Daytime micro-naps in a nocturnal migrant: an EEG analysis

Many species of typically diurnal songbirds experience sleep loss during the migratory seasons owing to their nocturnal migrations. However, despite substantial loss of sleep, nocturnally migrating songbirds continue to function normally with no observable effect on their behaviour. It is unclear if and how avian migrants compensate for sleep loss. Recent behavioural evidence suggests that some species may compensate for lost night-time sleep with short, uni- and bilateral 'micro-naps' during the day. We provide electrophysiological evidence that short episodes of sleep-like daytime behaviour (approx. 12s) are accompanied by sleep-like changes in brain activity in an avian migrant. Furthermore, we present evidence that part of this physiological brain response manifests itself as unihemispheric sleep, a state during which one brain hemisphere is asleep while the other hemisphere remains essentially awake. Episodes of daytime sleep may represent a potent adaptation to the challenges of avian migration and offer a plausible explanation for the resilience to sleep loss in nocturnal migrants.

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.

Dissociation of place and cue learning by telencephalic ablation in goldfish

This study examined the spatial strategies used by goldfish (Carassius auratus) to find a goal in a 4-arm maze and the involvement of the telencephalon in this spatial learning. Intact and telencephalon-ablated goldfish were trained to find food in an arm placed in a constant room location and signaled by a local visual cue (mixed place-cue procedure). Both groups learned the task, but they used different learning strategies. Telencephalon-ablated goldfish learned the task more quickly and made fewer errors to criterion than controls. Probe trials revealed that intact goldfish could use either a place or a cue strategy, whereas telencephalon-ablated goldfish learned only a cue strategy. The results offer additional evidence that place and cue learning in fish are subserved by different neural substrates and that the telencephalon of the teleost fish, or some unspecified structure within it, is important for spatial learning and memory in a manner similar to the hippocampus of mammals and birds.

Dissociation of place and cue learning by telencephalic ablation in goldfish

This study examined the spatial strategies used by goldfish (Carassius auratus) to find a goal in a 4-arm maze and the involvement of the telencephalon in this spatial learning. Intact and telencephalon-ablated goldfish were trained to find food in an arm placed in a constant room location and signaled by a local visual cue (mixed place-cue procedure). Both groups learned the task, but they used different learning strategies. Telencephalon-ablated goldfish learned the task more quickly and made fewer errors to criterion than controls. Probe trials revealed that intact goldfish could use either a place or a cue strategy, whereas telencephalon-ablated goldfish learned only a cue strategy. The results offer additional evidence that place and cue learning in fish are subserved by different neural substrates and that the telencephalon of the teleost fish, or some unspecified structure within it, is important for spatial learning and memory in a manner similar to the hippocampus of mammals and birds.

Partial experience with the arc of the sun is sufficient for all-day sun compass orientation in homing pigeons, Columba livia

The ability of animals to learn to use the sun for orientation has been explored in numerous species. In birds, there is conflicting evidence about the experience needed for sun compass orientation to develop. The prevailing hypothesis is that birds need entire daytime exposure to the arc of the sun to use the sun as an orientation cue. However, there is also some evidence indicating that, even with limited exposure to the arc of the sun, birds, like insects, can use the sun to orient at any time of day. We re-examine this issue in a study of compass orientation in a cue-controlled arena. Two groups of young homing pigeons received different exposure to the sun. The control group experienced the sun throughout the day; the experimental group experienced only the apparent descent of the sun. After 8 weeks of sun exposure, we trained both groups in the afternoon to find food in a specific compass direction in an outdoor arena that provided a view of the sun but not landmarks. We then tested the pigeons in the morning for their ability to use the morning sun as an orientation cue. The control group and the experimental group, which was exposed to the morning sun for the first time, succeeded in orienting in the training direction during test 1. The orientation of the experimental group was no different from that of the control group, although the experimental first trial directional response latencies were greater than the control latencies. Subsequently, we continued training both groups in the afternoon and then tested the pigeons during the morning under complete cloud cover. Both groups displayed random directional responses under cloud cover, indicating that the observed orientation was based on the visibility of the sun. The data indicate that pigeons with limited exposure to the arc of the sun can, like insects, use the sun for orientation at any time of day.

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.

