Coding of Azimuthal Directions via Time-Compensated Combination of Celestial Compass Cues

Common features in diverse insect clocks

This review describes common features among diverse biological clocks in insects, including circadian, circatidal, circalunar/circasemilunar, and circannual clocks. These clocks control various behaviors, physiological functions, and developmental events, enabling adaptation to periodic environmental changes. Circadian clocks also function in time-compensation for celestial navigation and in the measurement of day or night length for photoperiodism. Phase response curves for such clocks reported thus far exhibit close similarities; specifically, the circannual clock in Anthrenus verbasci shows striking similarity to circadian clocks in its phase response. It is suggested that diverse biological clocks share physiological properties in their phase responses irrespective of period length. Molecular and physiological mechanisms are best understood for the optic-lobe and mid-brain circadian clocks, although there is no direct evidence that these clocks are involved in rhythmic phenomena other than circadian rhythms in daily events. Circadian clocks have also been localized in peripheral tissues, and research on their role in various rhythmic phenomena has been started. Although clock genes have been identified as controllers of circadian rhythms in daily events, some of these genes have also been shown to be involved in photoperiodism and possibly in time-compensated celestial navigation. In contrast, there is no experimental evidence indicating that any known clock gene is involved in biological clocks other than circadian clocks.

Amplitude and dynamics of polarization-plane signaling in the central complex of the locust brain

The polarization pattern of skylight provides a compass cue that various insect species use for allocentric orientation. In the desert locust, Schistocerca gregaria, a network of neurons tuned to the electric field vector (E-vector) angle of polarized light is present in the central complex of the brain. Preferred E-vector angles vary along slices of neuropils in a compasslike fashion (polarotopy). We studied how the activity in this polarotopic population is modulated in ways suited to control compass-guided locomotion. To this end, we analyzed tuning profiles using measures of correlation between spike rate and E-vector angle and, furthermore, tested for adaptation to stationary angles. The results suggest that the polarotopy is stabilized by antagonistic integration across neurons with opponent tuning. Downstream to the input stage of the network, responses to stationary E-vector angles adapted quickly, which may correlate with a tendency to steer a steady course previously observed in tethered flying locusts. By contrast, rotating E-vectors corresponding to changes in heading direction under a natural sky elicited nonadapting responses. However, response amplitudes were particularly variable at the output stage, covarying with the level of ongoing activity. Moreover, the responses to rotating E-vector angles depended on the direction of rotation in an anticipatory manner. Our observations support a view of the central complex as a substrate of higher-stage processing that could assign contextual meaning to sensory input for motor control in goal-driven behaviors. Parallels to higher-stage processing of sensory information in vertebrates are discussed.

Receptive fields of locust brain neurons are matched to polarization patterns of the sky

Many animals, including insects, are able to use celestial cues as a reference for spatial orientation and long-distance navigation [1]. In addition to direct sunlight, the chromatic gradient of the sky and its polarization pattern are suited to serve as orientation cues [2-5]. Atmospheric scattering of sunlight causes a regular pattern of E vectors in the sky, which are arranged along concentric circles around the sun [5, 6]. Although certain insects rely predominantly on sky polarization for spatial orientation [7], it has been argued that detection of celestial E vector orientation may not suffice to differentiate between solar and antisolar directions [8, 9]. We show here that polarization-sensitive (POL) neurons in the brain of the desert locust Schistocerca gregaria can overcome this ambiguity. Extracellular recordings from POL units in the central complex and lateral accessory lobes revealed E vector tunings arranged in concentric circles within large receptive fields, matching the sky polarization pattern at certain solar positions. Modeling of neuronal responses under an idealized sky polarization pattern (Rayleigh sky) suggests that these "matched filter" properties allow locusts to unambiguously determine the solar azimuth by relying solely on the sky polarization pattern for compass navigation.

