Contrast sensitivity, visual acuity and the effect of behavioural state on optokinetic gain in fiddler crabs.

Most animals rely on visual information for a variety of everyday tasks. The information available to a visual system depends in part on its spatial resolving power and contrast sensitivity. Because of their competing demands for physical space within an eye, these traits cannot simultaneously be improved without increasing overall eye size. The contrast sensitivity function is an integrated measure of visual performance that measures both resolution and contrast sensitivity. Its measurement helps us identify how different species have made a trade-off between contrast sensitivity and spatial resolution. It further allows us to identify the evolutionary drivers of sensory processing and visually mediated behaviour. Here, we measured the contrast sensitivity function of the fiddler crab Gelasimus dampieri using its optokinetic responses to wide-field moving sinusoidal intensity gratings of different orientations, spatial frequencies, contrasts and speeds. We further tested whether the behavioural state of the crabs (i.e. whether crabs are actively walking or not) affects their optokinetic gain and contrast sensitivity. Our results from a group of five crabs suggest a minimum perceived contrast of 6% and a horizontal and vertical visual acuity of 0.4 cyc deg-1 and 0.28 cyc deg-1, respectively, in the crabs' region of maximum optomotor sensitivity. Optokinetic gain increased in moving crabs compared with restrained crabs, adding another example of the importance of naturalistic approaches when studying the performance of animals.

Descending neurons of the hoverfly respond to pursuits of artificial targets.

Many animals use motion vision information to control dynamic behaviors. Predatory animals, for example, show an exquisite ability to detect rapidly moving prey, followed by pursuit and capture. Such target detection is not only used by predators but is also important in conspecific interactions, such as for male hoverflies defending their territories against conspecific intruders. Visual target detection is believed to be subserved by specialized target-tuned neurons found in a range of species, including vertebrates and arthropods. However, how these target-tuned neurons respond to actual pursuit trajectories is currently not well understood. To redress this, we recorded extracellularly from target-selective descending neurons (TSDNs) in male Eristalis tenax hoverflies. We show that they have dorso-frontal receptive fields with a preferred direction up and away from the visual midline. We reconstructed visual flow fields as experienced during pursuits of artificial targets (black beads). We recorded TSDN responses to six reconstructed pursuits and found that each neuron responded consistently at remarkably specific time points but that these time points differed between neurons. We found that the observed spike probability was correlated with the spike probability predicted from each neuron's receptive field and size tuning. Interestingly, however, the overall response rate was low, with individual neurons responding to only a small part of each reconstructed pursuit. In contrast, the TSDN population responded to substantially larger proportions of the pursuits but with lower probability. This large variation between neurons could be useful if different neurons control different parts of the behavioral output.

Dual Receptive Fields Underlying Target and Wide-Field Motion Sensitivity in Looming-Sensitive Descending Neurons.

Responding rapidly to visual stimuli is fundamental for many animals. For example, predatory birds and insects alike have amazing target detection abilities, with incredibly short neural and behavioral delays, enabling efficient prey capture. Similarly, looming objects need to be rapidly avoided to ensure immediate survival, as these could represent approaching predators. Male Eristalis tenax hoverflies are nonpredatory, highly territorial insects that perform high-speed pursuits of conspecifics and other territorial intruders. During the initial stages of the pursuit, the retinal projection of the target is very small, but this grows to a larger object before physical interaction. Supporting such behaviors, E. tenax and other insects have both target-tuned and loom-sensitive neurons in the optic lobes and the descending pathways. We here show that these visual stimuli are not necessarily encoded in parallel. Indeed, we describe a class of descending neurons that respond to small targets, to looming and to wide-field stimuli. We show that these descending neurons have two distinct receptive fields where the dorsal receptive field is sensitive to the motion of small targets and the ventral receptive field responds to larger objects or wide-field stimuli. Our data suggest that the two receptive fields have different presynaptic input, where the inputs are not linearly summed. This novel and unique arrangement could support different behaviors, including obstacle avoidance, flower landing, and target pursuit or capture.

Physiological properties of the visual system in the Green Weaver ant, Oecophylla smaragdina.

