Memory and the hippocampus in food-storing birds: a comparative approach

American crows that excel at tool use activate neural circuits distinct from less talented individuals

Tools enable animals to exploit and command new resources. However, the neural circuits underpinning tool use and how neural activity varies with an animal's tool proficiency, are only known for humans and some other primates. We use 18F-fluorodeoxyglucose positron emission tomography to image the brain activity of naïve vs trained American crows (Corvus brachyrhynchos) when presented with a task requiring the use of stone tools. As in humans, talent affects the neural circuits activated by crows as they prepare to execute the task. Naïve and less proficient crows use neural circuits associated with sensory- and higher-order processing centers (the mesopallium and nidopallium), while highly proficient individuals increase activity in circuits associated with motor learning and tactile control (hippocampus, tegmentum, nucleus basorostralis, and cerebellum). Greater proficiency is found primarily in adult female crows and may reflect their need to use more cognitively complex strategies, like tool use, to obtain food.

Parental favoritism in a wild bird population

In most taxa with altricial young, offspring solicit food from their parents using a combination of visual and acoustic stimuli, but exactly what these young are communicating, and how selection shapes parental responses, remains unresolved. Theory posits that parents' interpretation and response to begging should vary with the likelihood of a return on their investment. We tested this in a wild population of prothonotary warblers (Protonotaria citrea), predicting that parents bias food non-randomly toward certain individuals within their broods depending on both the size and number of offspring. We observed parent-offspring interactions and detected strong dependence between brood size and nestling size in shaping parental responses to begging. Larger siblings were less likely to solicit food during feeding events than their smaller siblings, but they received a disproportionate share from parents in nests containing fewer-than-average young, whereas the smaller-than-average nestlings were disproportionately fed in broods containing a greater-than-average number of young. These findings suggest that parents respond to begging signals according to multiple social cues, favoring the stronger siblings with greater survival prospects when few copies of their genes are present, but overtly favoring runts to ensure whole-brood survival when capable of fledging more young. Future experimental studies may shed light on the contributions of parental decision-making and memory, how young nestlings learn in parent-offspring communication systems, and the adaptive significance of these behaviors.

Linking ecology and cognition: does ecological specialisation predict cognitive test performance?

Variation in cognitive abilities is thought to be linked to variation in brain size, which varies across species with either social factors (Social Intelligence Hypothesis) or ecological challenges (Ecological Intelligence Hypothesis). However, the nature of the ecological processes invoked by the Ecological Intelligence Hypothesis, like adaptations to certain habitat characteristics or dietary requirements, remains relatively poorly known. Here, we review comparative studies that experimentally investigated interspecific variation in cognitive performance in relation to a species’ degree of ecological specialisation. Overall, the relevant literature was biased towards studies of mammals and birds as well as studies focusing on ecological challenges related to diet. We separated ecological challenges into those related to searching for food, accessing a food item and memorising food locations. We found interspecific variation in cognitive performance that can be explained by adaptations to different foraging styles. Species-specific adaptations to certain ecological conditions, like food patch distribution, characteristics of food items or seasonality also broadly predicted variation in cognitive abilities. A species’ innovative problem-solving and spatial processing ability, for example, could be explained by its use of specific foraging techniques or search strategies, respectively. Further, habitat generalists were more likely to outperform habitat specialists. Hence, we found evidence that ecological adaptations and cognitive performance are linked and that the classification concept of ecological specialisation can explain variation in cognitive performance only with regard to habitat, but not dietary specialisation.

The ability of oriental magpies (Pica serica) to solve baited multiple-string problems

Background

Baited multiple-string problems are commonly used in avian laboratory studies to evaluate complex cognition. Several bird species possess the ability to use a string pull for obtaining food.

Methods

We initially tested and trained 11 magpies to determine whether the oriental magpie (Pica sericia) possesses the ability to solve baited multiple-string problems. Eight of the birds obtained the bait by pulling, and were selected for formal multiple-string tasks in the second stage. Second stage tests were divided into seven tasks based on string configurations.

Results

Only two magpies were able to solve two tasks: one solved the task of parallel strings, and the other solved the task of slanted strings with the bait farther from the middle point between the two strings and selected the short string in the task of long-short strings. When faced with more difficult tasks (i.e., the task of slanted strings with the bait closer to the middle point between the two strings, the task with two crossing strings, and the task of continuity and discontinuity), the birds initially observed the tasks and chose instead to adopt simpler strategies based on the proximity principle, side bias strategies and trial-and-error learning. Our results indicate that the oriental magpie had a partial understanding of the principle of multiple-string problems but adopted simpler strategies.

