Revisiting the maturation of medial temporal lobe memory functions in primates

Age-related episodic memory decline and the role of amyloid-β: a systematic review

Aging has been associated with the functional decline of episodic memory (EM). Unanswered questions are whether the decline of EM occurs even during healthy aging and whether this decline is related to amyloid-β (Aβ) deposition in the hippocampus.

Objective

The main purpose of this study was to investigate data on the relationship between the age-related EM decline and Aβ deposition.

Methods

We searched the Cochrane, MEDLINE, Scopus, and Web of Science databases and reference lists of retrieved articles that were published in the past 10 years. The initial literature search identified 517 studies. After screening the title, abstract, key words, and reference lists, 56 studies met the inclusion criteria.

Results

The overall results revealed that increases in Aβ are related to lower hippocampal volume and worse performance on EM tests. The results of this systematic review revealed that high levels of Aβ may be related to EM deficits and the progression to Alzheimer's disease.

Conclusions

We discussed the strengths and pitfalls of various tests and techniques used for investigating EM and Aβ deposition, methodological issues, and potential directions for future research.

A Multivariate Assessment of Age-Related Cognitive Impairment in Octodon degus

Aging is a progressive functional decline characterized by a gradual deterioration in physiological function and behavior. The most important age-related change in cognitive function is decline in cognitive performance (i.e., the processing or transformation of information to make decisions that includes speed of processing, working memory, and learning). The purpose of this study is to outline the changes in age-related cognitive performance (i.e., short-term recognition memory and long-term learning and memory) in long-lived Octodon degus. The strong similarity between degus and humans in social, metabolic, biochemical, and cognitive aspects makes it a unique animal model for exploring the mechanisms underlying the behavioral and cognitive deficits related to natural aging. In this study, we examined young adult female degus (12- and 24-months-old) and aged female degus (38-, 56-, and 75-months-old) that were exposed to a battery of cognitive-behavioral tests. Multivariate analyses of data from the Social Interaction test or Novel Object/Local Recognition (to measure short-term recognition memory), and the Barnes maze test (to measure long-term learning and memory) revealed a consistent pattern. Young animals formed a separate group of aged degus for both short- and long-term memories. The association between the first component of the principal component analysis (PCA) from short-term memory with the first component of the PCA from long-term memory showed a significant negative correlation. This suggests age-dependent differences in both memories, with the aged degus having higher values of long-term memory ability but poor short-term recognition memory, whereas in the young degus an opposite pattern was found. Approximately 5% of the young and 80% of the aged degus showed an impaired short-term recognition memory; whereas for long-term memory about 32% of the young degus and 57% of the aged degus showed decreased performance on the Barnes maze test. Throughout this study, we outlined age-dependent cognitive performance decline during natural aging in degus. Moreover, we also demonstrated that the use of a multivariate approach let us explore and visualize complex behavioral variables, and identified specific behavioral patterns that allowed us to make powerful conclusions that will facilitate further the study on the biology of aging. In addition, this study could help predict the onset of the aging process based on behavioral performance.

Critical aspects of neurodevelopment

Organisms have the unique ability to adapt to their environment by making use of external inputs. In the process, the brain is shaped by experiences that go hand-in-hand with optimisation of neural circuits. As such, there exists a time window for the development of different brain regions, each unique for a particular sensory modality, wherein the propensity of forming strong, irreversible connections are high, referred to as a critical period of development. Over the years, this domain of neurodevelopmental research has garnered considerable attention from many scientists, primarily because of the intensive activity-dependent nature of development. This review discusses the cellular, molecular, and neurophysiological bases of critical periods of different sensory modalities, and the disorders associated in cases the regulators of development are dysfunctional. Eventually, the neurobiological bases of the behavioural abnormalities related to developmental pathologies are discussed. A more in-depth insight into the development of the brain during the critical period of plasticity will eventually aid in developing potential therapeutics for several neurodevelopmental disorders that are categorised under critical period disorders.

Developmental onset distinguishes three types of spontaneous recognition memory in mice

Spontaneous recognition memory tasks build on an animal’s natural preference for novelty to assess the what, where and when components of episodic memory. Their simplicity, ethological relevance and cross-species adaptability make them extremely useful to study the physiology and pathology of memory. Recognition memory deficits are common in rodent models of neurodevelopmental disorders, and yet very little is known about the expression of spontaneous recognition memory in young rodents. This is exacerbated by the paucity of data on the developmental onset of recognition memory in mice, a major animal model of disease. To address this, we characterized the ontogeny of three types of spontaneous recognition memory in mice: object location, novel object recognition and temporal order recognition. We found that object location is the first to emerge, at postnatal day (P)21. This was followed by novel object recognition (24 h delay), at P25. Temporal order recognition was the last to emerge, at P28. Elucidating the developmental expression of recognition memory in mice is critical to improving our understanding of the ontogeny of episodic memory, and establishes a necessary blueprint to apply these tasks to probe cognitive deficits at clinically relevant time points in animal models of developmental disorders.

Long-term alterations in brain and behavior after postnatal Zika virus infection in infant macaques

Zika virus (ZIKV) infection has a profound impact on the fetal nervous system. The postnatal period is also a time of rapid brain growth, and it is important to understand the potential neurobehavioral consequences of ZIKV infection during infancy. Here we show that postnatal ZIKV infection in a rhesus macaque model resulted in long-term behavioral, motor, and cognitive changes, including increased emotional reactivity, decreased social contact, loss of balance, and deficits in visual recognition memory at one year of age. Structural and functional MRI showed that ZIKV-infected infant rhesus macaques had persistent enlargement of lateral ventricles, smaller volumes and altered functional connectivity between brain areas important for socioemotional behavior, cognitive, and motor function (e.g. amygdala, hippocampus, cerebellum). Neuropathological changes corresponded with neuroimaging results and were consistent with the behavioral and memory deficits. Overall, this study demonstrates that postnatal ZIKV infection in this model may have long-lasting neurodevelopmental consequences.