The Green Weaver ants, Oecophylla smaragdina are iconic animals known for their extreme cooperative behaviour where they bridge gaps by linking to each other to build living chains. They are visually oriented animals, build chains towards closer targets, use celestial compass cues for navigation and are visual predators. Here, we describe their visual sensory capacity. The major workers of O. smaragdina have more ommatidia (804) in each eye compared to minor workers (508), but the facet diameters are comparable between both castes. We measured the impulse responses of the compound eye and found their response duration (42 ms) was similar to that seen in other slow-moving ants. We determined the flicker fusion frequency of the compound eye at the brightest light intensity to be 132 Hz, which is relatively fast for a walking insect suggesting the visual system is well suited for a diurnal lifestyle. Using pattern-electroretinography we identified the compound eye has a spatial resolving power of 0.5 cycles deg-1 and reached peak contrast sensitivity of 2.9 (35% Michelson contrast threshold) at 0.05 cycles deg-1. We discuss the relationship of spatial resolution and contrast sensitivity, with number of ommatidia and size of the lens.

Hoverfly (Eristalis tenax) pursuit of artificial targets.

The ability to visualize small moving objects is vital for the survival of many animals, as these could represent predators or prey. For example, predatory insects, including dragonflies, robber flies and killer flies, perform elegant, high-speed pursuits of both biological and artificial targets. Many non-predatory insects, including male hoverflies and blowflies, also pursue targets during territorial or courtship interactions. To date, most hoverfly pursuits have been studied outdoors. To investigate hoverfly (Eristalis tenax) pursuits under more controlled settings, we constructed an indoor arena that was large enough to encourage naturalistic behavior. We presented artificial beads of different sizes, moving at different speeds, and filmed pursuits with two cameras, allowing subsequent 3D reconstruction of the hoverfly and bead position as a function of time. We show that male E. tenax hoverflies are unlikely to use strict heuristic rules based on angular size or speed to determine when to start pursuit, at least in our indoor setting. We found that hoverflies pursued faster beads when the trajectory involved flying downwards towards the bead. Furthermore, we show that target pursuit behavior can be broken down into two stages. In the first stage, the hoverfly attempts to rapidly decreases the distance to the target by intercepting it at high speed. During the second stage, the hoverfly's forward speed is correlated with the speed of the bead, so that the hoverfly remains close, but without catching it. This may be similar to dragonfly shadowing behavior, previously coined 'motion camouflage'.

Nocturnal Myrmecia ants have faster temporal resolution at low light levels but lower adaptability compared to diurnal relatives.

Nocturnal insects likely have evolved distinct physiological adaptations to enhance sensitivity for tasks, such as catching moving prey, where the signal-noise ratio of visual information is typically low. Using electroretinogram recordings, we measured the impulse response and the flicker fusion frequency (FFF) in six congeneric species of Myrmecia ants with different diurnal rhythms. The FFF, which measures the ability of an eye to respond to a flickering light, is significantly lower in nocturnal ants (∼125 Hz) compared to diurnal ants (∼189 Hz). However, the nocturnal ants have faster eyes at very low light intensities than the diurnal species. During the day, nocturnal ants had slower impulse responses than their diurnal counterparts. However, at night, both latency and duration significantly shortened in nocturnal species. The characteristics of the impulse responses varied substantially across all six species and did not correlate well with the measured flicker fusion frequency.

Ocellar spatial vision in Myrmecia ants.

In addition to compound eyes, insects possess simple eyes known as ocelli. Input from the ocelli modulates optomotor responses, flight-time initiation, and phototactic responses - behaviours that are mediated predominantly by the compound eyes. In this study, using pattern electroretinography (pERG), we investigated the contribution of the compound eyes to ocellar spatial vision in the diurnal Australian bull ant Myrmecia tarsata by measuring the contrast sensitivity and spatial resolving power of the ocellar second-order neurons under various occlusion conditions. Furthermore, in four species of Myrmecia ants active at different times of the day, and in European honeybee Apis mellifera, we characterized the ocellar visual properties when both visual systems were available. Among the ants, we found that the time of activity had no significant effect on ocellar spatial vision. Comparing day-active ants and the honeybee, we did not find any significant effect of locomotion on ocellar spatial vision. In M. tarsata, when the compound eyes were occluded, the amplitude of the pERG signal from the ocelli was reduced 3 times compared with conditions when the compound eyes were available. The signal from the compound eyes maintained the maximum contrast sensitivity of the ocelli as 13 (7.7%), and the spatial resolving power as 0.29 cycles deg-1. We conclude that ocellar spatial vison improves significantly with input from the compound eyes, with a noticeably larger improvement in contrast sensitivity than in spatial resolving power.