Integration of polarization and chromatic cues in the insect sky compass

Animals relying on a celestial compass for spatial orientation may use the position of the sun, the chromatic or intensity gradient of the sky, the polarization pattern of the sky, or a combination of these cues as compass signals. Behavioral experiments in bees and ants, indeed, showed that direct sunlight and sky polarization play a role in sky compass orientation, but the relative importance of these cues are species-specific. Intracellular recordings from polarization-sensitive interneurons in the desert locust and monarch butterfly suggest that inputs from different eye regions, including polarized-light input through the dorsal rim area of the eye and chromatic/intensity gradient input from the main eye, are combined at the level of the medulla to create a robust compass signal. Conflicting input from the polarization and chromatic/intensity channel, resulting from eccentric receptive fields, is eliminated at the level of the anterior optic tubercle and central complex through internal compensation for changing solar elevations, which requires input from a circadian clock. Across several species, the central complex likely serves as an internal sky compass, combining E-vector information with other celestial cues. Descending neurons, likewise, respond both to zenithal polarization and to unpolarized cues in an azimuth-dependent way.

Opsin expression, physiological characterization and identification of photoreceptor cells in the dorsal rim area and main retina of the desert locust, Schistocerca gregaria

For compass orientation many insects rely on the pattern of sky polarization but some species also exploit the sky chromatic contrast. Desert locusts, Schistocerca gregaria, detect polarized light through a specialized dorsal rim area (DRA) in their compound eye. To better understand retinal mechanisms underlying visual navigation, we compared opsin expression, spectral and polarization sensitivities and response-stimulus intensity functions in the DRA and main retina of the locust. In addition to previously characterized opsins of long-wavelength-absorbing (Lo1) and blue-absorbing visual pigments (Lo2), we identified an opsin of a UV-absorbing visual pigment (LoUV). DRA photoreceptors exclusively expressed Lo2, had peak spectral sensitivities at 441 nm and showed high polarization sensitivity (PS 1.3-31.7). In contrast, ommatidia in the main eye coexpressed Lo1 and Lo2 in five photoreceptors, expressed Lo1 in two proximal photoreceptors, and Lo2 or LoUV in one distal photoreceptor. Correspondingly, we found broadband blue- and green-peaking spectral sensitivities in the main eye and one narrowly tuned UV peaking receptor. Polarization sensitivity in the main retina was low (PS 1.3-3.8). V-log I functions in the DRA were steeper than in the main retina supporting a role in polarization vision. Desert locusts occur as two morphs, a day-active gregarious and a night-active solitarious form. In solitarious locusts sensitivities in the main retina were generally shifted to longer wavelengths, particularly in ventral eye regions, supporting a nocturnal life style at low light levels. The data support the role of the DRA in polarization vision and suggest trichromatic colour vision in the desert locust.

Interactions of the polarization and the sun compass in path integration of desert ants

Desert ants, Cataglyphis fortis, perform large-scale foraging trips in their featureless habitat using path integration as their main navigation tool. To determine their walking direction they use primarily celestial cues, the sky's polarization pattern and the sun position. To examine the relative importance of these two celestial cues, we performed cue conflict experiments. We manipulated the polarization pattern experienced by the ants during their outbound foraging excursions, reducing it to a single electric field (e-)vector direction with a linear polarization filter. The simultaneous view of the sun created situations in which the directional information of the sun and the polarization compass disagreed. The heading directions of the homebound runs recorded on a test field with full view of the natural sky demonstrate that none of both compasses completely dominated over the other. Rather the ants seemed to compute an intermediate homing direction to which both compass systems contributed roughly equally. Direct sunlight and polarized light are detected in different regions of the ant's compound eye, suggesting two separate pathways for obtaining directional information. In the experimental paradigm applied here, these two pathways seem to feed into the path integrator with similar weights.

Chromatic processing in the anterior optic tubercle of the honey bee brain

Color vision in honey bees (Apis mellifera) has been extensively studied at the behavioral level and, to a lesser degree, at the physiological level by means of electrophysiological intracellular recordings of single neurons. Few visual neurons have been so far characterized in the lateral protocerebrum of bees. Therefore, the possible implication of this region in chromatic processing remains unknown. We performed in vivo calcium imaging of interneurons in the anterior optic tubercle (AOTu) of honey bees upon visual stimulation of the compound eye to analyze chromatic response properties. Stimulation with distinct monochromatic lights (ultraviolet [UV], blue, and green) matching the sensitivity of the three photoreceptor types of the bee retina induced different signal amplitudes, temporal dynamics, and spatial activity patterns in the AOTu intertubercle network, thus revealing intricate chromatic processing properties. Green light strongly activated both the dorsal and ventral lobes of the AOTu's major unit; blue light activated the dorsal lobe more while UV light activated the ventral lobe more. Eye stimulation with mixtures of blue and green light induced suppression phenomena in which responses to the mixture were lower than those to the color components, thus concurring with color-opponent processing. These data provide evidence for a spatial segregation of color processing in the AOTu, which may serve for navigation purposes.