Nonhuman primate models of hippocampal development and dysfunction

Nonhuman primates provide highly valuable animal models that have significantly advanced our understanding of numerous behavioral and biological phenomena in humans. Here, we reviewed a series of developmental neuropsychological studies that informed us on the timing of development of the hippocampus and of hippocampal-dependent cognitive functions in primates. Data indicate that, in primates, the emergence of adult-like proficiency on behavioral tasks sensitive to hippocampal dysfunction is a stepwise process and reflects the gradual maturation of different hippocampal circuits and their connections with other neural structures. Profound and persistent memory loss resulting from insult to the hippocampus in infancy was absent in early infancy but became evident later in childhood and persisted in adulthood, indicating very little sparing or recovery of function. Finally, the early hippocampal insult resulted in both adaptive and maladaptive neuroplasticity: i.e., sparing contextual memory, but affecting working memory processes as well as emotional reactivity and hypothalamic-pituitary-adrenal (HPA) axis functioning. The results provide significant information on the emergence of hippocampal-dependent functions in humans, on the time course of memory impairment in human cases with early hippocampal insult, and on the clinical implication of the hippocampus in developmental neuropsychiatric disorders.

Neural Mechanisms of High-Level Vision

The last three decades have seen major strides in our understanding of neural mechanisms of high-level vision, or visual cognition of the world around us. Vision has also served as a model system for the study of brain function. Several broad insights, as yet incomplete, have recently emerged. First, visual perception is best understood not as an end unto itself, but as a sensory process that subserves the animal's behavioral goal at hand. Visual perception is likely to be simply a side effect that reflects the readout of visual information processing that leads to behavior. Second, the brain is essentially a probabilistic computational system that produces behaviors by collectively evaluating, not necessarily consciously or always optimally, the available information about the outside world received from the senses, the behavioral goals, prior knowledge about the world, and possible risks and benefits of a given behavior. Vision plays a prominent role in the overall functioning of the brain providing the lion's share of information about the outside world. Third, the visual system does not function in isolation, but rather interacts actively and reciprocally with other brain systems, including other sensory faculties. Finally, various regions of the visual system process information not in a strict hierarchical manner, but as parts of various dynamic brain-wide networks, collectively referred to as the "connectome." Thus, a full understanding of vision will ultimately entail understanding, in granular, quantitative detail, various aspects of dynamic brain networks that use visual sensory information to produce behavior under real-world conditions. © 2017 American Physiological Society. Compr Physiol 8:903-953, 2018.

Therapeutic hypothermia and hypoxia-ischemia in the term-equivalent neonatal rat: characterization of a translational preclinical model

BACKGROUND

Hypoxic-ischemic encephalopathy (HIE) is a major cause of morbidity in survivors. Therapeutic hypothermia (TH) is the only available intervention, but the protection is incomplete. Preclinical studies of HIE/TH in the rodent have relied on the postnatal day (P) 7 rat whose brain approximates a 32-36 wk gestation infant, less relevant for these studies. We propose that HIE and TH in the term-equivalent P10 rat will be more translational.

METHODS

P10-11 rat pups were subjected to unilateral hypoxia-ischemia (HI) and 4 h recovery in normothermic (N) or hypothermic (TH) conditions. Brain damage was assessed longitudinally at 24 h, 2 wk, and 12 wk. Motor function was assessed with the beam walk; recognition memory was measured by novel object recognition.

RESULTS

Neuroprotection with TH was apparent at 2 and 12 wk in both moderately and severely damaged animals. TH improved motor function in moderate, but not severe, damage. Impaired object recognition occurred with severe damage with no evidence of protection of TH.

CONCLUSION

This adaptation of the immature rat model of HI provides a reproducible platform to further study HIE/TH in which individual animals are followed up longitudinally to provide a useful translational preclinical model.

Responses of monkey prefrontal neurons during the execution of transverse patterning

Recent functional imaging studies have suggested that the prefrontal cortex (PF) is engaged in the performance of transverse patterning (TP), which consists of 3 conflicting discriminations (A+/B-, B+/C-, C+/A-). However, the roles of PF in TP are still unclear. To address this issue, we examined the neuronal responses in 3 regions [the principal sulcus (PS), dorsal convexity (DC), and medial prefrontal cortex (MPF)] of the macaque PF during the performance of an oculomotor version of TP. A delayed matching-to-sample (DMS) task was used as a control task. The TP task-responsive neurons were most abundant in MPF. We analyzed the dependency of each neuronal response on the task type (TP or DMS), target shape (A, B, or C), and target location (left or right). Immediately after the choice cue presentation, many MPF neurons showed task dependency. Interestingly, some of them already exhibited differential activity between the 2 tasks before the choice cue presentation. Immediately before the saccade, the number of target location-dependent neurons increased in MPF and PS. Among them, many MPF neurons were also influenced by the task type, whereas PS neurons tended to show location dependency without task dependency. These results suggest that MPF and PS are involved in the execution of TP: MPF appears to be more important in the target selection based on the TP rule, whereas PS is apparently more related to the response preparation. In addition, some neurons showed a postsaccadic response, which may be related to the feedback mechanism.

The hippocampus is not a geometric module: processing environment geometry during reorientation

The hippocampus has long been known to play a role in allocentric spatial coding, but its specific involvement in reorientation, or the recalibration of a disrupted egocentric spatial representation using allocentric spatial information, has received less attention. Initially, the cognitive literature on reorientation focused on a “geometric module” sensitive to the shape formed by extended surfaces in the environment, and the neuroscience literature followed with proposals that particular MTL regions might be the seat of such a module. However, with behavioral evidence mounting that a modular cognitive architecture is unlikely, recent work has begun to directly address the issue of the neural underpinnings of reorientation. In this review, we describe the reorientation paradigm, initial proposals for the role of the MTL when people reorient, our recent work on the neural bases of reorientation, and finally, how this new information regarding neural mechanism helps to re-interpret and clarify the original behavioral reorientation data.

The development of object recognition memory in rhesus macaques with neonatal lesions of the perirhinal cortex

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Assessed recognition memory in infant monkeys with neonatal perirhinal lesions using the visual paired comparison task.