Chiroptical activity of Au13 clusters: experimental and theoretical understanding of the origin of helical charge movements.

Ligand-protected gold clusters with an asymmetric nature have emerged as a novel class of chiral compounds, but the origins of their chiroptical activities associated with helical charge movements in electronic transitions remain unexplored. Herein, we perform experimental and theoretical studies on the structures and chiroptical properties of Au13 clusters protected by mono- and di-phosphine ligands. Based on the experimental reevaluation of diphosphine-ligated Au13 clusters, we show that these surface ligands slightly twist the Au13 cores from a true icosahedron to generate intrinsic chirality in the gold frameworks. Theoretical investigation of a monophosphine-ligated cluster model reproduced the experimentally observed circular dichroism (CD) spectrum, indicating that such a torsional twist of the Au13 core, rather than the surrounding chiral environment by helically arranged diphosphine ligands, contributes to the appearance of the chiroptical response. We also show that the calculated CD signals are dependent on the degree of asymmetry (torsion angle between the two equatorial Au5 pentagons), and provide a visual understanding of the origin of helical charge movements with transition-moment and transition-density analyses. This work provides novel insights into the chiroptical activities of ligand-protected metal clusters with intrinsically chiral cores.

Photoreceptors and diurnal variation in spectral sensitivity in the fiddler crab Gelasimus dampieri.

Colour signals, and the ability to detect them, are important for many animals and can be vital to their survival and fitness. Fiddler crabs use colour information to detect and recognise conspecifics, but their colour vision capabilities remain unclear. Many studies have attempted to measure their spectral sensitivity and identify contributing retinular cells, but the existing evidence is inconclusive. We used electroretinogram (ERG) measurements and intracellular recordings from retinular cells to estimate the spectral sensitivity of Gelasimus dampieri and to track diurnal changes in spectral sensitivity. G. dampieri has a broad spectral sensitivity and is most sensitive to wavelengths between 420 and 460 nm. Selective adaptation experiments uncovered an ultraviolet (UV) retinular cell with a peak sensitivity shorter than 360 nm. The species' spectral sensitivity above 400 nm is too broad to be fitted by a single visual pigment and using optical modelling, we provide evidence that at least two medium-wavelength sensitive (MWS) visual pigments are contained within a second blue-green sensitive retinular cell. We also found a ∼25 nm diurnal shift in spectral sensitivity towards longer wavelengths in the evening in both ERG and intracellular recordings. Whether the shift is caused by screening pigment migration or changes in opsin expression remains unclear, but the observation shows the diel dynamism of colour vision in this species. Together, these findings support the notion that G. dampieri possesses the minimum requirement for colour vision, with UV and blue/green receptors, and help to explain some of the inconsistent results of previous research.

Behavior of Japanese women after being informed about the benefits and disadvantages of breast cancer screening: a questionnaire survey.

BACKGROUND: The US Preventative Services Task Force assessed the efficacy of breast cancer screening according to the sum of its benefits and disadvantages. We estimate that the balance of the benefits and disadvantages varies among women depending on their demographic background. METHODS: Between March 2016 and March 2017, we conducted a questionnaire survey among Japanese women who underwent population-based or opportunistic breast cancer screening at our multicenter institutions. We investigated the behavior modification among women after being informed about the benefits and disadvantages of breast cancer screening depending on their demographic background. RESULTS: Out of 3032 questionnaires that were returned, 2936 (96.8%) were evaluated. The percentage of women with prior knowledge about the benefits and disadvantages of breast cancer screening before reading the leaflets that we created was 24%. However, 95% of the women were willing to undergo screening next time, despite knowing the disadvantages. Regarding overdiagnosis, the young women tended to choose usual treatment, and the elderly women tended to choose active surveillance. In response to the question on the significance of screening, the young women wished to avoid death by breast cancer; whereas, the elderly women wished to live a safe life. CONCLUSION: Our results indicate that the information of disadvantages does not lead to a reduction in screening rates. Additionally, we found that the balance between the benefits and disadvantages of breast cancer screening varies among women depending on their demographic background, especially age.