Receptive field properties and intensity-response functions of polarization-sensitive neurons of the optic tubercle in gregarious and solitarious locusts

Many migrating insects rely on the plane of sky polarization as a cue to detect spatial directions. Desert locusts (Schistocerca gregaria), like other insects, perceive polarized light through specialized photoreceptors in a dorsal eye region. Desert locusts occur in two phases: a gregarious swarming phase, which migrates during the day, and a solitarious nocturnal phase. Neurons in a small brain area, the anterior optic tubercle (AOTu), are critically involved in processing polarized light in the locust brain. While polarization-sensitive intertubercle cells [lobula-tubercle neuron 1 (LoTu1) and tubercle-tubercle neuron 1 (TuTu1)] interconnect the AOTu of both hemispheres, tubercle-lateral accessory lobe tract (TuLAL1) neurons transmit sky compass signals to a polarization compass in the central brain. To better understand the neural network underlying polarized light processing in the AOTu and to investigate possible adaptations of the polarization vision system to a diurnal versus nocturnal lifestyle, we analyzed receptive field properties, intensity-response relationships, and daytime dependence of responses of AOTu neurons in gregarious and solitarious locusts. Surprisingly, no differences in the physiology of these neurons were found between the two locust phases. Instead, clear differences were observed between the different types of AOTu neurons. Whereas TuTu1 and TuLAL1 neurons encoded E-vector orientation independent of light intensity and would thus be operational in bright daylight, LoTu1 neurons were inhibited by high light intensity and provided strong polarization signaling only under dim light conditions. The presence of high- and low-intensity polarization channels might, therefore, allow solitarious and gregarious locusts to use the same polarization coding system despite their different activity cycles.

Segregation of visual inputs from different regions of the compound eye in two parallel pathways through the anterior optic tubercle of the bumblebee (Bombus ignitus)

Visually guided behaviors require the brain to extract features of the visual world and to integrate them in a context-specific manner. Hymenopteran insects have been prime models for ethological research into visual behaviors for decades but knowledge about the underlying central processing is very limited. This is particularly the case for sky-compass navigation. To learn more about central processing of visual information in general and specifically to reveal a possible polarization vision pathway in the bee brain, we used tracer injections to investigate the pathways through the anterior optic tubercle, a prominent output target of the insect optic lobe, in the bumblebee Bombus ignitus. The anterior optic tubercle of the bumblebee is a small neuropil of 200 μm width and is located dorsolateral to the antennal lobe at the anterior surface of the brain. It is divided into a larger upper and a smaller lower subunit, both of which receive input from the optic lobe and connect to the lateral accessory lobe, and the contralateral tubercle, via two parallel pathways. The lower subunit receives input from the dorsal rim area (DRA) of the compound eye. The bumblebee DRA shares structural similarities with polarization-sensitive DRAs of other insects and looks similar to that of honeybees. We identified several neurons within this pathway that could be homologous to identified polarization-sensitive neurons in the locust brain. We therefore conclude that the pathway through the lower subunit of the anterior optic tubercle could carry polarization information from the periphery to the central brain.

Visual control of navigation in insects and its relevance for robotics

Flying insects display remarkable agility, despite their diminutive eyes and brains. This review describes our growing understanding of how these creatures use visual information to stabilize flight, avoid collisions with objects, regulate flight speed, detect and intercept other flying insects such as mates or prey, navigate to a distant food source, and orchestrate flawless landings. It also outlines the ways in which these insights are now being used to develop novel, biologically inspired strategies for the guidance of autonomous, airborne vehicles.

