The avian hippocampus: Evidence for a role in the development of the homing pigeon navigational map

The quantitative anatomy of the hippocampal formation in homing pigeons and other pigeon breeds: implications for spatial cognition

Artificial selection for specific behavioural and physical traits in domesticated animals has resulted in a wide variety of breeds. One of the most widely recognized examples of behavioural selection is the homing pigeon (Columba livia), which has undergone intense selection for fast and efficient navigation, likely resulting in significant anatomical changes to the hippocampal formation. Previous neuroanatomical comparisons between homing and other pigeon breeds yielded mixed results, but only focused on volumes. We completed a more systematic test for differences in hippocampal formation anatomy between homing and other pigeon breeds by measuring volumes, neuron numbers and neuron densities in the hippocampal formation and septum across homing pigeons and seven other breeds. Overall, we found few differences in hippocampal formation volume across breeds, but large, significant differences in neuron numbers and densities. More specifically, homing pigeons have significantly more hippocampal neurons and at higher density than most other pigeon breeds, with nearly twice as many neurons as feral pigeons. These findings suggest that neuron numbers may be an important component of homing behaviour in homing pigeons. Our data also provide the first evidence that neuronal density can be modified by artificial selection, which has significant implications for the study of domestication and interbreed variation in anatomy and behaviour.

Interactions between neural representations of the social and spatial environment

Even in our highly interconnected modern world, geographic factors play an important role in human social connections. Similarly, social relationships influence how and where we travel, and how we think about our spatial world. Here, we review the growing body of neuroscience research that is revealing multiple interactions between social and spatial processes in both humans and non-human animals. We review research on the cognitive and neural representation of spatial and social information, and highlight recent findings suggesting that underlying mechanisms might be common to both. We discuss how spatial factors can influence social behaviour, and how social concepts modify representations of space. In so doing, this review elucidates not only how neural representations of social and spatial information interact but also similarities in how the brain represents and operates on analogous information about its social and spatial surroundings.This article is part of the theme issue 'The spatial-social interface: a theoretical and empirical integration'.

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.

Dim artificial light at night alters immediate early gene expression throughout the avian brain

Artificial light at night (ALAN) is a pervasive pollutant that alters physiology and behavior. However, the underlying mechanisms triggering these alterations are unknown, as previous work shows that dim levels of ALAN may have a masking effect, bypassing the central clock. Light stimulates neuronal activity in numerous brain regions which could in turn activate downstream effectors regulating physiological response. In the present study, taking advantage of immediate early gene (IEG) expression as a proxy for neuronal activity, we determined the brain regions activated in response to ALAN. We exposed zebra finches to dim ALAN (1.5 lux) and analyzed 24 regions throughout the brain. We found that the overall expression of two different IEGs, cFos and ZENK, in birds exposed to ALAN were significantly different from birds inactive at night. Additionally, we found that ALAN-exposed birds had significantly different IEG expression from birds inactive at night and active during the day in several brain areas associated with vision, movement, learning and memory, pain processing, and hormone regulation. These results give insight into the mechanistic pathways responding to ALAN that underlie downstream, well-documented behavioral and physiological changes.

Virtual Morris water maze: opportunities and challenges

The ability to accurately recall locations and navigate our environment relies on multiple cognitive mechanisms. The behavioural and neural correlates of spatial navigation have been repeatedly examined using different types of mazes and tasks with animals. Accurate performances of many of these tasks have proven to depend on specific circuits and brain structures and some have become the standard test of memory in many disease models. With the introduction of virtual reality (VR) to neuroscience research, VR tasks have become a popular method of examining human spatial memory and navigation. However, the types of VR tasks used to examine navigation across laboratories appears to greatly differ, from open arena mazes and virtual towns to driving simulators. Here, we examined over 200 VR navigation papers, and found that the most popular task used is the virtual analogue of the Morris water maze (VWM). Although we highlight the many advantages of using the VWM task, there are also some major difficulties related to the widespread use of this behavioural method. Despite the task's popularity, we demonstrate an inconsistency of use - particularly with respect to the environmental setup and procedures. Using different versions of the virtual water maze makes replication of findings and comparison of results across researchers very difficult. We suggest the need for protocol and design standardisation, alongside other difficulties that need to be addressed, if the virtual water maze is to become the 'gold standard' for human spatial research similar to its animal counterpart.

A cognitive map in a poison frog

A fundamental question in cognitive science is whether an animal can use a cognitive map. A cognitive map is a mental representation of the external world, and knowledge of one's place in this world, that can be used to determine efficient routes to any destination. Many birds and mammals are known to employ a cognitive map, but whether other vertebrates can create a cognitive map is less clear. Amphibians are capable of using beacons, gradients and landmarks when navigating, and many are proficient at homing. Yet only one prior study directly tested for a cognitive map in amphibians, with negative results. Poison frogs exhibit unusually complex social and spatial behaviors and are capable of long-distance homing after displacement, suggesting that they may be using complex spatial navigation strategies in nature. Here, we trained the poison frog Dendrobates auratus in a modified Morris water maze that was designed to suppress thigmotaxis to the maze wall, promoting exploration of the arena. In our moat maze, the poison frogs were able to use a configuration of visual cues to find the hidden platform. Moreover, we demonstrate that they chose direct paths to the goal from multiple random initial positions, a hallmark of a cognitive map. The performance of the frogs in the maze was qualitatively similar to that of rodents, suggesting that the potential to evolve a cognitive map is an evolutionarily conserved trait of vertebrates.

