Animal navigation: the evolution of magnetic orientation

Animals have several types of magnetic organ, often separately specialized for determining direction versus location. Recent results offer hints about how these once-unimaginable detectors may have evolved.     

Plasma catecholamines and plasma corticosterone following restraint stress in juvenile alligators

Ten juvenile alligators, mean body mass 793 g, hatched from artificially incubated eggs and raised under controlled conditions, were held out of water with their jaws held closed for 48 hr. An initial blood sample was taken and further samples collected at 1, 2, 4, 8, 24, and 48 hr. Epinephrine, norepinephrine, and dopamine were measured in plasma aliquots of 1.5 ml using high pressure liquid chromatography with electrochemical detection. Corticosterone was measured by radioimmunoassay. Plasma glucose was measured using the Trinder method and plasma calcium, cholesterol, and triglycerides were measured in an autoanalyzer. Epinephrine was about 4 ng/ml at the initial bleed, but declined steadily to < 0.4 ng/ml by 24 hr. Norepinephrine was also about 4 ng/ml at the initial bleed, but rose to over 8 ng/ml at 1 hr, and then declined to < 0.2 ng/ml at 24 hr. A second, but smaller increase in plasma norepinephrine was seen at 48 hr. Plasma dopamine was low at the initial bleed (< 0.7 ng/ml), rose to over 8 ng/ml at 1 hr, then declined to < 0.2 ng/ml. Plasma corticosterone rose progressively for the first 4 hr, declined at 8 hr and 24 hr, then rose again at 48 hr. Plasma glucose rose significantly by 24 hr and remained elevated for 48 hr. Plasma calcium increased at 1, 2, and 4 hr then returned to levels not significantly different from the initial sample at 24 and 48 hr. The white blood cells showed changes indicating immune system suppression. By the end of the treatment the hetorophil/lymphocyte ratio increased to 4.7. These results suggest that handling alligators, taking multiple blood samples, and keeping them restrained for more than 8 hr is a severe stress to the animals.     

Field evidence of compass orientation in migrating juvenile grunts (Haemulidae)

Juvenile French and white grunts, Haemulon flavolineatum (Desmarest) and H. plumieri (Lacé-pède), were captured during their daily migrations between diurnal resting sites on coral patch reefs and nocturnal feeding grounds in seagrass beds. Grunts captured during morning and evening migrations were released on the route and after displacement up to 100 m or 5 km away. Grunts generally moved in the direction which would have taken them back towards their home reef or to their accustomed feeding sites, indicating that familiar landmarks are not essential for orientation. The spatial precision of migration may serve to partition the feeding area most efficiently. The timing of migrations is also very precise, and appears to be adapted to reduce the vulnerability of grunts to predation near their home reef.     

Experimental evidence for geomagnetic orientation in juvenile salmon, Oncorhynchus tschawytscha Walbaum

Juvenile chinook salmon, Oncorhynchus tschawytscha , kept under artificial light in a rectangular holding tank aligned east/west for 18 months, showed a preferred temporal and directional orientation of 270° with respect to water flow and the source of food.
Individual fish transferred from the holding/training tank to an unfamiliar circular test arena in another room devoid of local directional cues showed a mean of means preferred unimodal orientation of 264°.
Controlled re-introduction of individual stimuli revealed a hierarchy of orientation cues; one of these was a response to magnetism. A 90° clockwise shift in the horizontal component of the earth's magnetic field was followed by a significant change in the mean of means axial orientation, for the fish under test, from 258°/78° to 354°/174°. After restoration of the normal magnetic field the mean of means axial orientation reverted to 274°/94°.     