The effects of hippocampal lesions in homing pigeons on a one-trial food association task

The role of the avian hippocampal formation in a one-trial food association task was investigated across various retention intervals. Control pigeons, lesioned controls, and pigeons with hippocampal formation lesions were allowed to find food hidden in one of four uniquely decorated bowls in a specific location in a room. After retention intervals of 10 min, 1 h, 7 h, and 24 h, pigeons were placed back in the room with the same bowl in the same location (unmanipulated trials) or with the previously rewarding bowl in a new location and a different bowl in the previously rewarding location (test trials). Although all groups chose the correct bowl during unmanipulated trials, hippocampal formation lesioned birds' choices to the bowl in the correct location decreased compared to the combined controls during the test trials. The results suggest that hippocampal formation lesions do not impair long-term memory of a goal after one experience but significantly decrease the use of spatial information to return to that goal.

Hippocampal lesions do not impair the geomagnetic orientation of migratory savannah sparrows

The avian hippocampal formation is known to participate in naturally occurring spatial behavior such as homing in pigeons and cache recovery in food storing passerines, but its participation in the often spectacular migrations of birds remains uncertain. As a first investigation into the possible role of hippocampal formation in migration, the effect of hippocampal formation lesions on the geomagnetic migratory orientation of Savannah sparrows was examined. When tested indoors, hippocampal formation-lesioned sparrows were able to orient in an appropriate migratory direction indicating no necessary role for hippocampal formation in geomagnetic migratory orientation. However, hippocampal formation-lesioned birds displayed significantly less migratory (nocturnal) activity, a result that inspires further study.

Reduced growth of intra- and infra-pyramidal mossy fibers is produced by continuous exposure to polychlorinated biphenyl

Exposure to polychlorinated biphenyl (PCB) has been shown to produce cognitive deficits in both humans and laboratory animals. However, no study to date has identified long-term brain changes which could account for these problems. This study employed Timm's silver sulfide staining to visualize the hippocampal mossy fibers in Sprague-Dawley rats continuously exposed to either 125 ppm Aroclor 1254 or untreated control food beginning in utero. Reduced growth of hippocampal intra-and infra-pyramidal (II-P) mossy fibers were found in PCB treated rats compared to controls. Other measured hippocampal subdivisions remained relatively unaffected by PCB treatment, as did cortical thickness. The changes observed in hippocampal morphology in response to PCB exposure are the first to provide a potential explanation for at least part of the long-term PCB-induced cognitive deficits.

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.

The effects of lesions to the caudolateral neostriatum on sun compass based spatial learning in homing pigeons

To better define the role of the avian caudolateral neostriatum (NCL) in spatial behavior, we used homing pigeons to explore the effects of NCL lesions on a sun compass based spatial learning task. Although NCL lesioned birds learned the task, they required more sessions to reach criterion than controls. NCL lesioned pigeons were also able to acquire a color discrimination task that was procedurally similar to the sun compass spatial learning task, but they made more errors than controls. Both the deficits observed in sun compass based spatial learning and color discrimination were correlated with the volume of lesion damage to dorsal rather than ventral portions of NCL. Overall, these findings suggest that the role of NCL in homing pigeon navigation from distant unfamiliar locations is not related to a bird's ability to learn stimulus-direction associations using a sun compass. However NCL does appear involved in a pigeon's ability to perform at least some behaviors common to both the color discrimination and the sun compass based spatial learning tasks.

Homing in pigeons: the role of the hippocampal formation in the representation of landmarks used for navigation

When given repeated training from a location, homing pigeons acquire the ability to use familiar landmarks to navigate home. Both control and hippocampal-lesioned pigeons succeed in learning to use familiar landmarks for homing. However, the landmark representations that guide navigation are strikingly different. Control and hippocampal-lesioned pigeons were initially given repeated training flights from two locations. On subsequent test days from the two training locations, all pigeons were rendered anosmic to eliminate use of their navigational map and were phase- or clock-shifted to examine the extent to which their learned landmark representations were dependent on the use of the sun as a compass. We show that control pigeons acquire a landmark representation that allows them to directly use landmarks without reference to the sun to guide their flight home, called "pilotage". Hippocampal-lesioned birds only learn to use familiar landmarks at the training location to recall the compass direction home, based on the sun, flown during training, called "site-specific compass orientation." The results demonstrate that for navigation of 20 km or more in a natural field setting, the hippocampal formation is necessary if homing pigeons are to learn a spatial representation based on numerous independent landmark elements that can be used to directly guide their return home.

The homing pigeon hippocampus and the development of landmark navigation

The role of the homing pigeon hippocampal formation was examined in the development of loft fidelity and landmark navigation. During the course of five summers, different groups of young pigeons (hippocampal-lesioned, control-lesioned, and unoperated controls) were given free flight experience followed by short distance training and experimental releases. In Experiment 1, a census of which loft each pigeon entered revealed that hippocampal lesioned pigeons displayed less loft fidelity than controls. In Experiment 2 and 3, the percent of young birds lost during their first summer of training and their first experimental release was examined. Despite displaying similarly good homeward-oriented vanishing bearings, significantly more hippocampal lesioned pigeons were lost compared to control groups. The results support the hypothesis that the homing pigeon hippocampal formation participates in the learning/operation of a spatial representation of local landmarks near the loft that can be used for loft recognition and navigation.