Unraveling navigational strategies in migratory insects

Long-distance migration is a strategy some animals use to survive a seasonally changing environment. To reach favorable grounds, migratory animals have evolved sophisticated navigational mechanisms that rely on a map and compasses. In migratory insects, the existence of a map sense (sense of position) remains poorly understood, but recent work has provided new insights into the mechanisms some compasses use for maintaining a constant bearing during long-distance navigation. The best-studied directional strategy relies on a time-compensated sun compass, used by diurnal insects, for which neural circuits have begun to be delineated. Yet, a growing body of evidence suggests that migratory insects may also rely on other compasses that use night sky cues or the Earth's magnetic field. Those mechanisms are ripe for exploration.

A distinct layer of the medulla integrates sky compass signals in the brain of an insect

Mass migration of desert locusts is a common phenomenon in North Africa and the Middle East but how these insects navigate is still poorly understood. Laboratory studies suggest that locusts are able to exploit the sky polarization pattern as a navigational cue. Like other insects locusts detect polarized light through a specialized dorsal rim area (DRA) of the eye. Polarization signals are transmitted through the optic lobe to the anterior optic tubercle (AOTu) and, finally, to the central complex in the brain. Whereas neurons of the AOTu integrate sky polarization and chromatic cues in a daytime dependent manner, the central complex holds a topographic representation of azimuthal directions suggesting a role as an internal sky compass. To understand further the integration of sky compass cues we studied polarization-sensitive (POL) neurons in the medulla that may be intercalated between DRA photoreceptors and AOTu neurons. Five types of POL-neuron were characterized and four of these in multiple recordings. All neurons had wide arborizations in medulla layer 4 and most, additionally, in the dorsal rim area of the medulla and in the accessory medulla, the presumed circadian clock. The neurons showed type-specific orientational tuning to zenithal polarized light and azimuth tuning to unpolarized green and UV light spots. In contrast to neurons of the AOTu, we found no evidence for color opponency and daytime dependent adjustment of sky compass signals. Therefore, medulla layer 4 is a distinct stage in the integration of sky compass signals that precedes the time-compensated integration of celestial cues in the AOTu.

Central neural coding of sky polarization in insects

Many animals rely on a sun compass for spatial orientation and long-range navigation. In addition to the Sun, insects also exploit the polarization pattern and chromatic gradient of the sky for estimating navigational directions. Analysis of polarization-vision pathways in locusts and crickets has shed first light on brain areas involved in sky compass orientation. Detection of sky polarization relies on specialized photoreceptor cells in a small dorsal rim area of the compound eye. Brain areas involved in polarization processing include parts of the lamina, medulla and lobula of the optic lobe and, in the central brain, the anterior optic tubercle, the lateral accessory lobe and the central complex. In the optic lobe, polarization sensitivity and contrast are enhanced through convergence and opponency. In the anterior optic tubercle, polarized-light signals are integrated with information on the chromatic contrast of the sky. Tubercle neurons combine responses to the UV/green contrast and e-vector orientation of the sky and compensate for diurnal changes of the celestial polarization pattern associated with changes in solar elevation. In the central complex, a topographic representation of e-vector tunings underlies the columnar organization and suggests that this brain area serves as an internal compass coding for spatial directions.

Honeybee navigation: following routes using polarized-light cues

While it is generally accepted that honeybees (Apis mellifera) are capable of using the pattern of polarized light in the sky to navigate to a food source, there is little or no direct behavioural evidence that they actually do so. We have examined whether bees can be trained to find their way through a maze composed of four interconnected tunnels, by using directional information provided by polarized light illumination from the ceilings of the tunnels. The results show that bees can learn this task, thus demonstrating directly, and for the first time, that bees are indeed capable of using the polarized-light information in the sky as a compass to steer their way to a food source.

Insect circadian clock outputs

Insects display an impressive variety of daily rhythms, which are most evident in their behaviour. Circadian timekeeping systems that generate these daily rhythms of physiology and behaviour all involve three interacting elements: the timekeeper itself (i.e. the clock), inputs to the clock through which it entrains and otherwise responds to environmental cues such as light and temperature, and outputs from the clock through which it imposes daily rhythms on various physiological and behavioural parameters. In insects, as in other animals, cellular clocks are embodied in clock neurons capable of sustained autonomous circadian rhythmicity, and those clock neurons are organized into clock circuits. Drosophila flies spend their entire lives in small areas near the ground, and use their circadian brain clock to regulate daily rhythms of rest and activity, so as to organize their behaviour appropriately to the daily rhythms of their local environment. Migratory locusts and butterflies, on the other hand, spend substantial portions of their lives high up in the air migrating long distances (sometimes thousands of miles) and use their circadian brain clocks to provide time-compensation to their sun-compass navigational systems. Interestingly, however, there appear to be substantial similarities in the cellular and network mechanisms that underlie circadian outputs in all insects.

