Memory in food-storing birds: from behaviour to brain

Using the sender-receiver framework to understand the evolution of languages-of-thought

This commentary seeks to supplement the case Quilty-Dunn et al. make for the psychological reality of languages-of-thought (LoTs) in two ways. First, it focuses on the reduced physical demands which LoT architectures often make compared to alternative architectures. Second, it embeds LoT research within a broader framework that can be leveraged to understand the evolution of LoTs.

Space, feature, and risk sensitivity in homing pigeons (Columba livia): Broadening the conversation on the role of the avian hippocampus in memory

David Sherry has been a pioneer in investigating the avian hippocampal formation (HF) and spatial memory. Following on his work and observations that HF is sensitive to the occurrence of reward (food), we were interested in carrying out an exploratory study to investigate possible HF involvement in the representation goal value and risk. Control sham-lesioned and hippocampal-lesioned pigeons were trained in an open field to locate one food bowl containing a constant two food pellets on all trials, and two variable bowls with one containing five pellets on 75% (High Variable) and another on 25% (Low Variable) of their respective trials (High-Variable and Low-Variable bowls were never presented together). One pairing of pigeons learned bowl locations (space); another bowl colors (feature). Trained to color, hippocampal-lesioned pigeons performed as rational agents in their bowl choices and were indistinguishable from the control pigeons, a result consistent with HF regarded as unimportant for non-spatial memory. By contrast, when trained to location, hippocampal-lesioned pigeons differed from the control pigeons. They made more first-choice errors to bowls that never contained food, consistent with a role of HF in spatial memory. Intriguingly, the hippocampal-lesioned pigeons also made fewer first choices to both variable bowls, suggesting that hippocampal lesions resulted in the pigeons becoming more risk averse. Acknowledging that the results are preliminary and further research is needed, the data nonetheless support the general hypothesis that HF-dependent memory representations of space capture properties of reward value and risk, properties that contribute to decision making when confronted with a choice.

Has the hippocampus really forgotten about space?

Several lines of evidence, including the discovery of place cells, have contributed to the notion that the hippocampus serves primarily to navigate the environment, as a repository of spatial memories, like a drawer full of charts; and that in some species it has exapted on this original one an episodic memory function. We argue that recent evidence questions the primacy of space, and points at memory load, whether spatial or not, as the challenge that mammalian hippocampal circuitry has evolved to meet.

Cowards or clever guys: an alternative nest defence strategy employed by shrikes against magpies

Red-backed shrikes (Lanius collurio) show a substantial variability in their nest defence behaviour, which usually follows the rules of optimal parental behaviour, vigorously attacking egg and chick predators and only passively guarding against harmless animals. Nevertheless, shrikes hesitate to attack the Eurasian magpie (Pica pica), which specializes in plundering passerine nests. Our previous studies have suggested that this behaviour may be the result of an alternative defence strategy, relying on nest crypsis. To test this hypothesis, at the shrike nests, we presented a magpie dummy associated with playbacks drawing the predators' attention to the presence of the nest. We predicted that the presentation of a magpie dummy associated with shrike alarm calls moves the parents to action, causing them to chase the magpie away from the nest. We showed that the presence of a magpie dummy associated with shrike alarm calls elicits a significantly more active response in shrike parents compared to a magpie dummy associated with neutral song. Parents actively moved around the dummy and produced alarm calls; nevertheless, most of the tested pairs hesitated to attack the dummy. We may conclude that the low nest defence activity of shrike parents towards magpie dummy was partly the result of an alternative strategy, which may be cancelled out by alerting the predator to the location of the nest; nevertheless, shrikes seem to be afraid of the magpie and hesitate to attack it physically.

Equal performance but distinct behaviors: sex differences in a novel object recognition task and spatial maze in a highly social cichlid fish

Sex differences in behavior and cognition can be driven by differential selection pressures from the environment and in the underlying neuromolecular mechanisms of decision-making. The highly social cichlid fish Astatotilapia burtoni exhibits dynamic and complex social hierarchies, yet explicit cognitive testing (outside of social contexts) and investigations of sex differences in cognition have yet to be fully explored. Here we assessed male and female A. burtoni in two cognitive tasks: a novel object recognition task and a spatial task. We hypothesized that males outperform females in a spatial learning task and exhibit more neophilic/exploratory behavior across both tasks. In the present study we find that both sexes prefer the familiar object in a novel object recognition task, but the time at which they exhibit this preference differs between the sexes. Females more frequently learned the spatial task, exhibiting longer decision latencies and quicker error correction, suggesting a potential speed-accuracy tradeoff. Furthermore, the sexes differ in space use in both tasks and in a principal component analysis of the spatial task. A model selection analysis finds that preference, approach, and interaction duration in the novel object recognition task reach a threshold of importance averaged across all models. This work highlights the need to explicitly test for sex differences in cognition to better understand how individuals navigate dynamic social environments.