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Performance was assessed at 4 developmental ages.

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Novelty preference deteriorated with age after neonatal perirhinal lesions.

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Presence of functional sparing.

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Memory deficits after perirhinal lesions occurred earlier than after hippocampal lesions.

To investigate the role of the perirhinal cortex on the development of recognition measured by the visual paired-comparison (VPC) task, infant monkeys with neonatal perirhinal lesions and sham-operated controls were tested at 1.5, 6, 18, and 48 months of age on the VPC task with color stimuli and intermixed delays of 10 s, 30 s, 60 s, and 120 s. Monkeys with neonatal perirhinal lesions showed an increase in novelty preference between 1.5 and 6 months of age similar to controls, although at these two ages, performance remained significantly poorer than that of control animals. With age, performance in animals with neonatal perirhinal lesions deteriorated as compared to that of controls. In contrast to the lack of novelty preference in monkeys with perirhinal lesions acquired in adulthood, novelty preference in the neonatally operated animals remained above chance at all delays and all ages. The data suggest that, although incidental recognition memory processes can be supported by the perirhinal cortex in early infancy, other temporal cortical areas may support these processes in the absence of a functional perirhinal cortex early in development. The neural substrates mediating incidental recognition memory processes appear to be more widespread in early infancy than in adulthood.

Annual research review: The neurobehavioral development of multiple memory systems--implications for childhood and adolescent psychiatric disorders

Extensive evidence indicates that mammalian memory is organized into multiple brains systems, including a 'cognitive' memory system that depends on the hippocampus and a stimulus-response 'habit' memory system that depends on the dorsolateral striatum. Dorsal striatal-dependent habit memory may in part influence the development and expression of some human psychopathologies, particularly those characterized by strong habit-like behavioral features. The present review considers this hypothesis as it pertains to psychopathologies that typically emerge during childhood and adolescence. These disorders include Tourette syndrome, attention-deficit/hyperactivity disorder, obsessive-compulsive disorder, eating disorders, and autism spectrum disorders. Human and nonhuman animal research shows that the typical development of memory systems comprises the early maturation of striatal-dependent habit memory and the relatively late maturation of hippocampal-dependent cognitive memory. We speculate that the differing rates of development of these memory systems may in part contribute to the early emergence of habit-like symptoms in childhood and adolescence. In addition, abnormalities in hippocampal and striatal brain regions have been observed consistently in youth with these disorders, suggesting that the aberrant development of memory systems may also contribute to the emergence of habit-like symptoms as core pathological features of these illnesses. Considering these disorders within the context of multiple memory systems may help elucidate the pathogenesis of habit-like symptoms in childhood and adolescence, and lead to novel treatments that lessen the habit-like behavioral features of these disorders.

Hippocampal lateralization and memory in children and adults

The neural organization of cognitive processes, particularly hemispheric lateralization, changes throughout childhood and adolescence. Differences in the neural basis of relational memory between children and adults are not well characterized. In this study we used magnetoencephalography to observe the lateralization differences of hippocampal activation in children and adults during performance of a relational memory task, transverse patterning (TP). The TP task was paired with an elemental control task, which does not depend upon the hippocampus. We contrasted two hypotheses; the compensation hypothesis would suggest that more bilateral activation in children would lead to better TP performance, whereas the maturation hypothesis would predict that a more adult-like right-lateralized pattern of hippocampal activation would lead to better performance. Mean-centered partial least squares analysis was used to determine unique patterns of brain activation specific to each task per group, while diminishing activation that is consistent across tasks. Our findings support the maturation hypothesis that a more adult-like pattern of increased right hippocampal lateralization in children leads to superior performance on the TP task. We also found dynamic changes of lateralization throughout the time course for all three groups, suggesting that caution is needed when interpreting conclusions about brain lateralization.

Development of memory for spatial locations and object/place associations in infant rhesus macaques with and without neonatal hippocampal lesions

This study traces the development of spatial memory abilities in monkeys and reports the effects of selective neonatal hippocampal lesions on performance across development. Two different versions of the visual paired-comparison (VPC) task were used. The VPC-Spatial-Location task tested memory for object-locations that could be solved using an egocentric spatial frame of reference and the VPC-Object-In-Place task taxed memory for spatial relations using an allocentric reference frame. Eleven rhesus macaques (6 neonatal sham-operated controls and 5 with neonatal neurotoxic hippocampal lesions) were tested on both tasks as infants (8 months), juveniles (18 months), and adults (5-6 years). Memory for spatial locations was present by 18 months of age, whereas memory for object-place relations was present only in adulthood. Also, neonatal hippocampal lesions delayed the emergence of memory for spatial locations and abolished memory for object-place associations, particularly in animals that had sustained extensive and bilateral hippocampal lesions. The differential developmental time course of spatial memory functions and of the effects of neonatal hippocampal lesions on these functions are discussed in relation to morphological maturation of the medial temporal lobe structures in monkeys. Implications of the findings for the neural basis of spatial memory development in humans are also considered.

Long-term effects of neonatal hippocampal lesions on novelty preference in monkeys

In a recent longitudinal study to assess the development of incidental recognition memory processes in monkeys, we showed that the effects of neonatal hippocampal lesions did alter incidental recognition memory only when the animals reached the juvenile period (Zeamer et al., ). The current follow-up study tested whether this incidental memory loss was long-lasting, i.e., present in adulthood, or only transitory, due to functional compensation with further brain maturation. The same animals with neonatal hippocampal lesions and their sham-operated controls were re-tested in the visual paired-comparison task when they reached adulthood (48 months). The results demonstrated that, at least for easily discriminable color pictures of objects, the involvement of the hippocampus was only transitory, given that when re-tested as adults, animals with neonatal hippocampal lesions performed as well as sham-operated controls at all delays. Yet, significant recognition memory impairment was re-instated when the discriminability of the stimuli was made more difficult (black/white pictures of similar objects). The data demonstrate profound functional remodeling within the hippocampus and its interactions with different medial temporal lobe structures from the juvenile period to adulthood, which is substantiated by a parallel morphological maturation of hippocampal intrinsic circuits (Lavenex et al., ; Jabès et al., ).