The buzz around spatial resolving power and contrast sensitivity in the honeybee, Apis mellifera.

Most animals rely on vision to perform a range of behavioural tasks and variations in the anatomy and physiology of the eye likely reflect differences in habitat and life history. Moreover, eye design represents a balance between often conflicting requirements for gathering different forms of visual information. The trade-off between spatial resolving power and contrast sensitivity is common to all visual systems, and European honeybees (Apis mellifera) present an important opportunity to better understand this trade-off. Vision has been studied extensively in A. mellifera as it is vital for foraging, navigation and communication. Consequently, spatial resolving power and contrast sensitivity in A. mellifera have been measured using several methodologies; however, there is considerable variation in estimates between methodologies. We assess pattern electroretinography (pERG) as a new method for assessing the trade-off between visual spatial and contrast information in A.mellifera. pERG has the benefit of measuring spatial contrast sensitivity from higher order visual processing neurons in the eye. Spatial resolving power of A.mellifera estimated from pERG was 0.54 cycles per degree (cpd), and contrast sensitivity was 16.9. pERG estimates of contrast sensitivity were comparable to previous behavioural studies. Estimates of spatial resolving power reflected anatomical estimates in the frontal region of the eye, which corresponds to the region stimulated by pERG. Apis mellifera has similar spatial contrast sensitivity to other hymenopteran insects with similar facet diameter (Myrmecia ant species). Our results support the idea that eye anatomy has a substantial effect on spatial contrast sensitivity in compound eyes.

Insect Vision: Novel Mechanism for Contrast Constancy in Dim Light.

A general problem of sensory systems is how to simultaneously encode prevailing input as well as deviations from this baseline. A new study shows how the fly visual system has solved this by using parallel processing.

A new method for mapping spatial resolution in compound eyes suggests two visual streaks in fiddler crabs.

Visual systems play a vital role in guiding the behaviour of animals. Understanding the visual information animals are able to acquire is therefore key to understanding their visually mediated decision making. Compound eyes, the dominant eye type in arthropods, are inherently low-resolution structures. Their ability to resolve spatial detail depends on sampling resolution (interommatidial angle) and the quality of ommatidial optics. Current techniques for estimating interommatidial angles are difficult, and generally require in vivo measurements. Here, we present a new method for estimating interommatidial angles based on the detailed analysis of 3D micro-computed tomography images of fixed samples. Using custom-made MATLAB software, we determined the optical axes of individual ommatidia and projected these axes into the 3D space around the animal. The combined viewing directions of all ommatidia, estimated from geometrical optics, allowed us to estimate interommatidial angles and map the animal's sampling resolution across its entire visual field. The resulting topographic representations of visual acuity match very closely the previously published data obtained from both fiddler and grapsid crabs. However, the new method provides additional detail that was not previously detectable and reveals that fiddler crabs, rather than having a single horizontal visual streak as is common in flat-world inhabitants, probably have two parallel streaks located just above and below the visual horizon. A key advantage of our approach is that it can be used on appropriately preserved specimens, allowing the technique to be applied to animals such as deep-sea crustaceans that are inaccessible or unsuitable for in vivo approaches.

Seminal fluid compromises visual perception in honeybee queens reducing their survival during additional mating flights.

Queens of social insects make all mate-choice decisions on a single day, except in honeybees whose queens can conduct mating flights for several days even when already inseminated by a number of drones. Honeybees therefore appear to have a unique, evolutionarily derived form of sexual conflict: a queen's decision to pursue risky additional mating flights is driven by later-life fitness gains from genetically more diverse worker-offspring but reduces paternity shares of the drones she already mated with. We used artificial insemination, RNA-sequencing and electroretinography to show that seminal fluid induces a decline in queen vision by perturbing the phototransduction pathway within 24-48 hr. Follow up field trials revealed that queens receiving seminal fluid flew two days earlier than sister queens inseminated with saline, and failed more often to return. These findings are consistent with seminal fluid components manipulating queen eyesight to reduce queen promiscuity across mating flights.