Continuous versus discrete quantity discrimination in dune snail (Mollusca: Gastropoda) seeking thermal refuges

The ability of invertebrates to discriminate quantities is poorly studied, and it is unknown whether other phyla possess the same richness and sophistication of quantification mechanisms observed in vertebrates. The dune snail, Theba pisana, occupies a harsh habitat characterised by sparse vegetation and diurnal soil temperatures well above the thermal tolerance of this species. To survive, a snail must locate and climb one of the rare tall herbs each dawn and spend the daytime hours in an elevated refuge position. Based on their ecology, we predicted that dune snails would prefer larger to smaller groups of refuges. We simulated shelter choice under controlled laboratory conditions. Snails’ acuity in discriminating quantity of shelters was comparable to that of mammals and birds, reaching the 4 versus 5 item discrimination, suggesting that natural selection could drive the evolution of advanced cognitive abilities even in small-brained animals if these functions have a high survival value. In a subsequent series of experiments, we investigated whether snails used numerical information or based their decisions upon continuous quantities, such as cumulative surface, density or convex hull, which co-varies with number. Though our results tend to underplay the role of these continuous cues, behavioural data alone are insufficient to determine if dune snails were using numerical information, leaving open the question of whether gastropod molluscans possess elementary abilities for numerical processing.

Refrigeration or anti-theft? Food-caching behavior of wolverines (Gulo gulo) in Scandinavia

Abstract

Food-caching animals can gain nutritional advantages by buffering seasonality in food availability, especially during times of scarcity. The wolverine (Gulo gulo) is a facultative predator that occupies environments of low productivity. As an adaptation to fluctuating food availability, wolverines cache perishable food in snow, boulders, and bogs for short- and long-term storage. We studied caching behavior of 38 GPS-collared wolverines in four study areas in Scandinavia. By investigating clusters of GPS locations, we identified a total of 303 food caches from 17 male and 21 female wolverines. Wolverines cached food all year around, from both scavenging and predation events, and spaced their caches widely within their home range. Wolverines cached food items on average 1.1 km from the food source and made between 1 and 6 caches per source. Wolverines cached closer to the source when scavenging carcasses killed by other large carnivores; this might be a strategy to optimize food gain when under pressure of interspecific competition. When caching, wolverines selected for steep and rugged terrain in unproductive habitat types or in forest, indicating a preference for less-exposed sites that can provide cold storage and/or protection against pilferage. The observed year-round investment in caching by wolverines underlines the importance of food predictability for survival and reproductive success in this species. Increasing temperatures as a consequence of climate change may provide new challenges for wolverines by negatively affecting the preservation of cached food and by increasing competition from pilferers that benefit from a warmer climate. It is however still not fully understood which consequences this may have for the demography and behavior of the wolverine.

Significance statement

Food caching is a behavioral strategy used by a wide range of animals to store food for future use. Choosing appropriate caching sites appears important for slowing down decomposition rates and minimizes competition. In this study, we demonstrate that the wolverine, an opportunistic predator and scavenger, utilizes available carrion to create caches all year around. By following wolverines with GPS collars, we registered that they carried food far away to cache it in secluded and cold places, which are often located on steep slopes or in forest. However, when scavenging other carnivores’ prey, they move food in shorter distances, possibly to be able to quickly return for more. The observed efficiency in wolverine caching behavior is likely vital for their survival and reproductive success in the harsh and highly seasonal environment in which they live.