Paired-associate learning is unaffected by combined hippocampal and parahippocampal lesions in homing pigeons

To examine whether the avian hippocampus-parahippocampus (HF) is necessary for nonspatial, paired-associate learning, as has been suggested for rodents, HF-lesioned and control homing pigeons were tested on a visual paired-associate learning task. Both groups learned equally well to discriminate trials that consisted of a stimulus preceded by its paired associate from trials that consisted of a stimulus preceded by stimuli from other paired associates (mispair trials), even when a mispair was experienced for the first time. The groups also learned equally well not to respond to 2 stimuli that were never rewarded. The results demonstrate that HF lesions do not impair nonspatial paired-associate learning in birds, suggesting that the role of HF in nonspatial cognition differs between birds and mammals.

Paired-associate learning is unaffected by combined hippocampal and parahippocampal lesions in homing pigeons

To examine whether the avian hippocampus-parahippocampus (HF) is necessary for nonspatial, paired-associate learning, as has been suggested for rodents, HF-lesioned and control homing pigeons were tested on a visual paired-associate learning task. Both groups learned equally well to discriminate trials that consisted of a stimulus preceded by its paired associate from trials that consisted of a stimulus preceded by stimuli from other paired associates (mispair trials), even when a mispair was experienced for the first time. The groups also learned equally well not to respond to 2 stimuli that were never rewarded. The results demonstrate that HF lesions do not impair nonspatial paired-associate learning in birds, suggesting that the role of HF in nonspatial cognition differs between birds and mammals.

Goal recognition and hippocampal formation in the homing pigeon (Columba livia)

Stimulus control of food-site recognition and role of the hippocampal formation (HF) were investigated. Control and HF-lesioned pigeons were trained to find food located in a colored bowl, near a landmark beacon, in a constant room location. During later test trials, the sources of information were individually removed and/or disassociated. For all test trial types, HF-lesioned pigeons consistently chose bowls associated with one of the training stimuli. Controls were more sensitive to the changes introduced during the test trials; choosing like HF-lesioned pigeons on some test trials but choosing randomly on others. The data identify a critical role of the avian HF in learning the spatial relationship among environmental stimuli.

Goal recognition and hippocampal formation in the homing pigeon (Columba livia)

Stimulus control of food-site recognition and role of the hippocampal formation (HF) were investigated. Control and HF-lesioned pigeons were trained to find food located in a colored bowl, near a landmark beacon, in a constant room location. During later test trials, the sources of information were individually removed and/or disassociated. For all test trial types, HF-lesioned pigeons consistently chose bowls associated with one of the training stimuli. Controls were more sensitive to the changes introduced during the test trials; choosing like HF-lesioned pigeons on some test trials but choosing randomly on others. The data identify a critical role of the avian HF in learning the spatial relationship among environmental stimuli.

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.

The neuroethology of cognitive maps: contributions from research on the hippocampus and homing pigeon navigation

The rich ethological tradition that characterizes the homing behavior of pigeons offers an excellent opportunity to examine the importance of the hippocampal formation for the regulation of spatial cognitive mechanisms. The present review summarizes both anatomical and behavioral data obtained in researches on the pigeon hippocampal formation that have been performed over the last 12 years. Pathway connection studies and investigations on the neurochemical organization of the avian hippocampal formation show that this structure shares many similarities with the mammalian hippocampus and provide the basis for structural as well as functional homology. The initial research on the role of the hippocampal formation in the homing behavior showed that this brain structure is likely to be involved in phenomena of spatial cognition. Therefore, the homing behavior of pigeons has been extensively used as an experimental model to investigate the role of the hippocampal formation in spatial cognition related to a naturally occurring behavior. These studies have revealed that the hippocampal formation plays a fundamental role in the learning of a navigational map based on atmospheric odors, but it doesn't seem to be involved in the operation of such a map. In contrast, both the learning and the operation of a navigational map based on the recognition of familiar landmarks require a functional hippocampal formation. Further investigations indicated that these functions of the hippocampal formation are mediated by its involvement in the use of the sun compass, and suggested that the hippocampal formation plays a fundamental role in a cognitive process in which the sun compass is specifically used to learn about the location of stimuli in space. The studies reviewed in the present paper have provided a considerable amount of experimental data both on the anatomical/neurochemical organization of the avian hippocampal formation and on the role played by this brain structure in spatial cognition. The future development of these researches will need to consider the contribution to hippocampal function of specific transmitter systems that are involved in hippocampal circuitry. In particular, the afferent cholinergic system and some of the peptidergic systems intrinsic to the hippocampal formation deserve particular attention in view of their possible involvement in the acquisition and/or operation of spatial cognitive abilities by homing pigeons.