T-maze learning in weanling lambs

A major advantage of sheep models in experimental studies of neurodevelopmental disorders (e.g., with prenatal neurotoxicant exposure) is that the equivalent of all three trimesters of human brain development occurs in sheep entirely in utero. However, studies of learning and memory in sheep are limited. The goal of this study was to extend the analysis of spatial learning and memory in adolescent sheep using several traditional T-maze tasks. Both 9- and 14-week-old lambs acquired a delayed nonmatching-to-place task, but the older lambs learned the task significantly faster. In contrast, acquisition of a matching-to-place task was significantly more difficult. Lambs, like rodents, appear to have a predisposition toward learning "win-shift" spatial problems in a T-maze under appetitive motivation. Lambs also rapidly acquired a position habit and showed typical reversal learning curves. These findings support the use of T-maze tasks to assess behavioral outcomes in various sheep models.

Behavioral toxicology of cognition: extrapolation from experimental animal models to humans: behavioral toxicology symposium overview

A variety of behavioral instruments are available for assessing important aspects of cognition in both animals and humans and, in many cases, the same instruments can be used in both. While nonhuman primates are phylogenetically closest to humans, rodents, pigeons and other animals also offer behaviors worthy of note. Delay Discounting procedures are as useful as any in studies of impulsivity and may have utility in shedding light on processes associated with drug abuse. Specific memory tests such as Visual Paired Comparisons tasks (similar to the Fagan test of infant intelligence) can be modified to allow for assessment of different aspects of memory such as spatial memory. Use of these and other specific memory tasks can be used to directly monitor aspects of cognitive development in infant animals, particularly in nonhuman primates such as monkeys, and children and to draw inferences with respect to possible neuroanatomical substrates sub-serving their functions. Tasks for assessing working memory such as Variable Delayed Response (VDR), modified VDR and Spatial Working Memory tasks are now known to be affected in Parkinson's disease (PD). These and other cognitive function tasks are being used in a monkey model of PD to assess the ability of anti-Parkinson's disease therapies to ameliorate these cognitive deficits without diminishing their therapeutic effects on motor dysfunction. Similarly, in a rat model of the cognitive deficits associated with perinatal exposure to polychlorinated biphenyls (PCBs), clear parallels with children can be seen in at least two areas of executive function: cognitive flexibility and response inhibition. In the rat model, discrimination reversal tasks were utilized to assess cognitive flexibility, a function often assessed in humans using the Wisconsin Card Sorting Task. Response inhibition was assessed using performance in a Differential Reinforcement of Low Response Rates (DRL) task. As the data continue to accumulate, it becomes more clear that our attempts to adapt animal-appropriate tasks for the study of important aspects of human cognition have proven to be very fruitful.

The effects of selective hippocampal damage on tests of oddity in rhesus macaques

The oddity task (e.g., A-, A-, B+) is classified as a conjunctive or relational task in which accurate performance depends upon learning to attend to stimulus relationships, not stimulus identity, and has no retention component as stimuli are presented simultaneously. It has been suggested that the hippocampus may play a particular role in learning this type of task in humans and animals. To test this, we trained adult rhesus macaques with selective neurotoxic damage to the hippocampal formation on their ability to learn and apply an oddity rule. The results suggest that the monkeys were able to adapt simple strategies to solve variations of the oddity task, however as the opportunity for such strategies was reduced, monkeys with hippocampal damage were increasingly impaired.

Perception of object-context relations: eye-movement analyses in infants and adults

Twenty-eight 4-month-olds' and twenty-two 20-year-olds' attention to object-context relations was investigated using a common eye-movement paradigm. Infants and adults scanned both objects and contexts. Infants showed equivalent preferences for animals and vehicles and for congruent and incongruent object-context relations overall, more fixations of objects in congruent object-context relations, more fixations of contexts in incongruent object-context relations, more fixations of objects than contexts in vehicle scenes, and more fixation shifts in incongruent than congruent vehicle scenes. Adults showed more fixations of congruent than incongruent scenes, vehicles than animals, and objects than contexts; equal fixations of animals and their contexts but more fixations of vehicles than their contexts; and more shifts of fixation when inspecting animals in context than vehicles in context. These findings for location, number, and order of eye movements indicate that object-context relations play a dynamic role in the development and allocation of attention.

Medial temporal lobe memory in childhood: developmental transitions

The medial temporal lobes (MTL) support declarative memory and mature structurally and functionally during the postnatal years in humans. Although recent work has addressed the development of declarative memory in early childhood, less is known about continued development beyond this period of time. The purpose of this investigation was to explore MTL-dependent memory across middle childhood. Children (6 -10 years old) and adults completed two computerized tasks, place learning (PL) and transitive inference (TI), that each examined relational memory, as well as the flexible use of relational learning. Findings suggest that the development of relational memory precedes the development of the ability to use relational knowledge flexibly in novel situations. Implications for the development of underlying brain areas and ideas for future neuroimaging investigations are discussed.

Persisting cognitive deficits induced by low-dose, subchronic treatment with MK-801 in adolescent rats

Cognitive impairments have been proposed as a core feature of schizophrenia. Studies have shown that chronic or subchronic treatment with N-methyl-d-aspartate (NMDA) antagonists could induce cognitive deficits that resemble the symptoms of schizophrenia, yet few studies have investigated the effects of repeated NMDA blockade during adolescence on cognition. In the current study, adolescent, male rats were treated with an intraperitoneal injection of MK-801 (0.05, 0.1, and 0.2mg/kg) once daily for 14days. They were then tested 24h and 14days after drug cessation, respectively, in a series of behavioural tasks, including the object recognition task, the object-in-context recognition task and the working memory task of the Morris water maze (MWM). Results showed that object-in-context recognition and spatial working memory in the MWM were significantly impaired by repeated MK-801 treatment when animals were tested 24h after drug cessation, but object recognition was left intact. In particular, such deficits were observed 14days after drug cessation in the 0.2mg/kg group. The cognition-impairing effect of MK-801 could not be attributed to malnutrition or alterations in motor functions. Taken together, this study may provide support for establishing an animal model of cognitive deficits of schizophrenia based on low-dose, repeated treatment of MK-801 during adolescence.