Latent cognitive effects from low-level polychlorinated biphenyl exposure in juvenile European starlings (Sturnus vulgaris)

Ecotoxicology research on polychlorinated biphenyl (PCB) mixtures has focused principally on short-term effects on reproduction, growth, and other physiological endpoints. Latent cognitive effects from early life exposure to low-level PCBs were examined in an avian model, the European starling (Sturnus vulgaris). Thirty-six birds, divided equally among 4 treatment groups (control = 0 µg, low = 0.35 µg, intermediate = 0.70 µg, and high = 1.05 µg Aroclor 1254/g body weight), were dosed 1 d through 18 d posthatch, then tested 8 mo to 9 mo later in captivity in an analog to an open radial arm maze. Birds were subject to 4 sequential experiments: habituation, learning, cue selection, and memory. One-half of the birds did not habituate to the test cage; however, this was not linked to a treatment group. Although 11 of the remaining 18 birds successfully learned, only 1 was from the high-dosed group. Control and low-dosed birds were among the only treatment groups to improve trial times throughout the learning experiment. High-dosed birds were slower and more error-prone than controls. Cue selection (spatial or color cues) and memory retention were not affected by prior PCB exposure. The results indicate that a reduction in spatial learning ability persists among birds exposed to Aroclor 1254 during development. This may have implications for migration ability, resource acquisition, and other behaviors relevant for fitness.

Seasonal change in the avian hippocampus

The hippocampus plays an important role in cognitive processes, including memory and spatial orientation, in birds. The hippocampus undergoes seasonal change in food-storing birds and brood parasites, there are changes in the hippocampus during breeding, and further changes occur in some species in association with migration. In food-storing birds, seasonal change in the hippocampus occurs in fall and winter when the cognitively demanding behaviour of caching and retrieving food occurs. The timing of annual change in the hippocampus of food-storing birds is quite variable, however, and appears not to be under photoperiod control. A variety of factors, including cognitive performance, exercise, and stress may all influence seasonal change in the avian hippocampus. The causal processes underlying seasonal change in the avian hippocampus have not been extensively examined and the more fully described hormonal influences on the mammalian hippocampus may provide hypotheses for investigating the control of hippocampal seasonality in birds.

Navigation outside of the box: what the lab can learn from the field and what the field can learn from the lab

Space is continuous. But the communities of researchers that study the cognitive map in non-humans are strangely divided, with debate over its existence found among behaviorists but not neuroscientists. To reconcile this and other debates within the field of navigation, we return to the concept of the parallel map theory, derived from data on hippocampal function in laboratory rodents. Here the cognitive map is redefined as the integrated map, which is a construction of dual mechanisms, one based on directional cues (bearing map) and the other on positional cues (sketch map). We propose that the dual navigational mechanisms of pigeons, the navigational map and the familiar area map, could be homologous to these mammalian parallel maps; this has implications for both research paradigms. Moreover, this has implications for the lab. To create a bearing map (and hence integrated map) from extended cues requires self-movement over a large enough space to sample and model these cues at a high resolution. Thus a navigator must be able to move freely to map extended cues; only then should the weighted hierarchy of available navigation mechanisms shift in favor of the integrated map. Because of the paucity of extended cues in the lab, the flexible solutions allowed by the integrated map should be rare, despite abundant neurophysiological evidence for the existence of the machinery needed to encode and map extended cues through voluntary movement. Not only do animals need to map extended cues but they must also have sufficient information processing capacity. This may require a specific ontogeny, in which the navigator's nervous system is exposed to naturally complex spatial contingencies, a circumstance that occurs rarely, if ever, in the lab. For example, free-ranging, flying animals must process more extended cues than walking animals and for this reason alone, the integrated map strategy may be found more reliably in some species. By taking concepts from ethology and the parallel map theory, we propose a path to directly integrating the three great experimental paradigms of navigation: the honeybee, the homing pigeon and the laboratory rodent, towards the goal of a robust, unified theory of animal navigation.

Magnetoreception in an avian brain in part mediated by inner ear lagena

Many animals use the Earth's geomagnetic field for orientation and navigation, but the neural mechanisms underlying that ability remain enigmatic. Support for at least two avian magnetoreceptors exists, including magnetically activated photochemicals in the retina and ferrimagnetic particles in the beak. The possibility of a third magnetoreceptor in the inner ear lagena organs has been suggested. The brain must process magnetic receptor information to derive constructs representing directional heading and geosurface location. Here, we used the c-Fos transcription factor, a marker for activated neurons, to discover where in the brain computations related to a specific set of magnetic field stimulations occur. We found that neural activations in discrete brain loci known to be involved in orientation, spatial memory, and navigation may constitute a major magnetoreception pathway in birds. We also found, through ablation studies, that much of the observed pathway appears to receive magnetic information from the pigeon lagena receptor organs.