Spatial Cognition and Range Use in Free-Range Laying Hens

Simple Summary

Individual free-range laying hens vary in their use of the outdoor range. The outdoor environment is typically more complex and variable than indoor housing and thus range use may be related to differences in spatial abilities. Individual adult hens that never went outside were slower to learn a T-maze task—which requires birds to repeatedly find a food reward in one arm of the maze, compared to outdoor-preferring hens. Pullets that were faster to learn the maze also showed more visits to the range in their first month of range access but only in one of two tested groups. Early enrichment improved learning of the maze but only when the birds were tested before onset of lay. Fear may play a role in inhibiting bird’s spatial learning and their range use. More studies of different enriched rearing treatments and their impacts on fear and learning would be needed to confirm these findings. Overall, these results contribute to our understanding of why some birds choose to never access the outdoor range area.

Abstract

Radio-frequency identification tracking shows individual free-range laying hens vary in range use, with some never going outdoors. The range is typically more environmentally complex, requiring navigation to return to the indoor resources. Outdoor-preferring hens may have improved spatial abilities compared to indoor-preferring hens. Experiment 1 tested 32 adult ISA Brown hens in a T-maze learning task that showed exclusively-indoor birds were slowest to reach the learning success criterion (p < 0.05). Experiment 2 tested 117 pullets from enriched or non-enriched early rearing treatments (1 pen replicate per treatment) in the same maze at 15–16 or 17–18 weeks. Enriched birds reached learning success criterion faster at 15–16 weeks (p < 0.05) but not at 17–18 weeks (p > 0.05), the age that coincided with the onset of lay. Enriched birds that were faster to learn the maze task showed more range visits in the first 4 weeks of range access. Enriched and non-enriched birds showed no differences in telencephalon or hippocampal volume (p > 0.05). Fear may reduce spatial abilities but further testing with more pen replicates per early rearing treatments would improve our understanding of the relationship between spatial cognitive abilities and range use.

Understanding the origin of number sense: a review of fish studies

The ability to use quantitative information is thought to be adaptive in a wide range of ecological contexts. For nearly a century, the numerical abilities of mammals and birds have been extensively studied using a variety of approaches. However, in the last two decades, there has been increasing interest in investigating the numerical abilities of teleosts (i.e. a large group of ray-finned fish), mainly due to the practical advantages of using fish species as models in laboratory research. Here, we review the current state of the art in this field. In the first part, we highlight some potential ecological functions of numerical abilities in fish and summarize the existing literature that demonstrates numerical abilities in different fish species. In many cases, surprising similarities have been reported among the numerical performance of mammals, birds and fish, raising the question as to whether vertebrates' numerical systems have been inherited from a common ancestor. In the second part, we will focus on what we still need to investigate, specifically the research fields in which the use of fish would be particularly beneficial, such as the genetic bases of numerical abilities, the development of these abilities and the evolutionary foundation of vertebrate number sense.This article is part of a discussion meeting issue 'The origins of numerical abilities'.

Facing a Clever Predator Demands Clever Responses - Red-Backed Shrikes (Lanius collurio) vs. Eurasian Magpies (Pica pica)

Red-backed shrikes (Lanius collurio) behave quite differently towards two common nest predators. While the European jay (Garrulus glandarius) is commonly attacked, in the presence of the Eurasian magpie (Pica pica), shrikes stay fully passive. We tested the hypotheses that this passive response to the magpie is an alternative defense strategy. Nesting shrikes were exposed to the commonly attacked European kestrel (Falco tinnunculus) in a situation in which i) a harmless domestic pigeon, ii) a commonly attacked European jay, and iii) a non-attacked black-billed magpie are (separately) presented nearby. The kestrel dummy presented together with the magpie dummy was attacked with a significantly lower intensity than when it was presented with the other intruders (pigeon, jay) or alone. This means that the presence of the magpie inhibited the shrike’s defense response towards the other intruder. These results support our previous hypotheses that shrikes use an alternative defense strategy in the magpie’s presence. We hypothesize that the magpie is able to associate the active defense of the shrikes with the close proximity of a nest and that shrikes try not to draw the magpie’s attention to the nest. The reason why this strategy is not used against the jay remains unanswered as jays as well as magpies show very similar cognitive and foraging skills enabling them to individuate the nest presence according to active parental defense.

