Anisotropic charge trapping in phototransistors unlocks ultrasensitive polarimetry

Being able to probe the polarization states of light is crucial for applications from medical diagnostics and bio-inspired navigation to information encryption and quantum computing. Current state-of-the-art polarimeters based on anisotropic semiconductors enable direct linear dichroism photodetection without the need for bulky and complex external optics. However, their polarization sensitivity is restricted by the inherent optical anisotropy, leading to low dichroic ratios of typically smaller than ten. Here, we unveil an effective and general design rule to achieve a more than 2,000-fold enhanced polarization sensitivity by exploiting a light-induced anisotropic gating effect in organic phototransistors. The polarization-dependent trapping of photogenerated charge carriers provides an anisotropic photo-induced gate for current amplification, which has resulted in an extremely high dichroic ratio of over 1.2×104, more than two orders of magnitude higher than any previous reports. These findings further enable the first demonstration of a novel miniaturized bionic celestial compass for skylight-based polarization navigation. Our results offer a fundamental design principle and a new route for the development of next-generation highly polarization-sensitive optoelectronics.

Role of the pheromone for orientation in the group foraging ant, Veromessor pergandei

Role of the pheromone for orientation in the group foraging ant, Veromessor pergandei Navigation is comprised of a variety of strategies which rely on multiple external cues to shape a navigator’s behavioral output. An additional navigational challenge is coping with forces such as wind and water currents that push navigators off-course. Here, we explore the cue interactions that dictate orientation and foragers’ ability to counter course altering rotational changes in the desert ant, Veromessor pergandei. We found a cross sensory interaction between the pheromone cue and the path integrator underlies correct orientation during the inbound journey. The celestial compass provides directional information while the presence of the trail pheromone acts as a critical context cue, triggering distinct behavioral responses (vector orientation, search and backtracking). A particularly interesting interaction occurs between the pheromone and the forager’s vector state. While exposed to the pheromone, foragers orient to the vector direction regardless of vector state, while in the pheromone’s absence the current vector triggers the switch between behaviors. Such interactions maximize the foragers’ return to the nest and inhibit movement off the trail. Finally, our manipulations continuously pushed foragers away from their desired heading, yet foragers were highly proficient at counteracting these changes, steering to maintain a correct heading even at rotational speeds of ~40°/s.

A lateralised design for the interaction of visual memories and heading representations in navigating ants

The navigational skills of ants, bees and wasps represent one of the most baffling examples of the powers of minuscule brains. Insects store long-term memories of the visual scenes they experience 1, and they use compass cues to build a robust representation of directions 2,3. We know reasonably well how long-term memories are formed, in a brain area called the Mushroom Bodies (MB) 4–8, as well as how heading representations are formed in another brain area called the Central Complex (CX) 9–12. However, how such memories and heading representations interact to produce powerful navigational behaviours remains unclear 7,13,14. Here we combine behavioural experiments with computational modelling that is strictly based on connectomic data to provide a new perspective on how navigation might be orchestrated in these insects. Our results reveal a lateralised design, where signals about whether to turn left or right are segregated in the left and right hemispheres, respectively. Furthermore, we show that guidance is a two-stage process: the recognition of visual memories – presumably in the MBs – does not directly drive the motor command, but instead updates a “desired heading” – presumably in the CX – which in turn is used to control guidance using celestial compass information. Overall, this circuit enables ants to recognise views independently of their body orientation, and combines terrestrial and celestial cues in a way that produces exceptionally robust navigation.

Regionalization in the compound eye of Talitrus saltator (Crustacea, Amphipoda)

The crustacean Talitrus saltator is known to use many celestial cues during its orientation along the sea-land axis of sandy shores. In this paper, we investigated the existence of the eye regionalization by morphological, electrophysiological and behavioural experiments. Each ommatidium possesses five radially arranged retinular cells producing a square fused rhabdom by R1-R4 cells; the smaller R5 exist between R1 and R4. The size of R5 rhabdomere is largest in dorsal part and becomes gradually smaller in median and ventral part of the eye. Spectral-sensitivity measurements were recorded from either dorsal or ventral parts of the compound eye to clarify the chromatic difference. Results show that the dorsal part is green and UV-blue dichromatic, whereas the ventral part is UV (390 nm) with a substantial population of 450 nm receptors with the responses in the longer wavelength region. To evaluate the orienting behaviour of individuals, their eyes were black painted either in the dorsal or ventral part, under natural sky or a blue filter with or without the vision of the sun. Results show that animals painted on the dorsal part of their eyes tested under the screened sun were more dispersed and in certain cases their directions deflected than other groups of individuals. Furthermore, sandhoppers subjected to the obscuring of this area met in any case high difficulties in their directional choices. Therefore, our present work indicates the existence of a regionalization of the compound eye of T. saltator.Summary statementThis work provides evidences of the morphological and electrophysiological regionalization of the compound eye and the visual capabilities for behaviour involved in the recognition of the celestial compass orienting factors in crustaceans.

From skylight input to behavioural output: a computational model of the insect polarised light compass

Many insects navigate by integrating the distances and directions travelled on an outward path, allowing direct return to the starting point. Fundamental to the reliability of this process is the use of a neural compass based on external celestial cues. Here we examine how such compass information could be reliably computed by the insect brain, given realistic constraints on the sky polarisation pattern and the insect eye sensor array. By processing the degree of polarisation in different directions for different parts of the sky, our model can directly estimate the solar azimuth and also infer the confidence of the estimate. We introduce a method to correct for tilting of the sensor array, as might be caused by travel over uneven terrain. We also show that the confidence can be used to approximate the change in sun position over time, allowing the compass to remain fixed with respect to ‘true north’ during long excursions. We demonstrate that the compass is robust to disturbances and can be effectively used as input to an existing neural model of insect path integration. We discuss the plausibility of our model to be mapped to known neural circuits, and to be implemented for robot navigation.Author summaryWe propose a new hypothesis for how insects process polarised skylight to extract global orientation information that can be used for accurate path integration. Our model solves the problem of solar/anti-solar meridian ambiguity by using a biologically constrained sensor array, and includes methods to deal with tilt and time, providing a complete insect celestial compass output. We analyse the performance of the model using a realistic sky simulation and various forms of disturbances, and compare the results to both engineering approaches and biological data.