Hippocampal participation in the sun compass orientation of phase-shifted homing pigeons

The orientation of phase-shifted control and hippocampal lesioned homing pigeons with previous homing experience was examined to investigate the possible participation of the hippocampal formation in sun compass orientation. Hippocampal lesioned pigeons displayed appropriate shifts in orientation indicating that such birds possess a functional sun compass that is used for orientation. However, their shift in orientation was consistently larger than in control pigeons revealing a difference in orientation never observed in pigeons that have not undergone a phase shift. Although alternative interpretations exist, the data suggest the intriguing possibility that following a change in the light-dark cycle, the hippocampal formation participates in the re-entrainment of a circadian rhythm that regulates sun compass orientation.

Olfaction and the homing ability of pigeons in the southeastern United States

The importance of atmospheric odors for homing pigeon navigation was tested using birds from a loft located in Savannah, GA, in the southeastern United States. When released from a familiar training site, control pigeons and pigeons given intranasal injections of zinc sulfate to produce anosmia both displayed good homeward orientation and homed quickly. When released from three unfamiliar release sites, in contrast, control birds tended to orient southeast, while zinc sulfate-treated birds were more likely to fly northwest. More importantly, while the majority of control pigeons returned to the home loft, few of the zinc sulfate-treated birds returned. The good performance of both groups from the familiar site indicates that zinc sulfate treatment does not impair the general motor ability or motivation of homing pigeons. Therefore, the observed impairment in homing success of the zinc sulfate-treated pigeons from the unfamiliar locations presumably reflects an impaired ability to use atmospheric odors to navigate home. As such, the data support the hypothesis that successful homing pigeon navigation is based on the perception of atmospheric odors and that olfactory navigation is the primary mechanism used by pigeons over a broad range of geographic areas to approximate their relative position with respect to home from unfamiliar locations.

The muscarinic acetylcholine antagonist scopolamine impairs short-distance homing pigeon navigation

The present study employed intramuscular (i.m.) injections of the acetylcholine (ACh) receptor antagonist scopolamine hydrobromide (0.10 mg/kg) to investigate the possible involvement of ACh in naturally occurring spatial navigation in homing pigeons (Columba livia). Control pigeons receiving injections of saline or scopolamine methylbromide, an ACh antagonist that does not cross the blood-brain barrier, were oriented in a homeward direction when released from a location 8 km from home. In contrast, pigeons injected with scopolamine hydrobromide (0.10 mg/kg, i.m.) were less well oriented and took more time to return home from the same location. These results suggest that homing pigeon navigation is regulated, in part, by central cholinergic mechanisms.

Telencephalic afferents to the caudolateral neostriatum of the pigeon

The pigeon caudolateral neostriatum (NCL) shares a dopaminergic innervation with mammalian frontal cortical areas and is implicated in the regulation of avian cognitive behavior. Retrograde tracing methods were used to identify forebrain projections to NCL and to suggest a possible role of this area in mediating spatial behavior. NCL receives telencephalic projections from the hyperstriatum accessorium, cells along the border of hyperstriatum dorsale and hyperstriatum ventrale, anterolateral hyperstriatum adjacent to the vallecula, confined cell groups within the anterior neostriatum, and subdivisions of the archistriatum. In addition, labeling of a small number of large cells near the fasciculus prosencephali lateralis was observed at the level of the anterior commissure. In accordance with previous studies, projections of subtelencephalic areas were revealed to originate from the thalamic posterior dorsolateral nucleus and nucleus subrotundus, as well as from the tegmental nucleus pedunculopontinus and locus coeruleus. Forebrain connections of NCL show that somatosensory, visual, and olfactory information can combine in this division of the neostriatum. NCL is therefore suited to participate in a neural circuit that regulates spatial behavior. Moreover, the present study reveals that NCL is reached by a limbic projection from the nucleus taeniae. This projection also suggests similarity between NCL and mammalian frontal cortical areas.