Honeybees as a model for the study of visually guided flight, navigation, and biologically inspired robotics

Research over the past century has revealed the impressive capacities of the honeybee, Apis mellifera, in relation to visual perception, flight guidance, navigation, and learning and memory. These observations, coupled with the relative ease with which these creatures can be trained, and the relative simplicity of their nervous systems, have made honeybees an attractive model in which to pursue general principles of sensorimotor function in a variety of contexts, many of which pertain not just to honeybees, but several other animal species, including humans. This review begins by describing the principles of visual guidance that underlie perception of the world in three dimensions, obstacle avoidance, control of flight speed, and orchestrating smooth landings. We then consider how navigation over long distances is accomplished, with particular reference to how bees use information from the celestial compass to determine their flight bearing, and information from the movement of the environment in their eyes to gauge how far they have flown. Finally, we illustrate how some of the principles gleaned from these studies are now being used to design novel, biologically inspired algorithms for the guidance of unmanned aerial vehicles.

Influence of angle of view on judges' evaluations of inverted cross in men's rings

The present study focuses on two factors that may influence the accuracy and precision of a judge's evaluation of the angular deviation error in "inverted cross" (angle of the arms away from the horizontal axis). 16 Greek judges of competitive gymnastics participated in an evaluation of the magnitude of the error in the skill. Photographs of the performance of 8 gymnasts were shown to the judges in triplicates in random order from three angles of observation (0 degrees, 45 degrees, and 90 degrees) on a computer screen. The precision in the judges' judgments was best at the 45 degrees angle of observation and at angular deviations of 16 degrees to 30 degrees. Accuracy was influenced both by the judge's angle of observation and the gymnast's angular deviation, as well as by the interaction. Overall, judges may need to improve their ability to evaluate performance through training. Training may incorporate a variety of exercises in judging the inverted cross through augmented feedback of knowledge of results.

Polarization-sensitive descending neurons in the locust: connecting the brain to thoracic ganglia

Many animal species, in particular insects, exploit the E-vector pattern of the blue sky for sun compass navigation. Like other insects, locusts detect dorsal polarized light via photoreceptors in a specialized dorsal rim area of the compound eye. Polarized light information is transmitted through several processing stages to the central complex, a brain area involved in the control of goal-directed orientation behavior. To investigate how polarized light information is transmitted to thoracic motor circuits, we studied the responses of locust descending neurons to polarized light. Three sets of polarization-sensitive descending neurons were characterized through intracellular recordings from axonal fibers in the neck connectives combined with single-cell dye injections. Two descending neurons from the brain, one with ipsilaterally and the second with contralaterally descending axon, are likely to bridge the gap between polarization-sensitive neurons in the brain and thoracic motor centers. In both neurons, E-vector tuning changed linearly with daytime, suggesting that they signal time-compensated spatial directions, an important prerequisite for navigation using celestial signals. The third type connects the suboesophageal ganglion with the prothoracic ganglion. It showed no evidence for time compensation in E-vector tuning and might play a role in flight stabilization and control of head movements.

Sun compass integration of skylight cues in migratory monarch butterflies

Migrating monarch butterflies (Danaus plexippus) use a time-compensated sun compass to navigate from eastern North America to their overwintering grounds in central Mexico. Here we describe the neuronal layout of those aspects of the butterfly's central complex likely to establish part of the internal sun compass and find them highly homologous to those of the desert locust. Intracellular recordings from neurons in the monarch sun compass network reveal responses tuned to specific E-vector angles of polarized light, as well as azimuth-dependent responses to unpolarized light, independent of spectral composition. The neural responses to these two stimuli in individual neurons are mediated through different regions of the compound eye. Moreover, these dual responses are integrated to create a consistent representation of skylight cues in the sun compass throughout the day. The results advance our understanding of how ambiguous sensory signals are processed by the brain to elicit a robust behavioral response.