Prenatal auditory stimulation alters the levels of CREB mRNA, p-CREB and BDNF expression in chick hippocampus

Prenatal auditory stimulation influences the development of the chick auditory pathway and the hippocampus showing an increase in various morphological parameters as well as expression of calcium-binding proteins. Calcium regulates the activity of cyclic adenosine monophosphate-response element binding (CREB) protein. CREB is known to play a role in development, undergo phosphorylation with neural activity as well as regulate transcription of BDNF. BDNF is important for the survival of neurons and regulates synaptic strength. Hence in the present study, we have evaluated the levels of CREB mRNA and protein along with p-CREB protein as well as BDNF mRNA and protein levels in the chick hippocampus at embryonic days (E) 12, E16, E20 and post-hatch day (PH) 1 following activation by prenatal auditory stimulation. Fertilized eggs were exposed to species-specific sound or sitar music (frequency range: 100-6300Hz) at 65dB levels for 15min/h over 24h from E10 till hatching. The control chick hippocampus showed higher CREB mRNA and p-CREB protein in the early embryonic stages, which later decline whereas BDNF mRNA and BDNF protein levels increase until PH1. The CREB mRNA and p-CREB protein were significantly increased at E12, E16 and PH1 in the auditory stimulated groups as compared to control group. A significant increase in the level of BDNF mRNA was observed from E12 and the protein expression from E16 onwards in both auditory stimulated groups. Therefore, enhanced phosphorylation of CREB during development following prenatal sound stimulation may be responsible for cell survival. Increased levels of p-CREB again at PH1 may trigger synthesis of proteins necessary for synaptic plasticity. Further, the increased levels of BDNF may also help in regulating synaptic plasticity.

Effect of prenatal auditory stimulation on numerical synaptic density and mean synaptic height in the posthatch Day 1 chick hippocampus

Previous studies on prenatal auditory stimulation by species-specific sound or sitar music showed enhanced morphological and biochemical changes in chick hippocampus, which plays an important role in learning and memory. Changes in the efficiency of synapses, synaptic morphology and de novo synapse formation affects learning and memory. Therefore, in the present study, we set out to investigate the mean synaptic density and mean synaptic height at posthatch Day 1 in dorsal and ventral part of chick hippocampus following prenatal auditory stimulation. Fertilized 0 day eggs of domestic chick incubated under normal conditions were exposed to patterned sounds of species-specific and sitar music at 65 dB levels for 15 min/h round the clock (frequency range: 100-6300 Hz) from embryonic Day 10 till hatching. The synapses identified under transmission electron microscope were estimated for their numerical density by physical disector method and also the mean synaptic height calculated. Our results demonstrate a significant increase in mean synaptic density with no alterations in the mean synaptic height following both types of auditory stimulation in the dorsal as well as ventral part of the hippocampus. The observed increase in mean synaptic density suggests enhanced synaptic substrate to strengthen hippocampal function.

Calbindin D-28K and parvalbumin expression in embryonic chick hippocampus is enhanced by prenatal auditory stimulation

Calcium-binding proteins (CaBPs) buffer excess of cytosolic Ca(2+), which accompanies neuronal activity following external stimuli. Prenatal auditory stimulation by species-specific sound and music influences early maturation of the auditory pathway and the behavioral responses in chicks. In this study, we determined the volume, total number of neurons, proportion of calbindin D-28K and parvalbumin-positive neurons along with their levels of expression in the developing chick hippocampus following prenatal auditory stimulation. Fertilized eggs of domestic chicks were exposed to sounds of either species-specific calls or sitar music at 65 dB for 15 min/h round the clock from embryonic day (E) 10 until hatching. Hippocampi of developmental stages (E12, E16 and E20) were examined. With an increase in embryonic age during normal development, the hippocampus showed an increase in its volume, total number of neurons as well as in the neuron proportions and levels of expression of calbindin D-28K and parvalbumin. A significant increase of volume at E20 was noted only in the music-stimulated group compared to that of their age-matched control (p<0.05). On the other hand, both auditory-stimulated groups showed a significant increase in the proportion of immunopositive neurons and the levels of expression of calbindin D-28K and parvalbumin as compared to the control at all developmental stages studied (p<0.003). The increase in proportions of CaBP neurons during development and in the sound-enriched groups suggests an activity-dependent increase in Ca(2+) influx. The enhanced expression of CaBPs may help in cell survival by preventing excitotoxic death of neurons during development and may also be involved in long-term potentiation.

