Homing behavior of hippocampus and parahippocampus lesioned pigeons following short-distance releases

Neural and sensory basis of homing behaviour in the invasive cane toad, Rhinella marina

The behavioural, sensory and neural bases of vertebrate navigation are primarily described in mammals and birds. While many studies have explored amphibian navigation, none have characterized brain activity associated with navigation in the wild. To address this knowledge gap, we conducted a study on navigation in the cane toad, Rhinella marina. First, we performed a translocation experiment to describe how invasive cane toads in Hawaii navigate home and observed homing following displacements of up to 1 km. Next, we tested the effect of olfactory and magnetosensory manipulations on homing, as these senses are most commonly associated with amphibian navigation. We found that neither ablation alone prevents homing, further supporting that toad navigation is multimodal. Finally, we tested the hypothesis that the medial pallium, the amphibian homologue to the hippocampus, is involved in homing. Our comparisons of neural activity revealed evidence supporting a conservation of neural structures associated with navigation across vertebrates consistent with neural models of amphibian spatial cognition from recent laboratory studies. Our work furthers our evolutionary understanding of spatial behaviour and cognition in vertebrates and lays a foundation for studying the behavioural, sensory and neural bases of navigation in an invasive amphibian.

Neural and sensory basis of homing behavior in the invasive cane toad, Rhinella marina

The behavioral, sensory, and neural bases of vertebrate navigation are primarily described in mammals and birds. While many studies have explored amphibian navigation, none have characterized brain activity associated with navigation in the wild. To address this knowledge gap, we conducted a study on navigation in the cane toad, Rhinella marina. First, we performed a translocation experiment to describe how invasive cane toads in Hawai'i navigate home and observed homing following displacements of up to one kilometer. Next, we tested the effect of olfactory and magnetosensory manipulations on homing, as these senses are most commonly associated with amphibian navigation. We found that neither ablation alone prevents homing, further supporting that toad navigation is multimodal. Finally, we tested the hypothesis that the medial pallium, the amphibian homolog to the hippocampus, is involved in homing. Our comparisons of neural activity revealed evidence supporting a conservation of neural structures associated with navigation across vertebrates consistent with neural models of amphibian spatial cognition from recent laboratory studies. Our work furthers our evolutionary understanding of spatial behavior and cognition in vertebrates and lays a foundation for studying the behavioral, sensory, and neural bases of navigation in an invasive amphibian.

GPS tracking technology and re-visiting the relationship between the avian visual Wulst and homing pigeon navigation

Within their familiar areas homing pigeons rely on familiar visual landscape features and landmarks for homing. However, the neural basis of visual landmark-based navigation has been so far investigated mainly in relation to the role of the hippocampal formation. The avian visual Wulst is the telencephalic projection field of the thalamofugal pathway that has been suggested to be involved in processing lateral visual inputs that originate from the far visual field. The Wulst is therefore a good candidate for a neural structure participating in the visual control of familiar visual landmark-based navigation. We repeatedly released and tracked Wulst-lesioned and control homing pigeons from three sites about 10-15 km from the loft. Wulst lesions did not impair the ability of the pigeons to orient homeward during the first release from each of the three sites nor to localise the loft within the home area. In addition, Wulst-lesioned pigeons displayed unimpaired route fidelity acquisition to a repeated homing path compared to the intact birds. However, compared to control birds, Wulst-lesioned pigeons displayed persistent oscillatory flight patterns across releases, diminished attention to linear (leading lines) landscape features, such as roads and wood edges, and less direct flight paths within the home area. Differences and similarities between the effects of Wulst and hippocampal lesions suggest that although the visual Wulst does not seem to play a direct role in the memory representation of a landscape-landmark map, it does seem to participate in influencing the perceptual construction of such a map.

A GPS assisted translocation experiment to study the homing behavior of red deer

Many animals return to their home areas (i.e., 'homing') after translocation to sites further away. Such translocations have traditionally been used in behavioral ecology to understand the orientation and migration behavior of animals. The movement itself can then be followed by marking and recapturing animals or by tracking, for example, using GPS systems. Most detailed studies investigating this behavior have been conducted in smaller vertebrates (e.g., birds, amphibians, and mice), whereas information on larger mammals, such as red deer, is sparse. We conducted GPS-assisted translocation experiments with red deer at two sites in the Czech Republic. Individuals were translocated over a distance of approximately 11 km and their home journey was tracked. Circular statistics were used to test for significant homeward orientation at distances of 100, 500, 1000, and 5000 m from the release site. In addition, we applied Lavielle trajectory segmentation to identify the different phases of homing behavior. Thirty-one out of 35 translocations resulted in successful homing, with a median time of 4.75 days (range 1.23-100 days). Animals were significantly oriented towards home immediately after release and again when they came closer to home; however, they did not show a significant orientation at the distances in between. We were able to identify three homing phases, an initial 'exploratory phase', followed by a 'homing phase' which sometimes was again followed by an 'arrival phase'. The 'homing phase' was characterized by the straightest paths and fastest movements. However, the variation between translocation events was considerable. We showed good homing abilities of red deer after translocation. Our results demonstrate the feasibility of conducting experiments with environmental manipulations (e.g., to impede the use of sensory cues) close to the release site. The homing behavior of red deer is comparable to that of other species, and might represent general homing behavior patterns in animals. Follow-up studies should further dissect and investigate the drivers of the individual variations observed and try to identify the sensory cues used during homing.