Brain organization of gorillas reflects species differences in ecology

Gorillas include separate eastern (Gorilla beringei) and western (Gorilla gorilla) African species that diverged from each other approximately 2 million years ago. Although anatomical, genetic, behavioral, and socioecological differences have been noted among gorilla populations, little is known about variation in their brain structure. This study examines neuroanatomical variation between gorilla species using structural neuroimaging. Postmortem magnetic resonance images were obtained of brains from 18 captive western lowland gorillas (Gorilla gorilla gorilla), 15 wild mountain gorillas (Gorilla beringei beringei), and 3 Grauer's gorillas (Gorilla beringei graueri) (both wild and captive). Stereologic methods were used to measure volumes of brain structures, including left and right frontal lobe gray and white matter, temporal lobe gray and white matter, parietal and occipital lobes gray and white matter, insular gray matter, hippocampus, striatum, thalamus, each hemisphere and the vermis of the cerebellum, and the external and extreme capsules together with the claustrum. Among the species differences, the volumes of the hippocampus and cerebellum were significantly larger in G. gorilla than G. beringei. These anatomical differences may relate to divergent ecological adaptations of the two species. Specifically, G. gorilla engages in more arboreal locomotion and thus may rely more on cerebellar circuits. In addition, they tend to eat more fruit and have larger home ranges and consequently might depend more on spatial mapping functions of the hippocampus.

Pattern of visuospatial lateralization in two corvid species, black-billed magpies and Clark's nutcrackers

Cerebral lateralization is widespread amongst vertebrate species suggesting advantages are gained by having one of the brain's hemispheres exert dominant control over certain cognitive functions. A recently devised task for assessing lateralization of visuospatial attention by birds (Diekamp et al., 2005) has allowed researchers to suggest the corpus callosum may not be necessary for the emergence of such asymmetries. More recently, this task has been adopted to examine the embryonic development of lateralization in birds, research which may provide important insights as to the underlying genetic mechanisms (Chiandetti, 2011; Chiandetti et al., 2013) of vertebrate cerebral lateralization. However, to date only chicks and pigeons have been used in this paradigm. Thus, it is unclear whether other avian species will also show lateralization of visuospatial attention during this task. Here, we examined the pattern of visuospatial lateralization in two corvid species: social black-billed magpies (Pica hudsonia) and non-social Clark's nutcrackers (Nucifraga columbiana). We find that neither the magpies nor the nutcrackers show evidence for population level lateralization or predictable individual level lateralization, as only a subset of individuals of each species showed a significant individual bias, which were rarely stable over repeated testing.

Sexual differences in memory in shiny cowbirds

Avian brood parasites depend on other species, the hosts, to raise their offspring. During the breeding season, parasitic cowbirds (Molothrus sp.) search for potential host nests to which they return for laying a few days after first locating them. Parasitic cowbirds have a larger hippocampus/telencephalon volume than non-parasitic species; this volume is larger in the sex involved in nest searching (females) and it is also larger in the breeding than in the non-breeding season. In nature, female shiny cowbirds Molothrus bonariensis search for nests without the male's assistance. Here we test whether, in association with these neuroanatomical and behavioural differences, shiny cowbirds display sexual differences in a memory task in the laboratory. We used a task consisting of finding food whose location was indicated either by the appearance or the location of a covering disk. Females learnt to retrieve food faster than males when food was associated with appearance cues, but we found no sexual differences when food was associated with a specific location. Our results are consistent with the view that parasitism and its neuroanatomical correlates affect performance in memory tasks, but the effects we found were not in the expected direction, emphasising that the nature of avian hippocampal function and its sexual differences are not yet understood.

Neuronal plasticity in hibernation and the proposed role of the microtubule-associated protein tau as a "master switch" regulating synaptic gain in neuronal networks

The present paper provides an overview of adaptive changes in brain structure and learning abilities during hibernation as a behavioral strategy used by several mammalian species to minimize energy expenditure under current or anticipated inhospitable environmental conditions. One cellular mechanism that contributes to the regulated suppression of metabolism and thermogenesis during hibernation is reversible phosphorylation of enzymes and proteins, which limits rates of flux through metabolic pathways. Reversible phosphorylation during hibernation also affects synaptic membrane proteins, a process known to be involved in synaptic plasticity. This mechanism of reversible protein phosphorylation also affects the microtubule-associated protein tau, thereby generating a condition that in the adult human brain is associated with aggregation of tau protein to paired helical filaments (PHFs), as observed in Alzheimer's disease. Here, we put forward the concept that phosphorylation of tau is a neuroprotective mechanism to escape NMDA-mediated hyperexcitability of neurons that would otherwise occur during slow gradual cooling of the brain. Phosphorylation of tau and its subsequent targeting to subsynaptic sites might, thus, work as a kind of "master switch," regulating NMDA receptor-mediated synaptic gain in a wide array of neuronal networks, thereby enabling entry into torpor. If this condition lasts too long, however, it may eventually turn into a pathological trigger, driving a cascade of events leading to neurodegeneration, as in Alzheimer's disease or other "tauopathies".

