice strain 8 (SAMP8), an animal model for aging, to examine the association between cholesterol loss and cognitive impairment and to test strategies to revert this process. We show that the hippocampus of SAMP8 mice presents reduced cholesterol levels and enhanced amount of its degrading enzyme Cyp46A1 (Cyp46) already at 6 months of age. Cholesterol loss accounts for the impaired long-term potentiation in these mice. Plant sterol (PSE)-enriched diet prevents long-term potentiation impairment and cognitive deficits in SAMP8 mice without altering cholesterol levels. PSE diet also reduces the abnormally high amyloid peptide levels in SAMP8 mice brains and restores membrane compartmentalization of presenilin1, the catalytic component of the amyloidogenic γ-secretase. These results highlight the influence of cholesterol loss in age-related cognitive decline and provide with a noninvasive strategy to counteract it. Our results suggest that PSE overtake cholesterol functions in the brain contributing to reduce deleterious consequences of cholesterol loss during aging.

When is a cause the "same"? Incoherent generalization across contexts.

A theory or model of cause such as Cheng's power (p) allows people to predict the effectiveness of a cause in a different causal context from the one in which they observed its actions. Liljeholm and Cheng demonstrated that people could detect differences in the effectiveness of the cause when causal power varied across contexts of different outcome base rates, but that they did not detect similar changes when only the cause-outcome contingency, ∆p, but not power, varied. However, their procedure allowed participants to simplify the causal scenarios and consider only a subsample of observations with a base rate of zero. This confounds p, ∆p, and the probability of an outcome (O) given a cause (C), P(O|C). Furthermore, the contingencies that they used confounded p and P(O|C) in the overall sample. Following the work of Liljeholm and Cheng, we examined whether causal induction in a wider range of situations follows the principles suggested by Cheng. Experiments 1a and 1b compared the procedure used by Liljeholm and Cheng with one that did not allow the sample of observations to be simplified. Experiments 2a and 2b compared the same two procedures using contingencies that controlled for P(O|C). The results indicated that, if the possibility of converting all contexts to a zero base rate situation was avoided, people were sensitive to changes in P(O|C), p, and ∆p when each of these was varied. This is inconsistent with Liljeholm and Cheng's conclusion that people detect only changes in p. These results question the idea that people naturally extract the metric or model of cause from their observation of stochastic events and then, reasonably exclusively, use this theory of a causal mechanism, or for that matter any simple normative theory, to generalize their experience to alternative contexts.

Blocking in rats, humans and snails using a within-subjects design.

The present research investigated the blocking effect in three different species, rats, humans and snails in formally equivalent tasks using a within-subjects design. Experiment 1 demonstrated the blocking effect in a context-flavour aversive conditioning preparation in rats: Animals failed to associate a flavour with an illness episode when it was presented in a context in which the illness was already predicted by other cues. Experiment 2 replicated this blocking effect in humans assessing their ability to learn a goal location in a virtual environment: Participants failed to learn the location of the goal in reference to a spatial cue presented alongside other pre-trained spatial cues that already indicated its location. Finally, in Experiment 3, snails failed to associate an odour with the presentation of food in the presence of other odours that already reliably predicted its presentation. The present study offers a start point for systematic comparisons between vertebrate and invertebrate species in formally equivalent tasks that produce univocal demonstrations of the blocking effect.

Distractor effects upon habituation of complex stimuli.

In two experiments, rats were given serial forward (the target followed by the distractor) or backward (the distractor followed by the target) exposure to two compound flavor stimuli that could be either similar (Salt-X/AX) or dissimilar (Salt-X/AY, Experiment 1; Salt/AX, Experiment 2). Following pre-exposure, the Salt element was presented in a compound with a novel flavor, N. The salience or effectiveness of the Salt element was then assessed by presenting the new flavor, N, under a state of salt appetite. Experiments 1 and 2 revealed that the order of presentation modulated the habituation of the Salt element only when the distractor was similar to the target: the Salt element was more salient after forward than backward pre-exposure. In the groups Dissimilar the order of pre-exposure was irrelevant; however, when the Salt element was presented in compound with a second element (Salt-X, Experiment 1), its salience was preserved, whereas when it was presented alone (Salt, Experiment 2) its salience was significantly reduced. These results, which are discussed in terms of Wagner (1981) theory of habituation, inform about the way in which stimuli presented closely in time are processed.

