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Visual Edge Detection in the Honeybee and its Chromatic Properties
Abstract
Free-flying bees were trained to collect a reward of sugar-water at one of several discs placed horizontally on a contrasting background. In subsequent tests the reward was removed and the behaviour of the bees was observed and recorded on video-tape while they landed on the experimental arrangement in search of the reward. The spatial distribution of the landings was analysed to measure the detectability of the discs and attractiveness of their boundaries, as compared to the interior regions. The results reveal that, while landing on figures, bees pay special attention to the edges. Even though the reward is placed at a randomly chosen location within the figure during training, the bees show a distinct preference for landing near the boundary of the figure. The bees' preference for edges is restricted to figures with boundaries that provide contrast to the green-sensitive receptors. When the boundaries contain no green contrast, the edge preference disappears. In this situation, landings continue to occur mainly within the figure, but they tend to be distributed randomly over its entire surface. Thus, whereas colour information can mediate the detection of objects per se, the detection of edges, at least in the context of landing on a figure, is a colour-blind performance that is driven primarily by signals from the bee's green-sensitive photoreceptors. This finding has interesting parallels in primate vision, where edge detection is also colour-blind. On the basis of these findings, we propose that edges provide cues that play an important role in guiding landing manouevres towards objects of interest, such as flowers.
... Generally, insects can use both intensity contrast (i.e. achromatic contrast) and colour contrast for object detection 43,44 . By adjusting LEDs to the same spectral radiance, we aimed to minimize the intensity effects as much as possible, but quantifying colour contrast and intensity contrast 45 for WFT is difficult because, contrary to bees and many other insect species, information about individual photoreceptor spectral sensitivity remains unclear 4 . ...
... Moreover, for UV-A treatments we added red light for tracking purposes, which effectively increased the overall radiance of the stimulus, although the controls here showed that it had little or no influence on our main findings. On the other hand, colour contrast or a visual edge between two colours could aid thrips to differentiate visual cues more easily and induce them to land near boundaries of visual targets, as shown with other insects 41,44 . In our results, we found that for yellow and green, the landing sites were concentrated around the edge of the LED near the light contrast border between the LED and the black lamp frame. ...
... To what extent intensity contrasts plays a role on flight and landings of WFT remains unknown. It is obvious though, that in the absence of intensity contrast, colour contrast and colour vision can enable insects to detect and approach visual targets 44 . ...
Real-time 3D tracking and high-speed videography was used to examine the behaviour of a worldwide greenhouse pest, the western flower thrips (WFT), in response to different colours in the context of improving trap design. Measurements were taken of the number of landings on, and flight activity near, a lamp containing two LEDs of either the same colour or a combination of two colours presented side by side. Main findings show that landing patterns of WFT are different between colours, with landings on UV(+ red) as highly attractive stimulus being mostly distributed at the bottom half of the lamp, while for yellow also as very attractive and green as a ‘neutral’ stimulus, landings were clearly on the upper rim of the lamp. Additionally, a positive interaction with the UV-A(+ red) and yellow combination elicited the highest number of landings and flight time in front of the LED lamp. Conversely, a negative interaction was observed with decreased landings and flight time found for yellow when blue was present as the adjacent colour. Overall, differences between treatments were less obvious for flight times compared to number of landings, with tracking data suggesting that WFT might use different colours to orientate at different distances as they approach a visual stimulus.
... In the honeybee, behavioural experiments have shown that only the green photoreceptor (often called long-wavelength-sensitive receptor) is used for achromatic tasks such as motion and shape detection (Srinivasan, 1985;Srinivasan and Lehrer, 1988;Hempel de Ibarra and Giurfa, 2003). Achromatic vision is also used to control landing, a behaviour studied extensively in flies (e.g., Tinbergen and Abeln, 1983;Van Breugel and Dickinson, 2012) and bees (e.g., Lehrer et al., 1990). Patterns are very poorly detected by the bee's achromatic channel if they only present contrast in the UV (short-wavelength-sensitive) or blue (medium-wavelengthsensitive) photoreceptor. ...
... That first task is often referred to as detection. In addition to colour cues, insects can rely on depth cues to discriminate between objects and their background, often using motion parallax, which relies on achromatic contrast between flower and background (see "Historical Background, " e.g., Lehrer et al., 1990). We have not included other references to this in Table 2, as little has been done on flower-visiting insects. ...
