Chromatic properties of identified interneurons in the optic lobes of the bee

Chromatic processing and receptive-field structure in neurons of the anterior optic tract of the honeybee brain

Color vision in honeybees is a well-documented perceptual phenomenon including multiple behavioral tests of trichromaticity and color opponency. Data on the combined color/space properties of high order visual neurons in the bee brain is however limited. Here we fill this gap by analyzing the activity of neurons in the anterior optic tract (AOT), a high order brain region suggested to be involved in chromatic processing. The spectral response properties of 72 units were measured using UV, blue and green light stimuli presented in 266 positions of the visual field. The majority of these units comprise combined chromatic-spatial processing properties. We found eight different neuron categories in terms of their spectral, spatial and temporal response properties. Color-opponent neurons, the most abundant neural category in the AOT, present large receptive fields and activity patterns that were typically opponent between UV and blue or green, particularly during the on-tonic response phase. Receptive field shapes and activity patterns of these color processing neurons are more similar between blue and green, than between UV and blue or green. We also identified intricate spatial antagonism and double spectral opponency in some receptive fields of color-opponent units. Stimulation protocols with different color combinations applied to 21 AOT units allowed us to uncover additional levels of spectral antagonism and hidden inhibitory inputs, even in some units that were initially classified as broad-band neurons based in their responses to single spectral lights. The results are discussed in the context of floral color discrimination and celestial spectral gradients.

Understanding insect colour constancy

Colour constancy is the ability to recognize the colour of objects despite spectral changes in the natural illumination. As such, this phenomenon is important for most organisms with good colour vision, and it has been intensively studied in humans and primates. Colour constancy is also documented for several species of insects, which is not surprising given the ecological importance of colour vision. But how do insects, with their small brains, solve the complex problem of colour vision and colour constancy? In an interspecies approach, this review reports on behavioural studies on colour constancy in bees, butterflies, moths and humans, corresponding computational models and possible neurophysiological correlates. This article is part of the theme issue 'Understanding colour vision: molecular, physiological, neuronal and behavioural studies in arthropods'.

Haze of glue determines preference of western flower thrips (Frankliniella occidentalis) for yellow or blue traps

In a wind tunnel we compared the colour preference for western flower thrips to four types of colour plates (clear, white, blue and yellow) applied with two types of glue (diffuse Stikem versus clear D41). Further the results for blue and yellow preference were validated in two greenhouses. In the wind tunnel, we found a clear preference of yellow over blue when a clear glue (D41) was used. However, with a more diffuse (whitish) glue (Stikem) the preference for yellow over blue disappeared, whereby the attraction to yellow decreased (58%) while the attraction to blue increased (65%). In the greenhouses, we found similar effects as in the wind tunnel with a decrease in attraction to yellow (35%) and increase in attraction to blue (32%) for Stikem compared to D41. Light measurements showed an increase of 18% of blue, 21% of violet light, 8% of yellow and 9% of green light reflected on the yellow Stikem trap versus the yellow D41 trap. On blue plates there was only 4% increase of blue light, 8% decrease of yellow light reflected when Stikem glue was used compared to D41 glue. It is not yet clear if the change of light reflection ratio blue/yellow caused by the glue type plays a role in the change of attraction. The reflective properties of glue are so far an unknown factor in colour choice and may explain partially the different results on colour preference. A small review on thrips colour preference is discussed to determine possible other factors of influence on colour choice.

Complexity and plasticity in honey bee phototactic behaviour

The ability to move towards or away from a light source, namely phototaxis, is essential for a number of species to find the right environmental niche and may have driven the appearance of simple visual systems. In this study we ask if the later evolution of more complex visual systems was accompanied by a sophistication of phototactic behaviour. The honey bee is an ideal model organism to tackle this question, as it has an elaborate visual system, demonstrates exquisite abilities for visual learning and performs phototaxis. Our data suggest that in this insect, phototaxis has wavelength specific properties and is a highly dynamical response including multiple decision steps. In addition, we show that previous experience with a light (through exposure or classical aversive conditioning) modulates the phototactic response. This plasticity is dependent on the wavelength used, with blue being more labile than green or ultraviolet. Wavelength, intensity and past experience are integrated into an overall valence for each light that determines phototactic behaviour in honey bees. Thus, our results support the idea that complex visual systems allow sophisticated phototaxis. Future studies could take advantage of these findings to better understand the neuronal circuits underlying this processing of the visual information.

Color vision in insects: insights from Drosophila

Color vision is an important sensory capability that enhances the detection of contrast in retinal images. Monochromatic animals exclusively detect temporal and spatial changes in luminance, whereas two or more types of photoreceptors and neuronal circuitries for the comparison of their responses enable animals to differentiate spectral information independent of intensity. Much of what we know about the cellular and physiological mechanisms underlying color vision comes from research on vertebrates including primates. In insects, many important discoveries have been made, but direct insights into the physiology and circuit implementation of color vision are still limited. Recent advances in Drosophila systems neuroscience suggest that a complete insect color vision circuitry, from photoreceptors to behavior, including all elements and computations, can be revealed in future. Here, we review fundamental concepts in color vision alongside our current understanding of the neuronal basis of color vision in Drosophila, including side views to selected other insects.

