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

Lessons from behavioral lateralization in olfaction

Sensory information, sampled by sensory organs positioned on each side of the body may play a crucial role in organizing brain lateralization. This question is of particular interest with regard to the growing evidence of alteration in lateralization in several psychiatric conditions. In this context, the olfactory system, an ancient, mostly ipsilateral and well-conserved system across phylogeny may prove an interesting model system to understand the behavioral significance of brain lateralization. Here, we focused on behavioral data in vertebrates and non-vertebrates, suggesting that the two hemispheres of the brain differentially processed olfactory cues to achieve diverse sensory operations, such as detection, discrimination, identification of behavioral valuable cues or learning. These include reports across different species on best performances with one nostril or the other or odorant active sampling by one nostril or the other, depending on odorants or contexts. In some species, hints from peripheral anatomical or functional asymmetry were proposed to explain these asymmetries in behavior. Instigations of brain activation or more rarely of brain connectivity evoked by odorants revealed a complex picture with regards to asymmetric patterns which is discussed with respect to behavioral data. Along the steps of the discussed literature, we propose avenues for future research.

Involvement of the Avian Dorsal Thalamic Nuclei in Homing Pigeon Navigation

The navigational ability of birds has been a focus of popular and scientific interest for centuries, but relatively little is known about the neuronal networks that support avian navigation. In the brain, regions like the piriform cortex, olfactory bulbs, hippocampal formation, vestibular nuclei, and the wulst, are among the brain regions often discussed as involved in avian navigation. However, despite large literature showing a prominent role of some anterior and dorsal thalamic nuclei in mammalian spatial navigation, little is known about the role of the thalamus in avian navigation. Here, we analyzed a possible role of the dorsal anterior thalamic nuclei in avian navigation by combining olfactory manipulations during the transport of young homing pigeons to a release site and c-Fos immunohistochemistry for the mapping brain activity. The results reveal that odor modulated neurons in the avian dorsolateral lateral (DLL) subdivision of the anterior thalamic nuclei are actively involved in processing outward journey, navigational information. Outward journey information is used by pigeons to correctly determine the homeward direction. DLL participation in acquiring path-based information, and its modulation by olfactory exposure, broadens our understanding of the neural pathways underlying avian navigation.

Asymmetrical Processing of Olfactory Input in the Piriform Cortex Mediates "Activation" of the Avian Navigation Circuitry

The role of odors in the long-distance navigation of birds has elicited intense debate for more than half a century. Failure to resolve many of the issues fueling this debate is due at least in part to the absence of controls for a variety of non-specific effects that odors have on the navigational process. The present experiments were carried out to investigate whether the olfactory inputs are involved only in "activation" of neuronal circuitry involved in navigation or are also playing a role in providing directional information. Experienced adult pigeons were exposed to controlled olfactory stimuli during different segments of the journey (release site vs. displacement + release site). Protein levels of IEGs (immediate early genes used to mark synaptic activity) were analyzed in areas within the olfactory/navigation avian circuitry. The results indicate that 1) exposure to natural odors at the release site (and not before) elicit greater activation across brain regions than exposure to filtered air, artificial odors, and natural odors along the entire outward journey (from home to the release site, inclusive); 2) activation of the piriform cortex in terms of odor discrimination is lateralized; 3) activation of the navigation circuitry is achieved by means of lateralized activation of piriform cortex neurons. Altogether, the findings provide the first direct evidence that activation of the avian navigation circuitry is mediated by asymmetrical processing of olfactory input occurring in the right piriform cortex.

Pigeon navigation: exposure to environmental odours prior to release is sufficient for homeward orientation, but not for homing

The role of environmental olfactory information in pigeon navigation has been extensively studied by analysing vanishing bearing distributions and homing performances of homing pigeons subjected to manipulation of their olfactory perception and/or the olfactory information they were exposed to during transportation and at the release site. However, their behaviour during the homing flight remains undocumented. In this experiment we report the analysis of tracks of birds made anosmic at the release site by washing their olfactory mucosa with zinc sulfate. We thus can assess the role of local odours at the release site as well as the role of environmental odours perceived on the way, far from the release site. We observed that pigeons transported and kept at the release site in purified air and made anosmic at the release site were unable to orient towards home and were impaired at homing. By contrast, pigeons allowed to smell environmental odours during transportation and at the release site, although made anosmic prior to release, displayed unimpaired homeward orientation, but nevertheless showed impaired homing performance. These results are consistent with the view that local odours at the release site are critical for determining the direction of displacement (olfactory map) and suggest that pigeons consult the olfactory map also during their homing flight in order to be able to find their way home.