Differential c-fos expression in the brain of male Japanese quail following exposure to stimuli that predict or do not predict the arrival of a female

We investigated the effects of presenting a sexual conditioned stimulus on the expression of c-fos in male Japanese quail. Eight brain sites were selected for analysis based on previous reports of c-fos expression in these areas correlated with sexual behaviour or learning. Males received either paired or explicitly unpaired presentations of an arbitrary stimulus and visual access to a female. Nine conditioning trials were conducted, one per day, for each subject. On the day following the ninth trial, subjects were exposed to the conditional stimulus (CS) for 5 min. Conditioning was confirmed by analysis of rhythmic cloacal sphincter movements (RCSM), an appetitive sexual behaviour, made in response to the CS presentation. Subjects in the paired condition performed significantly more RCSM than subjects in the unpaired group. Brains were collected 90 min following the stimulus exposure and stained by immunohistochemistry for the FOS protein. Significant group differences in the number of FOS-immunoreactive (FOS-ir) cells were found in two brain regions, the nucleus taeniae of the amygdala (TnA) and the hippocampus (Hp). Subjects in the paired condition had fewer FOS-ir cells in both areas than subjects in the unpaired condition. These data provide additional support to the hypothesis that TnA is implicated in the expression of appetitive sexual behaviours in male quail and corroborate numerous previous reports of the involvement of the hippocampus in conditioning. Further, these data suggest that conditioned and unconditioned sexual stimuli activate different brain regions but have similar behavioural consequences.

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.

Passive avoidance training is correlated with decreased cell proliferation in the chick hippocampus

One-trial passive avoidance learning (PAL), where the aversive stimulus is the bitter-tasting substance methylanthranilate (MeA), affects neuronal and synaptic plasticity in learning-related areas of day-old domestic chicks (Gallus domesticus). Here, cell proliferation was examined in the chick forebrain by using 5-bromo-2-deoxyuridine (BrdU) at 24 h and 9 days after PAL. At 24 h post-BrdU injection, there was a significant reduction in labelling in MeA-trained chicks in both the dorsal hippocampus and area parahippocampalis, in comparison to controls. Moreover, double-immunofluorescence labelling for BrdU and the nuclear neuronal marker (NeuN) showed a reduction of neuronal cells in the dorsal hippocampus of the MeA-trained group compared with controls (35 and 49%, respectively). There was no difference in BrdU labelling in hippocampal regions between trained and control groups of chicks at 9 days post-BrdU injection; however, the number of BrdU-labelled cells was considerably lower than at 24 h post-BrdU injection, possibly due to migration of cells within the telencephalon rather than cell loss as apoptotic analyses at 24 h and 9 days post-BrdU injection did not demonstrate differences in cell death between treatment groups. Cortisol levels increased in the chick hippocampus of MeA-trained birds 20 min after PAL, suggesting the possibility of a stress-related mechanism of cell proliferation reduction in the hippocampus. In contrast to hippocampal areas, the olfactory bulb, an area strongly stimulated by the strong-smelling MeA, showed increased cell genesis in comparison to controls at both 24 h and 9 days post-training.

Prenatal acoustic stimulation influences neuronal size and the expression of calcium-binding proteins (calbindin D-28K and parvalbumin) in chick hippocampus

Prenatal auditory enrichment by species-specific sounds and sitar music enhances the expression of immediate early genes, synaptic proteins and calcium binding proteins (CaBPs) as well as modifies the structural components of the brainstem auditory nuclei and auditory imprinting area in chicks. There is also facilitation of postnatal auditory preference of the chicks to maternal calls following both types of sound stimulation indicating prenatal perceptual learning. To examine whether the sound enrichment protocol also affects the areas related to learning and memory, we assessed morphological changes in the hippocampus at post-hatch day 1 of control and prenatally sound-stimulated chicks. Additionally, the proportions of neurons containing calbindin D-28K and parvalbumin immunoreactivity as well as their protein levels were determined. Fertilized eggs of domestic chick were incubated under normal conditions of temperature, humidity, forced draft of air as well as light and dark (12:12h) photoperiods. They were exposed to patterned sounds of species-specific and sitar music at 65 dB for 15 min per hour over a day/night cycle from day 10 of incubation till hatching. The hippocampal volume, neuronal nuclear size and total number of neurons showed a significant increase in the music-stimulated group as compared to the species-specific sound-stimulated and control groups. However, in both the auditory-stimulated groups the protein levels of calbindin and parvalbumin as well as the percentage of the immunopositive neurons were increased. The enhanced proportion of CaBPs in the sound-enriched groups suggests greater Ca(2+) influx, which may influence long-term potentiation and short-term memory.

Deficits in acquisition of spatial learning after dorsomedial telencephalon lesions in goldfish

Acquisition of spatial learning is an important function of mammalian hippocampus. In order to identify the brain areas in teleost fish that are homologous to mammalian hippocampus, the present study examined the effects of lesions in the dorsal area of the caudal telencephalon of goldfish (Carassius auratus) on the acquisition of spatial learning. An open-field maze that was similar to the dry version of the Morris water maze was used. The task consisted of habituation and postoperative training to reach the position of the bait. Extramaze cues were visible in the habituation sessions in experiment 1, while they were blocked and not visible in the habituation sessions in experiment 2. Only in experiment 2, there was a significant deficit in the performance in the training sessions in the goldfish with damage to the dorsomedial area of the caudal telencephalon (DM). These data showed that blocking of the extramaze cues in the habituation sessions caused deficits in postoperative acquisition of spatial learning in the training sessions in the goldfish with DM lesions. Latent learning in the habituation sessions, however, eliminated the effects of the DM lesions on spatial learning. The present study suggests that the DM plays a critical role in acquisition of spatial learning.