Randomly weighted receptor inputs can explain the large diversity of colour-coding neurons in the bee visual system

True colour vision requires comparing the responses of different spectral classes of photoreceptors. In insects, there is a wealth of data available on the physiology of photoreceptors and on colour-dependent behaviour, but less is known about the neural mechanisms that link the two. The available information in bees indicates a diversity of colour opponent neurons in the visual optic ganglia that significantly exceeds that known in humans and other primates. Here, we present a simple mathematical model for colour processing in the optic lobes of bees to explore how this diversity might arise. We found that the model can reproduce the physiological spectral tuning curves of the 22 neurons that have been described so far. Moreover, the distribution of the presynaptic weights in the model suggests that colour-coding neurons are likely to be wired up to the receptor inputs randomly. The perceptual distances in our random synaptic weight model are in agreement with behavioural observations. Our results support the idea that the insect nervous system might adopt partially random wiring of neurons for colour processing.

Insect-Inspired Sequential Inspection Strategy Enables an Artificial Network of Four Neurons to Estimate Numerosity

Varying levels of numerical cognition have been found in several animal species. Bees, in particular, have been argued to be able to count up to four items and solve complex numerical tasks. Here we present an exceedingly simple neural circuit that, when provided with the actual visual input that the bee is receiving while carrying out the task, can make reliable estimates on the number of items in the display. Thus we suggest that the elegance of numerical problem solving in bees might not lie in the formation of numerical concepts (such as “more,” “less,” or “zero”), but in the use of specific flight movements to scan targets, which streamlines the visual input and so renders the task of counting computationally inexpensive. Careful examination of the actual inspection strategies used by animals might reveal that animals often employ active scanning behaviors as shortcuts to simplify complex visual pattern discrimination tasks.

•

Small-brained animals such as bees can solve counting tasks

•

Exceedingly small neural circuits can mediate numerosity estimations

•

The method requires a sequential inspection strategy to generate the visual input

•

Active scanning behavior is suggested to play a role in complex cognitive tasks

Why colour is complex: Evidence that bees perceive neither brightness nor green contrast in colour signal processing

Honey bees ( Apis mellifera Linnaeus, 1758) potentially rely on a variety of visual cues when searching for flowers in the environment. Both chromatic and achromatic (brightness) components of flower signals have typically been considered simultaneously to understand how flower colours have evolved. However, it is unclear whether honey bees actually use brightness information in their colour perception. We investigated whether free-flying honey bees can process brightness cues in achromatic stimuli when presented at a large visual angle of 28° to ensure colour processing. We found that green contrast (modulation of the green receptor against the background) and brightness contrast (modulation of all three receptors against the background) did not have a significant effect on the proportion of correct choices made by bees, indicating that they did not appear to use brightness cues in a colour processing context. Our findings also reveal that, even at a small visual angle, honeybees do not reliably process single targets solely based on achromatic information, at least considering values up to 60% modulation of brightness. We discuss these findings in relation to proposed models of bee colour processing. Therefore, caution should be taken when interpreting elemental components of complex flower colours as perceived by different animals.

Toward a Mechanistic Understanding of Color Vision in Insects

Many visual animals exploit spectral information for seeking food and mates, for identifying preys and predators, and for navigation. Animals use chromatic information in two ways. "True color vision," the ability to discriminate visual stimuli on the basis of their spectral content independent of brightness, is thought to play an important role in object identification. In contrast, "wavelength-specific behavior," which is strongly dependent on brightness, often associates with foraging, navigation, and other species-specific needs. Among animals capable of chromatic vision, insects, with their diverse habitats, stereotyped behaviors, well-characterized anatomy and powerful genetic tools, are attractive systems for studying chromatic information processing. In this review, we first discuss insect photoreceptors and the relationship between their spectral sensitivity and animals' color vision and ecology. Second, we review recent studies that dissect chromatic circuits and explore neural mechanisms of chromatic information processing. Finally, we review insect behaviors involving "true color vision" and "wavelength-specific behaviors," especially in bees, butterflies, and flies. We include examples of high-order color vision, such as color contrast and constancy, which are shared by vertebrates. We focus on Drosophila studies that identified neuronal correlates of color vision and innate spectral preferences. We also discuss the electrophysiological studies in bees that reveal color encoding. Despite structural differences between insects' and vertebrates' visual systems, their chromatic vision appears to employ the same processing principles, such as color opponency, suggesting convergent solutions of neural computation to common problems.