Studies on animal colour vision typically focus on the chromatic aspect of colour, which is related to the spectral distribution, and disregard the achromatic aspect, which is related to the intensity (“brightness”) of a stimulus. Although the chromatic component of vision is often most reliable for object recognition because it is fairly context independent, the achromatic component may provide a reliable signal under specific conditions, for example at night when light intensity is low. Here we make a case for the importance of achromatic cues in plant-pollinator signalling, based on experimental data on naïve Deilephila elpenor and Macroglossum stellatarum hawkmoths, optical modelling and synthesising published experiments on bees, flies, butterflies and moths. Our experiments show that in ecologically relevant light levels hawkmoths express a strong preference for brighter stimuli. Published experiments suggest that for flower-visiting bees, butterflies, moths and flies, achromatic cues may be more important for object detection than often considered. Our optical modelling enabled disentangling the contribution of pigments and scattering structures to the flower’s achromatic contrast, and illustrates how flower anatomy and background are important mediating factors. We discuss our findings in the context of the often-assumed dichotomy between detection and discrimination, chromatic versus achromatic vision, and the evolution of floral visual signals.
... For instance, when flying along narrow corridors, insects use the magnitude of visual motion experienced in each eye to control their position, height and speed [47][48][49] . Motion cues can be extracted at the edge of objects through parallax and allow evaluating the distance of targets with respect to their background based on differences in their relative retinal speed [50][51][52][53][54] . Edges are therefore contrasting regions in terms of motion-parallax cues and are privileged by flying insects in their detection and landing strategies 51 . ...
... Edges are therefore contrasting regions in terms of motion-parallax cues and are privileged by flying insects in their detection and landing strategies 51 . Numerous experiments have documented this fact in honey bees [50][51][52][53][54] . An interesting example is provided by experiments in which bees were trained to solve a discrimination between a plain black disk and a black ring positioned a few centimeters in front of a white background. ...
Honey bees exhibit remarkable visual learning capacities, which can be studied using virtual reality (VR) landscapes in laboratory conditions. Existing VR environments for bees are imperfect as they provide either open-loop conditions or 2D displays. Here we achieved a true 3D environment in which walking bees learned to discriminate a rewarded from a punished virtual stimulus based on color differences. We included ventral or frontal background cues, which were also subjected to 3D updating based on the bee movements. We thus studied if and how the presence of such motion cues affected visual discrimination in our VR landscape. Our results showed that the presence of frontal, and to a lesser extent, of ventral background motion cues impaired the bees’ performance. Whenever these cues were suppressed, color discrimination learning became possible. We analyzed the specific contribution of foreground and background cues and discussed the role of attentional interference and differences in stimulus salience in the VR environment to account for these results. Overall, we show how background and target cues may interact at the perceptual level and influence associative learning in bees. In addition, we identify issues that may affect decision-making in VR landscapes, which require specific control by experimenters.
... It is particularly notable that the number of L receptors is larger than what was originally assumed (6 per ommatidium instead of 4); this underlines the importance of the L channel not only for chromatic vision but also for achromatic vision. In particular, numerous studies (Lehrer et al., 1990;Si et al., 2003) have indicated that movement perception and parallax contrasts occur via the L channel. ...