Ocellar structure of African and Australian desert ants.

Few walking insects possess simple eyes known as the ocelli. The role of the ocelli in walking insects such as ants has been less explored. Physiological and behavioural evidence in the desert ant, Cataglyphis bicolor, indicates that ocellar receptors are polarisation sensitive and are used to derive compass information from the pattern of polarised skylight. The ability to detect polarised skylight can also be inferred from the structure and the organisation of the ocellar retina. However, the functional anatomy of the desert ant ocelli has not been investigated. Here we characterised the anatomical organisation of the ocelli in three species of desert ants. The two congeneric species of Cataglyphis we studied had a fused rhabdom, but differed in their organisation of the retina. In Cataglyphis bicolor, each retinula cell contributed microvilli in one orientation enabling them to compare e-vector intensities. In Cataglyphis fortis, some retinula cells contributed microvilli in more than one orientation, indicating that not all cells are polarisation sensitive. The desert ant Melophorus bagoti had an unusual ocellar retina with a hexagonal or pentagonal rhabdomere arrangement forming an open rhabdom. Each retinula cell contributed microvilli in more than one orientation, making them unlikely to be polarisation detectors.

Miniaturisation reduces contrast sensitivity and spatial resolving power in ants.

Vision is crucial for animals to find prey, locate conspecifics and navigate within cluttered landscapes. Animals need to discriminate objects against a visually noisy background. However, the ability to detect spatial information is limited by eye size. In insects, as individuals become smaller, the space available for the eyes reduces, which affects the number of ommatidia, the size of the lens and the downstream information-processing capabilities. The evolution of small body size in a lineage, known as miniaturisation, is common in insects. Here, using pattern electroretinography with vertical sinusoidal gratings as stimuli, we studied how miniaturisation affects spatial resolving power and contrast sensitivity in four diurnal ants that live in a similar environment but vary in their body and eye size. We found that ants with fewer and smaller ommatidial facets had lower spatial resolving power and contrast sensitivity. The spatial resolving power was maximum in the largest ant Myrmecia tarsata at 0.60 cycles deg-1 compared with that of the ant with smallest eyes Rhytidoponera inornata at 0.48 cycles deg-1 Maximum contrast sensitivity (minimum contrast threshold) in M. tarsata (2627 facets) was 15.51 (6.4% contrast detection threshold) at 0.1 cycles deg-1, while the smallest ant R. inornata (227 facets) had a maximum contrast sensitivity of 1.34 (74.1% contrast detection threshold) at 0.05 cycles deg-1 Miniaturisation thus dramatically decreases maximum contrast sensitivity and also reduces spatial resolution, which could have implications for visually guided behaviours. This is the first study to physiologically investigate contrast sensitivity in the context of insect allometry.

Moving in Dim Light: Behavioral and Visual Adaptations in Nocturnal Ants.

Visual navigation is a benchmark information processing task that can be used to identify the consequence of being active in dim-light environments. Visual navigational information that animals use during the day includes celestial cues such as the sun or the pattern of polarized skylight and terrestrial cues such as the entire panorama, canopy pattern, or significant salient features in the landscape. At night, some of these navigational cues are either unavailable or are significantly dimmer or less conspicuous than during the day. Even under these circumstances, animals navigate between locations of importance. Ants are a tractable system for studying navigation during day and night because the fine scale movement of individual animals can be recorded in high spatial and temporal detail. Ant species range from being strictly diurnal, crepuscular, and nocturnal. In addition, a number of species have the ability to change from a day- to a night-active lifestyle owing to environmental demands. Ants also offer an opportunity to identify the evolution of sensory structures for discrete temporal niches not only between species but also within a single species. Their unique caste system with an exclusive pedestrian mode of locomotion in workers and an exclusive life on the wing in males allows us to disentangle sensory adaptations that cater for different lifestyles. In this article, we review the visual navigational abilities of nocturnal ants and identify the optical and physiological adaptations they have evolved for being efficient visual navigators in dim-light.