Synaptic Organization of Microglomerular Clusters in the Lateral and Medial Bulbs of the Honeybee Brain

The honeybee Apis mellifera is an established model for the study of visual orientation. Yet, research on this topic has focused on behavioral aspects and has neglected the investigation of the underlying neural architectures in the bee brain. In other insects, the anterior optic tubercle (AOTU), the lateral (LX) and the central complex (CX) are important brain regions for visuospatial performances. In the central brain of the honeybee, a prominent group of neurons connecting the AOTU with conspicuous microglomerular synaptic structures in the LX has been recently identified, but their neural organization and ultrastructure have not been investigated. Here we characterized these microglomerular structures by means of immunohistochemical and ultrastructural analyses, in order to evaluate neurotransmission and synaptic organization. Three-dimensional reconstructions of the pre-synaptic and post-synaptic microglomerular regions were performed based on confocal microscopy. Each pre-synaptic region appears as a large cup-shaped profile that embraces numerous post-synaptic profiles of GABAergic tangential neurons connecting the LX to the CX. We also identified serotonergic broad field neurons that probably provide modulatory input from the CX to the synaptic microglomeruli in the LX. Two distinct clusters of microglomerular structures were identified in the lateral bulb (LBU) and medial bulb (MBU) of the LX. Although the ultrastructure of both clusters is very similar, we found differences in the number of microglomeruli and in the volume of the pre-synaptic profiles of each cluster. We discuss the possible role of these microglomerular clusters in the visuospatial behavior of honeybees and propose research avenues for studying their neural plasticity and synaptic function.

Mapping chromatic pathways in the Drosophila visual system

In Drosophila, color vision and wavelength-selective behaviors are mediated by the compound eye's narrow-spectrum photoreceptors R7 and R8 and their downstream medulla projection (Tm) neurons Tm5a, Tm5b, Tm5c, and Tm20 in the second optic neuropil or medulla. These chromatic Tm neurons project axons to a deeper optic neuropil, the lobula, which in insects has been implicated in processing and relaying color information to the central brain. The synaptic targets of the chromatic Tm neurons in the lobula are not known, however. Using a modified GFP reconstitution across synaptic partners (GRASP) method to probe connections between the chromatic Tm neurons and 28 known and novel types of lobula neurons, we identify anatomically the visual projection neurons LT11 and LC14 and the lobula intrinsic neurons Li3 and Li4 as synaptic targets of the chromatic Tm neurons. Single-cell GRASP analyses reveal that Li4 receives synaptic contacts from over 90% of all four types of chromatic Tm neurons, whereas LT11 is postsynaptic to the chromatic Tm neurons, with only modest selectivity and at a lower frequency and density. To visualize synaptic contacts at the ultrastructural level, we develop and apply a "two-tag" double-labeling method to label LT11's dendrites and the mitochondria in Tm5c's presynaptic terminals. Serial electron microscopic reconstruction confirms that LT11 receives direct contacts from Tm5c. This method would be generally applicable to map the connections of large complex neurons in Drosophila and other animals.

Mechanisms, functions and ecology of colour vision in the honeybee

Research in the honeybee has laid the foundations for our understanding of insect colour vision. The trichromatic colour vision of honeybees shares fundamental properties with primate and human colour perception, such as colour constancy, colour opponency, segregation of colour and brightness coding. Laborious efforts to reconstruct the colour vision pathway in the honeybee have provided detailed descriptions of neural connectivity and the properties of photoreceptors and interneurons in the optic lobes of the bee brain. The modelling of colour perception advanced with the establishment of colour discrimination models that were based on experimental data, the Colour-Opponent Coding and Receptor Noise-Limited models, which are important tools for the quantitative assessment of bee colour vision and colour-guided behaviours. Major insights into the visual ecology of bees have been gained combining behavioural experiments and quantitative modelling, and asking how bee vision has influenced the evolution of flower colours and patterns. Recently research has focussed on the discrimination and categorisation of coloured patterns, colourful scenes and various other groupings of coloured stimuli, highlighting the bees' behavioural flexibility. The identification of perceptual mechanisms remains of fundamental importance for the interpretation of their learning strategies and performance in diverse experimental tasks.