Keynote papers on sandhopper orientation and navigation

This article analyses the relevant studies that have made sandhoppers a model subject for the study of orientation, and traces the development of the paradigm through innovative hypotheses and empirical evidence. Sandhoppers are able to maintain their direction without sensorial contact with the goal, which is their burrowing zone extended along the beach, but very narrow across it. They actively determine the direction of their movements, according to their internal state and the environmental features encountered. Each population shows an 'innate directional tendency' adapted to the shoreline of origin, and the inexpert laboratory-born young behave in a similar way to the adults. Genetic differences have been demonstrated between, as well as within natural populations. The question of the calibration of the sun compass to orientation on a particular shoreline implies a redundancy of mechanisms of orientation. Orientation mechanisms may involve environmental cues perceived through diverse sensory modalities, and range from simple orientation reflexes to sun compass navigational systems. These include scototaxis and geotaxis, and the response to the silhouette of the dune, in addition to sun and moon orientation, which is dependent on the time of the day and orientates daily migrations on the beach. Different modalities of orientation may operate singly, or in conjunction with each other, and their ecological significance may vary according to the habitat and lifestyle of the animals. Taken collectively, the orientation behaviour of the group appears to be a most accommodating phenotype, with considerable adaptive potential. The evidence from comparative studies of different populations promotes consideration of behavioural plasticity as an adaptation to changing coastlines.     

Advances in the neural bases of orientation and navigation

The ability to locomote in one direction (oriented movement),and the ability to navigate toward a distant goal are relatedbehaviors that are phylogenetically widespread. Orientationbehaviors include finding the source of an odor or acousticsignal, using a sun-compass for guidance, and moving relativeto fluid-dynamic cues. Such abilities might require little morethan directionally selective sensors coupled to a turning mechanism.This type of behavior, therefore, can be implemented by relativelysimple circuits. In contrast, navigation involves both the abilityto detect direction, as well as a map-sense that provides position.Navigation is less common and arguably requires greater braincomputation than does simple orientation, but may be presentin arthropods as well as in vertebrates. Great progress hasbeen made in exploring the biophysical and sensory bases forthese behaviors, and in recent years the locations and the identityof the cellular transducers of the sensory stimuli (for example,geomagnetic fields) have been narrowed in some taxa. Similarly,neurons within the central nervous that most likely functionin higher order computational processes have been identified.For example, direction-selective and position-responsive braincells have been located in the brains of mammals and birds,and these cells might contribute to a cognitive map that canenable navigation. One model organism in which orientation andnavigation has been extensively studied is the sea slug Tritoniadiomedea. This animal orients its crawling to chemical, hydrodynamic,and geomagnetic cues. The brain of Tritonia has 7000 relativelylarge neurons that facilitate circuit analysis. Recent workhas characterized both peripheral and central neural correlatesof orientation signals, as well as the control of thrust andturning, and studies of their field behavior have suggestedhow these disparate orientation systems may be integrated. Thesefindings provide the basis for future studies to determine howthe nervous system combines multiple sensory cues into a complexhierarchy of signals that can direct motor output and thereforeguide navigational tasks.     

Remote magnetic navigation vs. manual navigation for ablation of ventricular tachycardia: a meta-analysis

Background

The purpose of this study was to prospectively evaluate the efficacy and safety of remote magnetic navigation (RMN) in comparison with manual catheter navigation (MCN) in performing ventricular tachycardia ablation.

Methods

An electronic search was performed using PubMed (1948–2013) and EMBASE (1974–2013) studies comparing RMN with MCN which were published prior to 31 December 2013. Outcomes of interest were as follows: acute success, recurrence rate, complications, total procedure and fluoroscopic times. Standard mean difference (SMD) and its 95 % confidence interval (CI) were used for continuous outcomes; odds ratios (OR) were reported for dichotomous variables.

Results

Four non-randomised studies, including a total of 328 patients, were identified. RMN was deployed in 191 patients. Acute success and long-term freedom from arrhythmias were not significantly different between the RMN and control groups (OR 1.845, 95 % CI 0.731–4.659, p = 0.195 and OR 0.676, 95 % CI 0.383–1.194, p = 0.177, respectively). RMN was associated with less peri-procedural complications (OR 0.279, 95 % CI 0.092–0.843, p = 0.024). Shorter procedural and fluoroscopy times were achieved (95 % CI -0.487 to -0.035, p = 0.024 and 95 % CI -1.467 to -0.984, p<0.001, respectively).