Salience modulation in serial preexposure: implications for perceptual learning.

In three experiments rats were given serial preexposure to two flavor stimuli. In Experiment 1, some animals were given exposure to AX followed by the presentation of BX, a forward schedule; the others were given backward preexposure (BX→AX). Conditioning and test trials with the A element showed that salience or effectiveness of A was better protected in the forward than in the backward condition. Experiments 2 and 3 assessed the relevance of this salience modulation mechanism for perceptual learning. In these experiments, generalization of a conditioned aversion from AX to BX was reduced in the forward (but not in the backward) condition only after prolonged exposure, indicating that the establishment of an inhibitory link from B to A is required for successful discrimination. However, generalization to a novel compound stimulus, NX, was reduced in the forward group both after short and long preexposure, suggesting the existence of salience modulation processes that work in parallel with associative inhibition. These results seem to support the existence of a salience modulation mechanism that seems to be beyond the scope of current theories of perceptual learning.

Salience modulation and associative inhibition interaction: short but not long exposure to similar stimuli protects the salience of the unique elements.

In three experiments, rats were given preexposure to two similar flavour compounds, AX and BX. Following preexposure, conditioning trials took place in which AX was paired with an illness-induced unconditioned stimulus. Animals that were given short alternated preexposure to AX and BX, showed higher generalization of conditioned aversion to AX to a new compound, AN, than animals that were given blocked preexposure (short and long) and long alternated preexposure (Experiments 1 and 2); and showed less preference for A when they were given a choice between A and X (Experiment 3). These results have been taken to indicate that the salience of the A element is well preserved after short alternated preexposure, but declines when preexposure goes on for some more trials. The results reported support the notion that perceptual learning is a multi-determined phenomenon that depends on salience modulation processes after relatively short preexposure, and on an associative inhibition mechanism after prolonged preexposure.

Choosing optimal causal backgrounds for causal discovery.

In two experiments, we studied the strategies that people use to discover causal relationships. According to inferential approaches to causal discovery, if people attempt to discover the power of a cause, then they should naturally select the most informative and unambiguous context. For generative causes this would be a context with a low base rate of effects generated by other causes and for preventive causes a context with a high base rate. In the following experiments, we used probabilistic and/or deterministic target causes and contexts. In each experiment, participants observed several contexts in which the effect occurred with different probabilities. After this training, the participants were presented with different target causes whose causal status was unknown. In order to discover the influence of each cause, participants were allowed, on each trial, to choose the context in which the cause would be tested. As expected by inferential theories, the participants preferred to test generative causes in low base rate contexts and preventative causes in high base rate contexts. The participants, however, persisted in choosing the less informative contexts on a substantial minority of trials long after they had discovered the power of the cause. We discuss the matching law from operant conditioning as an alternative explanation of the findings.

Is contextual-potentiated eating dependent on caloric density of food?

Se puso a prueba en un experimento con ratas si un contexto específico podría provocar la conducta de comer como resultado del condicionamiento Pavloviano, y si este efecto dependía de la densidad calórica del alimento. Treinta y dos ratas privadas de comida experimentaron dos contextos. Los animales tenían acceso al alimento en el contexto A, pero nunca encontraron alimento en el contexto B. La mitad delos animales recibió un alimento de alta densidad calórica (grupos HD) mientras que la otra mitad recibió un alimento de baja densidad calórica (grupos LD) durante las sesiones de condicionamiento. Posteriormente, la mitad de los animales en cada tipo de alimento se puso a prueba en el contexto A y la otra mitad en el contexto B. Los resultados mostraron un efecto de condicionamiento contextual solamente en los grupos HD. Estos resultados sugieren la importancia del condicionamiento del contexto y de la densidad calórica del alimento en la conducta de comer. Se discuten las implicaciones que estos hallazgos pueden tener en la etiología de la conducta de atracón (AU)