Long term effects of aversive reinforcement on colour discrimination learning in free-flying bumblebees

The results of behavioural experiments provide important information about the structure and information-processing abilities of the visual system. Nevertheless, if we want to infer from behavioural data how the visual system operates, it is important to know how different learning protocols affect performance and to devise protocols that minimise noise in the response of experimental subjects. The purpose of this work was to investigate how reinforcement schedule and individual variability affect the learning process in a colour discrimination task. Free-flying bumblebees were trained to discriminate between two perceptually similar colours. The target colour was associated with sucrose solution, and the distractor could be associated with water or quinine solution throughout the experiment, or with one substance during the first half of the experiment and the other during the second half. Both acquisition and final performance of the discrimination task (measured as proportion of correct choices) were determined by the choice of reinforcer during the first half of the experiment: regardless of whether bees were trained with water or quinine during the second half of the experiment, bees trained with quinine during the first half learned the task faster and performed better during the whole experiment. Our results confirm that the choice of stimuli used during training affects the rate at which colour discrimination tasks are acquired and show that early contact with a strongly aversive stimulus can be sufficient to maintain high levels of attention during several hours. On the other hand, bees which took more time to decide on which flower to alight were more likely to make correct choices than bees which made fast decisions. This result supports the existence of a trade-off between foraging speed and accuracy, and highlights the importance of measuring choice latencies during behavioural experiments focusing on cognitive abilities.

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.

Colour processing in complex environments: insights from the visual system of bees

Colour vision enables animals to detect and discriminate differences in chromatic cues independent of brightness. How the bee visual system manages this task is of interest for understanding information processing in miniaturized systems, as well as the relationship between bee pollinators and flowering plants. Bees can quickly discriminate dissimilar colours, but can also slowly learn to discriminate very similar colours, raising the question as to how the visual system can support this, or whether it is simply a learning and memory operation. We discuss the detailed neuroanatomical layout of the brain, identify probable brain areas for colour processing, and suggest that there may be multiple systems in the bee brain that mediate either coarse or fine colour discrimination ability in a manner dependent upon individual experience. These multiple colour pathways have been identified along both functional and anatomical lines in the bee brain, providing us with some insights into how the brain may operate to support complex colour discrimination behaviours.

Fast learning but coarse discrimination of colours in restrained honeybees

Colours are quickly learnt by free-moving bees in operant conditioning settings. In the present study, we report a method using the classical conditioning of the proboscis extension response (PER) in restrained honeybees(Apis mellifera), which allows bees to learn colours after just a few training trials. We further analysed how visual learning and discrimination is influenced by the quality of a stimulus by systematically varying the chromatic and achromatic properties of the stimuli. Using differential conditioning, we found that faster colour discrimination learning was correlated with reduced colour similarity between stimuli. In experiments with both absolute and differential conditioning, restrained bees showed poor colour discrimination and broad generalisation. This result is in strong contrast to the well-demonstrated ability of bees to finely discriminate colours under free-flight conditions and raises further questions about the temporal and perceptual processes underlying the ability of bees to discriminate and learn colours in different behavioural contexts.

Color processing in the medulla of the bumblebee (Apidae: Bombus impatiens)

The mechanisms of processing a visual scene involve segregating features (such as color) into separate information channels at different stages within the brain, processing these features, and then integrating this information at higher levels in the brain. To examine how this process takes place in the insect brain, we focused on the medulla, an area within the optic lobe through which all of the visual information from the retina must pass before it proceeds to central brain areas. We used histological and immunocytochemical techniques to examine the bumblebee medulla and found that the medulla is divided into eight layers. We then recorded and morphologically identified 27 neurons with processes in the medulla. During our recordings we presented color cues to determine whether response types correlated with locations of the neural branching patterns of the filled neurons among the medulla layers. Neurons in the outer medulla layers had less complex color responses compared to neurons in the inner medulla layers and there were differences in the temporal dynamics of the responses among the layers. Progressing from the outer to the inner medulla, neurons in the different layers appear to process increasingly complex aspects of the natural visual scene.

Higher order visual input to the mushroom bodies in the bee, Bombus impatiens

To produce appropriate behaviors based on biologically relevant associations, sensory pathways conveying different modalities are integrated by higher-order central brain structures, such as insect mushroom bodies. To address this function of sensory integration, we characterized the structure and response of optic lobe (OL) neurons projecting to the calyces of the mushroom bodies in bees. Bees are well known for their visual learning and memory capabilities and their brains possess major direct visual input from the optic lobes to the mushroom bodies. To functionally characterize these visual inputs to the mushroom bodies, we recorded intracellularly from neurons in bumblebees (Apidae: Bombus impatiens) and a single neuron in a honeybee (Apidae: Apis mellifera) while presenting color and motion stimuli. All of the mushroom body input neurons were color sensitive while a subset was motion sensitive. Additionally, most of the mushroom body input neurons would respond to the first, but not to subsequent, presentations of repeated stimuli. In general, the medulla or lobula neurons projecting to the calyx signaled specific chromatic, temporal, and motion features of the visual world to the mushroom bodies, which included sensory information required for the biologically relevant associations bees form during foraging tasks.