Large quantity discrimination by North Island robins (Petroica longipes)

While numerosity-representation and enumeration of different numbers of objects-and quantity discrimination in particular have been studied in a wide range of species, very little is known about the numerical abilities of animals in the wild. This study examined spontaneous relative quantity judgments (RQJs) by wild North Island robins (Petroica longipes) of New Zealand. In Experiment 1, robins were tested on a range of numerical values of up to 14 versus 16 items, which were sequentially presented and hidden. In Experiment 2, the same numerical contrasts were tested on a different group of subjects but quantities were presented as whole visible sets. Experiment 3 involved whole visible sets that comprised of exceedingly large quantities of up to 56 versus 64 items. While robins shared with other species a ratio-based representation system for representing very large values, they also appeared to have developed an object indexing system with an extended upper limit (well beyond 4) that may be an evolutionary response to ecological challenges faced by scatter-hoarding birds. These results suggest that cognitive mechanism influencing an understanding of physical quantity may be deployed more flexibly in some contexts than previously thought, and are discussed in light of findings across other mammalian and avian species.

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.

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.

The hippocampal system: Dissociating its functional components and recombining them in the service of declarative memory

Continuing commentary raised several issues concerning our proposal that the hippocampus, parahippocampal region, and cortical association areas mediate different aspects of memory function. Recent relevant findings strengthen our argument that neocortical areas and the parahippocampal region maintain persistent encodings of specific single items and that the hippocampus mediates representations of the relations among these items. The reciprocally and closely interconnected structures that compose the hippocampal memory system work interactively to support flexible memory expression that is relevant to the natural behavior of animals and to conscious recollection in humans.

The hunting of the hippocampal function

Eichenbaum et al.'s (1994a) theory suffers from a lack of ecological validation. It is not at all clear why the hypothesized faculties would have evolved and what their adaptive value would be. I argue that hippocampal function can only be understood if the animal is seen in its natural context.

Sequential processing of “items” and “relations”

Eichenbaum et al. (1994a) hypothesized that perceptually distinct items and the relations among them are processed sequentially by the parahippocampal region and the hippocampal formation, respectively. Predictions based solely on their model's sequential-processing feature might prove easier to disconfirm than those based on its representational features. Two such predictions are discussed: (1) double dissociations should be impossible following hippocampal vs. parahippocampal lesions, and (2) hippocampal lesions should not exacerbate impairments that follow complete parahippocampal lesions.

Recovered consciousness: A proposal for making consciousness integral to neuropsychological theories of memory in humans and nonhumans

Why is consciousness associated with recovery of memories that are initially dependent on the hippocampal system? A hypothesis is proposed that the medial temporal lobe/hippocampal complex (MTL/H) receives as its input only information that is consciously apprehended. By a process termed “cohesion,” the MTL/H binds into a memory trace those neural elements that mediated the conscious experience so that effectively, “consciousness” is an integral part of the memory trace. It is the phenomenological records of events (Conway 1992), integrated consciousness-content packets, that are recovered when memory traces are retrieved.

Hippocampus, delay neurons, and sensory heterogeneity

We raise three issues concerning the Eichenbaum, Otto & Cohen (1994) model. (1) We argue against the strict division of labor that Eichenbaum et al. attribute to neocortical and limbic regions. (2) We raise the possibility that the anterior and posterior portions of the hippocampus may be important for different types of information processing. (3) We argue that, rather than reflecting relational processing, different neural responses to “match” and “nonmatch” trials may relate to different required spatial responses.

The hippocampus seen in the context of declarative and procedural control

Various apparently incompatible theories of hippocampal function have been proposed but integration is now needed. It is argued that the involvement of the hippocampus is most clearly seen when the animal needs to extrapolate beyond current sensory information. Such control can involve both the initiation of behaviour in the absence of appropriate sensory input and the inhibition of behaviour that might otherwise be triggered by current sensory input.