Honey bees are endowed with the capacity of color vision as they possess three types of photoreceptors in their retina that are maximally sensitive in the ultraviolet, blue and green domains owing to the presence of corresponding opsin types. While the behavioral aspects of color vision have been intensively explored based on the easiness by which free-flying bee foragers are trained to color stimuli paired with sucrose solution, the molecular underpinnings of this capacity have been barely explored. Here we developed studies that spanned the exploration of opsin properties and changes of gene expression in the bee brain during color learning and retention in controlled laboratory protocols to fill this void. We characterized opsin distribution in the honey bee visual system, focusing on the presence of two types of green opsins (Amlop1 and Amlop2), one of which (Amlop2) was discovered upon sequencing of the bee genome. We confirmed that Amlop1 is present in ommatidia of the compound eye but not in the ocelli, while Amlop2 is confined to the ocelli. We developed a CRISPR/Cas9 approach to determine possible functional differences between these opsins. We successfully created Amlop1 and Amlop2 adult mutant bees by means of the CRISPR/Cas9 technology and we also produced white-gene mutants as a control for the efficiency of our method. We tested our mutants using a conditioning protocol in which bees learn to inhibit attraction to chromatic light based on electric-shock punishment (Icarus protocol). White and Amlop2 mutants learned to inhibit spontaneous attraction to blue light while Amlop1 mutants failed to do so. These results indicate that responses to blue light, which is also partially sensed by green receptors, are mediated mainly by compound-eye photoreceptors containing Amlop1 but not by the ocellar system in which photoreceptors contain Amlop2. Accordingly, 24 hours later, white and Amlop2 mutants exhibited an aversive memory for the punished color that was comparable to control bees but Amlop1 mutants exhibited no memory. We discuss these findings based on controls with eyes or ocelli covered by black paint and interpret our results by discussing use of chromatic vs. achromatic vision via the compound eyes and the ocelli, respectively. Finally, we analyzed immediate early gene (IEG) expression in specific areas of the bee brain following color vision learning in a virtual reality (VR) environment. We changed the degrees of freedom of this environment and subjected bees to a 2D VR in which only lateral movements of the stimuli were possible and to a 3D VR which provided a more immersive sensation. We analyzed levels of relative expression of three IEGs (kakusei, Hr38, and Egr1) in the calyces of the mushroom bodies, the optic lobes and the rest of the brain after color discrimination learning. In the 3D VR, successful learners exhibited Egr1 upregulation only in the calyces of the mushroom bodies, thus uncovering a privileged involvement of these brain regions in associative color learning. Yet, in the 2D VR, Egr1 was downregulated in the OLs while Hr38 and kakusei were coincidently downregulated in the calyces of the MBs in the learned group. Although both VR scenarios point towards specific activations of the calyces of the mushroom bodies (and of the visual circuits in the 2D VR), the difference in the type of expression detected suggests that the different constraints of the two VRs may lead to different kinds of neural phenomena. While 3D VR scenarios allowing for navigation and exploratory learning may lead to IEG upregulation, 2D VR scenarios in which movements are constrained may induce higher levels of inhibitory activity in the bee brain. Overall, we provide a series of new explorations of the visual system, including new functional analyses and the development of novel methods to study opsin function, which advances our understanding of honey bee vision and visual learning.
... On each side, the screw holds a green painted rod (8 mm in diameter) that runs through the whole tunnel length at each floor-wall junction. These rods are painted green because of the importance of green contrast in motion detection [24,26]. Turning, the screw makes the rods converge ( Fig.1-B.v) or diverge ( Fig.1-B.iv), ...
Bees outperform pilots in navigational tasks, despite having 100,000 times fewer neurons. It is commonly accepted in the literature that optic flow is a key parameter used by flying insects to control their altitude. The ambition of the present work was to design an innovative experimental setup that would make it possible to determine whether bees could rely simultaneously on several optical invariants, as pilots do. We designed a flight tunnel to enable manipulation of an optical invariant, the Splay Angle Rate of Change (SARC) and the restriction of the Optical Speed Rate of Change (OSRC) in the optic flow. It allows us to determine if bees use the SARC to control their altitude and to identify the integration process combining these two optical invariants. Access to the OSRC can be restricted by using different textures. The SARC can be biased thanks to motorized rods. This device allows to record bees’ trajectories in different visual configurations, including impoverished conditions and conditions containing contradictory information. The comparative analysis of the recorded trajectories provides first time evidence of SARC use in a ground-following task by a non-human animal. This new tunnel allows a precise experimental control of the visual environment in ecological experimental conditions. Therefore, it could pave the way for a new type of ecologically based studies examining the simultaneous use of several information sources for navigation by flying insects.
... In addition to this general trend, Figure 1 illustrates how flight paths were more streamlined toward bicolour stimuli than to single-colour stimuli, presumably aided by the central contrast line. It is known that contrast lines and edges are salient visual features for bees, used to steer flight behaviour (Lehrer et al., 1985(Lehrer et al., , 1990. Bees approached the lower edges of the single colour discs very closely before ascending steeply toward the centre, whereas those trained to bicolour discs began to ascend slightly earlier, generally before 2 cm horizontal distance from the target, and cluster more around the target centre (Figure 1). ...