The relative importance of location and feature cues for homing pigeon (Columba livia) goal recognition

Homing pigeons were raised and trained in two lofts that differed with respect to their color features and location in space. During training, pigeons displayed accurate site preference for a particular loft. When tested for loft preference with the feature cues switched between the 2 lofts, the pigeons returned to the loft that occupied the correct location. In a 2nd experiment, pigeons were trained to find food hidden in 1 of 4 color bowls (feature cues) located next to a landmark beacon (proximal spatial cue) in a constant location in a room (distal spatial cues). On test trials, pigeons chose the bowl at the correct location in the room if either the color bowl or the beacon was moved by itself but chose the correct color bowl next to the beacon if they were moved together. Together, the data suggest that the importance of location and feature information for goal recognition may be context specific.

Evidence for muscarinic acetylcholine receptor subtypes in the pigeon telencephalon

At least five subtypes of muscarinic acetylcholine receptors are expressed in various mammalian tissue preparations. The following experiment, through the use of direct binding assays (using tritiated quinuclidinyl benzilate), competitive binding assays (using tritiated quinuclidinyl benzilate and unlabeled pirenzepine or AF-DX 116), and autoradiographic techniques, examined whether two of these five putative muscarinic acetylcholine receptor subtypes can be found in avian brain. Accordingly, autoradiographic mapping of pirenzepine-sensitive (M1-like) and AF-DX 116-sensitive (M2-like) muscarinic acetylcholine receptor subtypes in the pigeon telencephalon was conducted. Although both ligands bound throughout the brain, most telencephalic regions, including the archistriatum, the neostriatum, and basal ganglia structures like lobus paraolfactorius, nucleus accumbens, and paleostriatum, showed a higher density of M1-like sites. The exception to this finding was the nucleus basalis which appeared as a region where M2-like sites predominated. Moreover, the telencephalic region with the largest ratio of M1-like to M2-like sites was the lateral portion of the parahippocampus; a characteristic shared with the mammalian dentate gyrus. The findings reported here are generally consistent with previous reports of mammalian M1/M2 receptor distributions.

Sun compass-based spatial learning impaired in homing pigeons with hippocampal lesions

The hippocampal formation is known to be critical for spatial cognition, for example, regulating the learning of environmental maps. But how is a spatial map learned, and what is the role of the hippocampal formation in the learning process? The sun compass is perhaps the most ubiquitous, naturally occurring spatial orientation mechanism found in the animal kingdom. The sun compass may also serve as a directional reference that supports spatial learning. We report that homing pigeons with hippocampal lesions were unable to use the sun compass to learn the directional location of food in an outdoor, experimental arena. Homing pigeons with lesions of the caudal neostriatum readily learned the same task, and showed appropriately shifted directional responses following a clock-shift manipulation demonstrating that they were indeed using the sun compass to learn the task. Finally, both hippocampal and control lesioned birds quickly learned a procedurally similar task where a color cue identified the location of food in the same experimental arena. The results indicate that hippocampal lesions impair sun compass use in the context of learning. As such, the results support the hypothesis that the importance of the hippocampal formation in spatial cognition may be related to its participation in a neural process in which information from a directional reference, in this case the sun compass, is used to learn the directional relationship among stimuli in space.

The NMDA-receptor antagonist MK-801 impairs navigational learning in homing pigeons

The present study employed the N-methyl-D-aspartate (NMDA) receptor antagonist MK-801 to investigate the possible importance of NMDA receptor activation for naturally occurring spatial learning in birds by exploiting the navigational ability of homing pigeons (Columba livia). Control pigeons released from two unfamiliar release sites displayed vanishing bearings that were poorly oriented. However, when released a second time from the same sites they displayed improved homeward orientation. The control birds apparently learned something about the spatial relationships of stimuli at the release sites on the first releases and used that information to orient better when released a second time from the same locations. Experimental pigeons given the NMDA receptor antagonist MK-801 (0.10 mg/kg) initially behaved as controls, orienting poorly when released for the first time from the two sites. In contrast to controls, the experimental birds failed to show significant improvement in orientation when released again from the same sites without MK-801. A second experiment revealed no state-dependent learning. Results of a position/color discrimination task showed that the impairments observed did not generalize to associative learning in an operant chamber, and together with field observations were not a result of sensory or motor drug effects. The data indicate that blocking NMDA receptors can disrupt navigational learning in homing pigeons. As such, the results are consistent with the hypothesis that NMDA receptor activation plays an important role in spatial learning in birds.