Conditional Perception Under Stimulus Ambiguity: Polarization- and Azimuth-Sensitive Neurons in the Locust Brain Are Inhibited by Low Degrees of Polarization

Sensory perception often relies on the integration and matching of multisensory inputs. In the brain of desert locusts, identified neurons that signal the sun's direction relative to the animal's head integrate information about the polarization pattern of the sky with information on the color and intensity contrast of the sky. The cloudless blue sky exhibits a gradient from unpolarized sunlight to strongly polarized light at 90° from the sun. Therefore the percentage of polarized light in the sky is highest at dusk and dawn and lowest when the sun is in the zenith. We investigated the effect of different degrees of polarization on neurons of the anterior optic tubercle of the desert locust through intracellular recordings. Whereas dorsal presentation of strongly polarized light largely excited the neurons, weakly polarized light, i.e., a blend of polarized light of many orientations, led to inhibition. The data suggest that the polarization input to these neurons is inhibited within a radius of 50° around the sun, thereby avoiding conflicting input from the polarization and direct sunlight channels. These properties can be regarded as sensory filters to avoid ambiguous signaling during sky compass orientation.

Studying sensorimotor processing with physiology in behaving Drosophila

The neural underpinnings of sensorimotor integration are best studied in the context of well-characterized behavior. A rich trove of Drosophila behavioral genetics research offers a variety of well-studied behaviors and candidate brain regions that can form the bases of such studies. The development of tools to perform in vivo physiology from the Drosophila brain has made it possible to monitor activity in defined neurons in response to sensory stimuli. More recently still, it has become possible to perform recordings from identified neurons in the brain of head-fixed flies during walking or flight behaviors. In this chapter, we discuss how experiments that simultaneously monitor behavior and physiology in Drosophila can be combined with other techniques to produce testable models of sensorimotor circuit function.

Navigational mechanisms of migrating monarch butterflies

Recent studies of the iconic fall migration of monarch butterflies have illuminated the mechanisms behind their southward navigation while using a time-compensated sun compass. Skylight cues, such as the sun itself and polarized light, are processed through both eyes and are probably integrated in the brain's central complex, the presumed site of the sun compass. Time compensation is provided by circadian clocks that have a distinctive molecular mechanism and that reside in the antennae. Monarchs might also use a magnetic compass because they possess two cryptochromes that have the molecular capability for light-dependent magnetoreception. Multiple genomic approaches are now being used with the aim of identifying navigation genes. Monarch butterflies are thus emerging as an excellent model organism in which to study the molecular and neural basis of long-distance migration.

Evidence for the possible existence of a second polarization-vision pathway in the locust brain

For spatial orientation and navigation, many insects derive compass information from the polarization pattern of the blue sky. The desert locust Schistocerca gregaria detects polarized light with a specialized dorsal rim area of its compound eye. In the locust brain, polarized-light signals are passed through the anterior optic tract and tubercle to the central complex which most likely serves as an internal sky compass. Here, we suggest that neurons of a second visual pathway, via the accessory medulla and posterior optic tubercle, also provide polarization information to the central complex. Intracellular recordings show that two types of neuron in this posterior pathway are sensitive to polarized light. One cell type connects the dorsal rim area of the medulla with the medulla and accessory medulla, and a second type connects the bilaterally paired posterior optic tubercles. Given the evidence for a role of the accessory medulla as the master clock controlling circadian changes in behavioral activity in flies and cockroaches, our data open the possibility that time-compensated polarized-light signals may reach the central complex via this pathway for time-compensated sky-compass navigation.

Are bigger brains better?

Attempts to relate brain size to behaviour and cognition have rarely integrated information from insects with that from vertebrates. Many insects, however, demonstrate that highly differentiated motor repertoires, extensive social structures and cognition are possible with very small brains, emphasising that we need to understand the neural circuits, not just the size of brain regions, which underlie these feats. Neural network analyses show that cognitive features found in insects, such as numerosity, attention and categorisation-like processes, may require only very limited neuron numbers. Thus, brain size may have less of a relationship with behavioural repertoire and cognitive capacity than generally assumed, prompting the question of what large brains are for. Larger brains are, at least partly, a consequence of larger neurons that are necessary in large animals due to basic biophysical constraints. They also contain greater replication of neuronal circuits, adding precision to sensory processes, detail to perception, more parallel processing and enlarged storage capacity. Yet, these advantages are unlikely to produce the qualitative shifts in behaviour that are often assumed to accompany increased brain size. Instead, modularity and interconnectivity may be more important.