Hippocampal Contribution to Ordinal Psychological Time in the Human Brain

The chronology of events in time-space is naturally available to the senses, and the spatial and temporal dimensions of events entangle in episodic memory when navigating the real world. The mapping of time-space during navigation in both animals and humans implicates the hippocampal formation. Yet, one arguably unique human trait is the capacity to imagine mental chronologies that have not been experienced but may involve real events-the foundation of causal reasoning. Herein, we asked whether the hippocampal formation is involved in mental navigation in time (and space), which requires internal manipulations of events in time and space from an egocentric perspective. To address this question, we reanalyzed a magnetoencephalography data set collected while participants self-projected in time or in space and ordered historical events as occurring before/after or west/east of the mental self [Gauthier, B., Pestke, K., & van Wassenhove, V. Building the arrow of time… Over time: A sequence of brain activity mapping imagined events in time and space. Cerebral Cortex, 29, 4398-4414, 2019]. Because of the limitations of source reconstruction algorithms in the previous study, the implication of hippocampus proper could not be explored. Here, we used a source reconstruction method accounting explicitly for the hippocampal volume to characterize the involvement of deep structures belonging to the hippocampal formation (bilateral hippocampi [hippocampi proper], entorhinal cortices, and parahippocampal cortex). We found selective involvement of the medial temporal lobes (MTLs) with a notable lateralization of the main effects: Whereas temporal ordinality engaged mostly the left MTL, spatial ordinality engaged mostly the right MTL. We discuss the possibility of a top-down control of activity in the human hippocampal formation during mental time (and space) travels.

MicroRNA-19b restricts Wnt7b to the hem, which induces aspects of hippocampus development in the avian forebrain

The functions of the hippocampus are conserved between birds and mammals; however, it is not known whether similar mechanisms are responsible for its development in these two classes. In mammals, hippocampus development is known to be regulated by the hem organizer. Here, we have identified that, in birds, Wnt7b secreted from the hem is sufficient for inducing the expression of hippocampal markers. Furthermore, we have demonstrated that a microRNA, miR-19b, which is selectively excluded from the hem region, is necessary and sufficient for restricting the expression of Wnt7b to the hem. This study suggests that the role of the Wnt signal emanating from the hem is conserved between birds and mammals, and that a microRNA-based mechanism is crucial for determining the position of the hippocampus.

Search Behavior in Goat (Capra hircus) Kids From Mothers Kept at Different Animal Densities Throughout Pregnancy

Individual differences in cognitive performance are often reported but factors related to variation within species are rarely addressed. Goats (Capra hircus) have been subjects of many cognitive studies recently but without focus on individual variation. Among others, factors such as prenatal stress and sex of the individual have been proposed as possible explanations for individual variation in cognitive skills. We aimed to study whether prenatal environment, prenatal stress, litter size, sex, and birth weight influences search behavior skills of goat kids. Pregnant Norwegian dairy goats were exposed to different spatial allowance (namely 1.0, 2.0, or 3.0 m2 per animal) within the commercially applied range during pregnancy and their serum cortisol levels were measured six times within this period. Twenty-six of the kids born entered a three-stage searching task with increasing difficulty when they were 6 weeks old. The tasks included finding a bucket of milk: while moving (stage 1), after moving and disappearing behind a curtain (stage 2), and moving behind a displacement device and the device moving behind a curtain while hiding the bucket (stage 3). We found that prenatal animal density had no effect on the search skills of the offspring, while kids with higher prenatal maternal cortisol levels performed better at the highest stage tested: finding an object after single invisible displacement. At this stage, singleton kids and males performed better than twins and females. Birth weight had no effect at this stage. The findings suggest that maternal cortisol in the observed range had a facilitating effect on cognitive development of goat kids.

Neuroethology of spatial cognition

A key challenge for animals is recognising locations and navigating between them. These capacities are varied: we can remember where our car is parked at the mall, rats are able to remember where their nest location is while foraging for food morsels, and bats are able to fly directly to a favourite fruit tree 20 kilometers from their home cave. These spatial abilities, whether commonplace or remarkable, raise fundamental questions. First, how do animals find their way? Second, how does the brain represent the outside world? In this Primer, we will address both questions from the perspective of rodent cognition and neuroscience.

Comparative Views of Adult Neurogenesis

Traditional views maintain that the generation of neurons within the mammalian brain is restricted to a discrete developmental period. This perspective has undergone significant revision during the later half of this century, culminating recently with the demonstration of neurogenesis in the brains of adult primates, including humans. Although it is becoming increasingly clear that adult neurogenesis represents an important mode of structural modification for the adult brain, its functional significance has not been determined. The production and survival of new neurons in the adult mammalian brain is regulated by both experiential and neuroendocrine factors, suggesting that adult-generated neurons may serve as a substrate by which these cues influence normal brain function. This article reviews significant advances that have led to the discovery of neurogenesis in adult mammals and examines comparative data suggesting that adult neurogenesis may play a role in certain forms of learning. Neural activity associated with behavioral experience is known to result in changes in brain structure and connectivity, for example, by modifying synapse number, axonal sprouting, dendrite length and branching, or synaptic strength. In the case of adult neurogenesis, experience may shape neural networks by directing the production and connectivity of whole cell populations.

Differential foraging success across a light level spectrum explains the maintenance and spatial structure of colour morphs in a polymorphic bird

Detectability of different colour morphs under varying light conditions has been proposed as an important driver in the maintenance of colour polymorphism via disruptive selection. To date, no studies have tested whether different morphs have selective advantages under differing light conditions. We tested this hypothesis in the black sparrowhawk, a polymorphic raptor exhibiting a discrete white and dark morph, and found that prey provisioning rates differ between the morphs depending on light condition. Dark morphs delivered more prey in lower light conditions, while white morphs provided more prey in brighter conditions. We found support for the role of breeding season light level in explaining the clinal pattern of variation in morph ratio across the species range throughout South Africa. Our results provide the first empirical evidence supporting the hypothesis that polymorphism in a species, and the spatial structuring of morphs across its distribution, may be driven by differential selective advantage via improved crypsis, under varying light conditions.

