Visual circuits in arthropod brains
Many arthropods, notably spiders, decapod crustaceans, and insects have excellent visual systems that allow them to evaluate self-movement and environmental features in many different ways. Objects can be discriminated based on luminance contrast, color contrast, and polarization contrast, and moving objects can be traced in front of moving backgrounds. Thus, various qualities are used for the identification and pursuit of conspecifics and prey, for the escape from predators, for monitoring self-movement, for maintaining orientation in space, and on a more basic level, for the entrainment of internal circadian clocks. The neuronal machinery evaluating the inputs from the eyes can be enormous and, in many species, encompasses more than three quarters of all brain neurons (Strausfeld 1976). In insects and crustaceans, primary visual processing takes place in a sequence of optic neuropils, termed lamina, medulla, and lobula complex that are retinotopically organized into layers and columns. Further processing, usually not organized retinotopically and including integration with other sensory modalities and motor control networks, involves large additional areas in the central brain and ventral nerve cord. Despite this complexity in visual signal processing, the repetitive nature of retinotopic modules in the optic lobes and the existence of individually identifiable neurons have favored arthropod, and in particular, insect visual systems as models for the analysis of visual functions at the level of single-identified neurons and neuronal networks. This special issue highlights particular areas of active research aimed at uncovering neuronal networks underlying visual functions such as motion vision, depth perception, color vision, polarization vision, and circadian clock entrainment. Owing to unique molecular genetic techniques and ongoing research toward establishing the complete connectome of the brain (Scheffer and Meinertzhagen 2019), research in these areas increasingly focusses on the fruit fly Drosophila melanogaster. This is also reflected in this special issue. Seven out of 13 articles focus on research in Drosophila. In addition, comparative studies in hoverfly, praying mantis, butterfly, grasshopper, and stomatopods elucidate neural mechanisms underlying particular visual adaptations such as target pursuit, predator approach, prey capture, and mechanisms of color vision. Neural networks underlying motion sensitivity are particularly well understood. Based on behavioral data in a beetle, Reichert and Hassenstein (1956) proposed a model for an elementary motion detection circuit in the insect brain. Research over the last 60 years largely supports this model and has now identified virtually every neuronal cell type involved in the motion vision circuitry in the fly brain. Haag and Borst (2020) summarize the current state of knowledge, provide a step-by-step guidance through the motion vision circuit, and address future directions of research, partly aimed at uncovering synaptic transmission mechanisms within the network. Research in various insect species, notably flies, bees, and locusts has shown that motion vision circuits in the optic lobe are modulated in various ways and at different levels of processing by nutritional status, current behavior, environmental changes, or novel sensory cues. Pharmacological experiments show that centrifugal neurons releasing biogenic amines, in particular serotonin and octopamine, mediate parts of these effects. Cheng and Frye (2020) review the state of knowledge on modulation of visual circuits in insects, address the limitations of current pharmacological evidence, and point out the benefits in future experiments of manipulating selected modulatory neurons in Drosophila using optogenetic tools. Feedback from large-field motion-sensitive neurons of the lobula plate on motor pathways is required in flies to control flight stability during flight maneuvers. Traditional electrophysiological studies have identified nearly 60 different types of these neurons in blowflies, but only a fraction of them in Drosophila. The paper by Wei et al. (2020) used systematic Gal4 driver screens to identify novel types of lobula tangential neurons in Drosophila that are morphologically * U. Homberg homberg@staff.uni-marburg.de