Patterns of chromatic information processing in the lobula of the honeybee, Apis mellifera L

The honeybee, Apis mellifera L., is one of the living creatures that has its colour vision proven through behavioural tests. Previous studies of honeybee colour vision has emphasized the relationship between the spectral sensitivities of photoreceptors and colour discrimination behaviour. The current understanding of the neural mechanisms of bee colour vision is, however, rather limited. The present study surveyed the patterns of chromatic information processing of visual neurons in the lobula of the honeybee, using intracellular recording stimulated by three light-emitting diodes, whose emission spectra approximately match the spectral sensitivity peaks of the honeybee. The recorded visual neurons can be divided into two groups: non-colour opponent cells and colour opponent cells. The non-colour opponent cells comprise six types of broad-band neurons and four response types of narrow-band neurons. The former might detect brightness of the environment or function as chromatic input channels, and the latter might supply specific chromatic input. Amongst the colour opponent cells, the principal neural mechanism of colour vision, eight response types were recorded. The receptive fields of these neurons were not centre surround as observed in primates. Some recorded neurons with tonic post-stimulus responses were observed, however, suggesting temporal defined spectral opponency may be part of the colour-coding mechanisms.

Anatomical organization of retinotopic motion-sensitive pathways in the optic lobes of flies

Anatomical methods have identified conserved neuronal morphologies and synaptic relationships among small-field retinotopic neurons in insect optic lobes. These conserved cell shapes occur across many species of dipteran insects and are also shared by Lepidoptera and Hymenoptera. The suggestion that such conserved neurons should participate in motion computing circuits finds support from intracellular recordings as well as older studies that used radioactive deoxyglucose labeling to reveal strata with motion-specific activity in an achromatic neuropil called the lobula plate. While intracellular recordings provide detailed information about the motion-sensitive or motion-selective responses of identified neurons, a full understanding of how arrangements of identified neurons compute and integrate information about visual motion will come from a multidisciplinary approach that includes morphological circuit analysis, the use of genetic mutants that exhibit specific deficits in motion processing, and biomimetic models. The latter must be based on the organization and connections of real neurons, yet provide output properties similar to those of more traditional theoretical models based on behavioral observations that date from the 1950s. Microsc. Res. Tech. 62:132-150, 2003.

Segregation of visual input to the mushroom bodies in the honeybee (Apis mellifera)

Insect mushroom bodies are brain regions that receive multisensory input and are thought to play an important role in learning and memory. In most neopteran insects, the mushroom bodies receive direct olfactory input. In addition, the calyces of Hymenoptera receive substantial direct input from the optic lobes. We describe visual inputs to the calyces of the mushroom bodies of the honeybee Apis mellifera, the neurons' dendritic fields in the optic lobes, the medulla and lobula, and the organization of their terminals in the calyces. Medulla neurons terminate in the collar region of the calyx, where they segregate into five layers that receive alternating input from the dorsal or ventral medulla, respectively. A sixth, innermost layer of the collar receives input from lobula neurons. In the basal ring region of the calyx, medulla neuron terminals are restricted to a small, distal part. Lobula neurons are more prominent in the basal ring, where they terminate in its outer half. Although the collar and basal ring layers generally receive segregated input from both optic neuropils, some overlap occurs at the borders of the layers. At least three different types of mushroom body input neurons originate from the medulla: (a) neurons with narrow dendritic fields mainly restricted to the vicinity of the medulla's serpentine layer and found throughout the medulla; (b) neurons restricted to the ventral half of the medulla and featuring long columnar dendritic branches in the outer medulla; and (c) a group of neurons whose dendrites are restricted to the most ventral part of the medulla and whose axons form the anterior inferior optic tract. Most medulla neurons (groups a and b) send their axons via the anterior superior optic tract to the mushroom bodies. Neurons connecting the lobula with the mushroom bodies have their dendrites in a defined dorsal part of the lobula. Their axons form a third tract to the mushroom bodies, here referred to as the lobula tract. Our findings match the anatomy of intrinsic mushroom body neurons (Strausfeld, 2002) and together indicate that the mushroom bodies may be composed of many more functional subsystems than previously suggested.

Insects could exploit UV-green contrast for Landmark navigation

Illumination-invariant detection of landmark features is a prerequisite for landmark navigation in insects. It is suggested that a contrast mechanism involving the UV and green receptors of insect eyes could guarantee a robust separation between natural objects as foreground and sky as background. Using a sensor with a UV and a green channel that in their spectral characteristics are close to the corresponding insect photoreceptors, data of natural objects and sky were collected. The data show that the two classes can be separated by a fixed threshold in the UV-green color space, offering an advantage over a purely UV-based separation that would require a dynamic threshold. Based on a numerical method, UV-green antagonism is shown to guarantee a more reliable discrimination than UV-blue antagonism.