Superior pattern processing is the essence of the evolved human brain

Humans have long pondered the nature of their mind/brain and, particularly why its capacities for reasoning, communication and abstract thought are far superior to other species, including closely related anthropoids. This article considers superior pattern processing (SPP) as the fundamental basis of most, if not all, unique features of the human brain including intelligence, language, imagination, invention, and the belief in imaginary entities such as ghosts and gods. SPP involves the electrochemical, neuronal network-based, encoding, integration, and transfer to other individuals of perceived or mentally-fabricated patterns. During human evolution, pattern processing capabilities became increasingly sophisticated as the result of expansion of the cerebral cortex, particularly the prefrontal cortex and regions involved in processing of images. Specific patterns, real or imagined, are reinforced by emotional experiences, indoctrination and even psychedelic drugs. Impaired or dysregulated SPP is fundamental to cognitive and psychiatric disorders. A broader understanding of SPP mechanisms, and their roles in normal and abnormal function of the human brain, may enable the development of interventions that reduce irrational decisions and destructive behaviors.

Comparison of learning ability and memory retention in altricial (Bengalese finch, Lonchura striata var. domestica) and precocial (blue-breasted quail, Coturnix chinensis) birds using a color discrimination task

The present study sought to assess the potential application of avian models with different developmental modes to studies on cognition and neuroscience. Six altricial Bengalese finches (Lonchura striata var. domestica), and eight precocial blue-breasted quails (Coturnix chinensis) were presented with color discrimination tasks to compare their respective faculties for learning and memory retention within the context of the two developmental modes. Tasks consisted of presenting birds with discriminative cues in the form of colored feeder lids, and birds were considered to have learned a task when 80% of their attempts at selecting the correctly colored lid in two consecutive blocks of 10 trials were successful. All of the finches successfully performed the required experimental tasks, whereas only half of the quails were able to execute the same tasks. In the learning test, finches required significantly fewer trials than quails to learn the task (finches: 13.5 ± 9.14 trials, quails: 45.8 ± 4.35 trials, P < 0.05), with finches scoring significantly more correct responses than quails (finches: 98.3 ± 4.08%, quails: 85.0 ± 5.77% at the peak of the learning curve). In the memory retention tests, which were conducted 45 days after the learning test, finches retained the ability to discriminate between colors correctly (95.0 ± 4.47%), whereas quails did not retain any memory of the experimental procedure and so could not be tested. These results suggested that altricial and precocial birds both possess the faculty for learning and retaining discrimination-type tasks, but that altricial birds perform better than precocial birds in both faculties. The present findings imply that developmental mode is an important consideration for assessing the suitability of bird species for particular experiments.

Morphometric analysis of telencephalic structure in a variety of neognath and paleognath bird species reveals regional differences associated with specific behavioral traits

Birds exhibit a huge array of behavior, ecology and physiology, and occupy nearly every environment on earth, ranging from the desert outback of Australia to the tropical rain forests of Panama. Some birds have adopted a fully nocturnal lifestyle, such as the barn owl and kiwi, while others, such as the albatross, spend nearly their entire life flying over the ocean. Each species has evolved unique adaptations over millions of years to function in their respective niche. In order to increase processing power or network efficiency, many of these adaptations require enlargements and/or specializations of the brain as a whole or of specific brain regions. In this study, we examine the relative size and morphology of 9 telencephalic regions in a number of Paleognath and Neognath birds and relate the findings to differences in behavior and sensory ecology. We pay particular attention to those species that have undergone a relative enlargement of the telencephalon to determine whether this relative increase in telencephalic size is homogeneous across different brain regions or whether particular regions have become differentially enlarged. The analysis indicates that changes in the relative size of telencephalic regions are not homogeneous, with every species showing hypertrophy or hypotrophy of at least one of them. The three-dimensional structure of these regions in different species was also variable, in particular that of the mesopallium in kiwi. The findings from this study provide further evidence that the changes in relative brain size in birds reflect a process of mosaic evolution.

Functional and evolutionary trade-offs co-occur between two consolidated memory phases in Drosophila melanogaster

Memory is a complex and dynamic process that is composed of different phases. Its evolution under natural selection probably depends on a balance between fitness benefits and costs. In Drosophila, two separate forms of consolidated memory phases can be generated experimentally: anaesthesia-resistant memory (ARM) and long-term memory (LTM). In recent years, several studies have focused on the differences between these long-lasting memory types and have found that, at the functional level, ARM and LTM are antagonistic. How this functional relationship will affect their evolutionary dynamics remains unknown. We selected for flies with either improved ARM or improved LTM over several generations, and found that flies selected specifically for improvement of one consolidated memory phase show reduced performance in the other memory phase. We also found that improved LTM was linked to decreased longevity in male flies but not in females. Conversely, males with improved ARM had increased longevity. We found no correlation between either improved ARM or LTM and other phenotypic traits. This is, to our knowledge, the first evidence of a symmetrical evolutionary trade-off between two memory phases for the same learning task. Such trade-offs may have an important impact on the evolution of cognitive capacities. On a neural level, these results support the hypothesis that mechanisms underlying these forms of consolidated memory are, to some degree, antagonistic.

