Despite their small brains, insects can navigate over long distances by orienting using visual landmarks [1], skylight polarization [2-9], and sun position [3, 4, 6, 10]. Although Drosophila are not generally renowned for their navigational abilities, mark-and-recapture experiments in Death Valley revealed that they can fly nearly 15 km in a single evening [11]. To accomplish such feats on available energy reserves [12], flies would have to maintain relatively straight headings, relying on celestial cues [13]. Cues such as sun position and polarized light are likely integrated throughout the sensory-motor pathway [14], including the highly conserved central complex [4, 15, 16]. Recently, a group of Drosophila central complex cells (E-PG neurons) have been shown to function as an internal compass [17-19], similar to mammalian head-direction cells [20]. Using an array of genetic tools, we set out to test whether flies can navigate using the sun and to identify the role of E-PG cells in this behavior. Using a flight simulator, we found that Drosophila adopt arbitrary headings with respect to a simulated sun, thus performing menotaxis, and individuals remember their heading preference between successive flights-even over several hours. Imaging experiments performed on flying animals revealed that the E-PG cells track sun stimulus motion. When these neurons are silenced, flies no longer adopt and maintain arbitrary headings relative to the sun stimulus but instead exhibit frontal phototaxis. Thus, without the compass system, flies lose the ability to execute menotaxis and revert to a simpler, reflexive behavior.

Although anatomy is often the first step in assigning functions to neural structures, it is not always clear whether architecturally distinct regions of the brain correspond to operational units. Whereas neuroarchitecture remains relatively static, functional connectivity may change almost instantaneously according to behavioral context. We imaged panneuronal responses to visual stimuli in a highly conserved central brain region in the fruit fly, Drosophila, during flight. In one substructure, the fan-shaped body, automated analysis revealed three layers that were unresponsive in quiescent flies but became responsive to visual stimuli when the animal was flying. The responses of these regions to a broad suite of visual stimuli suggest that they are involved in the regulation of flight heading. To identify the cell types that underlie these responses, we imaged activity in sets of genetically defined neurons with arborizations in the targeted layers. The responses of this collection during flight also segregated into three sets, confirming the existence of three layers, and they collectively accounted for the panneuronal activity. Our results provide an atlas of flight-gated visual responses in a central brain circuit.is a system of unpaired neuropils in the adult insect brain consisting of the protocerebral bridge (PB), the fan-shaped body (FB), and the ellipsoid body (EB) (Movie S1). The architecture of these neuropils is remarkably conserved across species (1, 2) such that systems of columnar and tangential neurons obey intricate wiring rules (3-5). In addition to the three primary structures, all winged insects possess paired noduli (NO) that are intimately connected to the rest of the CX (2). Despite the stereotyped structure of the CX, there is as yet no consensus regarding its function. In the desert locust Schistocerca gregaria, numerous CX neurons respond to the angle of linearly polarized light (6, 7) and other visual features (8). In the cockroach Blaberus discoidalis, some units in the CX respond to visual motion (9) and mechanosensation (10), whereas others fire during motor actions such as running and turning (11,12). Studies in fruit flies suggest that the CX is important for locomotion (13), certain types of climbing (14), visual learning and memory (15-17), and various other tasks involved in visual processing (18-21).To gain a more coherent understanding of CX function, we sought an unbiased approach to investigate its role systematically in different behavioral contexts and in a variety of stimulus conditions. We designed a broad panel of visual stimuli that are known to be important for guidance and stability during flight. Recent advances in the sensitivity of genetically encoded calcium indicators made it possible to record panneuronal responses to these stimuli during tethered flight using multiphoton imaging. Although all three primary neuropils of the CX contained neurons that responded to the stimuli, only in the FB did we observe responses during flight that were absent when the animals...