Passive avoidance training decreases synapse density in the hippocampus of the domestic chick

The bird hippocampus (Hp), although lacking the cellular lamination of the mammalian Hp, possesses comparable roles in spatial orientation and is implicated in passive avoidance learning. As in rodents it can be divided into dorsal and ventral regions based on immunocytochemical, tracing and electrophysiological studies. To study the effects of passive avoidance learning on synapse morphometry in the Hp, spine and shaft synapse densities of 1-day-old domestic chicks were determined in dorsal and ventral Hp of each hemisphere by electron microscopy, 6 and 24 h following training to avoid pecking at a bead coated with a bitter-tasting substance, methyl anthranilate (MeA). The density of asymmetric spine and shaft synapses in MeA-trained birds at 6 h post-training was significantly lower in the dorsal and ventral Hp of the right hemisphere relative to control (untrained) chicks, but by 24 h this difference was absent. A hemispheric asymmetry was apparent in the ventral Hp where the water-trained group showed enhanced shaft and spine synapse density in the left hemisphere, whilst in the MeA-trained group only asymmetric shaft synapses follow the same pattern in relation to the right hemisphere. There were no differences in asymmetric shaft synapses in the dorsal Hp at 6 h post-training, but at 24 h post-training there was a reduction in the density of shaft synapses in the right hemisphere in MeA compared with control birds. These data are discussed in relation to the pruning effects of stress and learning on synapse density in chick Hp.

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

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

A comparison of ventral tegmental neurons projecting to optic flow regions of the inferior olive vs. the hippocampal formation

The ventral tegmental area (catecholaminergic group A10) is a midbrain region characterized by concentrated dopaminergic immunoreactivity. Previous studies in pigeons show that the ventral tegmental area provides a robust projection to the hippocampal formation and to the medial column of the inferior olive. However, the distribution, morphology, and neurochemical content of the neurons that constitute these projections have not been resolved. In this study, we used a combination of retrograde tracing techniques and immunofluorohistochemistry to address these issues. Retrograde tracers were used to demonstrate that the distribution of ventral tegmental area neurons projecting to the hippocampus and the inferior olive overlap in the caudo-ventral ventral tegmental area. The hippocampus- and inferior olive-projecting ventral tegmental area neurons could not be distinguished based on morphology: most neurons had small- to medium-sized multipolar or fusiform soma. Double-labeling with fluorescent retrograde tracers revealed that the hippocampus- and medial column of the inferior olive-projecting neurons were found intermingled in the ventral tegmental area, but no cells were double labeled; i.e. individual ventral tegmental area neurons do not project to both the hippocampal formation and medial column of the inferior olive. Finally, we found that a minority (8.2%) of ventral tegmental area neurons providing input to the hippocampus were tyrosine hydroxylase-immunoreactive, whereas none of the inferior olive-projecting neurons were tyrosine hydroxylase positive. Combined, our findings show that the projections to the hippocampus and olivocerebellar pathway arise from intermixed subpopulations of ventral tegmental area neurons with indistinguishable morphology but only the hippocampal projection involves dopaminergic neurons. We suggest that equivalent projections from the ventral tegmental area to the hippocampal formation and inferior olive exist in mammals and discuss their potential role in the processing of optic flow and the analysis of self-motion.

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.

Functional asymmetry of left and right avian piriform cortex in homing pigeons' navigation

It has been shown that homing pigeons rely on olfactory cues to navigate over unfamiliar areas and that any kind of olfactory impairment produces a dramatic reduction of navigational performance from unfamiliar sites. The avian piriform cortex is the main projection field of olfactory bulbs and it is supposed to process olfactory information; not surprisingly bilateral lesions to this telencephalic region disrupt homing pigeon navigation. In the present study, we attempted to assess whether the left and right piriform cortex are differentially involved in the use of the olfactory navigational map. Therefore, we released from unfamiliar locations pigeons subjected, when adult, to unilateral ablation of the piriform cortex. After being released, the pigeons lesioned to the right piriform cortex orientated similarly to the intact controls. On the contrary, the left lesioned birds were significantly more scattered than controls, showing a crucial role of the left piriform cortex in processing the olfactory cues needed for determining the direction of displacement. However, both lesioned groups were significantly slower than controls in flying back to the home loft, showing that the integrity of both sides of the piriform cortex is necessary to accomplish the whole homing process.

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.

The evolution of the cognitive map

The hippocampal formation of mammals and birds mediates spatial orientation behaviors consistent with a map-like representation, which allows the navigator to construct a new route across unfamiliar terrain. This cognitive map thus appears to underlie long-distance navigation. Its mediation by the hippocampal formation and its presence in birds and mammals suggests that at least one function of the ancestral medial pallium was spatial navigation. Recent studies of the goldfish and certain reptile species have shown that the medial pallium homologue in these species can also play an important role in spatial orientation. It is not yet clear, however, whether one type of cognitive map is found in these groups or indeed in all vertebrates. To answer this question, we need a more precise definition of the map. The recently proposed parallel map theory of hippocampal function provides a new perspective on this question, by unpacking the mammalian cognitive map into two dissociable mapping processes, mediated by different hippocampal subfields. If the cognitive map of non-mammals is constructed in a similar manner, the parallel map theory may facilitate the analysis of homologies, both in behavior and in the function of medial pallium subareas.