Conclusion

The acute and long-term success rates for VT ablation are equal between RMN and MCN, whereas the RMN-guided procedure can be performed with a lower complication rate and less procedural and fluoroscopic times. More prospective randomised trials will be needed to better evaluate the superior role of RMN for catheter ablation of ventricular tachycardia.     

Mechanisms of magnetic orientation in birds

Behavior and electrophysiological studies have demonstrateda sensitivity to characteristics of the Geomagnetic field thatcan be used for navigation, both for direction finding (compass)and position finding (map). The avian magnetic compass receptorappears to be a light-dependent, wavelength-sensitive systemthat functions as a polarity compass (i.e., it distinguishespoleward from equatorward rather than north from south) andis relatively insensitive to changes in magnetic field intensity.The receptor is within the retina and is based on one or morephotopigments, perhaps cryptochromes. A second receptor systemappears to be based on magnetite and might serve to transducelocation information independent of the compass system. Thisreceptor is associated with the ophthalmic branch of the trigeminalnerve and is sensitive to very small (<50 nanotesla) changesin the intensity of the magnetic field. In neither case hasa neuron that responded to changes in the magnetic field beentraced to a structure that can be identified to be a receptor.Almost nothing is known about how magnetic information is processedwithin the brain or how it is combined with other sensory informationand used for navigation. These remain areas of future research.     

Magnetic orientation and navigation in marine turtles, lobsters, and molluscs: concepts and conundrums

The Earth's magnetic field provides a pervasive source of directionalinformation used by phylogenetically diverse marine animals.Behavioral experiments with sea turtles, spiny lobsters, andsea slugs have revealed that all have a magnetic compass sense,despite vast differences in the environment each inhabits andthe spatial scale over which each moves. For two of these animals,the Earth's field also serves as a source of positional information.Hatchling loggerhead sea turtles from Florida responded to themagnetic fields found in three widely separated regions of theAtlantic Ocean by swimming in directions that would, in eachcase, facilitate movement along the migratory route. Thus, foryoung loggerheads, regional magnetic fields function as navigationalmarkers and elicit changes in swimming direction at crucialgeographic boundaries. Older turtles, as well as spiny lobsters,apparently acquire a "magnetic map" that enables them to usemagnetic topography to determine their position relative tospecific goals. Relatively little is known about the neuralmechanisms that underlie magnetic orientation and navigation.A promising model system is the marine mollusc Tritonia diomedea,which possesses both a magnetic compass and a relatively simplenervous system. Six neurons in the brain of T. diomedea havebeen identified that respond to changes in magnetic fields.At least some of these appear to be ciliary motor neurons thatgenerate or modulate the final behavioral output of the orientationcircuitry. These findings represent an encouraging step towarda holistic understanding of the cells and circuitry that underliemagnetic orientation behavior in one model organism.     

The radial bias: a different slant on visual orientation sensitivity in human and nonhuman primates

It is generally assumed that sensitivity to different stimulus orientations is mapped in a globally equivalent fashion across primate visual cortex, at a spatial scale larger than that of orientation columns. However, some evidence predicts instead that radial orientations should produce higher activity than other orientations, throughout visual cortex. Here, this radial orientation bias was robustly confirmed using (1) human psychophysics, plus fMRI in (2) humans and (3) behaving monkeys. In visual cortex, fMRI activity was at least 20% higher in the retinotopic representations of polar angle which corresponded to the radial stimulus orientations (relative to tangential). In a global demonstration of this, we activated complementary retinotopic quadrants of visual cortex by simply changing stimulus orientation, without changing stimulus location in the visual field. This evidence reveals a neural link between orientation sensitivity and the cortical retinotopy, which have previously been considered independent.     