Summary Insects maintain a constant bearing across a wide range of spatial scales. Monarch butterflies and locusts traverse continents [1, 2], foraging bees and ants travel hundreds of meters to return to their nest [1, 3, 4], whereas many other insects fly straight for only a few centimeters before changing direction. Despite this variation in spatial scale, the brain region thought to underlie long-distance navigation is remarkably conserved [5, 6], suggesting that the use of celestial cues for navigation is a general and perhaps ancient behavioral capability of insects. Laboratory studies of Drosophila have identified a local search mode in which short straight segments are interspersed with rapid turns [7, 8]. Such flight modes, however, are inconsistent with measures of gene flow between geographically-separated populations [9-11], and individual Drosophila have been observed to travel 10 km across desert terrain in a single night [9, 12, 13] – a feat that would be impossible without prolonged periods of straight flight. To directly examine orientation behavior under outdoor conditions, we built a portable flight arena in which a fly viewed the natural sky through a liquid crystal device that could experimentally rotate the angle of polarization. Our findings indicate that flying Drosophila actively orient using the sky's natural polarization pattern.

Many insects exploit skylight polarization as a compass cue for orientation and navigation. In the fruit fly, Drosophila melanogaster, photoreceptors R7 and R8 in the dorsal rim area (DRA) of the compound eye are specialized to detect the electric vector (e-vector) of linearly polarized light. These photoreceptors are arranged in stacked pairs with identical fields of view and spectral sensitivities, but mutually orthogonal microvillar orientations. As in larger flies, we found that the microvillar orientation of the distal photoreceptor R7 changes in a fan-like fashion along the DRA. This anatomical arrangement suggests that the DRA constitutes a detector for skylight polarization, in which different e-vectors maximally excite different positions in the array. To test our hypothesis, we measured responses to polarized light of varying e-vector angles in the terminals of R7/8 cells using genetically encoded calcium indicators. Our data confirm a progression of preferred e-vector angles from anterior to posterior in the DRA, and a strict orthogonality between the e-vector preferences of paired R7/8 cells. We observed decreased activity in photoreceptors in response to flashes of light polarized orthogonally to their preferred e-vector angle, suggesting reciprocal inhibition between photoreceptors in the same medullar column, which may serve to increase polarization contrast. Together, our results indicate that the polarization-vision system relies on a spatial map of preferred e-vector angles at the earliest stage of sensory processing.

Sensory feedback is a ubiquitous feature of guidance systems in both animals and engineered vehicles. For example, a common strategy for moving along a straight path is to turn such that the measured rate of rotation is zero. This task can be accomplished by using a feedback signal that is proportional to the instantaneous value of the measured sensory signal. In such a system, the addition of an integral term depending on past values of the sensory input is needed to eliminate steady-state error [proportional-integral (PI) control]. However, the means by which nervous systems implement such a computation are poorly understood. Here, we show that the optomotor responses of flying Drosophila follow a time course consistent with temporal integration of horizontal motion input. To investigate the cellular basis of this effect, we performed whole-cell patch-clamp recordings from the set of identified visual interneurons [horizontal system (HS) cells] thought to control this reflex during tethered flight. At high stimulus speeds, HS cells exhibit steady-state responses during flight that are absent during quiescence, a state-dependent difference in physiology that is explained by changes in their presynaptic inputs. However, even during flight, the membrane potential of the large-field interneurons exhibits no evidence for integration that could explain the behavioral responses. However, using a genetically encoded indicator, we found that calcium accumulates in the terminals of the interneurons along a time course consistent with the behavior and propose that this accumulation provides a mechanism for temporal integration of sensory feedback consistent with PI control.feedback control | insect vision | lobula plate tangential cells | fruit fly A common strategy for maintaining a straight course is to steer such that the apparent rotation of the visual scene on the retina is continuously minimized. This stabilization can be achieved with a simple control system in which the rate of turning is proportional to the instantaneous value of the measured error signal. In the case of a yaw control system, the relevant error signal is the rotational optic flow on the retina. However, control systems that rely solely on a proportional response cannot achieve zero steady-state error (1). In the case of a yaw regulator, this small uncorrected error would result in the animal rotating in one direction. This steady-state bias could be corrected either by orienting to an absolute positional cue (landmark) or by incorporating a feedback component commensurate to the temporal integral of the error signal. In engineering applications, integral feedback is almost always included in the control signal to the actuator system, creating a proportional-integral (PI) controller (1). The integral term speeds convergence to zero error and avoids small steady-state offsets that plague systems relying on proportional feedback alone. Here, we provide evidence for temporal integration of visual feedback during flight in the fruit fly, Drosophila, ...