Gaze direction is closely coupled with body movement in insects and other animals. If movement patterns interfere with the acquisition of visual information, insects can actively adjust them to seek relevant cues. Alternatively, where multiple visual cues are available, an insect’s movements may influence how it perceives a scene. We show that the way a foraging bumblebee approaches a floral pattern could determine what it learns about the pattern. When trained to vertical bicoloured patterns, bumblebees consistently approached from below centre in order to land in the centre of the target where the reward was located. In subsequent tests, the bees preferred the colour of the lower half of the pattern that they predominantly faced during the approach and landing sequence. A predicted change of learning outcomes occurred when the contrast line was moved up or down off-centre: learned preferences again reflected relative frontal exposure to each colour during the approach, independent of the overall ratio of colours. This mechanism may underpin learning strategies in both simple and complex visual discriminations, highlighting that morphology and action patterns determines how animals solve sensory learning tasks. The deterministic effect of movement on visual learning may have substantially influenced the evolution of floral signals, particularly where plants depend on fine-scaled movements of pollinators on flowers.
... These levels of processing are also commonly described in the literature on predation and camouflage as low-and high-level figure-ground processing (Troscianko et al. 2009), and the existence of such processing distinctions is supported by behavioral evidence across animal systems. First, behavioral orientation toward visual edges (such as those created by chromatic or achromatic contrasts) has been found across arthropod and vertebrate species (Lehrer et al. 1990;Bhagavatula et al. 2009), and computational models have shown how camouflage that obscures edges disrupts prey recognition (Stevens and Cuthill 2006). Second, local feature detection (e.g., of visual "parts," as in parts-based processing) was even reported in Tinbergen's (1951) classic studies using three-spined sticklebacks, where males readily attacked crude, unrealistic models of conspecific males with the distinct red belly (a territorial signal) over more detailed and realistic models. ...
Synopsis
The term ‘cognitive template’ originated from work in human-based cognitive science to describe a literal, stored, neural representation used in recognition tasks. As the study of cognition has expanded to non-human animals, the term has diffused to describe a wider range of animal cognitive tools and strategies that guide action through the recognition of and discrimination between external states. One potential reason for this non-standardized meaning and variable employment is that researchers interested in the broad range of animal recognition tasks enjoy the simplicity of the cognitive template concept and have allowed it to become shorthand for many dissimilar or unknown neural processes without deep scrutiny of how this metaphor might comport with underlying neurophysiology. We review the functional evidence for cognitive templates in fields such as perception, navigation, communication, and learning, highlighting any neural correlates identified by these studies. We find that the concept of cognitive templates has facilitated valuable exploration at the interface between animal behavior and cognition, but the quest for a literal template has failed to attain mechanistic support at the level of neurophysiology. This may be the result of a misled search for a single physical locus for the ‘template’ itself. We argue that recognition and discrimination processes are best treated as emergent and, as such, may not be physically localized within single structures of the brain. Rather, current evidence suggests that such tasks are accomplished through synergies between multiple distributed processes in animal nervous systems. We thus advocate for researchers to move towards a more ecological, process-oriented conception, especially when discussing the neural underpinnings of recognition-based cognitive tasks.
... The edges of objects are important for recognition by many animals, including humans (Shapley and Tolhurst, 1973) and honeybees (Lehrer et al., 1990), so it is reasonable to hypothesize that mantis shrimps do the same. Shape recognition is likely to be critically important to mantis shrimp when they are recognizing landmarks, which they use to locate their home burrow during navigation (Patel and Cronin, 2020c Therefore, edge detection of objects may be critical during navigation as well as for other aspects of a mantis shrimp's life, such as signal recognition, food identification, and recognition of predatory threats. ...
Mantis shrimp commonly inhabit seafloor environments with an abundance of visual features including conspecifics, predators, prey, and landmarks used for navigation. While these animals are capable of discriminating color and polarization, it is unknown what specific attributes of a visual object are important during recognition. Here we show that mantis shrimp of the species Neogonodactylus oerstedii are able to learn the shape of a trained target (p=0.048). Further, when the shape and color of a target which they had been trained to identify were placed in conflict, N. oerstedii tended to choose the target of the trained shape over the target of the trained color (p=0.054). Thus, we conclude that the shape of the target was more salient than its color during recognition by N. oerstedii , suggesting that the shapes of objects, such as landmarks or other animals, are important for their identification by the species.