From Perceptual Categories to Concepts: What Develops?

People are remarkably smart: they use language, possess complex motor skills, make non-trivial inferences, develop and use scientific theories, make laws, and adapt to complex dynamic environments. Much of this knowledge requires concepts and this paper focuses on how people acquire concepts. It is argued that conceptual development progresses from simple perceptual grouping to highly abstract scientific concepts. This proposal of conceptual development has four parts. First, it is argued that categories in the world have different structure. Second, there might be different learning systems (sub-served by different brain mechanisms) that evolved to learn categories of differing structures. Third, these systems exhibit differential maturational course, which affects how categories of different structures are learned in the course of development. And finally, an interaction of these components may result in the developmental transition from perceptual groupings to more abstract concepts. This paper reviews a large body of empirical evidence supporting this proposal.

From Perceptual Categories to Concepts: What Develops?

People are remarkably smart: they use language, possess complex motor skills, make non-trivial inferences, develop and use scientific theories, make laws, and adapt to complex dynamic environments. Much of this knowledge requires concepts and this paper focuses on how people acquire concepts. It is argued that conceptual development progresses from simple perceptual grouping to highly abstract scientific concepts. This proposal of conceptual development has four parts. First, it is argued that categories in the world have different structure. Second, there might be different learning systems (sub-served by different brain mechanisms) that evolved to learn categories of differing structures. Third, these systems exhibit differential maturational course, which affects how categories of different structures are learned in the course of development. And finally, an interaction of these components may result in the developmental transition from perceptual groupings to more abstract concepts. This paper reviews a large body of empirical evidence supporting this proposal.

Maturation of the hippocampal formation and amygdala in Macaca mulatta: a volumetric magnetic resonance imaging study

Malformations of the hippocampal formation and amygdala have been implicated in several neurodevelopmental disorders; yet relatively little is known about their normal structural development. The purpose of this study was to characterize the early developmental trajectories of the hippocampus and amygdala in the rhesus macaques (Macaca mulatta) using noninvasive MRI techniques. T1-weighted structural scans of 22 infant and juvenile monkeys (11 male, 11 female) were obtained between 1 week and approximately 2 yrs of age. Ten animals (five males, five females) were scanned multiple times and 12 monkeys (six males, six females) were scanned once between 1 and 4 weeks of age. Both structures exhibited significant age-related changes throughout the first 2 yrs of life that were not explained by overall brain development. The hippocampal formation increased 117.05% in males and 110.86% in females. No sex differences were evident, but the left hemisphere was significantly larger than the right. The amygdala increased 86.49% in males and 72.94% in females with males exhibiting a larger right than left amygdala. For both structures, the most substantial volumetric increases were seen within the first month, but the hippocampal formation appeared to develop more slowly than the amygdala with the rate of hippocampal maturation stabilizing around 11 months and that of amygdala maturation stabilizing around 8 months. Differences in volumetric developmental trajectories of the hippocampal formation and amygdala largely mirror differences in the timing of the functional development of these structures. The current results emphasize the importance of including early postnatal ages when assessing developmental trajectories of neuroanatomical structures and reinforces the utility of nonhuman primates in the assessment of normal developmental patterns.

Developmental trajectory of object recognition memory in infant rhesus macaques with and without neonatal hippocampal lesions

To examine the developmental trajectory of object recognition memory and its neural substrate, 10-12-d-old monkeys (Macaca mulatta) received sham operations or neurotoxic hippocampal lesions and were tested at the ages of 1.5, 6, and 18 months on the visual paired-comparison task using delays of 10, 30, 60, and 120 s. In sham-operated controls, incidental recognition memory was present at 1.5 months, became more robust at 6 months, and was delay-dependent by 18 months of age, suggesting that the brain structures mediating these early developing recognition abilities may undergo significant modifications after 6 months of age in monkeys. A similar developmental progression was also observed in animals with neonatal hippocampal lesions, although the delay-dependent effect at 18 months was significantly more pronounced after the neonatal hippocampal lesions, suggesting that with maturation animals with neonatal hippocampal lesions grow into a recognition-memory deficit. These findings suggest not only that the medial temporal cortical areas, known to mediate incidental recognition memory processes in adulthood, could support these processes in early infancy even when long delays are used, but also that later in development, after reaching functional maturity, the hippocampus begins to interact with the medial temporal cortical areas to mediate this function.

Evidence for intact memory-guided attention in school-aged children

Visual scenes contain many statistical regularities such as the likely identity and location of objects that are present; with experience, such regularities can be encoded and can ultimately facilitate the deployment of spatial attention to important locations. Memory-guided attention has been extensively examined in adults with the 'contextual cueing' paradigm and has been linked to specific neural substrates - a medial temporal lobe (MTL)-frontoparietal network. However, it currently remains unknown when this ability comes 'online' during development. Thus, we examined the performance of school-aged children on an age-appropriate version of the contextual cueing paradigm. Children searched for a target fish among distractor fish in new displays and in 'old' displays on a touchscreen computer. Old displays repeated across blocks of trials and thus provided an opportunity for prior experience with the invariant configuration of the stimuli to guide attentional deployment. We found that over time children searched old displays significantly faster than new displays, thus revealing intact memory-guided attention and presumed function of an MTL-frontoparietal network in 5- to 9-year-olds. More generally, our findings suggest that children are remarkably sensitive to the inherent structure of their visual environment and this enables attentional deployment to become more efficient with experience.