The sex ratio of wild Chinese alligators Alligator sinensis

The Chinese alligator Alligator sinensis is one of the most endangered crocodilian species, and typically exhibits temperature-dependent sex determination. It is extremely important to clarify the sex structure of Chinese alligators to implement recovery projects successfully. However, the sex ratio of wild Chinese alligators remains unknown. In this study, we collected 28 years of sex ratio data from Chinese alligators residing in the natural and artificial habitats of Changxing Nature Reserve, China, and examined the differences in the sex ratio dynamics between these two populations. We observed that the sex ratio of wild Chinese alligators is 1 male to 4.507 females, which was significantly lower compared to that of the captive population （1 to 2.040; P 〈 0.001）, and is significantly different to previously documented sex ratios for this species （all P 〈 0.01）. Furthermore, we documented an annually stable （P = 1.000） female-biased sex ratio for wild alligators at hatching [1 male to 4.747 females; 0.174 （0.167~）.182）], in contrast to a dramatically fluctuating sex ratio （P 〈 0.001） in captivity [1 male to 1.674 females; 0.374 （0.246-0.593）], showing a potential mechanism for adjusting the sex structure. Finally, we found that the hatchling sex ratios were similar to that of the population sex ratio （P = 0.748）, with little correlation to air temperature values in the 60-70 day incubation period during the breeding season （July and August; both P 〉 0.05）. Overall, this study indicates that the stabilized female-biased sex ratio of Changxing Chinese alligators might result from selection pressure caused by local mate competition and major inbreeding .     

Dissociation of exteroceptive and idiothetic orientation cues: effect on hippocampal place cells and place navigation.

Navigation by means of cognitive maps appears to require the hippocampus; hippocampal place cells (PCs) appear to store spatial memories because their discharge is confined to cell-specific places called firing fields (FFs). Experiments with rats manipulated idiothetic and landmark-related information to understand the relationship between PC activity and spatial cognition. Rotating a circular arena in the light caused a discrepancy between these cues. This discrepancy caused most FFs to disappear in both the arena and room reference frames. However, FFs persisted in the rotating arena frame when the discrepancy was reduced by darkness or by a card in the arena. The discrepancy was increased by ''field clamping'' the rat in a room-defined FF location by rotations that countered its locomotion. Most FFs dissipated and reappeared an hour or more after the clamp. Place-avoidance experiments showed that navigation uses independent idiothetic and exteroceptive memories. Rats learned to avoid the unmarked footshock region within a circular arena. When acquired on the stable arena in the light, the location of the punishment was learned by using both room and idiothetic cues; extinction in the dark transferred to the following session in the light. If, however, extinction occurred during rotation, only the arena-frame avoidance was extinguished in darkness; the room-defined location was avoided when the lights were turned back on. Idiothetic memory of room-defined avoidance was not formed during rotation in light; regardless of rotation, there was no avoidance when the lights were turned off, but room-frame avoidance reappeared when the lights were turned back on. The place-preference task rewarded visits to an allocentric target location with a randomly dispersed pellet. The resulting behaviour alternated between random pellet searching and target-directed navigation, making it possible to examine PC correlates of these two classes of spatial behaviour. The independence of idiothetic and exteroceptive spatial memories and the disruption of PC firing during rotation suggest that PCs may not be necessary for spatial cognition; this idea can be tested by recordings during the place-avoidance and preference tasks.     

视觉皮层方位敏感性的皮层下起源

自Hubel和Wiesel关于视皮层研究的开拓性工作以来,视觉方位敏感性一直被认为是视皮层细胞独有的功能。本文综述了80年代以来的最新研究成果,表明视觉方位敏感性起源于视网膜、丘脑外膝体等皮层下结构,而在视皮层方位功能柱形成之前,丘脑外膝体已对具有相似最优方位的外膝体神经元作了初步的编组或预安排。     

Feasibility of remote magnetic navigation for epicardial ablation

Percutaneous epicardial mapping and ablation is an emerging method to treat ventricular tachycardias (VT), premature ventricular complexes (PVC), and accessory pathways. The use of a remote magnetic navigation system (MNS) could enhance precision and maintain safety. This multiple case history demonstrates the feasibility and safety of the MNS-guided epicardial approach in mapping and ablation of ischaemic VT, outflow tract PVCs, and a left-sided accessory pathway. All patients had previously undergone endocardial mapping for the same arrhythmia. MNS could present an advantage from more precise navigation for mapping and maintaining catheter stability during energy application.     