One experiment tested whether a specific context could elicit eating in rats as a result of Pavlovian conditioning and whether this effect depended on the caloric density of food. Thirty two deprived rats experienced two contexts. They had access to food in context A, but no food was available in context B. During conditioning, half of the animals received high density caloric food (HD groups) whereas the other half, low density caloric food (LD groups). Then, half of the rats in each type of food group was tested in context A and the other half in context B. The results demonstrated an effect of context conditioning only in HD groups. These findings suggest the relevance of both contextual conditioning and caloric density of food I neating behaviour. Implications for the a etiology of binge eating will be discussed (AU)

Is contextual-potentiated eating dependent on caloric density of food?

One experiment tested whether a specific context could elicit eating in rats as a result of Pavlovian conditioning and whether this effect depended on the caloric density of food. Thirty two deprived rats experienced two contexts. They had access to food in context A, but no food was available in context B. During conditioning, half of the animals received high density caloric food (HD groups) whereas the other half, low density caloric food(LD groups). Then, half of the rats in each type of food group was tested in context A and the other half in context B. The results demonstrated an effect of context conditioning only in HD groups. These findings suggest the relevance of both contextual conditioning and caloric density of food in eating behaviour. Implications for the aetiology of binge eating will be discussed (AU)

Se puso a prueba en un experimento con ratas si un contexto específico podría provocar la conducta de comer como resultado del condicionamiento Pavloviano, y si este efecto dependía de la densidad calórica del alimento. Treinta y dos ratas privadas de comida experimentaron dos contextos. Los animales tenían acceso al alimento en el contexto A, pero nunca encontraron alimento en el contexto B. La mitad delos animales recibió un alimento de alta densidad calórica (grupos HD) mientras que la otra mitad recibió un alimento de baja densidad calórica (grupos LD) durante las sesiones de condicionamiento. Posteriormente, la mitad de los animales en cada tipo de alimento se puso a prueba en el contexto A y la otra mitad en el contexto B. Los resultados mostraron un efecto de condicionamiento contextual solamente en los grupos HD. Estos resultados sugieren la importancia del condicionamiento del contexto y de la densidad calórica del alimento en la conducta de comer. Se discuten las implicaciones que estos hallazgos pueden tener en la etiología de la conducta de atracón (AU)

Partial reinforcement effects on learning and extinction of place preferences in the water maze.

In two experiments, two groups of rats were trained in a navigation task according to either a continuous or a partial schedule of reinforcement. In Experiment 1, animals that were given continuous reinforcement extinguished the spatial response of approaching the goal location more readily than animals given partial reinforcement-a partial reinforcement extinction effect. In Experiment 2, after partially or continuously reinforced training, animals were trained in a new task that made use of the same reinforcer according to a continuous reinforcement schedule. Animals initially given partial reinforcement performed better in the novel task than did rats initially given continuous reinforcement. These results replicate, in the spatial domain, well-known partial reinforcement phenomena typically observed in the context of Pavlovian and instrumental conditioning, suggesting that similar principles govern spatial and associative learning. The results reported support the notion that salience modulation processes play a key role in determining partial reinforcement effects.