Seven experiments on pattern vision of the honeybee, with a model

When vertically presented patterns are fixed in relation to the point of choice of the bees, the locations of areas of colour or black can be discriminated in the vertical direction, and in the horizontal direction when the bees use some mark with green contrast on which to stabilize. The bees can fixate on a radial pattern, a spot, or a ring of spots. Resolution depends on fixation, which depends on green contrast, but the discrimination of locations then depends on the photon flux at green and blue receptors. The model proposes that, when the eye is stabilised, a tonic channel from all receptor types is activated in the region of the eye that looks at the cue. This channel generates a perceptual space in which location, colour and size contribute to a signature for each cue in each eye. In other channels of processing, the stimulus is the phasic modulation of green receptors by edges, and these pathways are colour blind. When the patterns are composed of bars, the cues are the radial and tangential edges and the integrated edge orientation, irrespective of shuffling of locations on the target. Orientation can be detected in fuzzy edges with a gradient of black to white over about 60 degrees subtended from the point of choice. These cues correspond to the response profiles of large-field, phasic, coarsely tuned filters. There is no evidence that cues from edges have a spatial tag. The model also proposes that each filter for average orientation, or radial or tangential edges, corresponds to an ascending succession of phasic neurones. Each processing channel generates a perceptual space where combinations of related cues form signatures, and at least one signature is passed to memory as an index item. A pattern may generate several signatures but patterns are discriminated only when their signatures differ. In the bee there is no evidence for the re-assembly of the pattern.

Two visual systems in one brain: neuropils serving the principal eyes of the spider Cupiennius salei

Principal (anterior median) eyes of the wandering spider Cupiennius are served by three successive neuropils, the organization of which is distinct from those serving secondary eyes. Photoreceptors terminate in the first optic neuropil amongst second order neurons with overlapping dendritic fields. Second order neurons terminate at various depths in anterior median eye medulla where they are visited by bush-like dendritic trees of third order projection neurons. These supply tracts which extend into the "central body." This crescent-shaped neuropil lies midsagittally in the rear of the brain near its dorsal surface. It is organized into columns and it supplies both columnar and tangential efferents to other brain centers. The supply to and organization of the "central body" neuropil is reminiscent of retinotopic neuropils supplying the lobula of insects. Channels to the "central body" comprise one of two concurrent visual pathways, the other provided by the secondary eyes supplying the "mushroom body." We suggest that principal eye pathways may be involved in form and texture perception whereas secondary eyes detect motion, as is known for jumping spiders. Our data do not support Hanström's classical view that the "central body" is specifically associated with web-building, nor that it is homologous to its namesake in insect brains.

The Bezold-Brücke effect in the color vision system of the honeybee

Evidence is presented that intensity dependent color shifts (Bezold-Brücke effect) occur in the color vision system of the honeybee. The evidence comes from a fit between the choices of monochromatic lights in training experiments (Menzel, R., 1981; Journal of Comparative Physiology A, 141, 389-393) and the choice percentages derived now from recently presented quantitative predictions from the color opponent coding (COC) model for the bee (Backhaus, W., 1991; Vision Research, 31, 1381-1397) for the Bezold-Brücke effect. The only open parameter in the simulations of the training experiments is an experiment type dependent factor describing the weighting of color differences (judgement values) in the choice behavior. The results show (1) that the Bezold-Brücke effect exists in the bee. The results (2) confirm the color opponent coding (COC) model which was developed to describe the physiological components of the color vision system in the bee, (3) the general psychophysical assumptions about the structure of the color space, (4) the color difference formula, and (5) the general psychophysical assumptions about the (triadic) structure of judgements as tested in color similarity experiments.

Color vision in honeybees

Theoretical and experimental investigations of the color vision system in honeybees are reviewed. Grassmann's model and receptor models of color vision are discussed with respect to the problem of color difference. A recent analysis of the bee's color opponent coding system is presented in brief. Predictions for the spectral sensitivity of color opponent coding neurons derived directly from the color opponent coding (COC) model and predictions for the Bezold-Brücke color shift and the spectral discrimination function derived from the model via color difference formula are presented. The predictions are compared with electrophysiological data and with choice proportions of behavioral experiments, respectively.

The optic lobe of Drosophila melanogaster. II. Sorting of retinotopic pathways in the medulla

We present a quantitative evaluation of Golgi-impregnated columnar neurons in the optic lobe of wild-type Drosophila melanogaster. This analysis reveals the overall connectivity pattern between the 10 neuropil layers of the medulla and demonstrates the existence of at least three major visual pathways. Pathway 1 connects medulla layer M10 to the lobula plate. Input layers of this pathway are M1 and M5. Pathway 2 connects M9 to shallow layers of the lobula, which in turn are tightly linked to the lobula plate. This pathway gets major input via M2. Pathways 1 and 2 receive input from retinula cells R1-6, either via the lamina monopolar cell L1 (terminating in M1 and M5) or via L2 and T1 (terminating in M2). Neurons of these pathways typically have small dendritic fields. We discuss evidence that pathways 1 and 2 may play a major role in motion detection. Pathway 3 connects M8 to deep layers of the lobula. In M8 information converges that is derived either from M3 (pathway 3a) or from M4 and M6 (pathway 3b), layers that get their major input from L3 and R8 or L4 and R7, respectively. Some neurons of pathway 3 have large dendritic fields. We suggest that they may be involved in the computation of form and colour. Possible analogies to the organization of pathways in the visual system of vertebrates are discussed.