Self-organization of multiple spatial and context memories in the hippocampus

One obstacle to understanding the exact processes unfolding inside the hippocampus is that it is still difficult to clearly define what the hippocampus actually does, at the system level. Associated for a long time with the formation of episodic and semantic memories, and with their temporary storage, the hippocampus is also regarded as a structure involved in spatial navigation. These two independent perspectives on the hippocampus are not necessarily exclusive: proposals have been put forward to make them fit into the same conceptual frame. We review both approaches and argue that three critical developments need consideration: (a) recordings of neuronal activity in rodents, revealing beautiful spatial codes expressed in entorhinal cortex, upstream of the hippocampus; (b) comparative behavioral results suggesting, in an evolutionary perspective, qualitative similarity of function across homologous structures with a distinct internal organization; (c) quantitative measures of information, shifting the focus from who does what to how much each neuronal population expresses each code. These developments take the hippocampus away from philosophical discussions of all-or-none cause-effect relations, and into the quantitative mainstream of modern neural science.

Problems faced by food-caching corvids and the evolution of cognitive solutions

The scatter hoarding of food, or caching, is a widespread and well-studied behaviour. Recent experiments with caching corvids have provided evidence for episodic-like memory, future planning and possibly mental attribution, all cognitive abilities that were thought to be unique to humans. In addition to the complexity of making flexible, informed decisions about caching and recovering, this behaviour is underpinned by a motivationally controlled compulsion to cache. In this review, we shall first discuss the compulsive side of caching both during ontogeny and in the caching behaviour of adult corvids. We then consider some of the problems that these birds face and review the evidence for the cognitive abilities they use to solve them. Thus, the emergence of episodic-like memory is viewed as a solution for coping with food perishability, while the various cache-protection and pilfering strategies may be sophisticated tools to deprive competitors of information, either by reducing the quality of information they can gather, or invalidating the information they already have. Finally, we shall examine whether such future-oriented behaviour involves future planning and ask why this and other cognitive abilities might have evolved in corvids.

The ecological relevance of sleep: the trade-off between sleep, memory and energy conservation

All animals in which sleep has been studied express signs of sleep-like behaviour, suggesting that sleep must have some fundamental functions that are sustained by natural selection. Those functions, however, are still not clear. Here, we examine the ecological relevance of sleep from the perspective of behavioural trade-offs that might affect fitness. Specifically, we highlight the advantage of using food-caching animals as a system in which a conflict might occur between engaging in sleep for memory/learning and hypothermia/torpor to conserve energy. We briefly review the evidence for the importance of sleep for memory, the importance of memory for food-caching animals and the conflicts that might occur between sleep and energy conservation in these animals. We suggest that the food-caching paradigm represents a naturalistic and experimentally practical system that provides the opportunity for a new direction in sleep research that will expand our understanding of sleep, especially within the context of ecological and evolutionary processes.

Are bigger brains better?

Attempts to relate brain size to behaviour and cognition have rarely integrated information from insects with that from vertebrates. Many insects, however, demonstrate that highly differentiated motor repertoires, extensive social structures and cognition are possible with very small brains, emphasising that we need to understand the neural circuits, not just the size of brain regions, which underlie these feats. Neural network analyses show that cognitive features found in insects, such as numerosity, attention and categorisation-like processes, may require only very limited neuron numbers. Thus, brain size may have less of a relationship with behavioural repertoire and cognitive capacity than generally assumed, prompting the question of what large brains are for. Larger brains are, at least partly, a consequence of larger neurons that are necessary in large animals due to basic biophysical constraints. They also contain greater replication of neuronal circuits, adding precision to sensory processes, detail to perception, more parallel processing and enlarged storage capacity. Yet, these advantages are unlikely to produce the qualitative shifts in behaviour that are often assumed to accompany increased brain size. Instead, modularity and interconnectivity may be more important.

Remarkable spatial memory in a migratory cardinalfish

The ability to orient and navigate within a certain environment is essential for all animals, and spatial memory enables animals to remember the locations of such markers as predators, home, and food. Here we report that the migratory marine cardinalfish Apogon notatus has the potential to retain long-term spatial memory comparable to that of other animals. Female A. notatus establish a small territory on a shallow boulder bottom to pair and spawn with males. We carried out field research in two consecutive breeding seasons on territory settlement by individually marked females. Females maintained a territory at the same site throughout one breeding season. After overwintering in deep water, many of them (82.1%) returned to their breeding ground next spring and most occupied the same site as in the previous season, with only a 0.56 m shift on average. Our results suggest that female A. notatus have long-distance homing ability to pinpoint the exact location of their previous territory, and retain spatial memory for as long as 6 months.