Connections of the piriform cortex in homing pigeons (Columba livia) studied with fast blue and WGA-HRP

The piriform cortex in homing pigeons receives a projection from the olfactory bulb and is necessary for the operation of those aspects of the navigational map based on olfactory stimuli in these animals. The afferent and efferent projections of the piriform cortex were studied using retrograde migration of wheat-germ agglutinin horseradish peroxidase (WGA-HRP) and Fast Blue, and anterograde migration of WGA-HRP. The piriform cortex was found to receive projections from, and send projections to, numerous regions and nuclei in the telencephalon, diencephalon and lower brainstem. A reciprocal connection with the parahippocampal region suggests that the piriform cortex and hippocampal formation may be part of a neural system that regulates navigational map learning. The piriform cortex also connects reciprocally with a large portion of the anterior telencephalon, including the cortex prepiriformis and hyperstriatum dorsale. In general, the pathway connections of the piriform cortex in homing pigeons are similar to those of the piriform cortex in mammals.

Visual performance of pigeons following hippocampal lesions

The effect of hippocampal lesions on performance in two psychophysical measures of spatial vision (acuity and size-difference threshold) was examined in 7 pigeons. No difference between the preoperative and postoperative thresholds of the experimental birds was found. The visual performance of pigeons in the psychophysical tasks failed to reveal a role of the hippocampal formation in vision. The results argue strongly that the behavioral deficits found in pigeons with hippocampal lesions when tested in a variety of memory-related spatial tasks is not based on a defect in spatial vision but impaired spatial cognition.

The importance of comparative studies and ecological validity for understanding hippocampal structure and cognitive function

Building from the premise that hippocampal cognitive function has been molded by natural selection under natural environmental conditions, it is argued that traditional laboratory studies likely do not reveal the richness and complexity of hippocampal function. Research on the role of the hippocampal formation in the navigational behavior of homing pigeons is offered as an example to illustrate the advantages of using an ecological approach to understand hippocampal function. It is further proposed that dissimilarities in hippocampal anatomy, physiology, and neurochemistry found between species reflect species differences in the range of functions served by the hippocampal formation, as well as possibly the molecular and cellular mechanisms that support such functions. These differences notwithstanding, it is suggested that, from an evolutionary perspective, the primary function of the hippocampal formation is a role in some aspect of spatial cognition. Dissimilarities in hippocampal structure and function among extant species are viewed as resulting from differences in evolutionary selective pressure and evolutionary history.

Hippocampal lesions impair navigational learning in experienced homing pigeons

Adult experienced homing pigeons from Maryland were subjected to hippocampal lesion and then transferred to a new loft in Ohio to examine what effect such treatment may have on learning to navigate to a new home loft. When subsequently released from an unfamiliar site, the hippocampal lesioned birds were impaired in taking up a vanishing bearing toward their new Ohio loft. This deficit is interpreted as an impairment in hippocampal-lesioned birds learning a new navigational map. Together with a previous study, the results suggest that an intact hippocampus is necessary if young naive or adult experienced homing pigeons are to learn a navigational map.

Hippocampal lesions impair navigational learning in experienced homing pigeons

Adult experienced homing pigeons from Maryland were subjected to hippocampal lesion and then transferred to a new loft in Ohio to examine what effect such treatment may have on learning to navigate to a new home loft. When subsequently released from an unfamiliar site, the hippocampal lesioned birds were impaired in taking up a vanishing bearing toward their new Ohio loft. This deficit is interpreted as an impairment in hippocampal-lesioned birds learning a new navigational map. Together with a previous study, the results suggest that an intact hippocampus is necessary if young naive or adult experienced homing pigeons are to learn a navigational map.

The distribution of neurotransmitters and neurotransmitter-related enzymes in the dorsomedial telencephalon of the pigeon (Columba livia)

Immunoreactivity to four neurotransmitters/transmitter-related enzymes was found in the dorsomedial telencephalon (hippocampal region) of the pigeon. Putative afferent fibers containing choline acetyltransferase-like, serotonin-like, and tyrosine hydroxylase-like immunoreactivity were seen in a fiber tract passing through the septo-hippocampal junction and along the medial wall of the hippocampal region. The most intensive labeling of neuropil and terminals of all four substances was found in the dorsomedial area of the hippocampal region. Glutamic acid decarboxylase-like immunoreactivity was seen in sparsely scattered cells throughout the region. These results are discussed in relation to hypotheses about the boundaries and subdivisions of the hippocampal region of the pigeon.