Odours stimulate neuronal activity in the dorsolateral area of the hippocampal formation during path integration

The dorsolateral area of the hippocampal formation of birds is commonly assumed to play a central role in processing information needed for geographical positioning and homing. Previous work has interpreted odour-induced activity in this region as evidence for an 'olfactory map'. Here, we show, using c-Fos expression as a marker, that neuronal activation in the dorsolateral area of the hippocampal formation of pigeons is primarily a response to odour novelty, not to the spatial distribution of odour sources that would be necessary for an olfactory map. Pigeons exposed to odours had significantly more neurons activated in this area of the brain than pigeons exposed to filtered air with odours removed. This increased activity was observed only in response to unfamiliar odours. No change in activity was observed when pigeons were exposed to home odours. These findings are consistent with non-home odours activating non-olfactory components of the pigeon's navigation system. The pattern of neuronal activation in the triangular and dorsomedial areas of the hippocampal formation was, by contrast, consistent with the possibility that odours play a role in providing spatial information.

Night-migratory songbirds possess a magnetic compass in both eyes

Previous studies on European robins, Erithacus rubecula, and Australian silvereyes, Zosterops lateralis, had suggested that magnetic compass information is being processed only in the right eye and left brain hemisphere of migratory birds. However, recently it was demonstrated that both garden warblers, Sylvia borin, and European robins have a magnetic compass in both eyes. These results raise the question if the strong lateralization effect observed in earlier experiments might have arisen from artifacts or from differences in experimental conditions rather than reflecting a true all-or-none lateralization of the magnetic compass in European robins. Here we show that (1) European robins having only their left eye open can orient in their seasonally appropriate direction both during autumn and spring, i.e. there are no strong lateralization differences between the outward journey and the way home, that (2) their directional choices are based on the standard inclination compass as they are turned 180° when the inclination is reversed, and that (3) the capability to use the magnetic compass does not depend on monocular learning or intraocular transfer as it is already present in the first tests of the birds with only one eye open.

Associations between motor, sensory and structural lateralisation and guide dog success

The aim of this study was to determine whether objective measures of laterality could be used to identify dogs with a high probability of successfully completing a Guide Dog Training Programme. Three categories of laterality (motor, sensory, and structural), were assessed in 114 dogs entering guide dog training. Significant predictors of success were identified: the direction of laterality (P=0.028), paw preference category in the 'Kong' test (P=0.043), hindpaw clearance height (P=0.002), laterality indices for a number of measures in the Sensory Jump test, and chest hair whorl direction (P=0.050). This is the first study to report a structural marker of canine behaviour. All three categories of laterality may be used to predict the suitability of dogs for guiding work, and by identifying predictors of success, resources can be more efficiently utilised on dogs with greater potential.

Telencephalic organization of the olfactory system in homing pigeons (Columba livia)

Pigeons use olfactory cues to navigate over unfamiliar areas, and any impairment of the olfactory system generates remarkable reduction of homing performance. Lesion and deprivation studies suggest a critical involvement of the right nostril and thus, the right olfactory bulb (OB) and the left piriform cortex (CPi) for initial orientation. This functional pattern suggests that OB and CPi are asymmetrically connected with a stronger projection from the right OB to the left CPi. However, the structural organization of the olfactory system is not unequivocally clarified yet. Thus, we re-analyzed the system by antero- and retrograde tract tracing with biotinylated dextran amine and choleratoxin subunit B, and we especially evaluated quantitative differences in the number of cells in the OB innervating the left and right CPi. Our anterograde tracing data verified a strong bilateral input to the CPi, and the prepiriform cortex (CPP), as well as small projections to the ipsilateral medial septum and the dorsolateral corticoid area and the nucleus taeniae of the amygdala in both hemispheres. Apart from the bilateral bulbar afferents, CPi in turn receives unequivocal input from the ipsilateral CPP, hyperpallium densocellulare, dorsal arcopallium, and from a cluster of cells located within the frontolateral nidopallium. Thus, an indirect connection between OB and CPi is only mediated by the CPP. For quantitative analysis of bulbar input to the CPi, we counted the number of ipsi- and contralaterally projecting neurons located in the OB after injections into the left or right CPi. Retrogradely labeled cells were found bilaterally in the OB with a higher number of ipsilaterally located cells. The bilaterality index did not differ after left- or right-sided CPi injections indicating that the functional lateralization of the olfactory system is not simply based on differences in the number of projecting axons of the major processing stream.

Olfactory lateralization in homing pigeons: a GPS study on birds released with unilateral olfactory inputs

A large body of evidence has shown that pigeons rely on an olfactory-based navigational map when homing from unfamiliar locations. Previous studies on pigeons released with one nostril occluded highlighted an asymmetry in favour of the right nostril, particularly concerning the initial orientation performance of naïve birds. Nevertheless, all pigeons experiencing only unilateral olfactory input showed impaired homing, regardless of the side of the occluded nostril. So far this phenomenon has been documented only by observing the birds' vanishing bearings. In the present work we recorded the flight tracks of pigeons with previous homing experience equipped with a GPS data logger and released from an unfamiliar location with the right or the left nostril occluded. The analysis of the tracks revealed that the flight path of the birds with the right nostril occluded was more tortuous than that of unmanipulated controls. Moreover, the pigeons smelling with the left nostril interrupted their journey significantly more frequently and displayed more exploratory activity than the control birds, e.g. during flights around a stopover site. These data suggest a more important involvement of the right olfactory system in processing the olfactory information needed for the operation of the navigational map.