Animals must use external cues to maintain a straight course over long distances. In this study, we investigated how the fruit fly selects and maintains a flight heading relative to the axis of linearly polarized light, a visual cue produced by the atmospheric scattering of sunlight. To track flies' headings over extended periods, we used a flight simulator that coupled the angular velocity of dorsally presented polarized light to the stroke amplitude difference of the animals' wings. In the simulator, most flies actively maintained a stable heading relative to the axis of polarized light for the duration of 15 min flights. We found that individuals selected arbitrary, unpredictable headings relative to the polarization axis, which demonstrates that can perform proportional navigation using a polarized light pattern. When flies flew in two consecutive bouts separated by a 5 min gap, the two flight headings were correlated, suggesting individuals retain a memory of their chosen heading. We found that adding a polarized light pattern to a light intensity gradient enhanced flies' orientation ability, suggesting use a combination of cues to navigate. For both polarized light and intensity cues, flies' capacity to maintain a stable heading gradually increased over several minutes from the onset of flight. Our findings are consistent with a model in which each individual initially orients haphazardly but then settles on a heading which is maintained via a self-reinforcing process. This may be a general dispersal strategy for animals with no target destination.

1. The discharge of neurons in the deeper layers of the monkey superior colliculus was recorded during saccades made to stationary and to smoothly moving visual targets. 2. All neurons that discharged for saccades made to stationary targets also discharged during saccades made to moving targets, but there was a systematic shift in the saccade vector yielding maximal activity (i.e. center of the movement field) of collicular neurons for the latter class of movements. The shift moved the center of the movement fields toward larger-amplitude pursuit saccades for target motion away from the fovea, in comparison with saccades made to stationary targets. However, the discharge at the center of the movement field for pursuit saccades was 14% lower when averaged over the sample of recorded cells. 3. The saccades made during pursuit tracking of moving visual stimuli have different dynamics than saccades made to stationary targets. At similar amplitudes pursuit saccades are slower, and their velocity profiles often show secondary velocity peaks or inflection points and have longer-duration decelerating phases. 4. The combined experimental observations of a change in saccade dynamics and the shift in movement fields in collicular neurons for pursuit saccades are compatible with the hypothesis that saccades made to moving targets are controlled by neural processing in two partially separate pathways. In this theory, one path is concerned with correction of a presaccadic retinal position error (a path that includes the colliculus) and another path is concerned with position extrapolations based on the velocity of the moving target (a path that does not include the colliculus).

To follow a straight course, animals must maintain a constant heading relative to a fixed, distant landmark, a strategy termed menotaxis. In experiments using a flight simulator, we found that Drosophila adopt arbitrary headings with respect to a simulated sun, and individuals remember their heading preference between successive flights—even over gaps lasting several hours. Imaging experiments revealed that a class of neurons within the central complex, which have been previously shown to act as an internal compass, track the azimuthal motion of a sun stimulus. When these neurons are silenced, flies no longer adopt and maintain arbitrary headings, but instead exhibit frontal phototaxis. Thus, without the compass system, flies lose the ability to execute menotaxis and revert to a simpler, reflexive behavior.One sentence summarySilencing the compass neurons in the central complex of Drosophila eliminates sun navigation but leaves phototaxis intact.