Providing height to pullets does not influence hippocampal dendritic morphology or brain-derived neurotrophic factor at the end of the rearing period

Pullets reared with diverse behavioral experiences are faster to learn spatial cognition tasks and acclimate more successfully to laying environments with elevated structures. However, the neural underpinnings of the improved spatial abilities are unclear. The objective of this study was to determine whether providing structural height in the rearing environment affected the development of the hippocampus and whether hippocampal neural metrics correlated with individual behavior on spatial cognition tasks. Female Dekalb White pullets were reared in a floor pen (FL), single-tiered aviary (ST), or two-tiered aviary (TT; 5 pens/treatment). Pullets completed floor-based Y-maze and elevated visual cliff tasks to evaluate depth perception at 15 and 16 wk, respectively. At 16 wk, brains were removed for Golgi-Cox staining (n = 12 for FL, 13 for ST, 13 total pullets for TT; 2 to 3 pullets/pen) and qPCR to measure gene expression of brain-derived neurotrophic factor (BDNF; n = 10 for FL, 11 for ST, and 9 pullets for TT). Rearing environment did not affect various morphometric outcomes of dendritic arborization, including Sholl profiles; mean dendritic length; sum dendritic length; number of dendrites, terminal tips, or nodes; soma size; or BDNF mRNA expression (P > 0.05). Hippocampal subregion did affect dendritic morphology, with multipolar neurons from the ventral subregion differing in several characteristics from multipolar neurons in the dorsomedial or dorsolateral subregions (P < 0.05). Neural metrics did not correlate with individual differences in behavior during the spatial cognition tasks. Overall, providing height during rearing did not affect dendritic morphology or BDNF at 16 wk of age, but other metrics in the hippocampus or other brain regions warrant further investigation. Additionally, other structural or social components or the role of animal personality are areas of future interest for how rearing environments influence pullet behavior.

Dissociation of spatial and object memory in the hippocampal formation of Japanese quail

The mammalian temporal cortex can be functionally segregated into regions that encode spatial information and others that are predominantly responsible for object recognition. In the present study, we report comparable functional segregation in the avian brain. Using Japanese quail, we find that bilateral lesions of the hippocampus (Hp) produce robust deficits in performance in a foraging array (FA) spatial memory task, while sparing spontaneous object recognition (SOR). In contrast, lesions to the adjacent area parahippocampalis (APH) compromise both SOR and FA. These observations demonstrate a functional dissociation between Hp and APH that is comparable to the distinctions seen in mammals between the hippocampus and surrounding temporal cortex.

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Are spatial and object information separable in the avian hippocampal formation?

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Quail with lesions to the hippocampus are impaired in a spatial foraging task

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Lesions to area parahippocampalis also selectively impair object recognition

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Like mammals, bird hippocampus shows functional gradients in information processing

GPS-profiling of retrograde navigational impairments associated with hippocampal lesion in homing pigeons

The avian hippocampal formation (HF) is homologous to the mammalian hippocampus and plays a central role in the control of spatial cognition. In homing pigeons, HF supports navigation by familiar landmarks and landscape features. However, what has remained relatively unexplored is the importance of HF for the retention of previously acquired spatial information. For example, to date, no systematic GPS-tracking studies on the retention of HF-dependent navigational memory in homing pigeons have been performed. Therefore, the current study was designed to compare the pre- and post-surgical navigational performance of sham-lesioned control and HF-lesioned pigeons tracked from three different sites located in different directions with respect to home. The pre- and post-surgical comparison of the pigeons' flight paths near the release sites and before reaching the area surrounding the home loft (4 km radius from the loft) revealed that the control and HF-lesioned pigeons displayed similarly successful retention. By contrast, the HF-lesioned pigeons displayed dramatically and consistently impaired retention in navigating to their home loft during the terminal phase of the homing flight near home, i.e., where navigation is supported by memory for landmark and landscape features. The data demonstrate that HF lesions lead to a dramatic loss of pre-surgically acquired landmark and landscape navigational information while sparing those mechanisms associated with navigation from locations distant from home.