Brain organization mirrors caste differences, colony founding and nest architecture in paper wasps (Hymenoptera: Vespidae)

The cognitive challenges that social animals face depend on species differences in social organization and may affect mosaic brain evolution. We asked whether the relative size of functionally distinct brain regions corresponds to species differences in social behaviour among paper wasps (Hymenoptera: Vespidae). We measured the volumes of targeted brain regions in eight species of paper wasps. We found species variation in functionally distinct brain regions, which was especially strong in queens. Queens from species with open-comb nests had larger central processing regions dedicated to vision (mushroom body (MB) calyx collars) than those with enclosed nests. Queens from advanced eusocial species (swarm founders), who rely on pheromones in several contexts, had larger antennal lobes than primitively eusocial independent founders. Queens from species with morphologically distinct castes had augmented central processing regions dedicated to antennal input (MB lips) relative to caste monomorphic species. Intraspecific caste differences also varied with mode of colony founding. Independent-founding queens had larger MB collars than their workers. Conversely, workers in swarm-founding species with decentralized colony regulation had larger MB calyx collars and optic lobes than their queens. Our results suggest that brain organization is affected by evolutionary transitions in social interactions and is related to the environmental stimuli group members face.

Cell proliferation and death in the brain of active and hibernating frogs

'Binomial' cell proliferation and cell death have been studied in only a few non-mammalian vertebrates, such as fish. We thought it of interest to map cell proliferation/apoptosis in the brain of the frog (Rana esculenta L.) as this animal species undergoes, during the annual cycle, physiological events that could be associated with central nervous system damage. Therefore, we compared the active period and the deep underground hibernation of the frog. Using western blot analysis for proliferating cell nuclear antigen (PCNA), we revealed a positive 36 kDa band in all samples and found higher optical density values in the hibernating frogs than in active frogs. In both active and hibernating frogs, we found regional differences in PCNA-immunoreactive cells and terminal transferase dUTP nick-end labelling apoptotic cells in the ventricular zones and parenchyma areas of the main encephalon subdivisions. During the active period of the frogs, the highest concentration of PCNA-immunoreactive cells was found in the ventricle dorsal zone of the cerebral hemispheres but only some of the cells were apoptotic. By contrast, the tectal and cerebellar ventricular zones had a small or medium amount of PCNA-immunoreactive cells, respectively, and a higher number of apoptotic cells. During hibernation, an increased PCNA-immunoreactive cell number was observed in both the brain ventricles and parenchyma compared with active frogs. This increase was primarily evident in the lateral ventricles, a region known to be a proliferation 'hot spot'. Although differences existed among the brain areas, a general increase of apoptotic cell death was found in hibernating frogs, with the highest number of apoptotic cells being detected in the parenchyma of the cerebral hemispheres and optic tectum. In particular, the increased number of apoptotic cells in the hibernating frogs compared with active frogs in the parenchyma of these brain areas occurred when cell proliferation was higher in the corresponding ventricular zones. We suggest that the high number of dying cells found in the parenchymal regions of hibernating frogs might provide the stimulus for the ventricular zones to proliferate. Hibernating frogs could utilize an increased cell proliferation in the brain areas as a neuroprotective strategy to face cell death and the onset of neurological damages. Therefore, the hibernator promises to be a valuable model for studying the mechanisms naturally carried out by the central nervous system in order to adapt itself or survive adverse conditions.

What is the mammalian dentate gyrus good for?

In the mammalian hippocampus, the dentate gyrus (DG) is characterized by sparse and powerful unidirectional projections to CA3 pyramidal cells, the so-called mossy fibers (MF). The MF form a distinct type of synapses, rich in zinc, that appear to duplicate, in terms of the information they convey, what CA3 cells already receive from entorhinal cortex layer II cells, which project both to the DG and to CA3. Computational models have hypothesized that the function of the MF is to enforce a new, well-separated pattern of activity onto CA3 cells, to represent a new memory, prevailing over the interference produced by the traces of older memories already stored on CA3 recurrent collateral connections. Although behavioral observations support the notion that the MF are crucial for decorrelating new memory representations from previous ones, a number of findings require that this view be reassessed and articulated more precisely in the spatial and temporal domains. First, neurophysiological recordings indicate that the very sparse dentate activity is concentrated on cells that display multiple but disorderly place fields, unlike both the single fields typical of CA3 and the multiple regular grid-aligned fields of medial entorhinal cortex. Second, neurogenesis is found to occur in the adult DG, leading to new cells that are functionally added to the existing circuitry, and may account for much of its ongoing activity. Third, a comparative analysis suggests that only mammals have evolved a DG, despite some of its features being present also in reptiles, whereas the avian hippocampus seems to have taken a different evolutionary path. Thus, we need to understand both how the mammalian dentate operates, in space and time, and whether evolution, in other vertebrate lineages, has offered alternative solutions to the same computational problems.

Hippocampal size positively correlates with verbal IQ in male children

Historically, there have been numerous proposals that the size of the brain correlates with its capacity to process information. Little is known, however, about which specific brain regions contribute to this correlation in children and adolescents. This study evaluated the relationship between intelligence and the size of various brain structures in typically developing male children 8-18 yrs of age. Magnetic resonance imaging (MRI) scans were used to measure the volume of the cerebrum, cerebral gray and white matter, cerebellum, amygdala, and hippocampus. Gray matter and hippocampal volume significantly correlated with full scale and verbal IQ. Since the hippocampus strongly correlated with verbal but not performance IQ, our findings reinforce the hypothesis that the hippocampus is involved in declarative and semantic learning, which contributes more notably to verbal IQ, than to performance IQ. Given the substantial evidence for environmentally induced changes in hippocampal structure, an unresolved issue is whether this relationship reflects genetically determined individual variation or learning induced plasticity.

