Kosuke Sawa

Causal Reasoning in Rats

Empirical research with nonhuman primates appears to support the view that causal reasoning is a key cognitive faculty that divides humans from animals. The claim is that animals approximate causal learning using associative processes. The present results cast doubt on that conclusion. Rats made causal inferences in a basic task that taps into core features of causal reasoning without requiring complex physical knowledge. They derived predictions of the outcomes of interventions after passive observational learning of different kinds of causal models. These competencies cannot be explained by current associative theories but are consistent with causal Bayes net theories.

Comparison of C-fos-like immunoreactivity in the brainstem following intraoral and intragastric infusions of chemical solutions in rats

To examine whether the activation of brainstem neurons during ingestion is due to orosensory afferents or post-ingestive factors, neuronal activation in response to intraoral and intragastric infusions of taste stimuli was compared in the area postrema (AP), nucleus tractus solitarius (NTS) and parabrachial nucleus (PBN) by the c-fos immunohistochemical method. An aliquot (7.5 ml) of 0.5 M sucrose, 5 mM sodium saccharin, 1 mM quinine hydrochloride and distilled water was delivered into the oral cavity or the stomach in each rat, which had been deprived of water and food overnight. Water induced little c-Fos-like immunoreactivity (c-FLI), but both intraoral and intragastric infusions of sucrose, but not non-caloric saccharin, induced strong c-FLI in the AP, caudal NTS and the external lateral subnucleus of the rostral PBN, suggesting that these areas receive general visceral inputs. Other areas in the NTS and PBN may receive gustatory inputs since more dominant c-FLI was detected by intraoral rather than intragastric infusions of the stimuli. Functional segregation of neurons reflecting qualitative and hedonic aspects of sweeteners (sucrose and saccharin) and bitter-tasting substance (quinine) was suggested in the PBN, but less evident in the NTS. These results indicate that c-fos induction in brainstem neurons during ingestion reflects gustatory inputs and postingestional factors depending on the kind of food ingested.

c-Fos-like immunoreactivity in the brainstem following gastric loads of various chemical solutions in rats.

The distribution of c-Fos-like immunoreactivity (c-FLI) in the lower brainstem especially in the area postrema (AP), nucleus of the tractus solitarius (NTS) and parabrachial nucleus (PBN) was examined following gastric loads of various chemical solutions in rats. An aliquot of 7.5 ml of each stimulus was intragastrically infused, and c-FLI was detected. The most remarkable c-FLI was induced by LiCl, lactose and ethanol which are known to be effective unconditioned stimuli in conditioned taste aversions. Polycose and disaccharides such as sucrose and maltose induced more c-FLI than monosaccharides such as glucose, fructose and galactose. Relatively low levels of c-FLI were observed for other sweeteners such as saccharin, glycine and alanine, and other basic taste stimuli such as NaCl, HCl, quinine and umami substances. Each stimulus induced a similar proportion of c-FLI among the subnuclei of the NTS, but not in the PBN, where chemicals effective in inducing conditioned taste aversions elicited stronger c-FLI in the external lateral subnucleus, and those in inducing conditioned taste preferences such as Polycose and glucose elicited stronger c-FLI in the dorsal lateral subnucleus. Vagotomy reduced c-FLI to about 50% for LiCl stimulation and to about 30% for sucrose stimulation, suggesting that LiCl has a larger proportion of extravagal inputs than sucrose.

Reward prediction based on stimulus categorization in primate lateral prefrontal cortex

To adapt to changeable or unfamiliar environments, it is important that animals develop strategies for goal-directed behaviors that meet the new challenges. We used a sequential paired-association task with asymmetric reward schedule to investigate how prefrontal neurons integrate multiple already-acquired associations to predict reward. Two types of reward-related neurons were observed in the lateral prefrontal cortex: one type predicted reward independent of physical properties of visual stimuli and the other encoded the reward value specific to a category of stimuli defined by the task requirements. Neurons of the latter type were able to predict reward on the basis of stimuli that had not yet been associated with reward, provided that another stimulus from the same category was paired with reward. The results suggest that prefrontal neurons can represent reward information on the basis of category and propagate this information to category members that have not been linked directly with any experience of reward.

Temporal integration in Pavlovian appetitive conditioning in rats

We used an appetitive sensory preconditioning procedure to investigate temporal integration in rats in two experiments. In Phase 1, rats were presented with simultaneous compound trials on which a 10-sec conditioned stimulus (CS) X was embedded within a 60-sec CS A. In Group Early, CS X occurred during the early portion of CS A, whereas in Group Late, CS X occurred during the latter portion of CS A. In Phase 2, CS X was paired simultaneously with sucrose. On a subsequent test with CS A, the rate of magazine entries peaked during the early portions of the stimulus in Group Early and during the latter portions of the stimulus in Group Late (Experiments 1 and 2). Similar response peaks were not observed on tests with a control stimulus that had been presented in compound with a stimulus that did not signal reward (Experiment 2).