Overshadowing and potentiation of illness-based context conditioning

En cinco experimentos, se investigó el condicionamientocompuesto sabor-contexto empleando ratas como sujetos. Los animalesfueron expuestos a un contexto novedoso en el que tenían acceso a unasolución de ácido cítrico ó de sacarina antes de ser inyectados con LiCl. Acontinuación medimos la aversión condicionada al contexto empleando unprocedimiento de bloqueo. Cuando el sabor que acompañaba al contextodurante el condicionamiento era relativamente aversivo (ácido cítrico)observamos un nivel relativamente bajo de condicionamiento contextual; deacuerdo con nuestra interpretación, el sabor ácido ensombreció al contexto.Por el contrario, cuando el sabor era agradable (solución de sacarina) seobservó una potenciación del condicionamiento contextual. Nuestradiscusión de los resultados toma en consideración las propiedadesmotivacionales del sabor que acompaña a contexto en el momento delcondicionamiento(AU)

In five experiments using rats, we investigated compound context-flavorconditioning. The subjects were allowed to spend time in the target context,where they had access to a flavored solution (either citric acid or saccharine)before receiving an injection of LiCl. Context aversion was then assessed byusing a blocking procedure. When the flavor accompanying the context wasa non-palatable one, citric acid, impaired learning about the context wasobserved, an instance of overshadowing. However, when we presentedsaccharine in the novel environment enhanced learning about the contextwas found, an instance of context potentiation. The role of the motivationalproperties of the flavor that accompanies the target context duringconditioning is discussed(AU)

Preexposure effects in the spatial domain: Dissociation between latent inhibition and perceptual learning.

In 4 experiments, rats were given intermixed or blocked preexposure to an array of landmarks that subsequently defined the location of a hidden goal in a Morris pool task. Previous research has shown that intermixed preexposure to pairs of adjacent landmarks retards learning whereas preexposure to individual landmarks facilitates subsequent learning (J. Prados, V. D. Chamizo, & N. J. Mackintosh, 1999). Accordingly, in Experiment 1, intermixed and blocked preexposure to pairs of adjacent landmarks was found to retard learning. In Experiment 2, however, a scheduling effect was found: Rats given intermixed preexposure to the individual landmarks learned faster than rats given blocked or no preexposure. Experiment 3 showed that intermixed (but not blocked) preexposure to pairs of landmarks resulted in a facilitatory effect when preexposure and test were carried out in different contexts. Experiment 4 replicated within a single experiment the main results observed in Experiments 1 and 3. This pattern of results suggests that intermixed preexposure engages learning processes other than latent inhibition that facilitate subsequent learning of the navigation task.

Enhanced discrimination between flavor stimuli: roles of salience modulation and inhibition.

Rats were given intermixed preexposure to the compound flavors AX and BX and to the compound CX in a separate block of trials (4 presentations of each compound). In Experiment 1, rats showed less generalization of conditioned aversion from AX to BX than from CX to BX, a perceptual learning effect. Experiment 2 showed that the formation of an excitatory association proceeded more readily between A and B than between C and B, suggesting that intermixed preexposure maintains the effective salience of A and B and does not establish inhibition between them, a process that would require prolonged preexposure. According to this analysis, salience modulation and associative inhibition may contribute to perceptual learning at different stages of preexposure.

The Espinet and the perceptual learning effects in flavour aversion conditioning: do they depend on a common inhibitory mechanism?

In three experiments rats were given short or long preexposure (4 or 10 sessions) to two compound flavours, AX and BX, according to an intermixed or a blocked schedule. Following preexposure, aversion conditioning trials were given with AX as the conditioned stimulus (CS). In Experiments 1 and 2, retardation and summation tests were then carried out to assess the inhibitory properties of B (an Espinet procedure). In Experiment 3, test trials evaluated generalization from AX to BX (the standard perceptual learning procedure). The results showed that B performed as an inhibitor of the unconditioned stimulus (US; an Espinet effect) only after long intermixed preexposure, whereas a reliable perceptual learning effect was observed both after short and after long preexposure. The observation that B had no detectable inhibitory properties after short preexposure casts doubt on the suggestion that inhibitory learning is responsible for perceptual learning after brief exposure to AX and BX.

Modulation of the effective salience of a stimulus by direct and associative activation of its representation.

