Martin Giurfa

Detection of coloured stimuli by honeybees: minimum visual angles and receptor specific contrasts

Honeybees Apis mellifera were trained to distinguish between the presence and the absence of a rewarded coloured spot, presented on a vertical, achromatic plane in a Y-maze. They were subsequently tested with different subtended visual angles of that spot, generated by different disk diameters and different distances from the decision point in the device. Bees were trained easily to detect bee-chromatic colours, but not an achromatic one. Chromatic contrast was not the only parameter allowing learning and, therefore, detection: αmin, the subtended visual angle at which the bees detect a given stimulus with a probability P0 = 0.6, was 5° for stimuli presenting both chromatic contrast and contrast for the green photoreceptors [i.e. excitation difference in the green photoreceptors, between target and background (green contrast)], and 15° for stimuli presenting chromatic but no green contrast. Our results suggest that green contrast can be utilized for target detection if target recognition has been established by means of the colour vision system. The green-contrast signal would be used as a far-distance signal for flower detection. This signal would always be detected before chromatic contrast during an approach flight and would be learned in compound with chromatic contrast, in a facilitation-like process.

Colour preferences of flower-naive honeybees

Flower-naive honeybees Apis mellifera L. flying in an enclosure were tested for their colour preferences. Bees were rewarded once on an achromatic (grey, aluminium or hardboard), or on a chromatic (ultraviolet) disk. Since naive bees never alighted on colour stimuli alone, a scent was given in combination with colour. Their landings on twelve colour stimuli were recorded. Results after one reward (“first test”) were analysed separately from those obtained after few rewards (“late tests”).1)After pre-training to achromatic signals, bees preferred, in the first test, bee-uv-blue and bee-green colours. With increasing experience, the original preference pattern persisted but the choice of bee-blue and bee-green colours increased.2)Neither colour distance of the test stimuli to the background or to the pre-training signal, nor their intensity, nor their green contrast, accounted for the colour choice of bees. Choices reflected innate preferences and were only associated with stimulus hue.3)Bees learned very quickly the pre-trained chromatic stimulus, the original colour preferences being thus erased.4)Colour preferences were strongly correlated with flower colour and its associated nectar reward, as measured in 154 flower species.5)Colour preferences also resemble the wavelength dependence of colour learning demonstrated in experienced bees.

Why are there so many and so few white flowers

Though the world we see appears to be rich in white flowers, this is not the case for animals, such as insects, with ultraviolet (UV) receptors. In fact, flowers that appear white to insects are very rare. We analyse this phenomenon to highlight new discoveries in the mechanisms of insect vision that may have influenced the evolution of flower colour. Our analysis reveals that an understanding of biological signalling requires a comprehensive understanding of sensory physiology and perceptual psychology. An evaluation of UV signals alone may not be helpful, as this can be as inaccurate as models based solely on the human visual system. We interpret floral colours and their frequency in nature from the more relevant perspective of insect colour vision.

Insect visual perception: complex abilities of simple nervous systems

Despite their relatively simple nervous systems, insects display a rich behavioural repertoire, in which vision plays a major role. In the past two years, much knowledge has been gained about how insects are capable of a variety of flexible, visually guided tasks that involve a high level of complexity. From long-range navigation to median-range orientation and close-up recognition, insects apply different strategies that complement each other, that are used sequentially during their approach flight towards their goals, and that may replace each other, depending on the salience of, and the attention towards, particular visual cues.

Revisiting olfactory classical conditioning of the proboscis extension response in honey bees: A step toward standardized procedures

The honey bee Apis mellifera has emerged as a robust and influential model for the study of classical conditioning thanks to the existence of a powerful Pavlovian conditioning protocol, the olfactory conditioning of the proboscis extension response (PER). In 2011, the olfactory PER conditioning protocol celebrated its 50 years since it was first introduced by Kimihisa Takeda in 1961. In this protocol, individually harnessed honey bees are trained to associate an odor with sucrose solution. The resulting olfactory learning is fast and induces robust olfactory memories that have been characterized at the behavioral, neuronal and molecular levels. Despite the success of this protocol for studying the bases of learning and memory at these different levels, innumerable procedural variants have arisen throughout the years, which render comparative analyses of behavioral performances difficult. Moreover, because even slight variations in conditioning procedures may introduce significant differences in acquisition and retention performances, we revisit olfactory PER conditioning and define here a standardized framework for experiments using this behavioral protocol. To this end, we present and discuss all the methodological steps and details necessary for successful implementation of olfactory PER conditioning.