Ontogeny of Rat Recognition Memory measured by the novel object recognition task

Detection of novelty is an essential component of recognition memory, which develops throughout cerebral maturation. To better understand the developmental aspects of this memory system, the novel object recognition task (NOR) was used with the immature rat and ontogenically profiled. It was hypothesized that object recognition would vary across development and be inferior to adult performance. The NOR design was made age-appropriate by downsizing the testing objects and arena. Weanling (P20-23), juvenile (P29-40), and adult (P50+) rats were tested after 0.25, 1, 24, and 48 hr delays. Weanlings exhibited novel object recognition at 0.25 and 1 hr, while older animals showed a preference for the novel object out to 24 hr. These findings are consistent with previous research performed in humans and monkeys, as well as to studies using the NOR after medial temporal lobe damage in adult rats.

Comparability of developmental cognitive assessments between standard and computer testing methods

Substantial questions have been raised about the validity of using computer-based testing to assess cognitive development with young children. However, little work has been done to assess the comparability of performance elicited using computerized methods with performance garnered using standard testing methods. The purpose of this study was to establish whether computerized testing resulted in performance that was different than established performance norms for infant monkeys (Macaca nemestrina) tested on four highly used cognitive tasks. Infants performed comparably on simple discrimination, reversal learning, and delayed nonmatch to sample rule learning. However, the infants tested in a computer testing-environment appeared to have difficulty on a task that required them to form response strategies. The results of the study reveal some apparent limitations of computer-based testing with infants, but do show that performance on several common cognitive tasks is comparable between the environments.

Cognitive development in macaques: Attentional set-shifting in juvenile and adult rhesus monkeys

In humans and nonhuman primates, the structure and function of frontal cortical regions of the brain are not completely developed until early adulthood. How this cortical development affects cognitive function continues to be elucidated. To that end, this experiment tested the ability of juvenile and adult rhesus monkeys to perform a cognitive task that is dependent upon intact frontal cortical function for optimal performance. Twenty-four juvenile (mean age 2.3 years) and 16 adult (mean age 10.3 years) rhesus monkeys were tested on the Cambridge Neuropsychological Test Automated Battery intradimensional/extradimensional set-shifting (ID/ED) task. Performance on the ID/ED task has been shown to be dependent upon frontal cortical function in both humans and nonhuman primates. Compared with adults, juveniles were impaired on the reversal of simple discrimination, intradimensional shift, reversal of intradimensional shift, and the extradimensional shift stages of the task. These results indicate juveniles committed more perseverative errors and more errors on the set-formation and set-shifting components of the ID/ED task. The developmental stage of the juvenile monkeys corresponds to roughly 5 to 6-year-old children, and these results are consistent with performance of human children and adults on similar ID/ED tests and on several other tests of attentional set-shifting or attentional flexibility. Furthermore, these results are consistent with the ongoing development of frontal cortical structures relating to ongoing cognitive development in nonhuman primates.

Selective impairment of cognitive performance in the young monkey following recovery from iron deficiency

OBJECTIVE

While poor nutrition during development is an obvious concern, the magnitude and duration of the neural and cognitive deficits that occur after moderate iron deficiency in infancy have remained controversial. A nonhuman primate model of infancy anemia was refined to investigate the effects on cognitive performance.

METHODS

Young rhesus monkeys that experienced a delimited period of iron deficiency were tested on a series of cognitive tasks following normalization of their hematological status. Beginning at 8 to 9 months of age, 2 months after weaning from their mothers and consumption of solid food, the previously iron-deficient (ID) monkeys (n = 17) were compared to age- and gender-matched, iron-sufficient (IS) (n = 27) monkeys on a series of three tests of cognitive performance. Using the Wisconsin General Testing Apparatus, a Black/White Discrimination task was followed by acquisition of Black/White Reversal (BWR).

RESULTS

ID monkeys were significantly slower at mastering the BWR task (p < .04), which required reversing and inhibiting the previously learned response. In addition, ID infants were significantly less object oriented (p < .017) and more distractible (p < .018). However, on two subsequent tests, the Concurrent Object Discrimination and Delayed Non-Match-to-Sample, there were no differences in acquisition, performance, or behavioral reactivity.

CONCLUSIONS

The initial cognitive and behavioral deficits are similar to those seen in follow-up evaluations of anemic children, but the limited extent of the impairment after this moderate iron deficiency that involved a select nutrient deficiency is encouraging for the benefits attainable through early identification and iron supplementation.

Effects of neonatal amygdala or hippocampus lesions on resting brain metabolism in the macaque monkey: a microPET imaging study

Longitudinal analysis of animals with neonatal brain lesions enables the evaluation of behavioral changes during multiple stages of development. Interpretation of such changes, however, carries the caveat that permanent neural injury also yields morphological and neurochemical reorganization elsewhere in the brain that may lead either to functional compensation or to exacerbation of behavioral alterations. We have measured the long-term effects of selective neonatal brain damage on resting cerebral glucose metabolism in nonhuman primates. Sixteen rhesus monkeys (Macaca mulatta) received neurotoxic lesions of either the amygdala (n=8) or hippocampus (n=8) when they were two weeks old. Four years later, these animals, along with age- and experience-matched sham-operated control animals (n=8), were studied with high-resolution positron emission tomography (microPET) and 2-deoxy-2[(18)F]fluoro-d-glucose ([(18)F]FDG) to detect areas of altered metabolism. The groups were compared using an anatomically-based region of interest analysis. Relative to controls, amygdala-lesioned animals displayed hypometabolism in three frontal lobe regions, as well as in the neostriatum and hippocampus. Hypermetabolism was also evident in the cerebellum of amygdala-lesioned animals. Hippocampal-lesioned animals only showed hypometabolism in the retrosplenial cortex. These results indicate that neonatal amygdala and hippocampus lesions induce very different patterns of long-lasting metabolic changes in distant brain regions. These observations raise the possibility that behavioral alterations in animals with neonatal lesions may be due to the intended damage, to consequent brain reorganization or to a combination of both factors.

Accounting for change in declarative memory: A cognitive neuroscience perspective

The medial temporal lobe memory system matures relatively early and supports rudimentary declarative memory in young infants. There is considerable development, however, in the memory processes that underlie declarative memory performance during infancy. Here we consider age-related changes in encoding, retention, and retrieval in the context of current knowledge about the brain systems that may underlie these memory processes. While changes in infants' encoding may be attributed to rapid myelination during the first year of life, improvements in long-term retention and flexible retrieval are likely due to the prolonged development of the dentate gyrus. Future studies combining measures of brain and behavior are critical in improving our understanding of how brain development drives memory development during infancy and early childhood.