A comparative framework for understanding the biological principles of adult neurogenesis

Adult neurogenesis has been identified in all vertebrate species examined thus far. However, an evolutionary trend towards a reduction in both the number of proliferation zones and the overall number of newborn cells has been revealed in more recent lineages of vertebrates, such as mammals. Adult neurogenesis, and in particular the characterization of adult neural stem cells in mammals has been the focus of intense research with the goal of developing new cell-based regenerative treatments for neurodegenerative diseases, spinal cord injury, and acute damage due to stroke. Conversely, most other vertebrate classes, which display widespread production of adult neurons, are not typically used as model systems in this context. A more profound understanding of the structural composition and the mechanisms that support proliferation zones in the mature brain have become critical for revealing how adult neural stem cells are maintained in these regions and how they regulate neurogenesis. In this review we argue that comprehensive analyses of adult neurogenesis in various vertebrate and invertebrate species will lead to a more complete understanding of the fundamental biology and evolution of adult neurogenesis and provide a better framework for testing hypotheses regarding the functional significance of this trait.

Dynamic changes of apoptosis and expression of Bcl-2 family members in the posthatch hippocampus of Bengalese finches

The hippocampus of songbirds plays an important role in spatial memory, and probably in song learning. Although prolonged neuronal generation and apoptosis are thought to be closely correlated with memory function, natural changes of the number of neurons and in apoptosis in the hippocampus of songbirds have not been fully investigated during development and in the adult. In the current study, we examined developmental changes in the volume and the number of neurons and apoptotic cells in the hippocampus of songbirds (Lonchura striata) from posthatch day (P5) to adulthood. Apoptotic cells were determined by Nissl staining and immunohistochemistry for cleaved caspase-3, a key apoptotic caspase executioner. The expression levels of Bcl-2 family member mRNA and protein, including Bcl-2, Bcl-xL and Bax, were also investigated. Our results indicated that: (1) the hippocampus volume significantly increased from P5 to P60, although the number of neurons remained stable in all studied stages; (2) the number of apoptotic cells was highest at P45, based either on the Nissl staining or on the immunohistochemistry for caspase-3; (3) Bcl-2 mRNA expression was high from P5 to adulthood, while Bax mRNA declined abruptly from P5 to adulthood, and Bcl-x mRNA was high after P45. Bcl-2 protein was only detected at P5 and P15, while detection of Bcl-xL and Bax proteins paralleled levels of mRNA expression. Our study provides detailed changes of apoptosis in the posthatch songbird hippocampus, suggesting an important role for caspase-3 and Bcl-2 family members in hippocampus apoptosis.

A Multi-disciplinary Approach to Understanding Hippocampal Function in Foodhoarding Birds

Spatial memory and the hippocampal formation (HF) of food-hoarding birds have been put forward as a prime example of how natural selection has shaped a cognitive system and its neural underpinnings. Here, I review what we know about the HF of hoarding birds and lay out the work that is currently underway to use this system to obtain a better understanding of hippocampal function in general. This interdisciplinary programme includes evolutionary, ecological, psychological, ethological, and neuroscientific approaches to the study of behaviour and cognition. Firstly, we need to understand the behaviour of the birds in their natural environment, and identify the aspects of cognition and behaviour that may be especially valuable for the species under study. Secondly, these cognitive and behavioural traits are compared to closely-related non-hoarding species. Thirdly, we also compare HF anatomy between closely-related hoarding and non-hoarding species, identifying possible neural mechanisms underlying behavioural differences. Finally, behavioural and neuroscientific approaches are combined in experiments directly investigating the involvement of the HF or any of its anatomical and physiological aspects in the behaviours under study. This process loops back upon itself in many different ways, with all the different approaches informing each other. In this way we are making progress in understanding the functioning of the HF, not only in food-hoarding birds, but in all vertebrates.

Distribution of substance P reveals a novel subdivision in the hippocampus of parasitic South American cowbirds

Parasitic cowbirds monitor potential hosts' nests and return to lay when appropriate, a task that is likely to involve spatial recall. Seasonal and sexual behavioral variations in the cowbirds correlate with anatomical changes in the hippocampal formation. During the breeding season, parasites have larger hippocampal formations than nonparasites. In parasitic species in which females alone perform nest bookkeeping, females have larger hippocampal formations than males. We investigated the distribution of the neuropeptide substance P (SP) in three sympatric cowbirds: two obligate parasites (shiny cowbird and screaming cowbird) and one nonparasite (bay-winged cowbird). Distribution of SP was similar to that in other songbirds, except for a previously undescribed field of dense SP-rich terminals within the hippocampus that we call the hippocampal SP terminal field (SPh). We found robust species differences in the volume of this new area, measured relative to the remainder of the telencephalon. SPh was largest in the generalist parasite (shiny cowbird) and smallest in the nonparasitic species (bay-winged cowbird). In the specialist parasite (screaming cowbird), SPh was smaller than in the generalist parasite but larger than in the nonparasitic species. SPh overlaps with two subdivisions described in the pigeon that have been related to the mammalian dentate gyrus and subiculum. The area containing SPh receives a major input from the lateral mammillary nucleus, which is probably the avian equivalent of the mammalian supramammillary nucleus (SUM), the main source of extrinsic SP input to mammalian hippocampus. SPh may be the termination of a pathway homologous to the SP-rich projection from SUM to the hippocampus in mammals.

