These cells respond specifically to those object sizes that correspond to a small prey or a large troublemaker in the world of the zebrafish larva. The researchers were now able to identify such cells in a central area of the fish's brain - the tectum. However, little is understood about how these different messages travel via the optic nerve to the brain and are processed. The retina in the eye contains a variety of different ganglion cells which respond specifically to colour, size, movement or contrast, for example. A well-studied predator-prey relationship is that of the snowy owl and the lemmings. Changes in one population will result in changes in the other. The relationships between predator and prey animals make up the delicate balance that is part of an ecosystem. The behaviour-related distinction in size thus begins in the ganglion cells of the eye. Predator species that are not prey for any other animals are known as the top predator. Researchers working with Johann Bollmann at the Max Planck Institute in Heidelberg have now been able to demonstrate that small and large stimuli, which trigger swimming movements in different directions, generate neural activity in neighbouring but different circuits in the fish's brain. The decision about whether the larva approaches or avoids the object is made on the basis of size. The larva's well-developed visual system allows it to catch small prey and avoid larger objects. The basic mechanisms of object classification can be studied using zebrafish larva as the model system. Evidently, the visual system manages to detect objects from the constantly changing distribution of light stimuli on the retina based on simple criteria and, if necessary, mobilise a rapid response directly. How does the brain decide which things in our complex environment require an immediate response from us? A key question in the animal kingdom is: "Is the object moving in my environment prey or predator?" - a question that requires a quick answer in an emergency. Scientists at the Max Planck Institute for Medical Research in Heidelberg have now shed light on how such circuits, which are likely to be crucial in classifying objects by size, function in the brain of the zebrafish larva. If they are activated, they trigger the "fight" or "flight" signal in the brain. The rapid speed of a response suggests that specialised neural circuits in the visual system are responsible for recognising important object properties. When the bird swooped down upon its prey, the team flashed one of five images on a computer monitor lying directly beneath the worm: an owl (a tit predator) with open eyes, an owl with its eyes closed, a butterfly with prominent owl-like eyespots on its wings, a butterfly with eyespots whose colors had been reversed, or a butterfly whose. The size of a moving object is obviously an important criterion. Decisions about how we best respond to moving objects in our environment are often made very quickly and unconsciously. Red or green? Small or large? Fast or slow? Humans and animals rely on their visual organs to classify objects in their environment. Credit: Max Planck Institute for Medical Research This separation begins in the eye and probably decides the direction of the swimming behaviour. Small and large objects activate various circuits in the visual system of zebrafish larvae.
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