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

Is There Visual Lateralisation of the Sun Compass in Homing Pigeons?

Functional lateralisation in the avian visual system can be easily studied by testing monocularly occluded birds. The sun compass is a critical source of navigational information in birds, but studies of visual asymmetry have focussed on cues in a laboratory rather than a natural setting. We investigate functional lateralisation of sun compass use in the visual system of homing pigeons trained to locate food in an outdoor octagonal arena, with a coloured beacon in each sector and a view of the sun. The arena was rotated to introduce a cue conflict, and the experimental groups, a binocular treatment and two monocular treatments, were tested for their directional choice. We found no significant difference in test orientation between the treatments, with all groups showing evidence of both sun compass and beacon use, suggesting no complete functional lateralisation of sun compass use within the visual system. However, reduced directional consistency of binocular vs. monocular birds may reveal a conflict between the two hemispheres in a cue conflict condition. Birds using the right hemisphere were more likely to choose the intermediate sector between the training sector and the shifted training beacon, suggesting a possible asymmetry in favour of the left eye/right hemisphere (LE/RH) when integrating different cues.

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.

Having the nerve to home: trigeminal magnetoreceptor versus olfactory mediation of homing in pigeons

The ability of pigeons to find their way home from unfamiliar sites located up to hundreds of kilometers away is well known, but the mechanisms underlying this ability remain controversial. One proposed mechanism is based on the suggestion that pigeons are equipped with magnetoreceptors that can enable the detection of either the earth's magnetic field and/or magnetic field anomalies in the local terrain over which the pigeons fly. Recent reports have suggested that these magnetoreceptors are located in the upper beak where they are innervated by the ophthalmic branch of the trigeminal nerve. Moreover, this nerve has been shown to mediate pigeons' ability to discriminate the presence versus the absence of a magnetic field anomaly in a conditioning situation. In the present study, however, we show that an intact ophthalmic branch of the trigeminal nerve is neither necessary nor sufficient for good homing performance from unfamiliar locations, but that an intact olfactory nerve is necessary.

A lateralized avian hippocampus: preferential role of the left hippocampal formation in homing pigeon sun compass-based spatial learning

The hippocampal formation (HF) plays a crucial role in amniote spatial cognition. There are also indications of functional lateralization in the contribution of the left and right HF in processes that enable birds to navigate space. The experiments described in this study were designed to examine left and right HF differences in a task of sun compass-based spatial learning in homing pigeons (Columba livia). Control, left (HFL) and right (HFR) HF lesioned pigeons were trained in an outdoor arena to locate a food reward using their sun compass in the presence or absence of alternative feature cues. Subsequent to training, the pigeons were subjected to test sessions to determine if they learned to represent the goal location with their sun compass and the relative importance of the sun compass vs. feature cues. Under all test conditions, the control pigeons demonstrated preferential use of the sun compass in locating the goal. By contrast, the HFL pigeons demonstrated no ability to locate the goal by the sun compass but an ability to use the feature cues. The behaviour of the HFR pigeons demonstrated that an intact left HF is sufficient to support sun compass-based learning, but in conflict situations and in contrast to controls, they often relied on feature cues. In conclusion, only the left HF is capable of supporting sun compass-based learning. However, preferential use of the sun compass for learning requires an intact right HF. The data support the hypothesis that the left and right HF make different but complementary contributions toward avian spatial cognition.

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.

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.

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.

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.