Spatial response properties of homing pigeon hippocampal neurons: correlations with goal locations, movement between goals, and environmental context in a radial-arm arena

The amniote hippocampal formation plays an evolutionarily-conserved role in the neural representation of environmental space. However, species differences in spatial ecology nurture the expectation of species differences in how hippocampal neurons represent space. To determine the spatial response properties of homing pigeon ( Columba livia) HFneurons, we recorded from isolated units in birds freely navigating a radial arena in search of food present at four goal locations. Fifty of 76 neurons displayed firing rate variations that could be placed into three response categories. Location cells ( n=25) displayed higher firing rates at restricted locations in the arena space, often in proximity to goal locations. Path cells ( n=13) displayed higher firing rates as a pigeon moved between a subset of goal locations. Arena-off cells ( n=12) were more active when a pigeon was in a baseline holding space compared to inside the arena. Overall, reliability and coherence scores of the recorded neurons were lower compared to rat place cells. The differences in the spatial response profiles of pigeon hippocampal formation neurons, when compared to rats, provide a departure point for better understanding the relationship between spatial behavior and how hippocampal formation neurons participate in the representation of space.

Bigger brains or bigger nuclei? Regulating the size of auditory structures in birds

Increases in the size of the neuronal structures that mediate specific behaviors are believed to be related to enhanced computational performance. It is not clear, however, what developmental and evolutionary mechanisms mediate these changes, nor whether an increase in the size of a given neuronal population is a general mechanism to achieve enhanced computational ability. We addressed the issue of size by analyzing the variation in the relative number of cells of auditory structures in auditory specialists and generalists. We show that bird species with different auditory specializations exhibit variation in the relative size of their hindbrain auditory nuclei. In the barn owl, an auditory specialist, the hindbrain auditory nuclei involved in the computation of sound location show hyperplasia. This hyperplasia was also found in songbirds, but not in non-auditory specialists. The hyperplasia of auditory nuclei was also not seen in birds with large body weight suggesting that the total number of cells is selected for in auditory specialists. In barn owls, differences observed in the relative size of the auditory nuclei might be attributed to modifications in neurogenesis and cell death. Thus, hyperplasia of circuits used for auditory computation accompanies auditory specialization in different orders of birds.

Separate processing mechanisms for encoding of geometric and landmark information in the avian hippocampus

Domestic chicks bilaterally or unilaterally lesioned to the hippocampus were trained to search for food hidden beneath sawdust by ground-scratching in the centre of a large enclosure, the correct position of food being indicated by a local landmark in the absence of any extra-enclosure visual cues. At test, the landmark was removed or displaced at a distance from its original position. Results showed that sham-operated chicks and chicks with a lesion of the left hippocampus searched in the centre, relying on large-scale geometric information provided by the enclosure, whereas chicks with a lesion of either the right hippocampus or both hippocampi were completely disoriented (landmark removed) or searched close to the landmark shifted from the centre (landmark displaced). These results indicate that encoding of geometric features of an enclosure occurs in the right hippocampus even when local information provided by a landmark would suffice to localize the goal; encoding based on local information, in contrast, seems to occur outside the hippocampus. These findings provide evidence that the left and right avian hippocampi play different roles in spatial cognition, a phenomenon which had been documented previously only for the human hippocampus.

Intrahippocampal connections in the pigeon (Columba livia) as revealed by stimulation evoked field potentials

The hippocampal formation (HF) of mammals and birds is crucial for spatial learning and memory. However, although the underlying synaptic organization and connectivity of the mammalian HF are well characterized, comparatively little is known about the avian HF. Localized regions of the homing pigeon HF were stimulated at 400-600 microA while evoked field potentials (EFPs) were recorded from adjacent and more distant HF areas relative to the stimulation site. The shortest discernible EFP latency was 12.2 msec. The emerging connectivity profile (using the location of peak EFP amplitude after stimulation and making no determination of the number of intervening synapses) was characterized by projections from the dorsolateral (DL) HF to the dorsomedial (DM) HF (15-msec latency) at the same anterior/posterior (A/P) level, DM to ventrolateral (VL) and ventromedial (VM; 15 msec) HF across A/P levels, VM to VL (12 msec) and contralateral VM (15 msec) at the same A/P level, and VL to ventral DL (DLv; 15 msec) across A/P levels posterior to the stimulation site. Using these data as a first approximation, connectivity through the avian HF appears to be characterized by a discernible feed-forward network starting with a projection from DL to DM, DM to VL, VM, and contralateral VM, VM to VL, and VL to posterior ventral DLv. Although still speculative, the results suggest that the internal connectivity of the avian HF is similar to that of the mammalian HF, despite the large evolutionary divergence between the two taxa.