Seasonal plasticity in the morphology and cytoarchitecture of the hippocampal complex of the Indian Roller, Coracias benghalensis

Coracias benghalensis, commonly known as Indian Roller, a subtropical seasonally breeding bird native to Prayagraj (25° 28' N, 81° 54' E), U.P., India, exhibits a specific rolling behavior (an ornate sexual display) to attract the female for courtship. We hypothesized that the emergence of the seasonal rolling behavior of C. benghalensis would coincide with seasonal neuronal morphology changes in the dorsomedial hippocampus (DMH) area of the hippocampal complex (HCC). To test this hypothesis, the present study aimed to reveal qualitative and quantitative changes in neuronal plasticity in various neuronal classes of DMH across the breeding (pre-breeding and breeding) and the non-breeding (quiescent and regression) phases of the reproductive cycle of C. benghalensis. Plasticity in the morphology of four neuronal types (unipolar, bipolar, pyramidal, and multipolar) in the DMH area of HCC during the breeding and the non-breeding phases was characterized by using Golgi-Colonnier staining for identification and characterization of neuronal morphology. As compared to the quiescent phase, a significant increase of soma diameter, dendritic field, dendritic thickness, length of spine neck, spine head diameter, number of visible spines, and spine density in all four types of neurons was observed during the breeding phase. In contrast, significant decreases were observed during the bird's non-breeding phase compared to the breeding phase. This study concludes that during the breeding phase of C. benghalensis, neuronal arborization was substantially increased in DMH, suggesting an enhanced capability for circuit plasticity possibly underlying rolling behavior. Our study establishes seasonal plasticity in DMH and will serve as a novel model for future studies investigating the molecular, physiological, and cellular mechanisms underlying complex, yet stereotyped, sensorimotor behavior.

Neural substrates involved in the cognitive information processing in teleost fish

Over the last few decades, it has been shown that fish, comprising the largest group of vertebrates and in many respects one of the least well studied, possess many cognitive abilities comparable to those of birds and mammals. Despite a plethora of behavioural studies assessing cognition abilities and an abundance of neuroanatomical studies, only few studies have aimed to or in fact identified the neural substrates involved in the processing of cognitive information. In this review, an overview of the currently available studies addressing the joint research topics of cognitive behaviour and neuroscience in teleosts (and elasmobranchs wherever possible) is provided, primarily focusing on two fundamentally different but complementary approaches, i.e. ablation studies and Immediate Early Gene (IEG) analyses. More recently, the latter technique has become one of the most promising methods to visualize neuronal populations activated in specific brain areas, both during a variety of cognitive as well as non-cognition-related tasks. While IEG studies may be more elegant and potentially easier to conduct, only lesion studies can help researchers find out what information animals can learn or recall prior to and following ablation of a particular brain area.

Neuronal circuits and the magnetic sense: central questions

Magnetoreception is the ability to sense the Earth's magnetic field, which is used for orientation and navigation. Behavioural experiments have shown that it is employed by many species across all vertebrate classes; however, our understanding of how magnetic information is processed and integrated within the central nervous system is limited. In this Commentary, we review the progress in birds and rodents, highlighting the role of the vestibular and trigeminal systems as well as that of the hippocampus. We reflect on the strengths and weaknesses of the methodologies currently at our disposal, the utility of emerging technologies and identify questions that we feel are critical for the advancement of the field. We expect that magnetic circuits are likely to share anatomical motifs with other senses, which culminates in the formation of spatial maps in telencephalic areas of the brain. Specifically, we predict the existence of spatial cells that encode defined components of the Earth's magnetic field.

Mountain chickadees return to their post-natal dispersal settlements following long-term captivity

There is little work investigating the relationship between environmental changes and associated hippocampal effects on animal homing. We took advantage of previous studies in which wild, non-migratory mountain chickadees spent six months in captivity prior to being released. Over the following three years, 45.8% of the birds were resighted, and in all cases birds were identified less than 300 m from their initial capture locations at their respective elevation, despite previous studies documentingca30% captivity-related reduction of the hippocampus. Reproductive success of birds that spent six months in captivity did not differ from control birds that did not experience captivity. Our findings suggest that chickadees are highly site faithful and can return to their original capture location after spending time in captivity. Our results also have important implications for animal welfare practices as birds held in captivity bred successfully and may not need to be sacrificed following captivity.

Navigating through digital folders uses the same brain structures as real world navigation

Efficient storage and retrieval of digital data is the focus of much commercial and academic attention. With personal computers, there are two main ways to retrieve files: hierarchical navigation and query-based search. In navigation, users move down their virtual folder hierarchy until they reach the folder in which the target item is stored. When searching, users first generate a query specifying some property of the target file (e.g., a word it contains), and then select the relevant file when the search engine returns a set of results. Despite advances in search technology, users prefer retrieving files using virtual folder navigation, rather than the more flexible query-based search. Using fMRI we provide an explanation for this phenomenon by demonstrating that folder navigation results in activation of the posterior limbic (including the retrosplenial cortex) and parahippocampal regions similar to that previously observed during real-world navigation in both animals and humans. In contrast, search activates the left inferior frontal gyrus, commonly observed in linguistic processing. We suggest that the preference for navigation may be due to the triggering of automatic object finding routines and lower dependence on linguistic processing. We conclude with suggestions for future computer systems design.