The development of caching and object permanence in Western scrub-jays (Aphelocoma californica): which emerges first?

Recent studies on the food-caching behavior of corvids have revealed complex physical and social skills, yet little is known about the ontogeny of food caching in relation to the development of cognitive capacities. Piagetian object permanence is the understanding that objects continue to exist even when they are no longer visible. Here, the authors focus on Piagetian Stages 3 and 4, because they are hallmarks in the cognitive development of both young children and animals. Our aim is to determine in a food-caching corvid, the Western scrub-jay, whether (1) Piagetian Stage 4 competence and tentative caching (i.e., hiding an item invisibly and retrieving it without delay), emerge concomitantly or consecutively; (2) whether experiencing the reappearance of hidden objects enhances the timing of the appearance of object permanence; and (3) discuss how the development of object permanence is related to behavioral development and sensorimotor intelligence. Our findings suggest that object permanence Stage 4 emerges before tentative caching, and independent of environmental influences, but that once the birds have developed simple object-permanence, then social learning might advance the interval after which tentative caching commences.

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.

The dynamic nature of spatial encoding in the hippocampus

Many hippocampal neurons (place cells) appear to represent a particular location within an environment (their place field). This property would appear to be central to hippocampal involvement in navigation based on spatial memory. Although a navigationally useful representation might also include information about distal goals, having a place field and being able to represent a distal goal would appear to be mutually exclusive place cell properties. Our simulations demonstrate, however, that information about goal direction can be simply derived from the changes in place field density that occur when place fields shift location in a goal-directed manner. Previous reports that place fields respond dynamically to shifts in goal location may, therefore, represent the operation of such a system.

Structure of the mushroom bodies of the insect brain

The past decade has produced an explosion of new information on the development, neuroanatomy, and possible functions of the mushroom bodies. This review provides a concise, contemporary overview of the structure of the mushroom bodies. Two topics are highlighted: the volume plasticity of mushroom body neuropils evident in the brains of some adult insects and a possible essential role for the gamma lobe in olfactory memory.

Festina lente: Late-night thoughts on high-throughput screening of mouse behavior

A recent perspective discussed high-throughput behavioral analysis using mice, giving the overall impression that this area is lagging behind in neuroscience and biomedical research. Not only are we more optimistic about the current state of the art in behavioral neuroscience and its promise, but we also have reservations about whether high-throughput analysis is always an appropriate goal for most behavioral studies. We argue that behavioral studies should be carried out with clear goals and more regard to the intellectual context in which they have developed. In addition, behavioral studies can be performed quite easily, but this does not ensure the required validity or reliability of the particular tests used. Finally, high throughput may not always be an appropriate goal. We discuss the role of automated data collection and unique data-mining algorithms, and the question of the ethological relevance of behavioral tests.

c-fos is induced in the hippocampus during consolidation of sexual imprinting in the zebra finch (Taeniopygia guttata)

c-fos was used to mark regions of enhanced neuronal activity during sexual imprinting, an early learning process by which information about the prospective sexual partner is acquired and consolidated. In the present study, we demonstrate that the hippocampus, already known for its specialized spatial memory capacities in navigating pigeons and in food-storing birds, depicts a selective differential c-fos induction in a situation shown to lead to sexual imprinting, that is, exposing previously isolated male birds to a female for 1 h. c-fos induction is lateralized, the left hippocampus showing more c-fos activity than the right. Our results would indicate a role for the hippocampus in the consolidation process of imprinting, probably in the transfer of information to the other telencephalic areas that show alterations in synaptic connectivity as a result of consolidation of sexual imprinting.

Presynaptic N-methyl-D-aspartate receptor expression is increased by estrogen in an aromatase-rich area of the songbird hippocampus