In 2 experiments, rats received exposure to presentations of a footshock preceded by a given cue. In the PRf (partial reinforcement) condition, this cue also occurred in the absence of the shock; in the CRf (continuous reinforcement) condition, it did not. Subsequent testing in which a new stimulus was used to signal the shock (Experiment 1) showed that the shock was more effective as a reinforcer for the PRf than for the CRf group. In Experiment 2, the shock was used as a conditioned stimulus signaling food delivery, and it was found that conditioning occurred more readily in the PRf than in the CRf group. These results accord with the hypothesis that preexposure to the shock results in a decline in its effective salience but that experience of a cue that signals shock in the absence of the shock itself attenuates this effect and helps maintain stimulus salience.

Recovery effects after extinction in the Morris swimming pool navigation task.

In three experiments in which rats were used as subjects, we developed an extinction procedure using a Morris pool. The animals were trained to find a hidden platform located at a fixed position and were then given extinction trials in which the platform was removed from the pool. When training and extinction were carried out in the same context and time was allowed to elapse between extinction and test, spontaneous recovery of learning was observed. On the other hand, those rats that received extinction in a context different from the one used for training failed to show spontaneous recovery of learning when tested in the extinction context after an interval of 96 h. However, they did show renewal of spatial learning when tested in the training context. These results show that extinction in the spatial domain behaves like extinction in standard conditioning preparations.

Ensombrecimiento entre puntos de referencia en una tarea de navegación / Overshadowing between landmarks in a navigation task

O'Keefe y Nadel (1978) consideran que un mapa cognitivo es una representación del entorno en la que se incluye toda la información disponible respecto a los puntos de referencia y sus relaciones espaciales. Una vez formado el mapa cognitivo, los animales pueden utilizar cualquier subconjunto de puntos de referencia para localizar una meta. El experimento que se presenta pretende analizar si efectivamente un mapa cognitivo incluye toda la información del medio o sólo la información que es más relevante para localizar una meta invisible. Para este objetivo se entrenó a dos grupos de ratas a localizar una plataforma invisible ubicada en una piscina de Morris. Uno de los grupos disponía de cuatro puntos de referencia (A, B, C, D) mientras que el segundo grupo aprendió la tarea en presencia de dos puntos de referencia (C, D). Después del entrenamiento, ambos grupos se sometieron a una prueba en la que se les permitía nadar en la piscina con sólo dos puntos de referencia (C, D) y sin la plataforma. Los resultados mostraron que el grupo que siempre había tenido dos claves presentes permaneció más tiempo en el área de la plataforma que los animales que habían sido entrenados con cuatro puntos de referencia y se pusieron a prueba con dos. Estos resultados se interpretan como un indicio de ensombrecimiento espacial entre claves ubicadas alrededor de la piscina, y se discuten a la luz de las teorías asociativas del aprendizaje (AU)

Transferencia de un aprendizaje relacional en una tarea de navegación / Relational learning transference in a navigation task

Se entrenó a tres grupos de ratas en la tarea de nadar hacia una plataforma invisible en una piscina circular. La plataforma estaba situada en una posición fija respecto a unos puntos de referencia que rodeaban la piscina. El grupo No Cambio recibió preexposición activa (Prados, Redhead & Pearce, 1999) a los mismos puntos de referencia (ABCD) que definían la posición de la plataforma durante el entrenamiento. El grupo Cambio fue preexpuesto a un conjunto de puntos de referencia (LMNO) diferente al que definía la posición de la meta en el entrenamiento (ABCD). Por último, el grupo Control fue preexpuesto al aparato y entrenado en presencia de ABCD. Los grupos preexpuestos en presencia de las claves mostraron una facilitación en el aprendizaje de la tarea espacial durante el entrenamiento respecto al grupo Control. La actuación del grupo Cambio puede interpretarse como evidencia de que el aprendizaje de la relevancia de la relación espacial entre los diferentes puntos de referencia presentes durante la preexposición se transfirió al nuevo conjunto de claves que se presentó en el entrenamiento. Estos resultados sugieren que los procesos atencionales juegan un importante papel en los fenómenos de preexposición en el dominio espacial (AU)