Distribution of neurotransmitter receptors and zinc in the pigeon (Columba livia) hippocampal formation: A basis for further comparison with the mammalian hippocampus

The avian hippocampal formation (HF) and mammalian hippocampus share a similar functional role in spatial cognition, but the underlying neuronal mechanisms allowing the functional similarity are incompletely understood. To understand better the organization of the avian HF and its transmitter receptors, we analyzed binding site densities for glutamatergic AMPA, NMDA, and kainate receptors; GABAA receptors; muscarinic M1 , M2 and nicotinic (nACh) acetylcholine receptors; noradrenergic α1 and α2 receptors; serotonergic 5-HT1A receptors; dopaminergic D1/5 receptors by using quantitative in vitro receptor autoradiography. Additionally, we performed a modified Timm staining procedure to label zinc. The regionally different receptor densities mapped well onto seven HF subdivisions previously described. Several differences in receptor expression highlighted distinct HF subdivisions. Notable examples include 1) high GABAA and α1 receptor expression, which rendered distinctive ventral subdivisions; 2) high α2 receptor expression, which rendered distinctive a dorsomedial subdivision; 3) distinct kainate, α2 , and muscarinic receptor densities that rendered distinctive the two dorsolateral subdivisions; and 4) a dorsomedial region characterized by high kainate receptor density. We further observed similarities in receptor binding densities between subdivisions of the avian and mammalian HF. Despite the similarities, we propose that 300 hundred million years of independent evolution has led to a mosaic of similarities and differences in the organization of the avian HF and mammalian hippocampus and that thinking about the avian HF in terms of the strict organization of the mammalian hippocampus is likely insufficient to understand the HF of birds.

The shark Chiloscyllium griseum can orient using turn responses before and after partial telencephalon ablation

This study assessed spatial memory and orientation strategies in Chiloscyllium griseum. In the presence of visual landmarks, six sharks were trained in a fixed turn response. Group 1 started from two possible compartments approaching two goal locations, while group 2 started from and approached only one location, respectively. The learning criterion was reached within 9 ± 5.29 (group 1) and 8.3 ± 3.51 sessions (group 2). Transfer tests revealed that sharks had applied a direction strategy, possibly in combination with some form of place learning. Without visual cues, sharks relied solely on the former. To identify the underlying neural substrate(s), telencephalic were lesioned and performance compared before and after surgery. Ablation of the dorsal and medial pallia only had an effect on one shark (group 1), indicating that the acquisition and retention of previously gained knowledge were unaffected in the remaining four individuals. Nonetheless, the shark re-learned the task. In summary, C. griseum can utilize fixed turn responses to navigate to a goal; there is also some evidence for the use of external visual landmarks while orienting. Probably, strategies can be used alone or in combination. Neither the dorsal nor medial pallium seems to be responsible for the acquisition and processing of egocentric information.

Place learning prior to and after telencephalon ablation in bamboo and coral cat sharks (Chiloscyllium griseum and Atelomycterus marmoratus)

This study assessed complex spatial learning and memory in two species of shark, the grey bamboo shark (Chiloscyllium griseum) and the coral cat shark (Atelomycterus marmoratus). It was hypothesized that sharks can learn and apply an allocentric orientation strategy. Eight out of ten sharks successfully completed the initial training phase (by locating a fixed goal position in a diamond maze from two possible start points) within 14.9 ± 7.6 sessions and proceeded to seven sets of transfer tests, in which sharks had to perform under altered environmental conditions. Transfer tests revealed that sharks had oriented and solved the tasks visually, using all of the provided environmental cues. Unintentional cueing did not occur. Results correspond to earlier studies on spatial memory and cognitive mapping in other vertebrates. Future experiments should investigate whether sharks possess a cognitive spatial mapping system as has already been found in several teleosts and stingrays. Following the completion of transfer tests, sharks were subjected to ablation of most of the pallium, which compromised their previously acquired place learning abilities. These results indicate that the telencephalon plays a crucial role in the processing of information on place learning and allocentric orientation strategies.

Activation changes in zebra finch (Taeniopygia guttata) brain areas evoked by alterations of the earth magnetic field

Many animals are able to perceive the earth magnetic field and to use it for orientation and navigation within the environment. The mechanisms underlying the perception and processing of magnetic field information within the brain have been thoroughly studied, especially in birds, but are still obscure. Three hypotheses are currently discussed, dealing with ferromagnetic particles in the beak of birds, with the same sort of particles within the lagena organs, or describing magnetically influenced radical-pair processes within retinal photopigments. Each hypothesis is related to a well-known sensory organ and claims parallel processing of magnetic field information with somatosensory, vestibular and visual input, respectively. Changes in activation within nuclei of the respective sensory systems have been shown previously. Most of these previous experiments employed intensity enhanced magnetic stimuli or lesions. We here exposed unrestrained zebra finches to either a stationary or a rotating magnetic field of the local intensity and inclination. C-Fos was used as an activity marker to examine whether the two treatments led to differences in fourteen brain areas including nuclei of the somatosensory, vestibular and visual system. An ANOVA revealed an overall effect of treatment, indicating that the magnetic field change was perceived by the birds. While the differences were too small to be significant in most areas, a significant enhancement of activation by the rotating stimulus was found in a hippocampal subdivision. Part of the hyperpallium showed a strong, nearly significant, increase. Our results are compatible with previous studies demonstrating an involvement of at least three different sensory systems in earth magnetic field perception and suggest that these systems, probably less elaborated, may also be found in nonmigrating birds.