... Many animals use the edges of objects for recognition, including humans (Shapley and 119 Tolhurst, 1973) and honeybees (Lehrer et al., 1990), so it is reasonable to hypothesize that 120 mantis shrimps do the same. Shape recognition is likely to be critically important to mantis 121 shrimp when they are recognizing landmarks, which they use to locate their home burrow during Reflectance measurements of the colored targets were taken in a dark room using an 195 Ocean Optics USB2000 spectrometer connected to a 3 m long, 400 µm diameter, fiber-optic 196 cable. ...
Mantis shrimp are predatory crustaceans that commonly occupy burrows in shallow, tropical waters worldwide. Most of these animals inhabit structurally complex, benthic environments with an abundance of visual features that are regularly observed, including conspecifics, predators, prey, and landmarks for use in navigation. While these animals are capable of learning and discriminating color and polarization, it is unknown what specific attributes of a visual object are important for its recognition. Here we show that mantis shrimp of the species Neogonodactylus oerstedii can learn the shape of a trained target. Furthermore, when the shape and color of a target which they had been trained to identify were placed in conflict, N. oerstedii significantly chose the target of the trained shape over the target of the trained color. Thus, we conclude that the shape of a target is more important than its color for its recognition by N. oerstedii . Our findings suggest that the shapes of learned structures, such as landmarks or other animals, are important for N. oerstedii during object recognition.
Many insects show by their behaviour that they detect visually the
existence of separate objects. The experimental material to analyse how
they perceive objects is provided by an insect that walks to the end of
a stick; then, because it has no alternative, it reaches with a foreleg
towards a neighbouring object that it perceives to be within range. Some
insects make horizontal peering movements as an aid to vision. The
peering motion is exactly appropriate for generating an apparent
velocity of nearby objects relative to the background. These
experiments, when put together with the known properties of optic lobe
neurons, suggest that a mechanism based on velocity parallax projected
to the horizontal plane accounts for much insect visual behavour.
Velocity parallax is defined as the discrepancy seen at the edge of an
object against a distant background when the eye moves laterally. On
this theory, perception of an object is inseparable from the local
detection of velocity differences. The background may not be `perceived'
at all when an object occurs in the foreground. The postulated mechanism
is a two- or three-stage feedback, in which the perceived velocity (or,
more accurately, the spatially correlated contrast frequency) in
small-field motion-perception units is reduced by the averaged contrast
frequency in larger fields, which feed back upon them. Contrast
frequency is defined as the frequency of the flicker that is generated
by a pattern moving across the eye. An alternative mechanism to the
feedback of the velocity signal with lateral spread is adaptation to the
local average background velocity, while sensitivity to a smaller local
change in velocity is retained. That idea comes from recent work on the
H1 neuron in the fly optic lobe, and could be the basis of a primitive
form vision that, if present in medium-field neurons, is adequate for
the whole of the normal visual behaviour of a freely moving insect.
These speculations invite a variety of experimental tests, ranging from
visual discrimination tests with bees that are shown the velocity
parallax situation, to appropriate stimulation of optic lobe neurons, to
simulation of a visual processing system that relies on velocity
parallax cues to detect objects.
To extract the third dimension from a two-dimensional retinal image most insects, including bees, cannot rely on mechanisms common in vertebrates such as accommodation, binocular convergence or stereoscopic vision1,2. Instead, they use the apparent size of familiar objects (the nearer the object, the larger its image), and objects' apparent motion (the nearer an object, the higher the speed of its image) 38. In several studies912 bees have been found to exploit size cues, whereas in others6,11,13 they seem to use both strategies. We have studied the influence of motion cues in isolation by excluding size cues. We report that bees can discriminate between objects at different distances irrespective of their size. This discrimination is mediated primarily by the green-sensitive visual channel and is therefore colour blind, like all of the motion-dependent behaviours investigated so far in the bee1417. The bee's ability to discriminate range by motion of the image explains how bees manage to manoeuvre in novel environments, where the size of objects is unknown.
1.
