Gavin J. Taylor

Selective attention in the honeybee optic lobes precedes behavioral choices

Significance Attention, observed in a wide variety of animals from insects to humans, involves selectively attending to behaviorally relevant stimuli while filtering out other stimuli. We designed a paradigm that allowed us to record brain activity in tethered, walking bees selecting virtual visual objects. We found that stimulus-specific brain activity increased when the bees controlled the position of the visual objects, and that activity decreased when bees were not in control. When bees were presented with competing objects, brain activity in the optic lobes preceded behavioral choices; this suggests that in animals with tiny brains, such as bees, attention-like processes are pushed far out into the sensory periphery. This trait is likely important for efficiently navigating complex visual environments. Attention allows animals to respond selectively to competing stimuli, enabling some stimuli to evoke a behavioral response while others are ignored. How the brain does this remains mysterious, although it is increasingly evident that even animals with the smallest brains display this capacity. For example, insects respond selectively to salient visual stimuli, but it is unknown where such selectivity occurs in the insect brain, or whether neural correlates of attention might predict the visual choices made by an insect. Here, we investigate neural correlates of visual attention in behaving honeybees (Apis mellifera). Using a closed-loop paradigm that allows tethered, walking bees to actively control visual objects in a virtual reality arena, we show that behavioral fixation increases neuronal responses to flickering, frequency-tagged stimuli. Attention-like effects were reduced in the optic lobes during replay of the same visual sequences, when bees were not able to control the visual displays. When bees were presented with competing frequency-tagged visual stimuli, selectivity in the medulla (an optic ganglion) preceded behavioral selection of a stimulus, suggesting that modulation of early visual processing centers precedes eventual behavioral choices made by these insects.

FicTrac: a visual method for tracking spherical motion and generating fictive animal paths.

Studying how animals interface with a virtual reality can further our understanding of how attention, learning and memory, sensory processing, and navigation are handled by the brain, at both the neurophysiological and behavioural levels. To this end, we have developed a novel vision-based tracking system, FicTrac (Fictive path Tracking software), for estimating the path an animal makes whilst rotating an air-supported sphere using only input from a standard camera and computer vision techniques. We have found that the accuracy and robustness of FicTrac outperforms a low-cost implementation of a standard optical mouse-based approach for generating fictive paths. FicTrac is simple to implement for a wide variety of experimental configurations and, importantly, is fast to execute, enabling real-time sensory feedback for behaving animals. We have used FicTrac to record the behaviour of tethered honeybees, Apis mellifera, whilst presenting visual stimuli in both open-loop and closed-loop experimental paradigms. We found that FicTrac could accurately register the fictive paths of bees as they walked towards bright green vertical bars presented on an LED arena. Using FicTrac, we have demonstrated closed-loop visual fixation in both the honeybee and the fruit fly, Drosophila melanogaster, establishing the flexibility of this system. FicTrac provides the experimenter with a simple yet adaptable system that can be combined with electrophysiological recording techniques to study the neural mechanisms of behaviour in a variety of organisms, including walking vertebrates.

The image-forming mirror in the eye of the scallop

Fine-tuned for image formation We typically think of eyes as having one or more lenses for focusing incoming light onto a surface such as our retina. However, light can also be focused using arrays of mirrors, as is commonly done in telescopes. A biological example of this is the scallop, which can have up to 200 reflecting eyes that focus light onto two retinas. Palmer et al. find that spatial vision in the scallop is achieved through precise control of the size, shape, and packing density of the tiles of guanine that together make up an image-forming mirror at the back of each of the eyes. Science, this issue p. 1172 The crystal morphology, organization, and three-dimensional shape of the scallop eye mirror are finely controlled for image formation. Scallops possess a visual system comprising up to 200 eyes, each containing a concave mirror rather than a lens to focus light. The hierarchical organization of the multilayered mirror is controlled for image formation, from the component guanine crystals at the nanoscale to the complex three-dimensional morphology at the millimeter level. The layered structure of the mirror is tuned to reflect the wavelengths of light penetrating the scallop’s habitat and is tiled with a mosaic of square guanine crystals, which reduces optical aberrations. The mirror forms images on a double-layered retina used for separately imaging the peripheral and central fields of view. The tiled, off-axis mirror of the scallop eye bears a striking resemblance to the segmented mirrors of reflecting telescopes.

Using an abstract geometry in virtual reality to explore choice behaviour: visual flicker preferences in honeybees

ABSTRACT Closed-loop paradigms provide an effective approach for studying visual choice behaviour and attention in small animals. Different flying and walking paradigms have been developed to investigate behavioural and neuronal responses to competing stimuli in insects such as bees and flies. However, the variety of stimulus choices that can be presented over one experiment is often limited. Current choice paradigms are mostly constrained as single binary choice scenarios that are influenced by the linear structure of classical conditioning paradigms. Here, we present a novel behavioural choice paradigm that allows animals to explore a closed geometry of interconnected binary choices by repeatedly selecting among competing objects, thereby revealing stimulus preferences in an historical context. We used our novel paradigm to investigate visual flicker preferences in honeybees (Apis mellifera) and found significant preferences for 20–25 Hz flicker and avoidance of higher (50–100 Hz) and lower (2–4 Hz) flicker frequencies. Similar results were found when bees were presented with three simultaneous choices instead of two, and when they were given the chance to select previously rejected choices. Our results show that honeybees can discriminate among different flicker frequencies and that their visual preferences are persistent even under different experimental conditions. Interestingly, avoided stimuli were more attractive if they were novel, suggesting that novelty salience can override innate preferences. Our recursive virtual reality environment provides a new approach to studying visual discrimination and choice behaviour in animals. Summary: Visual flicker preferences in honeybees is shown by allowing bees to walk their way through multiple decision points in a recursive closed-loop environment.