Reward Inference by Primate Prefrontal and Striatal Neurons

The brain contains multiple yet distinct systems involved in reward prediction. To understand the nature of these processes, we recorded single-unit activity from the lateral prefrontal cortex (LPFC) and the striatum in monkeys performing a reward inference task using an asymmetric reward schedule. We found that neurons both in the LPFC and in the striatum predicted reward values for stimuli that had been previously well experienced with set reward quantities in the asymmetric reward task. Importantly, these LPFC neurons could predict the reward value of a stimulus using transitive inference even when the monkeys had not yet learned the stimulus–reward association directly; whereas these striatal neurons did not show such an ability. Nevertheless, because there were two set amounts of reward (large and small), the selected striatal neurons were able to exclusively infer the reward value (e.g., large) of one novel stimulus from a pair after directly experiencing the alternative stimulus with the other reward value (e.g., small). Our results suggest that although neurons that predict reward value for old stimuli in the LPFC could also do so for new stimuli via transitive inference, those in the striatum could only predict reward for new stimuli via exclusive inference. Moreover, the striatum showed more complex functions than was surmised previously for model-free learning.

The diurnal variation of performance of the novel location recognition task in male rats.

Circadian changes of performance have been reported in various kinds of learning task. However, the diurnal variation of performance in hippocampus-dependent learning tasks remains unclear. In the present study, rats were subjected to the novel location recognition (NLR) task as well as the novel object recognition (NOR) task to examine whether the circadian pattern of hippocampus-dependent task performance was similar to that in tasks in which brain regions other than the hippocampus contribute. The performance in the NOR task was relatively constant irrespective of the time of day, while the performance in the NLR task was higher at night than during the daytime. When the pineal hormone melatonin was injected into rats before the training phase in order to examine its effects on the pattern of circadian changes of NLR performance, rats showed improvement of performance in the daytime, but impairment at night. These results suggest that the pattern of circadian variation of memory performance depends on the type of task, and that the effects of exogenous melatonin on learning performance vary with the time of day.

Factors that influence negative summation in a spatial-search task with pigeons

A variant of the standard conditioned inhibition procedure was used to evaluate landmark-based spatial search in a touchscreen preparation. Pigeons were given compound trials with one landmark (A) positioned in a consistent spatial relationship to a hidden goal and another landmark (B) positioned randomly with respect to A and the hidden goal (AB+). On half of the non-reinforced inhibitory trials, A was paired with landmark X (AX-) and on the remaining trials B was paired with Y (BY-). All subjects were also given reinforced trials with a transfer excitor (T+). During conditioned inhibition training, subjects showed no change in overall responding during AX- trials but did show a decrease in the number of pecks to the goal location signaled by A. During non-reinforced summation tests with landmark T, X had a greater suppressive effect than did Y on overall responding but the percentage of pecks at the goal did not differ unless X was positioned near the expected goal signaled by T. These data demonstrate that the effectiveness of a stimulus trained as an inhibitor is dependent on the strength of the association between its training excitor (A) and the US, as well as, the spatial arrangement of stimuli during testing.

Context and the renewal of conditioned taste aversion: the role of rat dorsal hippocampus examined by electrolytic lesion

An extinguished conditioned response can sometimes be restored. Previous research has shown that this renewal effect depends on the context in which conditioning versus extinction takes place. Here we provide evidence that the dorsal hippocampus is critically involved in the representation of context that underscores the renewal effect. We performed electrolytic lesions in dorsal hippocampus, before or after extinction, in a conditioned taste aversion paradigm with rats. Rats that underwent all conditioning, extinction and testing procedures in the same experimental context showed no renewal during testing in the original context. In contrast, rats that underwent extinction procedures in a different experimental context than the one in which they had acquired the conditioned response, showed a reliable renewal effect during testing in the original context. When electrolytic lesion was performed prior to extinction, the context-dependent renewal effect was disrupted. When electrolytic lesion was undertaken after extinction, we observed a complex pattern of data including the blockage of the conventional renewal effect, and the appearance of an unconventional renewal effect. The implications of these results are discussed with respect to current views on the role of the dorsal hippocampus in processing context information.

Acquired equivalence of flavour cues with a common antecedent in rats.

Equivalence of flavour cues, each of which had been paired with a common antecedent, was demonstrated with rats in a three-stage design. In the first stage, a group of thirsty rats were given each of two target flavour cues after a common antecedent flavour (Xright arrowA and Xright arrowB), while a second group of rats were given A and B after differential antecedent flavours (Xright arrowA and Yright arrowB). Another group of rats was allowed to drink A and B after familiar tap water. In the second stage, aversion to A was established by a lithium chloride injection after drinking A. The acquired equivalence effect was verified in the third stage by strong aversion to B in the group trained with the common antecedent compared with the remaining two groups. The representation-mediation, rather than response-mediation, hypothesis seems to fit the backward acquired equivalence effect obtained here.