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.

Maps in birds: representational mechanisms and neural bases

The often extraordinary navigational behavior of birds is based in part on their ability to learn map-like representations of the heterogeneous distribution of environmental stimuli in space. Whether navigating small-scale laboratory environments or large-scale field environments, birds appear to be reliant on a directional framework, for example that provided by the sun, to learn how stimuli are distributed in space and to represent them as a map. The avian hippocampus plays a critical role in some aspects of map learning. Recent results from electrophysiological studies hint at the possibility that different aspects of space may be represented in the activity of different neuronal types in the avian hippocampus.

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

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

Bilateral participation of the hippocampus in familiar landmark navigation by homing pigeons

Recent findings indicate a different role of the left and right hippocampal formation (RHF) in homing pigeon navigational map learning. However, it remains uncertain whether the left or the RHF may play a more important role in navigation based on familiar landmarks. In the present study, we attempted to answer this question by experimentally releasing control and left and right hippocampal ablated pigeons from familiar training sites under anosmia, to render their navigational map dysfunctional, and after a phase-shift of the light-dark cycle, to place into conflict a pilotage-like landmark navigational strategy and a site-specific compass orientation landmark navigational strategy. Both left and right hippocampal ablated birds succeeded in learning to navigate by familiar landmarks, and both preferentially relied on sun-compass based, site-specific compass orientation to home. Like bilateral hippocampal lesioned birds, and in contrast to intact controls, neither ablation group adopted a pilotage-like strategy. We conclude that both the left and RHF are necessary if pilotage-like, familiar landmark navigation is to be learned or preferentially used for navigation.

Intratelencephalic connections of the hippocampus in pigeons (Columba livia)

Behavioral experiments using ablation of the hippocampus are increasingly being used to address the hypothesis that the avian hippocampus plays a role in memory, as in mammals. However, the morphological basis of the avian hippocampus has been poorly understood. In the present study, the afferent and efferent connections of the hippocampus in the pigeon telencephalon were defined by injections, at various rostrocaudal sites, of neuronal tracers mainly into the triangular part located between its V-shaped layer of densely packed neurons. The major results obtained in the present study were as follows. 1) A topographical organization of the commissural projections was confirmed. These projections had two courses that projected to the contralateral side, one traveling through the fiber wall of the ventromedial telencephalon, which was the main path from neurons in the caudal hippocampus, and the other running down through the septohippocampal junction, which was the main path from neurons in the middle to rostral hippocampus. Both courses passed through the pallial commissure. 2) The hippocampus projected bilaterally to the septum, parahippocampal area (APH), and dorsolateral cortical area (CDL). These projections were also distributed topographically, with contralateral efferents crossing through the pallial commissure. 3) The hippocampus had ipsilateral reciprocal connections with APH, CDL, and the dorsal hyperstriatum. Septal afferents to the ipsilateral hippocampus were very small. 4) Intrinsic connections were found between the triangular part of the hippocampus and the lateral limb of the V-shaped layer of neurons. 5) The hippocampus projected ipsilaterally to the ventral basal ganglia and the fasciculus diagonalis Brocae. In sum, these connections of the hippocampus may form a neuronal circuit for the processing of spatial memory in pigeons.

Effects of hippocampal lesions on repeated acquisition of spatial discrimination in pigeons

Anatomical studies of avian hippocampus suggest this structure is a counterpart of that of mammals, and allometric studies of food storing birds support the idea that the avian hippocampus has spatial cognitive functions. In the present study, the spatial cognitive function of hippocampus in pigeons was examined by lesion experiments. Pigeons were trained on either a spatial discrimination, or a spatial discrimination with an added color cue, using a repeated acquisition procedure. In the spatial task, the pigeons were trained to discriminate the position of three keys. Each time the subjects reached the criterion, they were trained on different discriminations in which one out of two previously incorrect keys became the correct key. In the task with color added, each key had its own color, so the subject had both spatial and color cues for the discrimination. The hippocampal lesions disturbed the acquisition of the spatial discrimination, but not in the task in which color cues were added. These results suggest that the avian hippocampus have a crucial role in acquisition of spatial discriminations.

Hippocampus and homing in pigeons: left and right hemispheric differences in navigational map learning

One-month-old, inexperienced homing pigeons, prior to any opportunity to learn a navigational map, were subjected to either right or left unilateral ablation of the hippocampal formation (HF). These pigeons were then held together with a group of age-matched control birds in an outdoor aviary, where they were kept for about 3 months with the opportunity to learn a navigational map. When subsequently tested for navigational map learning at about 4 months of age posthatching, control and right HF-ablated pigeons were equally good at orienting homeward from distant, unfamiliar locations, indicating successful navigational map learning. By contrast, left HF-ablated pigeons were impaired in orienting homeward, indicating a failure to learn a navigational map. Interestingly, both right and left HF-ablated pigeons displayed impaired homing performance relative to controls. These results suggest that different aspects of homing pigeon navigation may be lateralized to different hemispheres, and in particular, the HF of the different hemispheres. The left HF appears critical for navigational map learning, i.e. determining an approximate direction home from distant, unfamiliar locations. The right HF, and possibly the left HF as well, appear to play an important role in local navigation near the loft, which is likely based on familiar landmarks.