Longitudinal magnetic resonance imaging study of rhesus monkey brain development

To examine early brain development, T1-weighted structural MRI scans of seven rhesus monkeys (Macaca mulatta) were obtained longitudinally between the ages of 1 week and 4 years at 12 age points. Total brain volume, calculated at each age point, increased significantly, by 56%, between 1 week and 4 years. The greatest increase of 22% occurred between 1 week and 1 month, followed by further significant increases between 1 and 2 months, and 3 and 4 months. Gradually smaller increases continued up to 3 years with no further significant changes thereafter. A robust maturation of white matter occurred between 1 week, at which the only easily identifiable fibre tracts were internal capsule and optic radiations, and 3 months, at which most large fibre tracts were visible; only at this age reproducible measurements were possible for all cases. White matter volume increased by 126% between 3 months and 4 years, with the biggest increase between 3 and 4 months (32%) followed by smaller but significant increases up to 4 years. The macaque brain development parallels that of humans by reaching the maximum in total brain volume around the age of sexual maturity (in macaques 3-4 years) and by the increases in white matter continuing beyond this age. The most rapid growth in both total brain volume and white matter from birth to approximately 4 months is consistent with the emergence of various cognitive abilities in macaques at that age.

Human memory development and its dysfunction after early hippocampal injury

Cognitive memory involves long-term memories for facts (semantic memory) and personal experiences (episodic memory) that can be brought to mind. There is consensus that the hippocampus and related medial temporal lobe (MTL) structures are crucial for adult cognitive memory, but much less is known about their contribution to memory during infancy and childhood. We argue that the MTL is involved in memory from early in life, supporting recognition memory within the first postnatal months and recall memory within the first year. We propose that normal development involves a sequence in which a form of semantic-like memory emerges first, whereas the characteristics of episodic memory develop only later with progressive development of the hippocampus. Early bilateral injury to the hippocampus disrupts this normal pattern such that memory skills cannot develop beyond the stage of semantic memories. This review is part of the INMED/TINS special issue "Nature and nurture in brain development and neurological disorders", based on presentations at the annual INMED/TINS symposium (http://inmednet.com/).

Primate errors in transitive ‘inference’: a two-tier learning model

Transitive performance (TP) is a learning-based behaviour exhibited by a wide range of species, where if a subject has been taught to prefer A when presented with the pair AB but to prefer B when presented with the pair BC, then the subject will also prefer A when presented with the novel pair AC. Most explanations of TP assume that subjects recognize and learn an underlying sequence from observing the training pairs. However, data from squirrel monkeys (Saimiri sciureus) and young children contradict this, showing that when three different items (a triad) are drawn from the sequence, subjects' performance degrades systematically (McGonigle and Chalmers, Nature 267:694-696, 1977; Chalmers and McGonigle, Journal of Experimental Child Psychology 37:355-377, 1984; Harris and McGonigle, The Quarterly Journal of Experimental Psychology 47B:319-348, 1994). We present here the two-tier model, the first learning model of TP which accounts for this systematic performance degradation. Our model assumes primate TP is based on a general-purpose task learning system rather than a special-purpose sequence-learning system. It supports the hypothesis of Heckers et al. (Hippocampus 14:153-162, 2004) that TP is an expression of two separate general learning elements: one for associating actions and contexts, another for prioritising associations when more than one context is present. The two-tier model also provides explanations for why phased training is important for helping subjects learn the initial training pairs and why some subjects fail to do so. It also supports the Harris and McGonigle (The Quarterly Journal of Experimental Psychology 47B:319-348, 1994) explanation of why, once the training pairs have been acquired, subjects perform transitive choice automatically on two-item diads, but not when exposed to triads from the same sequence.

Do neonatal bilateral ibotenic acid lesions of the hippocampal formation or of the amygdala impair HPA axis responsiveness and regulation in infant rhesus macaques (Macaca mulatta)?

In response to stressful events, the HPA axis is activated triggering the successive release of CRF, ACTH, and glucocorticoids. The glucocorticoids in turn provide a negative feedback signal to terminate the stress response. The amygdala and the hippocampus are involved in the regulation of the HPA axis. In rodents, their respective roles have been identified; the amygdala exerts a stimulatory effect, whereas the hippocampus provides negative feedback control. In primates, however, their regulatory roles are still not well defined. The present study compared HPA axis responsiveness and regulation in 3- to 5-month-old rhesus macaques that received neonatal (15 +/- 3 days old) bilateral ibotenic acid lesions of the hippocampus or amygdala, or sham lesions. Group differences in plasma cortisol response to separation from the mother and relocation in a novel environment were assessed as well as response to dexamethasone suppression and ACTH challenge. Results revealed that the initial cortisol levels after separation/relocation did not differ between groups. Subjects with hippocampus lesions did not show a suppression of cortisol in response to dexamethasone, suggesting a loss of negative feedback control of HPA regulation. Subjects with amygdala and sham lesions did not differ in response to dexamethasone. Indeed, bilateral neonatal lesions of the amygdala have little impact on HPA axis responsiveness and regulation in contrast to lesions in adult monkeys. Finally, females displayed higher cortisol levels than males, independently of their lesion, indicating that the development of sex differences in the regulation of the HPA axis does not involve the amygdala or hippocampus.