Hippocampal memory consolidation during sleep: a comparison of mammals and birds

The transition from wakefulness to sleep is marked by pronounced changes in brain activity. The brain rhythms that characterize the two main types of mammalian sleep, slow-wave sleep (SWS) and rapid eye movement (REM) sleep, are thought to be involved in the functions of sleep. In particular, recent theories suggest that the synchronous slow-oscillation of neocortical neuronal membrane potentials, the defining feature of SWS, is involved in processing information acquired during wakefulness. According to the Standard Model of memory consolidation, during wakefulness the hippocampus receives input from neocortical regions involved in the initial encoding of an experience and binds this information into a coherent memory trace that is then transferred to the neocortex during SWS where it is stored and integrated within preexisting memory traces. Evidence suggests that this process selectively involves direct connections from the hippocampus to the prefrontal cortex (PFC), a multimodal, high-order association region implicated in coordinating the storage and recall of remote memories in the neocortex. The slow-oscillation is thought to orchestrate the transfer of information from the hippocampus by temporally coupling hippocampal sharp-wave/ripples (SWRs) and thalamocortical spindles. SWRs are synchronous bursts of hippocampal activity, during which waking neuronal firing patterns are reactivated in the hippocampus and neocortex in a coordinated manner. Thalamocortical spindles are brief 7-14 Hz oscillations that may facilitate the encoding of information reactivated during SWRs. By temporally coupling the readout of information from the hippocampus with conditions conducive to encoding in the neocortex, the slow-oscillation is thought to mediate the transfer of information from the hippocampus to the neocortex. Although several lines of evidence are consistent with this function for mammalian SWS, it is unclear whether SWS serves a similar function in birds, the only taxonomic group other than mammals to exhibit SWS and REM sleep. Based on our review of research on avian sleep, neuroanatomy, and memory, although involved in some forms of memory consolidation, avian sleep does not appear to be involved in transferring hippocampal memories to other brain regions. Despite exhibiting the slow-oscillation, SWRs and spindles have not been found in birds. Moreover, although birds independently evolved a brain region--the caudolateral nidopallium (NCL)--involved in performing high-order cognitive functions similar to those performed by the PFC, direct connections between the NCL and hippocampus have not been found in birds, and evidence for the transfer of information from the hippocampus to the NCL or other extra-hippocampal regions is lacking. Although based on the absence of evidence for various traits, collectively, these findings suggest that unlike mammalian SWS, avian SWS may not be involved in transferring memories from the hippocampus. Furthermore, it suggests that the slow-oscillation, the defining feature of mammalian and avian SWS, may serve a more general function independent of that related to coordinating the transfer of information from the hippocampus to the PFC in mammals. Given that SWS is homeostatically regulated (a process intimately related to the slow-oscillation) in mammals and birds, functional hypotheses linked to this process may apply to both taxonomic groups.

Spatial learning-induced egr-1 expression in telencephalon of gold fish Carassius auratus

The immediate-early gene (egr-1) expression was used to examine the neuron's response in telencephalon of goldfish during spatial learning in small space. Fishes were pre-exposed in the experimental apparatus and trained to pick food from the tray in a rectangular-shaped arena. The apparatus was divided into identical compartments comprising three gates to provide different spatial tasks. After the fish learned to pass through the gate one, two more gates were introduced one by one. Fish made more number of attempts and took longer time (P < 0.05) to pass through the first gate than the gate two or three. This active learning induces the expression of egr-1 in telencephalon as established by western blot analysis. Subsequently, the fish learn quickly to cross the similar type of second and third gate and make fewer errors with a corresponding decline in the level of egr-1 expression. As the fish learned to pass through all the three gates, third gate was replaced by modified gate three. Interestingly, the level of egr-1 expression increased again, when the fish exhibit a high exploratory behavior to cross the modified gate three. The present study shows that egr-1 expression is induced in the telencephalon of goldfish while intensively acquiring geometric spatial information to pass through the gates.

Navigation-induced ZENK expression in the olfactory system of pigeons (Columba livia)

A large body of evidence indicates that pigeons use olfactory cues to navigate over unfamiliar areas with a differential contribution of the left and right hemispheres. In particular, the right nostril/olfactory bulb (OB) and left piriform cortex (Cpi) have been demonstrated to be crucially involved in navigation. In this study we analysed behaviour-induced activation of the olfactory system, indicated by the expression of the immediate early gene ZENK, under different homing conditions. One experimental group was released from an unfamiliar site, the second group was transported to the unfamiliar site and back to the loft, and the third group was released in front of the loft. To evaluate the differential contribution of the left and/or right olfactory input, the nostrils of the pigeons were either occluded unilaterally or not. Released pigeons revealed the highest ZENK cell density in the OB and Cpi, indicating that the olfactory system is activated during navigation from an unfamiliar site. The groups with no plug showed the highest ZENK cell density, supporting the activation of the olfactory system probably being due to sensory input. Moreover, both Cpis seem to contribute differently to the navigation process. Only occlusion of the right OB resulted in a decreased ZENK cell expression in the Cpi, whereas occlusion of the left nostril had no effect. This is the first study to reveal neuronal activation patterns in the olfactory system during homing. Our data show that lateralized processing of olfactory cues is indeed involved in navigation over unfamiliar areas.

