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(l to r) Alex Schier, Martin Haesemeyer and Florian Engert

As we explore our environment we constantly experience new sensations – changes in the visual scene, different sounds or changes in temperature. Many of these cues can be safely ignored but our brain continuously needs to process incoming information to decide whether to act on it or not. Depending on the stimulus it can be advantageous to act quickly or to accumulate evidence over longer time-scales before executing a behavioral action. By challenging model organisms in the laboratory with stimuli and recording their behavioral actions we can understand how brains process sensory information to decide on appropriate behavioral actions.
Larval zebrafish live in shallow waters, which means that the environmental temperature abruptly changes with changes in sunlight intensity. As cold-blooded animals the body temperature of zebrafish is determined by the temperature of the surrounding water. Accordingly, larval zebrafish rely on their ability to detect external temperature and execute appropriate behaviors to be able to navigate away from areas that are either too hot or too cold. Indeed, from three days post fertilization zebrafish have been shown to increase activity with increasing temperature (Prober et al., 2008) and to actively avoid both hot and cold waters (Gau et al., 2013). However, how the zebrafish nervous system interprets temperature stimuli to execute changes in behavior and whether zebrafish accumulate temperature information over long or short timescales to make behavioral decisions is poorly understood.
In a recent paper Martin Haesemeyer, Alexander Schier, Florian Engert and colleagues investigate the timescales over which freely swimming larval zebrafish integrate sensory heat information to control swim initiation. Probing these timescales requires delivery of heat stimuli with high temporal precision, but quickly heating or cooling the water contained in an arena in which fish freely swim is highly impractical. The researchers solved this problem by tracking fish in real time, and using a set of scan-mirrors (as found in a confocal microscope) to center the beam of a high-power infrared laser directly on swimming larval zebrafish. This setup allows precise delivery of stimuli, directly heating the larvae without having to heat all water in the arena. Zebrafish robustly avoid the laser stimulus in a manner similar to navigating away from heat-sources, and increase their swim activity with increasing laser power.
With this behavioral setup in hand, they developed a random heat stimulation protocol changing the temperature experienced by larval zebrafish at short time intervals. This type of stimulation paradigm is an excellent tool to identify stimuli that effectively trigger behaviors without assuming any prior knowledge of what stimuli an animal is most sensitive to. Importantly, it allows to identify the timescales over which a stimulus is integrated and to derive the stimulus features that effect behavior. Using their stimulation paradigm, Haesemeyer and colleagues show that larval zebrafish are sensitive to temperature information in the short interval of 400 ms preceding a swim. This allows zebrafish to quickly react to temperature stimuli without “much deliberation”. The results furthermore revealed that not only absolute temperature levels but also increases in heat effectively drive swimming. This added sensitivity to change is wide-spread in biology – most animals including us are considerably more sensitive to moving objects than static scenes. This added sensitivity to changes means that larval zebrafish can effectively react to temperature increases independent of the current environmental temperature a feature that likely aides in heat avoidance.
Understanding how the zebrafish brain transforms temperature sensation into behaviors together with brain wide imaging techniques available in larval zebrafish (Ahrens et al., 2012; 2013; Portugues et al., 2014) will make it possible to understand the computations the brain performs to translate heat sensation into action.
Read more in Cell Systems or download PDF; a new journal with a focus on systems level analysis across scientific disciplines.

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