Some visual interneurons in the medulla of the locust (Locusta migratoria) optic lobe, give highly phasic responses, typically a single spike, to any suprathreshold intensity change. Although forming a distinctive class, these cells vary in receptive field area and their relative sensitivities to intensity increments and decrements.
2.
The timing of responses is quite precise. Typically the standard deviation of the spike latency is 2 ms. The responses to intensity increments and decrements are very similar in waveform and latency, so the cells not signal stimulus polarity. Spike timing depends upon stimulus contrast, below contrasts of about 0.4.
3.
Spiking responses adapt so that cells do not give a steady state response to stimulus frequencies of over 10 Hz. Graded potential responses to sinusoidal flicker exhibit a powerful second harmonic component.
4.
A description is given of a simple way in which the responses of this group of cells can be obtained by linear and nonlinear operations on the photoreceptor input.
During shape discrimination experiments bees are seen to fly slowly around in front of a shape before landing. The significance of this behaviour was investigated by recording flight paths (Figs. 1, 5) and landing patterns during choice experiments. The analysis consisted of two steps.1.
To show that changing the shape changes the flight path of the bee in a related way (Figs. 2, 3):
a.
the overall pattern of flight traces out the form of the shape,
b.
the bee tends to fly to novel areas of a shape directly,
c.
the bee spends an increased amount of time flying directly in front of novel areas.
2.
To show that different flight paths to the same shape are reliably associated with different frequencies of choosing the shape. Flight paths were divided into categories and it was demonstrated that the bees frequency of landing is related to the area of the shape it flies across, bees encountering novel areas having an altered frequency of landing (Table 1). This change results from a changed frequency in the novel area itself coupled with a tendency to fly away from the shape directly from these changed areas rather than going on to investigate other parts of the shape (Fig. 4).
In addition it was noted that the entrance hole is an important locus, overall most flights passing to this region wherever they begin and many landings being recorded here.As bees fly to novel areas of a shape directly and the subsequent flight in front of them affects the choice frequency, the flight may be regarded as a visual exploration or scanning of the shape and one part of a sequence of behaviours in which assessment of shape similarity is made. It is discussed how, for different shapes, different parts of this sequence may be of different importance in determining the overall choice frequency.
1.
Temporal resolving power of freely flying bees is measured by training the bees to choose between two simultaneously-presented visual timuli, one of which is rewarded. Each stimulus consists of a multisectored black-and-white or two-coloured disc. The rewarded stimulus rotates rapidly enough that the intensity fluctuations produced by its sectors are beyond the bees' flickerfusion frequency. The unrewarded stimulus differs from the rewarded one only in rotational speed: It is either stationary (in some experiments) or rotates at a slower speed, producing a temporal frequency well below flicker-fusion. After training, the rewarded stimulus is tested against a stimulus whose temporal frequency (f) is varied from test to test. The bees' relative preference (%) for the rewarded stimulus is measured as a function off to obtain a frequency-response curve (FRC).
2.
In the frequency range 60–90 Hz, the FRC attains a peak value of 100% (i.e. the rewarded stimulus is chosen consistently). At ca. 200 Hz the FRC drops to 50% (i.e. the bees choose randomly between the two stimuli) (Figs. 4, 5, 9). This suggests that movement fusion occurs at ca. 200 Hz, under our experimental conditions.
3.
Video films taken of the bees during flight indicate that the bees experience difficulty in approaching the rotating disc when it presents a temporal frequency higher than 8 Hz, and that this difficulty persists up to ca. 200 Hz, beyond which movement is evidently not perceived (Figs. 6 and 7). This movement-avoidance response is a largely inflexible phenomenon, apparently innate to the bees. It appears to be the main factor determining the shape of the FRC. However, at frequencies below 8 Hz, the FRC can be modified by appropriate training (Fig. 9).
4.
Experiments with coloured patterns reveal that the green receptors play a dominant role in the bees' performance in their assigned task (Figs. 10 and 11).
5.
Reducing the intensity of ambient light to below that normally used in our experiments, does not greatly affect the FRC at low (9 Hz) or intermediate (30 Hz) frequencies, but depresses it at high frequencies (144 Hz), suggesting a reduced fusion frequency at low light-levels (Fig. 12).
6.
In our experimental situation, the temporal acuity of the bees appears to be limited by the receptors rather than by slower processes at higher nervous levels.
1.