Neurobiology of the homing pigeon--a review

Homing pigeons are well known as good homers, and the knowledge of principal parameters determining their homing behaviour and the neurological basis for this have been elucidated in the last decades. Several orientation mechanisms and parameters-sun compass, earth's magnetic field, olfactory cues, visual cues-are known to be involved in homing behaviour, whereas there are still controversial discussions about their detailed function and their importance. This paper attempts to review and summarise the present knowledge about pigeon homing by describing the known orientation mechanisms and factors, including their pros and cons. Additionally, behavioural features like motivation, experience, and track preferences are discussed. All behaviour has its origin in the brain and the neuronal basis of homing and the neuroanatomical particularities of homing pigeons are a main topic of this review. Homing pigeons have larger brains in comparison to other non-homing pigeon breeds and particularly show increased size of the hippocampus. This underlines our hypothesis that there is a relationship between hippocampus size and spatial ability. The role of the hippocampus in homing and its plasticity in response to navigational experience are discussed in support of this hypothesis.

Finding home: the final step of the pigeons' homing process studied with a GPS data logger

Experiments have shown that homing pigeons are able to develop navigational abilities even if reared and kept confined in an aviary, provided that they are exposed to natural winds. These and other experiments performed on inexperienced birds have shown that previous homing experiences are not necessary to determine the direction of displacement. While the cues used in the map process for orienting at the release site have been extensively investigated, the final step of the homing process has received little attention by researchers. Although there is general agreement on the relevance of visual cues in navigation within the home area, there is a lack of clear evidence. In order to investigate the final step of the homing process, we released pigeons raised under confined conditions and others that had been allowed to fly freely around the loft and compared their flight paths recorded with a Global-Positioning-System logger. Our data show that a limited view of the home area impairs the pigeons' ability to relocate the loft at their first homing flight, suggesting that the final step of the homing process is mediated via recognition of familiar visual landmarks in the home area.

Effects of Partial Hippocampal Lesions by IbotenicAcid on Repeated Acquisition of Spatial Discrimination in Pigeons

Pigeons were trained on a spatial discrimination task using a repeated acquisition procedure. In this procedure, the pigeons were trained to discriminate between the positions of three keys. One of them was designated the correct key. When the subjects reached the criterion, the discrimination task was changed, with one of two previously incorrect keys now being made the correct key. This procedure was repeated at least 15 times. Then, lesions to the whole hippocampus, the medial hippocampus or to the lateral hippocampus were made by injections of ibotenic acid (Experiment 1). Only the subjects with damage to the whole hippocampus showed deficits in learning after the lesions. The deficits were similar to those caused by aspiration lesions /37/. Knife cuts separating the medial and lateral hippocampi were made in Experiment 2. The subjects did not show deficits in the spatial discrimination task after the sections. Although studies of the connectivity in the avian hippocampus suggested functional differences between the medial and lateral hippocampi, the present results show that pigeons can learn spatial discrimination with the medial and lateral parts of hippocampus separated.

Spatial-specificity of single-units in the hippocampal formation of freely moving homing pigeons

The importance of space-specific single-unit activity for hippocampal formation (HF)-mediated learning and memory in rodents has been extensively studied, yet little is known about how the unit findings in rodents generalize to other vertebrate species. We report a first assessment of the space-specific single-unit activity recorded from the HF of homing pigeons as they moved through a plus maze for food reward. Rate maps of pigeon HF single-unit activity typically revealed multiple regions (2-5 per cell) of increased activity (on average, 2.5 times higher than other regions of the maze) that in 27% of slow-firing cells was reliably space-specific over time. The qualitative appearance of rate maps and the degree of spatial-specificity observed for most all pigeon HF cells suggests more modest space-specific activity than typically reported for rat hippocampal cells. The nature of space-specific activity in the pigeon HF includes (1) often transiently reliable regions of increased activity for many cells, (2) multiple patches of activity that were sometimes observed in analogous maze areas, and (3) cells displaying substantial decreases in firing rate between baseline and maze-run conditions that could not be explained by a simple relationship between firing rate and a pigeon's speed. These observations suggest that pigeon HF cells may be coding for an unspecified behavioral/motivational/environmental factors in addition to a pigeon's momentary location. The data further suggest that the spatial ecology and evolutionary history of different species may be a critical feature shaping how HF neurons capture properties of space.