The vertebrate hippocampus (HP) is sensitive to estrogens, in part via effects on N-methyl-D-aspartate (NMDA)-type glutamate receptors (NR). Although the precise mechanism of this interaction is unclear, it constitutes a key interface in the plasticity of the adult vertebrate HP. The songbird HP expresses high levels of aromatase (estrogen synthase), suggesting that locally generated steroid may affect excitatory pathways. By using light, confocal, and electron microscopy with antibodies that specifically recognize aromatase and NR, we have 1) mapped their distribution in the zebra finch brain, 2) documented their coexpression in HP neurons, 3) studied the ultrastructure of NR-expressing cells in the HP, and 4) tested the influence of estrogen on the cellular and subcellular characteristics of NR-positive HP neurons. Aromatase and NR are coexpressed in HP neurons. NRs are detectable in presynaptic boutons of the songbird HP in addition to postsynaptic loci. Treatment with estrogen increased the somal size and innervation of NR-positive neurons and the frequency of presynaptic NR. Autoreception of excitatory neurotransmission via presynaptic NR may promote the strengthening of activity-dependent, excitatory synapses, thereby enhancing learning. NR-mediated autoreception may underlie estrogenic enhancement of HP structural and functional plasticity.

Limits on volume changes in the mushroom bodies of the honey bee brain

The behavioral maturation of adult worker honey bees is influenced by a rising titer of juvenile hormone (JH), and is temporally correlated with an increase in the volume of the neuropil of the mushroom bodies, a brain region involved in learning and memory. We explored the stability of this neuropil expansion and its possible dependence on JH. We studied the volume of the mushroom bodies in adult bees deprived of JH by surgical removal of the source glands, the corpora allata. We also asked if the neuropil expansion detected in foragers persists when bees no longer engage in foraging, either because of the onset of winter or because colony social structure was experimentally manipulated to cause some bees to revert from foraging to tending brood (nursing). Results show that adult exposure to JH is not necessary for growth of the mushroom body neuropil, and that the volume of the mushroom body neuropil in adult bees is not reduced if foraging stops. These results are interpreted in the context of a qualitative model that posits that mushroom body neuropil volume enlargement in the honey bee has both experience-independent and experience-dependent components.

Agonistic encounters in aged male mouse potentiate the expression of endogenous brain NGF and BDNF: possible implication for brain progenitor cells' activation

The condition of dominance or submission following agonistic encounters in the adult male mouse is known to differentially affect brain nerve growth factor, a neurotrophin playing a role in brain remodeling, in the fine tuning of behaviour and in the regulation of the basal forebrain cholinergic neurons. During development and adult life nerve growth factor regulates brain expression of neurotransmitters and the stimulation of progenitor cells (stem cells) which, under different external stimuli, may differentiate into neuronal and/or glial cells promoting the recovery of the injured brain. However, little information is available for the aged brain. Thus in the present study we investigated the effect of the social status ('dominance' vs. 'submission') in the aged mouse on the presence of nerve growth factor, brain-derived neurotrophic factor, choline acetyltransferase, neuropeptide Y and progenitor cells of selected brain regions. We found that aged dominant mice showed increased brain-derived neurotrophic factor in the subventricular zone and hippocampus and increased choline acetyltransferase in the septum and basal nuclei, which were associated with increased presence of progenitor cells in the subventricular zone. Conversely, in aged subordinate mice the data showed a marked brain increase in nerve growth factor in the subventricular zone and hippocampus, choline acetyltransferase in the septum and basal nuclei and neuropeptide Y in the hippocampus and parietal cortex. The possible functional implications of these findings are discussed.

The proliferative ventricular zone in adult vertebrates: a comparative study using reptiles, birds, and mammals

Although evidence accumulated during the last decades has advanced our understanding of adult neurogenesis in the vertebrate brain, many aspects of this intriguing phenomenon remain controversial. Here we review the organization and cellular composition of the ventricular wall of reptiles, birds, and mammals in an effort to identify differences and commonalities among these vertebrate classes. Three major cell types have been identified in the ventricular zone of reptiles and birds: migrating (Type A) cells, radial glial (Type B) cells, and ependymal (Type E) cells. Cells similar anatomically and functionally to Types A, B, and E have also been described in the ventricular wall of mammals, which contains an additional cell type (Type C) not found in reptiles or birds. The bulk of the evidence points to a role of Type B cells as primary neural precursors (stem cells) in the three classes of living amniotic vertebrates. This finding may have implications for the development of strategies for the possible treatment of human neurological disorders.

A critique of the neuroecology of learning and memory

Recent years have seen the emergence of neuroecology, the study of the neural mechanisms of behaviour guided by functional and evolutionary principles. This research has been of enormous value for our understanding of the evolution of brain- and species-specific behaviour. However, we question the validity of the neuroecological approach when applied to the analysis of learning and memory, given its arbitrary assumption that different 'problems' engage different memory mechanisms. Differences in memory-based performance in 'natural' tasks do not prove differences in memory capacity; similarly, differences in the use of memory in the natural environment do not provide a sound basis for expecting differences in anatomical structures that subserve learning and memory. This critique is illustrated with examples taken from the study of the neurobiology of food storing and song learning in birds.