The distribution of neuropeptides in the dorsomedial telencephalon of the pigeon (Columba livia): a basis for regional subdivisions

The distribution of six neuropeptides [substance P (SP), leucine (leu5-) enkephalin (LENK), vasoactive intestinal polypeptide (VIP), cholecystokinin (CCK), neuropeptide Y (NPY), and somatostatin (SS)] in the dorsomedial telencephalon (hippocampal region) of the pigeon was studied by immunohistochemistry. All six peptides were found in fibers passing through the septo-hippocampal junction and along the medial wall of the hippocampal region. NPY-, SS-, and VIP-like staining of fibers was seen in the hippocampal commissure. NPY and SS had similar distributions within the hippocampal region, both being most conspicuous in cell bodies, terminals, and fibers of the medial hippocampal region. VIP-positive cells were found in an area dorsal to the SS/NPY cell region. CCK-like immunoreactivity was found in terminal baskets surrounding large cells of a v-shaped structure in the ventromedial hippocampal region. SP- and LENK-like immunoreactivity was found in neuropils in a lateral-dorsal region, the two substances showing similar distributions. This region is thought to lie lateral to the limit of the hippocampal region. Parallels with the distribution of immunoreactivity in the mammalian hippocampus are used to suggest possible equivalent subdivisions of the avian and mammalian hippocampal regions.

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

Young homing pigeons were subjected to hippocampal lesion before being placed in their permanent loft to examine what effect such treatment may have on the development of their navigational map, which supports homing from distant unfamiliar locations. When later released from 3 distant unfamiliar locations, the hippocampal-lesioned pigeons were impaired in taking up a homeward bearing. The results identify a deficit in the acquisition of navigational ability after hippocampal ablation in homing pigeons. The results strongly suggest a deficit in navigational map acquisition, but alternative interpretations cannot be excluded. The findings offer the first insight into the central neural structures involved in the acquisition of the pigeon navigational map. Further, the results identify the hippocampus as a structure critical for the regulation of navigational behavior that manifests itself in a natural setting.

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

The avian hippocampal formation has been proposed to play a critical role in the neural regulation of a navigational system used by homing pigeons to locate their loft once in the familiar area near home. In support of this hypothesis, the homing performance of pigeons with target lesions of either the hippocampus or parahippocampus was found to be impaired compared to controls following releases of about 10 km. Further, radio tracking revealed that the in-flight behavior of the hippocampal lesioned homing pigeons was characterized by numerous direction changes and generally poor orientation with respect to the home loft. The results identify a local navigational impairment on the part of the hippocampal lesioned pigeons in the vicinity of the loft where landmark cues are thought to be important. Additionally, target lesions of the hippocampus or parahippocampus were found to be similarly effective in causing homing deficits.

Unimpaired acquisition of spatial reference memory, but impaired homing performance in hippocampal-ablated pigeons

Hippocampal ablated homing pigeons have been shown to suffer a retrograde spatial reference memory deficit involving a preoperatively acquired homeward orientation response based on local cues around a previously visited release site. Here we report that the postoperative acquisition of such a response is unimpaired. Initially, 25 hippocampal ablated and 11 sham-operated controls were given 5 training releases from each of two sites. In the subsequent experimental releases from the two training sites, the controls and half the hippocampal-ablated pigeons had their navigational maps rendered dysfunctional via an anosmic procedure. Nonetheless, both groups successfully oriented homeward, indicating that the hippocampal-ablated pigeons were unimpaired in the acquisition and implementation of directionally useful information around the training sites to direct a homeward orientation response. The remaining half of the hippocampal-ablated pigeons who were not rendered anosmic, and thus served as controls, also oriented homeward. The data indicate that, for hippocampal-ablated homing pigeons, postoperative acquisition is unimpaired in the same spatial reference memory task where a robust retrograde impairment was observed. However, the hippocampal-ablated pigeons were impaired in the time required to return home, indicating a deficit in homing performance beyond the initial orientation stage.

Impaired retention of preoperatively acquired spatial reference memory in homing pigeons following hippocampal ablation

Hippocampal-parahippocampal-ablated homing pigeons have been shown to suffer a retrograde loss of information used in the recognition of their home loft. Here we report that the range of retrograde deficits includes spatial reference memory in the form of information gained from repeated training sites that can be used to direct a homeward orientation response. Following 8 training releases from each of two sites, 28 of 42 homing pigeons underwent hippocampal-parahippocampal ablation. In the subsequent test releases from the two sites, untreated controls whose navigational map was rendered temporarily dysfunctional by an anosmic procedure showed no impairment in determining the home direction, indicating the successful retention and utilization of directionally useful information around the release sites. Hippocampal-ablated controls who were not rendered anosmic and thus had access to their navigational map also showed no impairment in determining the home direction, indicating no general impairment in initial orientation as a result of hippocampal ablation. In contrast, hippocampal-ablated pigeons whose navigational map was rendered temporarily dysfunctional failed to successfully orient homeward from the training sites, indicating impairment in the retention and/or implementation of directional information gathered at the release sites during training. The data reveal a spatial reference memory deficit involving pre-ablation acquired directional information in homing pigeons following hippocampal ablation.