The ontogeny of the homing pigeon navigational map: evidence for a sensitive learning period

Homing pigeons can learn a navigational map by relying on the heterogeneous distribution of atmospheric odours in the environment. To test whether there might be a sensitive period for learning an olfactory-based navigational map, we maintained a group of young pigeons in an aviary screened from the winds until the age of three to four months post-fledging. Subsequently, the screens were removed and the pigeons were exposed to the winds and the environmental odours they carry for three months. One control group of pigeons was held in a similar aviary but exposed to the winds immediately upon Hedging, while another control group of pigeons was allowed free-flight. When the pigeons from the three groups were released from two distant release sites at about six months of age post-fledging, the two control groups were found to be equally good at orientating and returning home, while the experimental pigeons held in the shielded aviary for the first three months post-fledging were unable to orientate homeward and they were generally unsuccessful in returning home. This result supports the hypothesis that environmental experience during the first three months post-fledging is critical for some aspect of navigational map learning and that navigational map learning displays sensitive period-like properties.

Effects of medial and dorsal cortex lesions on spatial memory in lizards

In mammals and birds, the hippocampus is a major learning and memory center that plays a prominent role in spatial memory, the use of distal cues to guide navigation. The role of reptilian hippocampal homologues, the medial and dorsal cortex, in spatial memory has not been thoroughly investigated. The medial and dorsal cortex of reptiles is known to play a role in learning both tasks that are hippocampally dependent and tasks that are not hippocampally dependent in mammals and birds. In order to examine the specific role of the medial and dorsal cortex in spatial memory, we trained medial cortex, dorsal cortex, and sham lesioned Cnemidophorus inornatus lizards to locate the one heated rock of four identical rocks spaced evenly around the perimeter of a circular, sand filled, arena in a cool room. We used probe trials to examine the strategies used by lizards to locate the goal. Medial cortex lesions and dorsal cortex lesions slowed acquisition and altered the strategies used to locate the goal. However, none of the lizards adopted a spatial strategy to locate the goal suggesting that the dorsal cortex and medial cortex are involved in using non-spatial strategies for navigation.

Is the avian hippocampus a functional homologue of the mammalian hippocampus?

The effects of hippocampal lesions on the processing and retention of visual and spatial information in birds and mammals is reviewed. Both birds and mammals with damage to the hippocampus are severely impaired on a variety of spatial tasks, such as navigation, maze learning, and the retention of spatial information. In contrast, both birds and mammals with damage to the hippocampus are not impaired on a variety of visual tasks, such as delayed matching-to-sample, concurrent discrimination, or retention of a visual discrimination. In addition, both birds and mammals with hippocampal damage display impairments in the acquisition of an autoshaped response, as well as alterations in response suppression. These findings suggest that the avian hippocampus is a functional homologue of the mammalian hippocampus, and that in both birds and mammals the hippocampus is important for the processing and retention of spatial, rather than purely visual information.

Further experiments on the relationship between hippocampus and orientation following phase-shift in homing pigeons

Following a clock- or phase-shift of the light dark cycle, hippocampal lesioned pigeons (Columba livia) consistently display a larger deviation in vanishing bearings away from the homeward direction compared to intact birds; an effect never seen in unshifted birds. In Experiment 1, control and hippocampal lesioned pigeons oriented similarly after being held 1 week under artificial lighting in the absence of a phase-shift. Housing under artificial light by itself does not result in between group orientation differences. In Experiment 2, control and hippocampal lesioned pigeons oriented equally well under overcast conditions, indicating that both groups had a functional magnetic compass. The between group difference in orientation following phase-shift does not appear to be a consequence of control birds being able to use both the sun and earth's magnetic field for orientation and the hippocampal lesioned pigeons only being able to use the sun. In Experiment 3, lengthening the time held under 6-h clock-shift from 1 to 2 weeks had no effect on the magnitude of the difference in orientation, but fast shifting produced clearer effects than slow shifting. Taken together, the data suggest that hippocampal lesions alter how a pigeon responds to a rapidly changing light-dark cycle, particularly following a fast-shift manipulation, suggesting an as yet unspecified relationship between the avian hippocampus and the circadian rhythm(s) that regulate sun compass orientation.