Common projection areas of antennal and visual pathways in the honeybee brain, Apis mellifera

The convergence of primary sensory neurons of the antennae, higher order visual interneurons, and antennal motoneurons was analysed with neuroanatomical techniques in the honeybee, Apis mellifera. The different modalities evoke specific antennal responses in this insect. Three different fluorescent dyes were applied successively in the same preparation in order to visualise the various fiber projections from the antennae and the lobula in the brain of the honeybee. Three neuropile areas where sensory fibers of the antennae overlap with visual projection neurons from the lobula were found. Within the posterior-median protocerebrum the antennal tract T6-1 comes in close vicinity to the lobula tract LoT-9 and to some other lobula fibers that cannot be assigned to a special tract. Antennal T6-3 fibers overlap with lobula LoT-7 neurons within the posterior protocerebrum more laterally. Antennal T5 fibers arborise in the dorsal lobe and show common projection sites with lobula LoT-3 neurons. The multimodal convergence in the three common neuropiles demonstrates that these areas are important centers for multimodal information processing between sensory, motor, and descending neurons in insects.

Neuronal basis for parallel visual processing in the fly

Behavioral and electrophysiological studies of insects demonstrate both spectrally independent and chromatically dependent behaviors and interneurons. This account describes the neuroanatomical identification of two parallel retinotropic subsystems, one supplying descending channels to spectrally independent neck and flight motor circuits, the other supplying polychromatic channels to neuropils associated with leg motor circuits in the thoracic ganglia. In the compound eye, two classes of photoreceptors contribute to each of several thousand sampling units. High-sensitivity, chromatically uniform short-axon photoreceptors (R1-R6) supply the lamina's external plexiform layer and are presynaptic to L1, L2 efferents. These project in parallel with a second system of trichromatic long-axon receptors and the L3 efferent. Both pathways supply columns of the medulla, equal in number to ommatidia. Golgi and cobalt-silver impregnation demonstrates that neurons from the medulla diverge to two deeper regions, the lobula plate and lobula, the former a thin tectum of neuropil dorsal to the more substantial lobula. Layer relationships between medulla neurons and their afferent supply suggest that the lobula plate and lobula are each supplied by one or the other, but not both, of the two parallel subsystems. Independence of the two parallel pathways is suggested by ablation of the photoreceptor layer leading to selective degeneration of the motion-sensitive lobula plate neuropil. In addition, octets of small-field neurons associated with the R1-R6/L1, L2 pathway give rise to synaptic complexes with motion-sensitive neurons of the lobula plate. A variety of behavioral and electrophysiological studies provide supporting evidence that certain insects possess parallel visual pathways comparable to the magnocellular and parvocellular subsystems of primates.

Color opponent coding in the visual system of the honeybee

A model is presented for the color vision system of the honeybee, which takes the nonlinear phototransduction process in the photoreceptors into account and assumes linear computations of the excitations of the photoreceptors. The model parameters are derived by a least squares fit of the scale values determined by multidimensional scaling analysis of the results of color choice experiments to the excitation values of two hypothetical spectral antagonistic coding cells. The psychophysical scale values are interpreted physiologically. Furthermore, a color difference formula is presented which is based on the color opponent coding (COC) model. The model explains quantitatively (1) the sensitivity of spectral antagonistic neurons measured by Kien and Menzel (1977; Journal of Comparative Physiology, 113, 17-34, 35-53), (2) the color discrimination function measured by von Helversen (1972; Journal of Comparative Physiology, 80, 439-472). The following predictions are derived from the model: (1) excitation/log (I) curves of the spectral antagonistic neurons; and from the model in conjunction with the color difference formula: (2) intensity dependent color shifts (Bezold-Brücke effect); (3) the intensity dependence of wavelength discrimination.

Descending neurons supplying the neck and flight motor of Diptera: organization and neuroanatomical relationships with visual pathways

In dipterous insects, a volume of behavioral and electrophysiological studies promote the contention that three wide-field motion-sensitive tangential neurons provide a necessary and sufficient input to specific channels that drive the torque motor during flight. The present studies describe the results of neuroanatomical investigations of the relationships between motion-sensitive neuropil in the fly optic lobes and descending neurons that arise from a restricted area of the brain and supply segmental neck and flight motor neuropil. The present observations resolve at least 50 pairs of descending neurons supplying flight motor centers in the thoracic ganglia. The majority of descending neurons receive a distributed output from horizontal motion-sensitive neurons. However, the same descending neurons are also visited by numerous small-field retinotopic neurons from the lobula plate as well as hitherto undescribed small tangential neurons. Neuroanatomical studies, using cobalt, Golgi, and Texas red histology, demonstrate that these smaller inputs onto descending neurons have dendrites that are organized at specific strata in retinotopic neuropil and that these correspond to horizontal and vertical motion sensitivity layers. Conclusions that only a restricted number of wide-field neurons are necessary and sufficient for visually stabilized flight may be premature. Rather, neuroanatomical evidence suggests that descending neurons to the flight motor may each be selectively tuned to specific combinations of wide- and small-field visual cues, so providing a cooperative descending network controlling the rich repertoire of visually evoked flight behavior.