The Locust Standard Brain: A 3D Standard of the Central Complex as a Platform for Neural Network Analysis

Many insects use the pattern of polarized light in the sky for spatial orientation and navigation. We have investigated the polarization vision system in the desert locust. To create a common platform for anatomical studies on polarization vision pathways, Kurylas et al. (2008) have generated a three-dimensional (3D) standard brain from confocal microscopy image stacks of 10 male brains, using two different standardization methods, the Iterative Shape Averaging (ISA) procedure and the Virtual Insect Brain (VIB) protocol. Comparison of both standardization methods showed that the VIB standard is ideal for comparative volume analysis of neuropils, whereas the ISA standard is the method of choice to analyze the morphology and connectivity of neurons. The central complex is a key processing stage for polarization information in the locust brain. To investigate neuronal connections between diverse central-complex neurons, we generated a higher-resolution standard atlas of the central complex and surrounding areas, using the ISA method based on brain sections from 20 individual central complexes. To explore the usefulness of this atlas, two central-complex neurons, a polarization-sensitive columnar neuron (type CPU1a) and a tangential neuron that is activated during flight, the giant fan-shaped (GFS) neuron, were reconstructed 3D from brain sections. To examine whether the GFS neuron is a candidate to contribute to synaptic input to the CPU1a neuron, we registered both neurons into the standardized central complex. Visualization of both neurons revealed a potential connection of the CPU1a and GFS neurons in layer II of the upper division of the central body.

Which coordinate system for modelling path integration?

Path integration is a navigation strategy widely observed in nature where an animal maintains a running estimate, called the home vector, of its location during an excursion. Evidence suggests it is both ancient and ubiquitous in nature, and has been studied for over a century. In that time, canonical and neural network models have flourished, based on a wide range of assumptions, justifications and supporting data. Despite the importance of the phenomenon, consensus and unifying principles appear lacking. A fundamental issue is the neural representation of space needed for biological path integration. This paper presents a scheme to classify path integration systems on the basis of the way the home vector records and updates the spatial relationship between the animal and its home location. Four extended classes of coordinate systems are used to unify and review both canonical and neural network models of path integration, from the arthropod and mammalian literature. This scheme demonstrates analytical equivalence between models which may otherwise appear unrelated, and distinguishes between models which may superficially appear similar. A thorough analysis is carried out of the equational forms of important facets of path integration including updating, steering, searching and systematic errors, using each of the four coordinate systems. The type of available directional cue, namely allothetic or idiothetic, is also considered. It is shown that on balance, the class of home vectors which includes the geocentric Cartesian coordinate system, appears to be the most robust for biological systems. A key conclusion is that deducing computational structure from behavioural data alone will be difficult or impossible, at least in the absence of an analysis of random errors. Consequently it is likely that further theoretical insights into path integration will require an in-depth study of the effect of noise on the four classes of home vectors.

Transformation of polarized light information in the central complex of the locust

Many insects perceive the E-vector orientation of polarized skylight and use it for compass navigation. In locusts, polarized light is detected by photoreceptors of the dorsal rim area of the eye. Polarized light signals from both eyes are integrated in the central complex (CC), a group of neuropils in the center of the brain. Thirteen types of CC neuron are sensitive to dorsally presented, polarized light (POL-neurons). These neurons interconnect the subdivisions of the CC, particularly the protocerebral bridge (PB), the upper and lower divisions of the central body (CBU, CBL), and the adjacent lateral accessory lobes (LALs). All POL-neurons show polarization-opponency, i.e., receive excitatory and inhibitory input at orthogonal E-vector orientations. To provide physiological evidence for the direction of information flow through the polarization vision network in the CC, we analyzed the functional properties of the different cell types through intracellular recordings. Tangential neurons of the CBL showed highest signal-to-noise ratio, received either ipsilateral polarized-light input only or, together with CL1 columnar neurons, had eccentric receptive fields. Bilateral polarized-light inputs with zenith-centered receptive fields were found in tangential neurons of the PB and in columnar neurons projecting to the LALs. Together with other physiological parameters, these data suggest a flow of information from the CBL (input) to the PB and from here to the LALs (output). This scheme is supported by anatomical data and suggests transformation of purely sensory E-vector coding at the CC input stage to position-invariant coding of 360 degrees -compass directions at the output stage.