Hippocampal participation in navigational map learning in young homing pigeons is dependent on training experience

The homing pigeon navigational map is perhaps one of the most striking examples of a naturally occurring spatial representation of the environment used to guide navigation. In a previous study, it was found that hippocampal lesions thoroughly disrupt the ability of young homing pigeons held in an outdoor aviary to learn a navigational map. However, since that study an accumulation of anecdotal data has hinted that hippocampal-lesioned young pigeons allowed to fly during their first summer could learn a navigational map. In the present study, young control and hippocampal-lesioned homing pigeons were either held in an outdoor aviary or allowed to fly during the time of navigational map learning. At the end of their first summer, the birds were experimentally released to test for navigational map learning. Independent of training experience, control pigeons oriented homeward during the experimental releases demonstrating that they learned a navigational map. Surprisingly, while the aviary-held hippocampal-lesioned pigeons failed to learn a navigational map as reported previously, hippocampal-lesioned birds allowed flight experience learned a navigational map indistinguishable from the two control groups. A subsequent experiment revealed that the navigational map learned by the three groups was based on atmospheric odours. The results demonstrate that hippocampal participation in navigational map learning depends on the type of experience a young bird pigeon has, and presumably, the type of navigational map learned.

Neurochemical evidence for at least two regional subdivisions within the homing pigeon (Columba livia) caudolateral neostriatum

The distributions of one neurotransmitter, two neurotransmitter-related substances, and five neuropeptides were examined within the homing pigeon caudolateral neostriatum (NCL). All eight neuroactive substances were found within a tyrosine hydroxylase (TH)-dense region that defines the NCL. Overall regional variation in the relative density of these substances suggested at least two neurochemically distinct portions of NCL. Dorsal NCL contained relatively dense staining for TH, choline acetyltransferase, and substance P, whereas vasoactive intestinal polypeptide was more abundant in ventral portions of NCL. Serotonin and cholecystokinin were found to be densest in intermediate portions of NCL. Somatostatin and leucine-enkephalin were homogeneously distributed throughout NCL. The results suggest that NCL may consist of multiple subdivisions. Investigations into the behavioral importance of these regions are necessary to clarify the role of this brain region in avian behavior.

Effects of hippocampal lesions on spatial operant discrimination in pigeons

In experiment 1, pigeons were trained on spatial or color autodiscrimination. Presentation of one of two keys or one of two colors was followed by food presentation. However, the other side of the keys or the other color was not. The hippocampal lesions disturbed the acquisition of spatial discrimination but not of color discrimination. In experiment 2, pigeons were preoperatively trained the spatial autodiscrimination, then received the hippocampal lesions. The subjects maintained the discrimination. These results suggest that the avian hippocampus plays a crucial role in acquisition of spatial discrimination.

Optic flow input to the hippocampal formation from the accessory optic system

Recent studies in rodents have implicated the hippocampal formation in "path integration": the ability to use self-motion cues (ideothesis) to guide spatial behavior. Such models of hippocampal function assume that self-motion information arises from the vestibular system. In the present study we used the retrograde tracer cholera toxin subunit B, the anterograde tracer biotinylated dextran amine, and standard extracellular recording techniques to investigate whether the hippocampal formation [which consists of the hippocampus proper and the area parahippocampalis (Hp/APH) in pigeons] receives information from the accessory optic system (AOS). The AOS is a visual pathway dedicated to the analysis of the "optic flow fields" that result from self-motion. Optic flow constitutes a rich source of ideothetic information that could be used for navigation. Both the nucleus of the basal optic root (nBOR) and nucleus lentiformis mesencephali of the AOS were shown to project to the area ventralis of Tsai (AVT), which in turn was shown to project to the Hp/APH. A smaller direct projection from the nBOR pars dorsalis to the hippocampus was also revealed. During extracellular recording experiments, about half of the cells within the AVT responded to optic flow stimuli. Together these results illustrate that the Hp/APH receives information about self-motion from the AOS. We postulate that this optic flow information is used for path integration. A review of the current literature suggests that an analogous neuronal circuit exists in mammals, but it has simply been overlooked.

Piriform cortex ablations block navigational map learning in homing pigeons

Young homing pigeons were subjected to ablations of the piriform cortex or left intact and allowed to learn a navigational map. Three months later, control and piriform cortex lesioned pigeons were released from three unfamiliar locations. Control pigeons oriented homeward indicating successful navigational map learning. In contrast, piriform cortex ablated pigeons consistently oriented east, took more time to return home and were more likely to get lost. The results demonstrate that piriform cortex ablations in young homing pigeons disrupt navigational learning. The data support the conclusion that participation of the piriform cortex is necessary for navigational map learning, and its role in navigational learning cannot be substituted for by other telencephalic olfactory processing regions. Further, the results show that the role of olfactory cues in building up the navigational map cannot be replaced by other non olfactory environmental stimuli.

Hippocampal lesion effects on learning strategies in homing pigeons

Several studies have shown that birds have a directional view of space and tend to use the sun compass over landmark beacons when both are available. Intact homing pigeons can use either the sun compass or colour beacons to locate a food reward, whereas pigeons with hippocampal lesions are unable to use the sun compass, but quickly learn to use colour beacons. We trained hippocampal ablated and intact pigeons to find a reward in an outdoor octagonal arena when both sun compass information (directional cues) and intramaze landmark beacons (colour cues) were available. The intact control pigeons learned the task by preferentially relying on directional cues while effectively ignoring the colour beacons. The behaviour of the hippocampal ablated birds, based on a clock-shift manipulation and after the rotation of the colour beacons, showed that they learned to locate the food reward in the arena only on the basis of the landmark beacons, ignoring the sun compass directional information.