Immunocytochemistry of GABA in the brain and suboesophageal ganglion of Manduca sexta

We have used specific antisera against protein-conjugated gamma-aminobutyric acid (GABA) in immunocytochemical preparations to investigate the distribution of putatively GABAergic neurons in the brain and suboesophageal ganglion of the sphinx moth Manduca sexta. About 20,000 neurons per brain hemisphere exhibit GABA-immunoreactivity. Most of these are optic-lobe interneurons, especially morphologically centrifugal neurons of the lamina and tangential neurons that innervate the medulla or the lobula complex. Many GABA-immunoreactive neurons, among them giant fibers of the lobula plate, project into the median protocerebrum. Among prominent GABA-immunoreactive neurons of the median protocerebrum are about 150 putatively negative-feedback fibers of the mushroom body, innervating both the calyces and lobes, and a group of large, fan-shaped neurons of the lower division of the central body. Several commissures in the supra- and suboesophageal ganglion exhibit GABA-like immunoreactivity. In the suboesophageal ganglion, a group of contralaterally descending neurons shows GABA-like immunoreactivity. The frontal ganglion is innervated by immunoreactive processes from the tritocerebrum but does not contain GABA-immunoreactive somata. With few exceptions the brain nerves do not contain GABA-immunoreactive fibers.

Bees perceive illusory colours induced by movement

Certain black-and-white patterns, when rotated at appropriate speeds, can create the artificial perception of hues. We report that this illusion is not confined to human vision, but is also perceived by an insect, the honeybee. The findings suggest that certain features underlying the processing of colour information are shared by man and bee.

Color and spatial antagonism in the visual system of the larval swallowtail butterfly

Many neurons exhibiting an antagonistic interaction between signals from one particular stemma (center) and neighboring stemmata (surround) were found in the second optic neuropil (medulla) of the larval swallowtail butterfly. Furthermore, most of the neurons showed a color opponent interaction in the center: combinations of opponent colors were blue vs green, UV vs green, and blue vs UV + green. Thus, photoreceptor signals are integrated into chromatically and spatially antagonistic mechanisms, even in the invertebrate.

Distribution of GABA-like immunoreactivity in the brain of the honeybee

The distribution of GABA-like immunoreactivity in the brain of the honeybee was investigated with antisera generated against GABA protein conjugates. The binding of the antisera in paraffin serial sections was studied with the peroxidase-antiperoxidase method. GABA-like immunoreactive fibers appeared in all main neuropile areas. The staining of the optic lobes showed pronounced stratification. The receptor cells of compound eyes, ocelli, and antennae were not labelled. Several prominent fiber tracts showed GABA-like immunoreactivity, whereas other tracts were devoid of staining. There are no major immunoreactive commissures linking the two brain hemispheres with the exception of small commissures that bridge short distances between the beta-lobes and the antennal lobes. Several fibers in the cervical connective were also labelled; some of those may descend from the suboesophageal ganglion to the thoracic ganglia. The dense reactivity seen in the optic and antennal neuropiles implies that GABA is more important in mediating local rather than more distant neural interactions.

The second and third optic ganglia of the worker bee: Golgi studies of the neuronal elements in the medulla and lobula

The gross morphology and the fine-structural characteristics of neurones of the second and third optic ganglia of the honeybee Apis mellifera were investigated light microscopically on the basis of Golgi (selective silver)- and reduced silver preparations. The second optic ganglion, the medulla, is ovoid in shape and has a slightly convex distal surface and a slightly concave proximal surface. The medullar outer levels are characteristically composed of neuronal arrangements showing strict precision of their geometrical spacing proximally as far as a pronounced layer of tangential fibre elements comprising the serpentine layer of the medulla. At the inner medullary levels retinotopic channels are again multiplied, and the arrangement of axons and dendrites contribute to a complex lattice. The third optic ganglion, the lobula, is interposed between the medulla and the protocerebrum. It is the site of termination of the third-order neurones. The lobula in hymenopterans appears, in contrast to dipterans, odonates and lepidopterans, as a single neuropilic mass. A short review of the electrophysiological data concerning these two ganglia has been tentatively correlated with some of the anatomical data.