Hippocampal lesions do not disrupt navigational map retention in homing pigeons under conditions when map acquisition is hippocampal dependent

In contrast to map-like navigation by familiar landmarks, understanding the relationship between the avian hippocampal formation (HF) and the homing pigeon navigational map has remained a challenge. With the goal of filling an empirical gap, we performed an experiment in which young homing pigeons learned a navigational map while being held in an outdoor aviary, and then half the birds were subjected to HF ablation. The question was whether HF lesion would impair retention of a navigational map learned under conditions known to require participation of HF. The pigeons, which had never flown from the aviary before, together with an additional control group that learned a navigational map with free-flight experience, were then released from two distant release sites. Contrary to expectation, the HF-lesioned birds oriented in a homeward direction in manner indistinguishable from the intact control pigeons raised in the same outdoor aviary. HF lesion did not result in a navigational map retention deficit. Together with previous results, it is now clear that regardless of the learning environment present during acquisition, HF plays no necessary role in the subsequent retention or operation of the homing pigeon navigational map.

Transitive Behavior in Hippocampal-Lesioned Pigeons

The hippocampus of birds and mammals is critical for the learning of map-like memory representations of environmental space. It has been suggested that the hippocampus of rats also participates in non-spatial relational learning, including the learning of non-spatial transitive relationships among odor stimuli [Bunsey and Eichenbaum, Nature 1996]. Although transitive-like learning has been demonstrated in a variety of vertebrate species, from a comparative perspective the role of the hippocampus in this form of learning has not been tested in other amniote groups. We trained control and hippocampal-lesioned homing pigeons on a series of visual, non-spatial, go/no-go conditional discriminations and then tested them on novel transitivity probe trials. The hippocampal-lesioned pigeons were as successful as control pigeons in responding appropriately to correct and incorrect transitivity pairs. The finding that the homing pigeon hippocampal formation is not necessary for solving this serial, conditional discrimination task is important for further understanding hippocampal function across species, and represents one of the few studies that have attempted to localize a brain region responsible for the phenomenon of transitive behavior learning.

What Does a Pigeon (Columba livia) Brain Look Like During Homing? Selective Examination of ZENK Expression

Lesion studies have shown that the avian hippocampus plays a crucial role in homing pigeon (Columba livia) navigation. Using the expression of the immediate early gene protein ZENK in intact pigeons, the authors found regional variation in hippocampal activation as a consequence of homing and, necessarily, the behavior and internal states that accompany it. Specifically, pigeons that homed displayed a significant increase in the number of ZENK-labeled cells in the lateral hippocampal formation compared with pigeons that did not home, whereas no difference was seen in the medial hippocampus. Significant changes in ZENK expression were also found in the medial striatum, which resembles the mammalian ventral striatum. The results identify portions of the hippocampal formation and the medial striatum as sites of plasticity associated with homing.

Evolution of forebrain and spatial cognition in vertebrates: conservation across diversity

Historically the dominant trend in comparative brain and behavior research has emphasized the differences in cognition and its neural basis among species. In fact, the vertebrate forebrain shows a remarkable range of diversity and specialized adaptations. Probably the major morphological variation is that observed in the telencephalon of the actinopterygian fish, which undergoes a process of eversion during embryonic development, relative to the telencephalon of non-actinopterygians (for instance, amniotes), which develops by a process of evagination. These different developmental processes produce notable variation, mainly two solid telencephalic hemispheres separated by a unique ventricle in the actinopterygian radiation that contrasts with the hemispheres with internal ventricles in other groups. However, an increasing amount of evidence reveals that the forebrain of vertebrates, whether everted or evaginated, presents a common pattern of basic organization that supports highly conserved cognitive functions. We analyze here recent data indicating a close functional similarity between spatial cognition mechanisms in different groups of vertebrates, mammals, birds, reptiles, and teleost fish, and we show in addition that they rely on homologous neural mechanisms. Thus, recent functional and behavioral comparative evidence is added to the developmental and neuroanatomical data suggesting that the evolution of cognitive capabilities and their neural basis in vertebrates could have been more conservative than previously realized.

The importance of hippocampus-dependent non-spatial tasks in analyses of homology and homoplasy

The hippocampus or a homologous region plays a role in spatial tasks in a large number of vertebrate species. This result, in combination with recent findings of adaptive specializations of the hippocampus for spatial demands, has led to the conclusion that the prominent selective force behind hippocampal evolution was a need for spatial abilities. However, a review of non-spatial hippocampus-dependent tasks shows that many vertebrate species also share non-spatial functions of the hippocampus. Placed in the appropriate phylogenetic context, it becomes clear that non-spatial facets of hippocampal function were just as likely to be present in our vertebrate ancestors as spatial ones. In addition, the absence of spatial strategy use in three lineages suggests divergence of this feature. Divergence in this character and other characteristics of hippocampal function are meaningful indicators of lineage specific functions. Studies of the evolution of the hippocampus must include examination of spatial and non-spatial functions of the hippocampus and consider both conserved, as well as derived, features.