Aversive Reinforcement Improves Visual Discrimination Learning in Free-Flying Honeybees

Background Learning and perception of visual stimuli by free-flying honeybees has been shown to vary dramatically depending on the way insects are trained. Fine color discrimination is achieved when both a target and a distractor are present during training (differential conditioning), whilst if the same target is learnt in isolation (absolute conditioning), discrimination is coarse and limited to perceptually dissimilar alternatives. Another way to potentially enhance discrimination is to increase the penalty associated with the distractor. Here we studied whether coupling the distractor with a highly concentrated quinine solution improves color discrimination of both similar and dissimilar colors by free-flying honeybees. As we assumed that quinine acts as an aversive stimulus, we analyzed whether aversion, if any, is based on an aversive sensory input at the gustatory level or on a post-ingestional malaise following quinine feeding. Methodology/Principal Findings We show that the presence of a highly concentrated quinine solution (60 mM) acts as an aversive reinforcer promoting rejection of the target associated with it, and improving discrimination of perceptually similar stimuli but not of dissimilar stimuli. Free-flying bees did not use remote cues to detect the presence of quinine solution; the aversive effect exerted by this substance was mediated via a gustatory input, i.e. via a distasteful sensory experience, rather than via a post-ingestional malaise. Conclusion The present study supports the hypothesis that aversion conditioning is important for understanding how and what animals perceive and learn. By using this form of conditioning coupled with appetitive conditioning in the framework of a differential conditioning procedure, it is possible to uncover discrimination capabilities that may remain otherwise unsuspected. We show, therefore, that visual discrimination is not an absolute phenomenon but can be modulated by experience.

Cognition with few neurons: higher-order learning in insects

Insects possess miniature brains but exhibit a sophisticated behavioral repertoire. Recent studies have reported the existence of unsuspected cognitive capabilities in various insect species that go beyond the traditionally studied framework of simple associative learning. Here, I focus on capabilities such as attentional modulation and concept learning and discuss their mechanistic bases. I analyze whether these behaviors, which appear particularly complex, can be explained on the basis of elemental associative learning and specific neural circuitries or, by contrast, require an explanatory level that goes beyond simple associative links. In doing this, I highlight experimental challenges and suggest future directions for investigating the neurobiology of higher-order learning in insects, with the goal of uncovering the basic neural architectures underlying cognitive processing.

Detection of coloured patterns by honeybees through chromatic and achromatic cues.

Abstract. We asked whether the detection range of two-coloured centre-surround patterns differs from that of single-coloured targets. Honeybees Apis mellifera were trained to distinguish between the presence and absence of a single-coloured disc or a coloured pattern at different visual angles. The patterns presented colours which were either different in chromatic and L-receptor contrasts to the background, equal in chromatic but different in L-receptor contrasts, or vice-versa. Patterns with colours presenting only chromatic contrast were also tested. Patterns with higher L-receptor contrast in its outer than in its inner element were better detected than patterns with a reversed L-contrast distribution. However, both were detected worse than single-coloured discs of the respective colours. When the L-receptor contrast was the same for both elements, the detection range of the two-coloured and single-coloured targets was the same. Patterns whose colours lacked L-receptor contrast were detected just as single-coloured targets of the same colours. These results demonstrate that both chromatic and L-receptor contrasts mediate the detection of coloured patterns and that particular distributions of L-receptor contrast within a target are better detected than others. This finding is consistent with the intervention of neurons with centre-surround receptive fields in the detection of coloured patterns.

Conceptualization of above and below relationships by an insect

Relational rules such as ‘same’ or ‘different’ are mastered by humans and non-human primates and are considered as abstract conceptual thinking as they require relational learning beyond perceptual generalization. Here, we investigated whether an insect, the honeybee (Apis mellifera), can form a conceptual representation of an above/below spatial relationship. In experiment 1, bees were trained with differential conditioning to choose a variable target located above or below a black bar that acted as constant referent throughout the experiment. In experiment 2, two visual stimuli were aligned vertically, one being the referent, which was kept constant throughout the experiment, and the other the target, which was variable. In both experiments, the distance between the target and the referent, and their location within the visual field was systematically varied. In both cases, bees succeeded in transferring the learned concept to novel stimuli, preserving the trained spatial relation, thus showing an ability to manipulate this relational concept independently of the physical nature of the stimuli. Absolute location of the referent into the visual field was not a low-level cue used by the bees to solve the task. The honeybee is thus capable of conceptual learning despite having a miniature brain, showing that such elaborated learning form is not a prerogative of vertebrates.

The angular range of achromatic target detection by honey bees

Abstract Honeybees Apis mellifera were trained to enter a Y-maze and choose the arm with a rewarded disc presented against a grey background. The alternative arm displayed the unrewarded grey background alone. Training and testing were performed with the rewarding disc subtending different visual angles. The training disc was either achromatic and provided green contrast, or chromatic and provided the same amount of green contrast as the achromatic one. The bee-achromatic disc could be learned and detected by the bees whenever it subtended 5° or 10°, but not if it subtended 30°. The chromatic disc was learned well and detected at all three visual angles. However, at 5° the maximum level of correct choices was ca. 75% with the achromatic disc whilst it was ca. 90% with the chromatic one. Thus, the presence of chromatic contrast enhances considerably the level of correct choices for the same amount of green contrast. The lower threshold of achromatic target detection lies between 3.7° and 5°; the upper threshold between 15° and 10°. At the upper threshold, detection switches from chromatic-based to achromatic-based. Thus, in the context of target detection, the achromatic green contrast channel specialises in the detection of objects of reduced angular size, whilst the chromatic channels are specialised for objects of large angular size. We suggest that achromatic detectors with a centre-surround organisation are involved in the task of detecting achromatic targets.