Midbrain dopamine neurons are key components of the brain’s reward system. These neurons are excited by reward and sensory stimuli predicting reward, while they are inhibited by reward omission. These excitatory and inhibitory responses have been shown to play important roles in reward learning and positive motivation. However, it was unknown which parts of the brain provide dopamine neurons with reward-related signals necessary for their responses. Recent studies showed evidence that the lateral habenula, part of the structure called the epithalamus, is a good candidate for a source of reward-related signals in dopamine neurons. The lateral habenula projects to midbrain structures such as the substantia nigra pars compacta and ventral tegmental area which contain dopamine neurons. Electrical stimulation of the lateral habenula inhibits dopamine neuron activity. Neurons in the lateral habenula also encode reward-related signals but in an opposite manner to dopamine neurons (i.e., they are inhibited by reward and sensory stimuli predicting reward, and excited by reward omission). These findings suggest that the lateral habenula transmits reward-related signals to dopamine neurons by inhibiting them.

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The field of animal learning and behavior has a long history and continues to contribute in important ways to the understanding of cognitive processes in different animal species as compared to human beings. Animals have considerable flexibility to optimize their behaviors in solving particular problems as well as coping with ever-changing circumstances. Research on learning mechanisms from a comparative perspective may deepen our understanding of functional significance of cognitive behaviors of both human and nonhuman species. Examples are taken from studies of list memory, categorization, formation and expansion of equivalence relations among physically different stimuli, and visual search for category.

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I first discuss the developmental and evolutionary implications of the temporal reorganization of individual development in human infants, which have resulted in unique human characteristics during early development and child rearing, for example, (1) the large size of neonates, (2) “trade-off” in mother-infant interaction, (3) self-contact behaviors, (4) various manipulations of objects, and (5) emergence of “childhood” and caregiving by multiple caregivers. I also discuss the results of our recent study on human and chimpanzee fetuses by using three-dimensional ultrasonography; this study showed that the growth velocity of the brain volumes of chimpanzee fetuses does not accelerate during late pregnancy, whereas that of human fetuses does accelerate through late pregnancy. Additional analysis and findings show that the timing of cessation in the increase of growth velocity of brain volume among species is crucial to clarify how much earlier infants are born and how retarded is the development of their postural reactions. Previously accumulated data suggest that further verification of temporally modified growth and development among species will help understand the effect of individual development on the evolution of human behavior.

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Gonadal steroid hormones, androgen and estrogen, regulate the expression of sex-typical behavior through organizational and activational action. To understand the biological basis of animal behavior, particularly social behavior, it is very important to focus on the life-long action of various hormones in the brain. In this article, we first summarize basic concepts in behavioral neuroendocrinology. We then overview our up-to-date findings in mice on the neuroendocrine basis of social behaviors such as sex and aggression, and effects of environmental and experiential manipulations during perinatal and pubertal periods. Based on these findings, we address the significance to study hormonal actions on formation, maintenance and modification of social bonding in both animals and humans, and propose an emerging field of translational social neuroendocrinology.

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This review shows the effects of oxytocin involved in bond formation and the manner of reciprocal communication leading to mother-infant bonding in rodents. Various social stimuli, such as tactile stimuli and ultrasonic vocalizations (USVs) from the pups to the mother, and feeding and tactile stimulation from the mother to the pups, reinforce the mother-infant bond formation in rats and mice. This suggests that the mother and infant are able to develop a cross-modal sensory recognition of each other. Although the mechanisms underlying bond formation in the brains of infants have not yet been clarified, oxytocin in the neural system plays a pivotal role in each sides of the mother-infant bonding process. The deprivation of social stimuli from the mother strongly influences the offspring’s sociality, including maternal behavior towards their own offspring, which implies the “non-genomic transmission of maternal environment.” The comparative understanding of cognitive functions between mother and infants, and the biological mechanisms involved in mother-infant bonding may help us understand psychiatric disorders associated with mother-infant relationships.

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Landmark cues are especially important in spatial cognition, because the processing of landmarks is seen at an early stage of cognitive map formation. Previous studies have indicated that animals are capable of conducting goal searches using multiple landmarks. We reviewed studies on the competition and integration of spatial information regarding multiple landmarks. It was considered that knowledge about the processing of landmarks would lead to an understanding about the competition that occurs among multiple landmarks and about the spatial cognition leading to the development of multiple landmarks in cognitive maps. These issues have been independently investigated in the contexts of cognitive mapping and associative learning. It is concluded that cue competitions among multiple landmarks are acquisition processes and it is proposed that competition for cues precedes the integration process in cognitive map formation.

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Three goldfish (Carassius auratus) were trained in a free-choice task of an aquatic version of an eight-arm radial maze. Extra-maze cues but not intra-maze cues were available in Phase 1, both extra-maze and intra-maze cues were removed in Phase 2, and intra-maze cues but not extra-maze cues were introduced in Phase 3. The fish could perform the task reliably without a stereotyped response pattern if they were allowed to utilize either the extra-maze or intra-maze cues. However, in Phase 2 where both extra-maze and intra-maze cues were not available, the fish showed a response strategy to choose the adjacent arms. These results suggest that goldfish can utilize multiple navigation strategies flexibly to perform a radial maze task depending on the availability of visual cues.

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The present study examined the visual discrimination learning of Japanese fire-bellied newts (Cynops pyrrhogaster). Four female newts were trained in a simultaneous visual bright and dark discrimination task using a dry T-maze. Two newts were assigned to a dark-positive condition and two to a bright-positive condition. Submersion in water was used as reinforcement. For correct responses, newts could enter into a water tank through an opening in the floor of the arm, whereas in the case of incorrect responses newts were dropped onto a net above the water. They learned to choose the positive arm and performed significantly better than chance during test trials when a net was put onto each goal water tank regardless of correctness of their responses. These results strongly suggest that newts can learn a simultaneous visual discrimination task and that submersion in water is effective reinforcement for Japanese fire-bellied newts.

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