Life-long stability of neurons: a century of research on neurogenesis, neuronal death and neuron quantification in adult CNS

In this chapter we provide an extensive review of 100 years of research on the stability of neurons in the mammalian brain, with special emphasis on humans. Although Cajal formulated the Neuronal Doctrine, he was wrong in his beliefs that adult neurogenesis did not occur and adult neurons are dying throughout life. These two beliefs became accepted "common knowledge" and have shaped much of neuroscience research and provided much of the basis for clinical treatment of age-related brain diseases. In this review, we consider adult neurogenesis from a historical and evolutionary perspective. It is concluded, that while adult neurogenesis is a factor in the dynamics of the dentate gyrus and olfactory bulb, it is probably not a major factor during the life-span in most brain areas. Likewise, the acceptance of neuronal death as an explanation for normal age-related senility is challenged with evidence collected over the last fifty years. Much of the problem in changing this common belief of dying neurons was the inadequacies of neuronal counting methods. In this review we discuss in detail implications of recent improvements in neuronal quantification. We conclude: First, age-related neuronal atrophy is the major factor in functional deterioration of existing neurons and could be slowed down, or even reversed by various pharmacological interventions. Second, in most cases neuronal degeneration during aging is a pathology that in principle may be avoided. Third, loss of myelin and of the white matter is more frequent and important than the limited neuronal death in normal aging.

Hippocampal growth and maintenance depend on food-caching experience in juvenile mountain chickadees (Poecile gambeli)

This experiment investigated the development of caching behavior and the hippocampus (HF) in postfledging mountain chickadees (Poecile gambeli). From Days 35 to 53, the number of seeds stored increased but the proportion recovered did not. Birds that stored and recovered during 3 or more trials had significantly enlarged HF but not telencephalon volumes (experienced) compared with those that stored but did not recover (store only) and those deprived of caching experience altogether (deprived). HF size did not increase linearly with the number of experience trials. Birds that received less than 3 experience trials did not differ from deprived birds in HF size, suggesting a threshold effect. Experienced birds prevented from caching for 1 month had significantly smaller HF volumes than those examined immediately after caching experience and did not differ from deprived birds. Experience of both storing and recovery is required to initiate growth and maintain HF size.

Hemispheric memories for the content and position of food caches in the domestic chick

From Day 2 to Day 4 of life, chicks were fed daily in a large enclosure with 2 identical food caches, each filled with a different type of seed. On Day 5, binocular and monocular chicks were fed in their home cages 1 type of seed exclusively for 30 min. At test, soon after this devaluation phase, both binocular and right-eyed chicks chose the food caches containing the seeds that had not been devalued; in contrast, left-eyed chicks did not show a clear choice. Experiments revealed that the asymmetry was not due to lack of motivation, worse spatial memory, or inability to remember the consequences of devaluation by left-eyed chicks. Results suggest that young chicks can form declarative-like memories of the content of food caches. However, chicks using their left eye (which provides a supply mainly to the right hemisphere) failed to integrate memory of the content of food caches with memory of the consequences of devaluation.

Extrahippocampal temporal lobe atrophy in temporal lobe epilepsy and mesial temporal sclerosis

Visual inspection and volumetric analysis of MRIs allow mesial temporal sclerosis (MTS) to be reliably identified in patients with temporal lobe epilepsy. The presence of unilateral MTS ipsilateral to the side of habitual seizure onset is an indicator for the prognosis of good outcome after temporal lobe resection. There is evidence to suggest that widespread temporal lobe pathology, leading to atrophy, may be associated with MTS and such abnormal tissue may play an important role in epileptogenesis. We have analysed quantitatively the volumes of the mesial and lateral temporal lobe substructures in MRIs from 62 patients with intractable mesial temporal lobe epilepsy and in 20 normal controls. We found significant atrophy in these structures in patients, ranging from 8.3 to 18.4% compared with controls. The degree of atrophy in the extrahippocampal structures correlated with the degree of hippocampal atrophy, suggesting that a common process may be responsible. There was no correlation between the degree of atrophy in the extrahippocampal structures and the duration of epilepsy, a history of febrile convulsions or of generalized seizures. These findings suggest that there may be widespread pathological abnormalities in the temporal lobe associated with MTS. The importance of extrahippocampal atrophy to surgical outcome and whether it occurs in temporal lobe epilepsy not associated with MTS remain to be investigated.

Temporal and spatial adaptation to food restriction in mice under naturalistic conditions

Free-living female laboratory mice, adapted to outdoor life in large pens providing a naturalistic environment, were tested for their ability to modify their foraging habits to controlled food supply. An automatic feeder box delivered a small portion of the daily quantity of seeds to each individual mouse. Eight such boxes were placed into an outdoor pen. Each day, mice had to visit all boxes to gather the daily amount of food and were rewarded only at the first visit to each box. Mice were individually recognised by an implanted microchip. Throughout a 16-day period, feeding activity concentrated in an interval time around the beginning of the daily session. During the same period, the number of different feeders visited every day by mice increased irrespective of variation in exploratory activity. The experimental set-up allowed detecting temporal and spatial adaptations to the food restriction, as well as behavioural differences due to territorial and social factors. These data permit the design of novel tests assessing behavioural changes, memory and learning in normal and genetically modified mice, both in the laboratory and in naturalistic settings.