Connections of the pigeon dorsomedial forebrain studied with WGA-HRP and 3H-proline

The afferent-efferent connections of the pigeon dorsomedial forebrain, which is composed of the "hippocampus" (Hp) and "parahippocampus" (APH), presumed homologues of the mammalian hippocampal complex, were studied. Afferent projections were identified by wheat germ agglutinin-conjugated horseradish peroxidase (WGA-HRP) and efferent projections were identified by 3H-proline and WGA-HRP. In addition to identified intrinsic connections within Hp and APH, both Hp and APH were found to be in receipt of ipsilateral forebrain afferents from each other, the hyperstriatum accessorium, nucleus of the diagonal band, nucleus taeniae, and area corticoidea dorsolateralis. Only Hp received input from the contralateral Hp while only APH received input from the ipsilateral hyperstriatum dorsale and archistriatum, pars ventralis. Both Hp and APH received ipsilateral diencephalic afferents from the nucleus mamillaris lateralis, stratum cellulare internum, nucleus lateralis hypothalami, and nucleus paramedianus internus thalami. Only APH received bilateral input from the nucleus superficialis parvicellularis (this nucleus may send a small projection to Hp) and nucleus dorsolateralis anterior thalami, pars medialis, and an ipsilateral projection from the nucleus subrotundus. Brainstem afferents to Hp and APH included ipsilateral projections from the area ventralis (Tsai) nucleus reticularis pontis oralis, nucleus raphes, nucleus subceruleus dorsalis, and nucleus centralis superior of Bechterew, and bilateral projections from the nucleus linearis caudalis and locus ceruleus, of which the nucleus subceruleus dorsalis, nucleus centralis superior of Bechterew, and locus ceruleus projected to APH only. Forebrain efferents from both Hp and APH were found to project ipsilaterally to the septum, the area of the fasciculus diagonalis Brocae, nucleus taeniae, and area corticoidea dorsolateralis. Only Hp appeared to send efferents to the contralateral septum and Hp, while only APH sent efferents to the hyperstriatum dorsale and the archistriatum. A hypothalamic projection from Hp and APH was found to partially terminate near the nucleus mamillaris lateralis. At the level of pathway connections, the results demonstrate a striking similarity between the avian dorsomedial forebrain and the dorsomedial cortex of reptiles and the mammalian hippocampus.

Dorsomedial forebrain ablations and home loft association behavior in homing pigeons

In a series of experiments which involved only short distance experimental releases (800 m or less and within view of the home loft), it was demonstrated that dorsomedial forebrain ablated pigeons generally failed to reassociate with their home loft if the postablation experimental release took place soon postablation or if during the time between ablation and experimental release they were kept away from their home loft. In contrast, if dorsomedial forebrain ablated pigeons were allowed to recover at their home loft prior to experimental release, they succeeded in associating with their home loft in a manner similar to controls. However, only postablation exposure to a pigeon's own loft was sufficient to permit continued home loft association. Pigeons from one loft failed to associate with a foreign postablation recovery loft when released within sight of it. The results show that dorsomedial forebrain ablations result in pigeons which no longer succeed in associating with their home loft; recovery from failed home loft association behavior is possible with postablation exposure to the home loft, and a pigeon's previous association with a loft was a precondition if postablation association was to be affected. The results suggest that dorsomedial forebrain ablated pigeons retain something like a 'home loft trace' which they can use to mediate retrieval and reformation of the recognition properties needed for proper home loft association.

Homing behavior of pigeons after telencephalic ablations

In a first experiment, dorsomedial forebrain ablated birds showed similar homeward orientation when compared to untreated controls independent of whether the birds were released from a previous training site or a site they had never been before. However, although all control birds returned to the home loft, only 2 of 28 birds with lesions homed successfully. In a subsequent experiment, both sham operated control birds and birds with lesions of the visual Wulst homed successfully when released only 800 m from and in full view of their respective home lofts. Pigeons with dorsomedial forebrain lesions, however, failed to return to their respective home lofts. The results show that the avian dorsomedial forebrain plays a critical role in that step of the homing process by which a pigeon returns to its home loft once in its vicinity, and that the failure to reassociate with the home loft is a likely result of deficient recognition of the home loft and/or its surrounding area. In an additional experiment, pigeons with Wulst lesions were shown to orient as controls and to successfully return to the home loft when released from two distant sites. This experiment demonstrated that the avian Wulst plays no necessary role in the homing behavior of pigeons.