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.

Effects of demanding foraging conditions on cache retrival accuracy in food-caching mountain chickadees (Poecile gambeli)

Birds rely, at least in part, on spatial memory for recovering previously hidden caches but accurate cache recovery may be more critical for birds that forage in harsh conditions where the food supply is limited and unpredictable. Failure to find caches in these conditions may potentially result in death from starvation. In order to test this hypothesis we compared the cache recovery behaviour of 24 wild-caught mountain chickadees (Poecile gambeli), half of which were maintained on a limited and unpredictable food supply while the rest were maintained on an ad libitum food supply for 60 days. We then tested their cache retrieval accuracy by allowing birds from both groups to cache seeds in the experimental room and recover them 5 hours later. Our results showed that birds maintained on a limited and unpredictable food supply made significantly fewer visits to non-cache sites when recovering their caches compared to birds maintained on ad libitum food. We found the same difference in performance in two versions of a one-trial associative learning task in which the birds had to rely on memory to find previously encountered hidden food. In a non-spatial memory version of the task, in which the baited feeder was clearly marked, there were no significant differences between the two groups. We therefore concluded that the two groups differed in their efficiency at cache retrieval. We suggest that this difference is more likely to be attributable to a difference in memory (encoding or recall) than to a difference in their motivation to search for hidden food, although the possibility of some motivational differences still exists. Overall, our results suggest that demanding foraging conditions favour more accurate cache retrieval in food-caching birds.

Development of object permanence in food-storing magpies (Pica pica)

The development of object permanence was investigated in black-billed magpies (Pica pica), a food-storing passerine bird. The authors tested the hypothesis that food-storing development should be correlated with object-permanence development and that specific stages of object permanence should be achieved before magpies become independent. As predicted, Piagetian Stages 4 and 5 were reached before independence was achieved, and the ability to represent a fully hidden object (Piagetian Stage 4) emerged by the age when magpies begin to retrieve food. Contrary to psittacine birds and humans, but as in dogs and cats, no "A-not-B error" occurred. Although magpies also mastered 5 of 6 invisible displacement tasks, evidence of Piagetian Stage 6 competence was ambiguous.

Hippocampal mossy fibers and swimming navigation learning in two vole species occupying different habitats

We showed previously for mice that size differences of the infrapyramidal hippocampal mossy fiber projection (IIP-MF) correlate with spatial learning abilities. In order to clarify the role of the IIP-MF in a natural environment, we studied the bank vole (Clethrionomys glareolus), adapted to a wide range of different habitats, and the root vole (Microtus oeconomus), living in homogenous grassland habitats with small home ranges. Morphometry on Timm-stained horizontal brain sections of six C. glareolus and six M. oeconomus revealed that the size of the entire mossy fiber projection was 42% larger in C. glareolus than M. oeconomus. C. glareolus had also an IIP-MF projection about 230% larger than that of the root vole. A sample of captured animals was then transferred to the laboratory (C. glareolus, n = 23; M. oeconomus, n = 15) and underwent testing for swimming navigation according to a standardized protocol used to assess water maze learning in about 2,000 normal and transgenic mice. Both species learned faster than laboratory mice. Overall escape times showed no differences, but path length was significantly reduced in C. glareolus, which also showed superior performance in a variety of scores assessing spatial search patterns. On the other hand, M. oeconomus showed faster swimming speed, and strong thigmotaxis combined with circular swimming. M. oeconomus also scored at chance levels during the probe trial, about as poorly as mutant knockout mice considered to be deficient in spatial memory. These differences probably reflect differential styles of water maze learning rather than spatial memory deficits: C. glareolus appears to be superior in inhibiting behavior interfering with proper spatial search behavior, while M. oeconomus succeeds in escaping by using rapid circular swimming. We assume that size variations of the IIP-MF correspond to a mechanism stabilizing hippocampal processing during spatial learning or complex activities. This corresponds to the ecological lifestyle of the two species and is in line with previous observations on the role of the IIP-MF.