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.

Lateralization of magnetic compass orientation in pigeons

The aim of our study was to test for lateralization of magnetic compass orientation in pigeons. Having shown that pigeons are capable of learning magnetic compass directions in an operant task, we wanted to know whether the brain hemispheres contribute differently and how the lateralization pattern relates to findings in other avian species. Birds that had learnt to locate food in an operant chamber by means of magnetic directions were tested for lateralization of magnetic compass orientation by temporarily covering one eye. Successful orientation occurred under all conditions of viewing. Thus, pigeons can perceive and process magnetic compass directions with the right eye and left brain hemisphere as well as the left eye and right brain hemisphere. However, while the right brain hemisphere tended to confuse the learned direction with its opposite (axial response), the left brain hemisphere specifically preferred the correct direction. Our findings demonstrate bilateral processing of magnetic information, but also suggest qualitative differences in how the left and the right brain deal with magnetic 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.

Evolution of hemispheric specialisation of antagonistic systems of management of the body's energy resources

Excellent and rich reviews of lateralised behaviour in animals have recently been published indexing renewed interest in biological theorising about hemispheric specialisation and yielding rich theory. The present review proposes a new account of the evolution of hemispheric specialisation, a primitive system of "management of the body's energy resources". This model is distinct from traditionally evoked cognitive science categories such as verbal/spatial, analytic/holistic, etc., or the current dominant neuroethological model proposing that the key is approach/avoidance behaviour. Specifically, I show that autonomic, immune, psychomotor, motivational, perceptual, and memory systems are similarly and coherently specialised in the brain hemispheres in rodents and man. This energy resource management model, extended to human neuropsychology, is termed here the "psychic tonus" model of hemispheric specialisation.

Olfactory lateralization in homing pigeons: initial orientation of birds receiving a unilateral olfactory input

It has been shown that homing pigeons (Columba livia) rely on olfactory cues to navigate from unfamiliar locations. In fact, the integrity of the olfactory system, from the olfactory mucosa to the piriform cortex, is required for pigeons to navigate over unfamiliar areas. Recently it has been shown that there is a functional asymmetry in the piriform cortex, with the left piriform cortex more involved in the use of the olfactory navigational map than the right piriform cortex. To investigate further the lateralization of the olfactory system in relation to navigational processes in carrier pigeons, we compared their homing performance after either their left or the right nostril was plugged. Contrary to our expectations, we observed an impairment in the initial orientation of the pigeons with their right nostril plugged. However, both groups released with one nostril plugged tended to be poorer than control pigeons in their homing performance. The observed asymmetry in favour of the right nostril might be due to projections from the olfactory bulbs to the contralateral globus pallidum, a structure involved in motor responses.

Behavioural lateralisation in the Australian magpie (Gymnorhina tibicen)

In many vertebrates, the brain is lateralised such that each hemisphere is specialised to serve specific functions. This may translate into lateralisation in behaviour through preferential use of receptors or appendages associated with a particular hemisphere (e.g., handedness) or in differential responses to stimuli perceived on the animal's left or right side. In this study, we investigated behavioural laterality in the Australian magpie, Gymnorhina tibicen. We found that, while the birds did not have a population bias for one antipredator behaviour (visual inspection of an approaching human), there were biases for another (alarm calling); those birds that used their left eye more relative to their right eye gave alarm calls more frequently. We also observed that juvenile birds begged for food on the right side of parents significantly more frequently than on the left side. These trends are consistent with trends in behavioural laterality that have been recorded in captive and lab-reared species. Ours is one of the few studies to observe patterns of laterality in a wild species.

Spared feature-structure discrimination but diminished salience of environmental geometry in hippocampal-lesioned homing pigeons (Columba livia)

Homing pigeons (Columba livia) were trained to locate a goal in one corner of a rectangular arena by either its shape (geometry) or the left-right configuration of colored features located in each corner (feature structure). Control and hippocampal-lesioned pigeons learned at a similar rate, but the control birds made proportionally more geometric errors during acquisition. On conflict probe trials, the control birds preferred geometrically correct corners, whereas the hippocampal-lesioned birds displayed a greater preference for the correct corner defined by feature structure. On geometry-only probe trials, both groups demonstrated an ability to identify the goal location. Hippocampal lesions do not interfere with goal recognition by the feature structure of local cues but diminish the salience of arena shape.