X-ray micro computed-tomography

Emily Baird and Gavin Taylor describe how you can make three-dimensional models of biological samples using x-ray micro-computed tomography.

Bumblebee visual allometry results in locally improved resolution and globally improved sensitivity

The quality of visual information that is available to an animal is limited by the size of its eyes. Differences in eye size can be observed even between closely related individuals but we understand little about how this affects visual quality. Insects are good models for exploring the effects of size on visual systems because many species exhibit size polymorphism, which modifies both the size and shape of their eyes. Previous work in this area has been limited, however, due to the challenge of determining the 3D structure of eyes. To address this, we have developed a novel method based on x-ray tomography to measure the 3D structure of insect eyes and calculate their visual capabilities. We investigated visual allometry in the bumblebee Bombus terrestris and found that size affects specific aspects of visual quality including binocular overlap, optical sensitivity across the field of view, and visual resolution in the dorsofrontal visual field. This holistic study on eye allometry reveals that differential scaling between different eye areas provides substantial flexibility for larger bumblebees to have improved visual capabilities.

Imaging the evolution of visual specializations in fungus gnats

Many insects use vision to inform their behavior, but visual information differs between habitats and the sensory demands vary with each species9 ecology. The small size of insects9 eyes constrains their optical performance, and so it is unsurprising that they have evolved specializations for optimizing the information they obtain from their habitat. Unraveling how behavioral, environmental, and phylogenetic factors influence the evolution of such specializations is difficult, however, because existing techniques to analyze insect eyes require specimens to be preserved beforehand. To facilitate broad comparative studies on insect eyes and the evolution of complex visual behavior, we developed a novel analysis technique that uses x-ray micro-computed tomography to quantify and recreate the visual world of insects. We use our methodology to investigate the eyes of fungus gnats (Orfeliini), a tribe of diminutive Dipterans, to identify the visual specializations they evolved for surviving in different forest habitats and to explore how this changed over 30 million years of evolutionary history. The specimens we studied were preserved in different ways (in ethanol, air dried, and as an endocast in amber), demonstrating that our method provides a new opportunity to quantitatively study and compare the vision of a wide range insects held in museum collections. Our analysis indicates that different visual specializations have evolved between fungus gnat species living in different forest types and that the eyes of gnats from a similar geographic location have evolved to match the changing environmental conditions. Despite the small size of fungus gnats, evolution has evidentially been able to exploit sensory specializations to meet the differing sensory demands of species from a variety of forest habitats.Animal eyes typically have specialized regions of high resolution and/or sensitivity that enable them to execute behavior effectively within their specific visual habitat. These specializations, and evolutionary changes to them, can be crucial for understanding an animal’s ecology. Techniques for analyzing visual specializations typically require fresh samples and are thus limited to studies on extant species. However, the cornea of invertebrate compound eyes is readily preserved, even in fossils. To compare and quantify vision in specimens from different time periods and habitats, and with different preservation states – fossilized in amber, dried or stored in alcohol – we developed a novel technique that uses X-ray microtomography to create high resolution 3D models of compound eyes from which their sensitivity, spatial resolution, and field of view can be estimated using computational geometry. We apply our technique to understanding how the visual systems of some of the smallest flying insects, fungus gnats, have adapted to different types of forest habitat over evolutionary time (~30 mya to today). Our results demonstrate how such investigations can provide critical insights into the evolution of visual specializations and the sensory ecology of animals in general.

Using micro-CT techniques to explore the role of sex and hair in the functional morphology of bumblebee (Bombus terrestris) ocelli

Many insects have triplets of camera type eyes, called ocelli, whose function remains unclear for most species. Here, we investigate the ocelli of the bumblebee, Bombus terrestris, using reconstructed 3D data from X-ray micro computed-tomography scans combined with computational ray-tracing simulations. This method enables us, not only to predict the visual fields of the ocelli, but to explore for the first time the effect that hair has on them as well as the difference between worker female and male ocelli. We find that bumblebee ocellar fields of view are directed forward and dorsally, incorporating the horizon as well as the sky. There is substantial binocular overlap between the median and lateral ocelli, but no overlap between the two lateral ocelli. Hairs in both workers and males occlude the ocellar field of view, mostly laterally in the worker median ocellus and dorsally in the lateral ocelli. There is little to no sexual dimorphism in the ocellar visual field, suggesting that in B. terrestris they confer no advantage to mating strategies. We compare our results with published observations for the visual fields of compound eyes in the same species as well as with the ocellar vision of other bee and insect species.