The sensitivity of the heart to static magnetic fields

Static magnetic fields induce flow potentials in arterial flows in and around the heart, that have been detected as distortions in the ECG. The resultant currents flowing through the myocardium could alter the rate or rhythm of the heart. No such changes have been seen in animal experiments, or with humans, in static fields up to 8 T. The possible effects of such currents induced by fields larger than 8 T on cardiac pacemaker rate, and arrhythmogenesis are reviewed, using virtual cardiac tissues—computational models of cardiac electrophysiology. Arrhythmogenesis can be by the initiation of ectopic beats, or by re-entry, whose probability of occurrence is increased by any increase in the electrical heterogeneity, in particular, the action potential duration heterogeneity of the ventricle. Focal ectopic activity would be readily detectable, but since re-entrant arrhythmias are very rare events, even a large increase in their probability of occurrence still leaves them unlikely to be observed. Both of these two arrhythmogenic mechanisms would show a steep sigmoidal, or threshold dependence on induced current intensity, with the threshold for increasing the vulnerability to re-entry less than the threshold for initiating activity. Failure to observe them at fields less than 8 T provides only a lower bound for any threshold for arrhythmogenesis.     

The visual pigment sensitivity hypothesis: further evidence from fishes of varying habitats

Summary Visual pigments were extracted from the retinas of 8 species of marine teleosts and 4 species of elasmobranchs and a comparison was made of the pigment properties from these fishes, some inhabiting surface waters, others from the mesopelagic zone, and a few migrating vertically between these two environments. An association was found between the spectral position of the absorbance curve and the habitat depth or habitat behavior, with the blue-shifted chrysopsins being the pigments of the twilight zone fishes and the rhodopsins with fishes living near the surface. The retina of the swell shark (Cephaloscyllium ventriosum) yielded extracts with two photopigments; one, a rhodopsin at 498 nm; the second, a chrysopsin at 478 nm. This fish has been reported to practice seasonal vertical migrations between the surface and the mesopelagic waters. In addition to the spectral absorbance, several properties of these visual pigments were examined, including the meta-III product of photic bleaching, regeneration with added 11-cis and 9-cis retinals, and the chromophoric photosensitivity. The chrysopsin properties were found to be fundamentally similar to those of typical vertebrate rhodopsins. Correlating the spectral data with the habitat and habitat behavior of our fishes gives us confidence in the idea that the scotopic pigments have evolved as adaptations to those aspects of their color environment that are critical to the survival of the species.     

Spatial orientation of rainbow trout to plane-polarized light: The ontogeny of E-vector discrimination and spectral sensitivity characteristics

Summary The results of this study demonstrate that trout (Salmo gairdneri) are capable of orienting to polarized light fields. The spectral composition of the polarized light fields can significantly influence the orientation of trout. Rainbow trout exhibit ontogenetic losses in orientation to polarized light fields which appears coincident with the ontogenetic loss of the UV-sensitive cones. Trout were trained to swim to a refuge located at one end of the training tank under a polarized light field. The E-vector of the polarized light field was oriented parallel or perpendicular to the long axis of the training tank. Trained fish were released in a circular test tank and their angular response scored. Under a white plus ultraviolet polarized light field, trout oriented in the trained E-vector orientation. For instance, fish trained under a parallel E-vector orientation exhibited angular responses close to parallel in the test tank. However, when the spectral composition of the polarized light field was manipulated, the accuracy of spatial orientation of the trout varied. Trout weighing about 30 g exhibited accurate orientation to the white plus UV polarized light field. The trout were incapable of orientation at a body weight of 50 to 60 g.