Linking the input to the output: new sets of neurons complement the polarization vision network in the locust central complex

Polarized light is a key feature of the blue sky, used by many animals as a sensory cue for compass navigation. Like other insects, locusts perceive the E-vector orientation of polarized light with a specialized region of their compound eye, the dorsal rim area. Neurons in the brain relay this information through several processing stages to the central complex. The central complex has a modular neuroarchitecture, composed of vertical columns and horizontal layers. Several types of central-complex neurons respond to dorsally presented, rotating E-vectors with tonic modulation of their firing frequency. These neurons were found at the input stage of the central complex, as well as near the proposed output stage, where neurons are tuned to form a compass-like representation of E-vector orientations underlying the columnar organization of the central complex. To identify neurons suited to link input and output elements, we recorded intracellularly from 45 neurons of the central complex. We report several novel types of polarization-sensitive neurons. One of these is suited to fill the gap between input and output stages of the central-complex polarization vision network. Three types of neurons were sensitive to polarized light in only 50% of experiments suggesting that they are recruited to the network depending on behavioral context. Finally, we identified two types of neurons suited to transfer information toward thoracic motor circuits. The data underscore the key role of two subunits of the central complex, the lower division of the central body and the protocerebral bridge, in sky compass orientation.

Primary processes in sensory cells: current advances

In the course of evolution, the strong and unremitting selective pressure on sensory performance has driven the acuity of sensory organs to its physical limits. As a consequence, the study of primary sensory processes illustrates impressively how far a physiological function can be improved if the survival of a species depends on it. Sensory cells that detect single-photons, single molecules, mechanical motions on a nanometer scale, or incredibly small fluctuations of electromagnetic fields have fascinated physiologists for a long time. It is a great challenge to understand the primary sensory processes on a molecular level. This review points out some important recent developments in the search for primary processes in sensory cells that mediate touch perception, hearing, vision, taste, olfaction, as well as the analysis of light polarization and the orientation in the Earth's magnetic field. The data are screened for common transduction strategies and common transduction molecules, an aspect that may be helpful for researchers in the field.

Polarization Vision: How Insects Find Their Way by Watching the Sky

Scientists have long studied how some animals exploit celestial cues to solve navigational tasks. Recent discoveries show how locusts obtain unambiguous information from time-dependent patterns of polarized and unpolarized light in the sky.

The ancestral circadian clock of monarch butterflies: role in time-compensated sun compass orientation

The circadian clock has a vital role in monarch butterfly (Danaus plexippus) migration by providing the timing component of time-compensated sun compass orientation, which contributes to navigation to the overwintering grounds. The location of circadian clock cells in monarch brain has been identified in the dorsolateral protocerebrum (pars lateralis); these cells express PERIOD, TIMELESS, and a Drosophila-like cryptochrome designated CRY1. Monarch butterflies, like all other nondrosophilid insects examined so far, express a second cry gene (designated insect CRY2) that encodes a vertebrate-like CRY that is also expressed in pars lateralis. An ancestral circadian clock mechanism has been defined in monarchs, in which CRY1 functions as a blue light photoreceptor for photic entrainment, whereas CRY2 functionswithin the clockwork as themajor transcriptional repressor of an intracellular negative transcriptional feedback loop. A CRY1-staining neural pathway has been identified that may connect the circadian (navigational) clock to polarized light input important for sun compass navigation, and a CRY2-positive neural pathway has been discovered that may communicate circadian information directly from the circadian clock to the central complex, the likely site of the sun compass. The monarch butterfly may thus use the CRY proteins as components of the circadian mechanism and also as output molecules that connect the clock to various aspects of the sun compass apparatus.