Spatial and non-spatial learning in turtles: the role of medial cortex

In mammals and birds, hippocampal processing is crucial for allocentric spatial learning. In these vertebrate groups, lesions to the hippocampal formation produce selective impairments in spatial tasks that require the encoding of relationships among environmental features, but not in tasks that require the approach to a single cue or simple non-spatial discriminations. In reptiles, a great deal of anatomical evidence indicates that the medial cortex (MC) could be homologous to the hippocampus of mammals and birds; however, few studies have examined the functional role of this structure in relation to learning and memory processes. The aim of this work was to study how the MC lesions affect spatial strategies. Results of Experiment 1 showed that the MC lesion impaired the performance in animals pre-operatively trained in a place task, and although these animals were able to learn the same task after surgery, probe test revealed that learning strategies used by MC lesioned turtles were different to that observed in sham animals. Experiment 2 showed that the MC lesion did not impair the retention of the pre-operatively learned task when a single intramaze visual cue identified the goal. These results suggest that the reptilian MC and hippocampus of mammals and birds function in quite similar ways, not only in relation to those spatial functions that are impaired, but also in relation to those learning processes that are not affected.

The role of visual landmarks in the avian familiar area map

The question of whether homing pigeons use visual landmarks for orientation from distant, familiar sites is an unresolved issue in the field of avian navigation. Where evidence has been found, the question still remains as to whether the landmarks are used independent of the map and compass mechanism for orientation that is so important to birds. Recent research has challenged the extent to which experiments that do not directly manipulate the visual sense can be used as evidence for compass-independent orientation. However, it is proposed that extending a new technique for research on vision in homing to include manipulation of the compasses used by birds might be able to resolve this issue. The effect of the structure of the visual sense of the homing pigeon on its use of visual landmarks is also considered.

How the viewing of familiar landscapes prior to release allows pigeons to home faster: evidence from GPS tracking

Providing homing pigeons with a 5 min preview of the landscape at familiar sites prior to release reliably improves the birds' subsequent homing speeds. This phenomenon has been taken to suggest that the visual panorama is involved in familiar-site recognition, yet the exact nature of the improvement has never been elucidated. We employed newly developed miniature Global Positioning System (GPS) tracking technology to investigate how access to visual cues prior to release affects pigeons' flight along the length of the homing route. By applying a variety of novel analytical techniques enabled by the high-resolution GPS data (track efficiency, virtual vanishing bearings,orientation threshold), we localised the preview effect to the first 1000 m of the journey. Birds denied preview of a familiar landscape for 5 min before take-off flew an initially more tortuous path, including a high incidence of circling, possibly as part of an information-gathering strategy to determine their position. Beyond the first 1000 m, no differences were found in the performance of birds with or without preview. That the effect of the visual treatment was evident only in the early part of the journey suggests that lack of access to visual cues prior to release does not result in a non-specific effect on behaviour that is maintained throughout the flight. Instead, it seems that at least some decisions regarding the direction of home can be made prior to release and that such decisions are delayed if visual access to the landscape is denied. Overall, the variety of approaches applied here clearly highlight the potential for future applications of GPS tracking technology in navigation studies.

Hippocampus lesions impair landmark array spatial learning in homing pigeons: a laboratory study

Hippocampal (HF)-lesioned pigeons display impaired homing ability when flying over familiar terrain, where they are presumably relying on a map-like representation of familiar landmarks to navigate. However, research carried out in the field precludes a direct test of whether hippocampal lesions compromise the ability of homing pigeons to navigate by familiar landmarks. To examine more thoroughly the relationship between hippocampus and landmark spatial learning, control, neostriatum-lesioned, and HF-lesioned homing pigeons were trained on two open field, laboratory, conditional discrimination tasks. One was a visual landmark array task, and the other was a room color discrimination task. For the tasks, the correct of three differently colored food bowls was determined by the spatial relationship among a group of five landmarks and room color, respectively. Intact control birds successfully learned both tasks, while neostriatum-lesioned birds successfully learned the landmark array task-the only task on which they were trained. By contrast, HF-lesioned birds successfully learned the room color task but were unable to learn the landmark array task. The data support the hypothesis that homing performance deficits observed in the field following hippocampal lesions are in part a consequence of an impairment in the ability of lesioned pigeons to use familiar visual landmarks for navigation.

Electrophysiological profile of avian hippocampal unit activity: a basis for regional subdivisions

Electrophysiological activity was recorded from single neurons (units) in the hippocampal formation (HF) of freely moving homing pigeons in order to provide a taxonomy of unit types found in the avian HF; a taxonomy that could be used to define regional subdivisions and be compared with unit types found in the mammalian hippocampus. Two distinct types of unit were observed in the avian HF. One type was uniformly characterized by relatively rapid firing rates and shorter spike widths, and was found throughout the HF. The other type was more variable in activity profile but, compared with the fast-firing units, was characterized by slower firing rates and longer spike widths. However, despite the variable nature of the slow-firing units, most slow-firing units recorded within a given anatomical region displayed similar firing rates, spike widths, and interspike intervals. In general, ventral HF units displayed activity patterns similar to projection cells found in the mammalian Ammon's horn. Most dorsocaudal units displayed activity patterns similar to presumed granular cells in the mammalian dentate gyrus. By contrast, most dorsorostral units displayed activity patterns similar to a type of unit found in the mammalian subiculum. Although different in some details, the overall activity profile of units found in the avian HF, and their regional distribution, is strikingly similar to unit types found in the mammalian hippocampus, suggesting that unit activity profile is one hippocampal dimension conserved through evolution.