Spatial relational memory in 9-month-old macaque monkeys

This experiment assesses spatial and nonspatial relational memory in freely moving 9-mo-old and adult (11-13-yr-old) macaque monkeys (Macaca mulatta). We tested the use of proximal landmarks, two different objects placed at the center of an open-field arena, as conditional cues allowing monkeys to predict the location of food rewards hidden in one of two sets of three distinct locations. Monkeys were tested in two different conditions: (1) when local visual cues marked the two sets of potentially baited locations, so that monkeys could use both local and spatial information to discriminate these locations from never-baited locations; and (2) when no local visual cues marked the two sets of potentially baited locations, so that monkeys had to rely on a spatial relational representation of the environment to discriminate these locations. No 9-mo-old or adult monkey associated the presence of the proximal landmarks, at the center of the arena, with the presence of food in one set of three distinct locations. All monkeys, however, discriminated the potentially baited locations in the presence of local visual cues, thus providing evidence of visual discrimination learning. More importantly, all 9-mo-old monkeys tested discriminated the potentially baited locations in absence of the local visual cues, thus exhibiting evidence of spatial relational learning. These findings indicate that spatial memory processes characterized by a relational representation of the environment are present as early as 9 mo of age in macaque monkeys.

The primate hippocampus: ontogeny, early insult and memory

Recent evidence suggests that in primates, as in rodents, the hippocampus shows a developmental continuum that affects memory abilities from infancy to adulthood. In primates relatively few hippocampal-dependent abilities (e.g. some aspects of recognition memory) are present in early infancy, whereas others (e.g. relational memory) begin to show adult-like characteristics around 2 years of age in monkeys and 5-7 years in humans. Profound and persistent memory loss resulting from insult to the hippocampus in infancy becomes evident in everyday behavior only later in childhood. This pattern of results suggests a maturational gradient within the medial temporal lobe memory system, with most abilities crucially dependent upon the hippocampus emerging in later stages of development, supporting a model of hierarchical organization of memory within the medial temporal lobe.

Short- and long-term spatial delayed response performance across the lifespan

Delayed response paradigms have been used to examine the neural basis of short and long-term memory in humans. However, limited information exists on how delayed response performance changes across the lifespan. Using a well-validated spatial delayed response (SDR) task, we examined performance at short and long delays in over 300 control participants, 7 to 80 years old. We found a significant nonlinear relation between age and short delay performance (children and older adults worse than young adults) and a significant effect of delay length across the entire lifespan (long worse than short; largest in the youngest children, diminishing nonlinearly with age). This study compares short and long-term spatial memory and suggests that the relation between these systems may alter across the lifespan.

Development of flexible visual recognition memory in human infants

Research using the visual paired comparison task has shown that visual recognition memory across changing contexts is dependent on the integrity of the hippocampal formation in human adults and in monkeys. The acquisition of contextual flexibility may contribute to the change in memory performance that occurs late in the first year of life. To assess this skill, the images are presented on a background of one colour during familiarization and on a different coloured background during the recognition test. Our research showed that recognition memory is impaired by a change in context at 6 and 12 months of age but is unaffected at 18 and 24 months of age. The findings are discussed in relation to hippocampal development and the proposed developmental step in memory at 9-10 months of age.

The neural mechanisms of object working memory: what is where in the infant brain?

The question of how representational capacities develop in humans has been engaging cognitive psychologists for decades. Looking time studies have explored when infants start to show signs of perceiving and remembering the properties of specific objects at specific locations. Here we integrate these findings into the neuroscientific framework of human visual working memory. We suggest that the development of a system involving the temporal cortex, thalamic and hippocampal structures and possibly the dorsolateral prefrontal cortex (later in development) can account for these behavioral results. Our explanation differs from most of the current approaches in developmental science as we put less emphasis on the contribution of lateral prefrontal areas. We discuss shortcomings of the theories that propose a functional subdivision of these areas and their difficulty in accounting for results from monkey lesion and infant studies. We believe that this shift in focus is desirable both in light of what recent results on medial temporal lobe processing reveal about object working memory, and given how well these results fit the behavioral developmental data.

Developmental psychobiology and response to threats: relevance to trauma in children and adolescents

Interest in developmental and psychobiological aspects of trauma has grown with recent research in adults with mood and anxiety disorders reporting histories of trauma during childhood. Studies conducted directly in children and adolescents could add much to ongoing research in this area. This review summarizes data in three areas of developmental science that might inform future studies. First, the review briefly summarizes current data on clinical aspects of trauma in juveniles, focusing on associations with psychopathology and moderators of outcome. Second, the review summarizes data from the basic sciences delineating experiential and developmental changes in brain systems involved in threat perception and response. This review incorporates knowledge gained from research examining the effects of rearing manipulations on regulation of the stress response in rodents and primates. Third, the review summarizes data from cognitive neuroscience studies among both adults and children, again focusing on studies examining aspects of the threat response. This summary includes a review from studies in patients with posttraumatic stress disorder.

Unpacking the cognitive map: the parallel map theory of hippocampal function

In the parallel map theory, the hippocampus encodes space with 2 mapping systems. The bearing map is constructed primarily in the dentate gyrus from directional cues such as stimulus gradients. The sketch map is constructed within the hippocampus proper from positional cues. The integrated map emerges when data from the bearing and sketch maps are combined. Because the component maps work in parallel, the impairment of one can reveal residual learning by the other. Such parallel function may explain paradoxes of spatial learning, such as learning after partial hippocampal lesions, taxonomic and sex differences in spatial learning, and the function of hippocampal neurogenesis. By integrating evidence from physiology to phylogeny, the parallel map theory offers a unified explanation for hippocampal function.

Ontogenetic dissociation between habit learning and recognition memory in capuchin monkeys (Cebus apella)

The performance of young and adult capuchin monkeys (Cebus apella) on a Concurrent Discrimination Learning (CDL) test and a Delayed Non-Matching to Sample (DNMS) task were investigated. Results indicate that all subjects were able to learn the CDL test with 20-pairs simultaneously and retain this stimulus/reward association within 24-h interval. In contrast, young subjects did not perform the DNMS task with the same proficiency as adults. While adults' scores were above chance across all memory test delays, the young capuchin monkeys performed the test by chance level. These results support the hypothesis that these two tasks require different cognitive processes mediated by two independent neural systems with a differentiated ontogenetic development. Moreover, they provide evidence that this dissociation occurs not only in humans and Old World monkeys but also in the New World capuchin monkeys indicating that this species can be a valuable alternative model for investigations of the neurobiological basis of memory.