Regional differences in the preferred e-vector orientation of honeybee ocellar photoreceptors.

In addition to compound eyes, honeybees (Apis mellifera) possess three single-lens eyes called ocelli located on the top of the head. Ocelli are involved in head-attitude control and in some insects have been shown to provide celestial compass information. Anatomical and early electrophysiological studies have suggested that UV and blue-green photoreceptors in ocelli are polarization sensitive. However, their retinal distribution and receptor characteristics have not been documented. Here, we used intracellular electrophysiology to determine the relationship between the spectral and polarization sensitivity of the photoreceptors and their position within the visual field of the ocelli. We first determined a photoreceptor's spectral response through a series of monochromatic flashes (340-600 nm). We found UV and green receptors, with peak sensitivities at 360 and 500 nm, respectively. We subsequently measured polarization sensitivity at the photoreceptor's peak sensitivity wavelength by rotating a polarizer with monochromatic flashes. Polarization sensitivity (PS) values were significantly higher in UV receptors (3.8±1.5, N=61) than in green receptors (2.1±0.6, N=60). Interestingly, most receptors with receptive fields below 35 deg elevation were sensitive to vertically polarized light while the receptors with visual fields above 35 deg were sensitive to a wide range of polarization angles. These results agree well with anatomical measurements showing differences in rhabdom orientations between dorsal and ventral retinae. We discuss the functional significance of the distribution of polarization sensitivities across the visual field of ocelli by highlighting the information the ocelli are able to extract from the bee's visual environment.

Three spectrally distinct photoreceptors in diurnal and nocturnal Australian ants.

Ants are thought to be special among Hymenopterans in having only dichromatic colour vision based on two spectrally distinct photoreceptors. Many ants are highly visual animals, however, and use vision extensively for navigation. We show here that two congeneric day- and night-active Australian ants have three spectrally distinct photoreceptor types, potentially supporting trichromatic colour vision. Electroretinogram recordings show the presence of three spectral sensitivities with peaks (λmax) at 370, 450 and 550 nm in the night-active Myrmecia vindex and peaks at 370, 470 and 510 nm in the day-active Myrmecia croslandi. Intracellular electrophysiology on individual photoreceptors confirmed that the night-active M. vindex has three spectral sensitivities with peaks (λmax) at 370, 430 and 550 nm. A large number of the intracellular recordings in the night-active M. vindex show unusually broad-band spectral sensitivities, suggesting that photoreceptors may be coupled. Spectral measurements at different temporal frequencies revealed that the ultraviolet receptors are comparatively slow. We discuss the adaptive significance and the probability of trichromacy in Myrmecia ants in the context of dim light vision and visual navigation.

Sex-specific retinal pigmentation results in sexually dimorphic long-wavelength-sensitive photoreceptors in the eastern pale clouded yellow butterfly, Colias erate.

The compound eyes of the eastern pale clouded yellow butterfly, Colias erate, contain three types of ommatidia (I, II and III), identifiable by the differing arrangements of pigment clusters around the rhabdoms. The pigment color is red in all ommatidial types except for type II ommatidia of females, where the pigment is orange. Intracellular recordings demonstrated that the spectral sensitivities of the proximal photoreceptors (R5-8) of all ommatidia in both sexes are strongly tuned by the perirhabdomal pigments. These pigments act as long-pass filters, shifting the peak sensitivities into the wavelength range above 600 nm. Due to the sex-specific pigments in type II ommatidia, the spectral sensitivities of the R5-8 photoreceptors of females peaked at 620 nm while those in males peaked at 660 nm. The measured spectral sensitivities could be well reproduced by an optical model assuming a long-wavelength-absorbing visual pigment with peak absorbance at 565 nm. Whereas the sexual dimorphism was unequivocally demonstrated for the ventral eye region, dimorphism in the dorsal region was not found. Presumably the ventral region is adapted for sexual behaviors such as courtship and oviposition.