In a series of behavioural experiments designed to measure spatial acuity, freely-flying honeybees were trained to discriminate between a horizontal and a vertical grating in a Y-shaped, dual-tunnel apparatus (Fig. 1). Each grating was placed at the entrance to a tunnel, and one of the gratings carried a reward of sugar solution. After training, the spatial frequency of the two gratings, as seen from the tunnel entrances, was varied by varying, symmetrically, their distances from the tunnel entrances. At each spatial frequency, the bees' response (percentage correct discriminations) was calculated from the number of entrances that they made into the tunnels associated with the rewarded and unrewarded gratings, to obtain Response-versus-spatial-frequency (RSF) curves (Figs. 3–5, 10–12).
2.
In general, response decreases with increasing spatial frequency. With black-and-white gratings, the RSF curve exhibits a corner spatial frequency (corresponding to a response level of 65%) of ca. 0.25 cycles per degree of visual angle (c/deg), and the response is statistically indistinguishable from the 50% level (corresponding to random choice) at a spatial frequency of ca. 0.34 c/deg (Figs. 3, 4).
3.
The shape of the RSF curve does not depend upon which grating is rewarded (horizontal or vertical). Similar RSF curves are also obtained when bees are trained to discriminate a horizontal or a vertical grating from a uniform grey field (Figs. 4, 5). These results imply that the acuities in the horizontal and vertical plane at the front of the eye are essentially equal for freely-flying bees.
4.
An independent estimate of spatial acuity was derived by analysing video-films of the bees' flight trajectories when they approached the incorrect stimulus (Figs. 6–9). This analysis estimates acuity to be slightly poorer (corner spatial frequency: ca. 0.18 c/deg) than that inferred from the RSF curves. The films also confirm that the bees use the frontal regions of their eyes in making the visual discrimination (Fig. 10).
5.
Detailed observation of the bees' decision behaviour indicates that, when the stimuli in the two tunnels cannot be distinguished at the tunnel entrances, each individual choice that a bee makes between the two tunnels tends to be independent of the outcome of the previous choice. That is, the success or failure of a choice is not memorized.
6.
Comparison of acuity measured behaviourally with that predicted from the optical characteristics of the compound eye, reveals that the bees' capacity to discriminate orientation of linear gratings is limited primarily by the size of the visual fields of individual photoreceptors.
7.
Experiments conducted using grey gratings, and dual-colour gratings which provide contrast exclusively to a single spectral class of receptors (blue or green) reveal that orientation discrimination of vertical and horizontal high-spatial-frequency gratings is mediated chiefly by the greenreceptor channel, and is therefore colour-blind (Figs. 10–13).
8.
Experiments investigating orientation discrimination of dual-colourradial gratings reveal an acuity that is poorer andnot colour-blind (Fig. 14). We suggest that, besides the high-acuity colour-blind mechanism, there is another mechanism that is not colour blind, works on the basis of a memorized, colour-coded spatial template, and has lower spatial acuity.
The visual fixation response of the mealworm beetle Tenebrio molitor, elicited by black stripes upon a bright background is studied in an arena and by means of the Y-maze technique. In the arena the distribution n() of the beetle's angular position is measured at different distances from the centre, which is also the starting point. If the black stripe is narrow, the maximum of n() coincides with the centre of the stripe (centre-fixation Figure 1a). If one half of the panorama is black, the distribution n() has two maxima, which are near the borders between the black and white regions (edge-fixation Figure 1b). In the Y-maze experiments the beetle is tethered, but its head is free to move. The black stripes elicit turning tendencies F(), the strength of which depends upon the angular distance between the centre of the stripe and the animal's body axis. If the black stripe is narrow, the stable zero crossing of F() lies at =0, in agreement with the centre fixation in the arena (Fig. 3). If the stripe is 180 wide, two stable zero crossings are obtained near the border lines between the black and white regions, provided that the panorama is rotated around the animal with an angular velocity w larger than about 0.08/s (Fig. 4). Below this value of w only one stable zero crossing at =0 exists (Fig. 6). Thus the tethered beetle's response underlies a transition between centre resp. edge fixation at a critical angular velocity of the drum. Some implications of this surprising phenomenon with respect to the mechanism of fixation and negative phototaxis are discussed but at present it is considered primarily a challenge for further investigation.