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Neural organisation of innate behaviour in zebrafish larvae
Neural organisation of innate behaviour in zebrafish larvae
Animals’ inner worlds are a hazy imitation of reality, shaped by evolution. Of the infinitude of stimuli that can arise in their natural environment, only a few will bear significance for an animal’s survival and reproductive success. Thus, neural circuits have evolved to extract only these relevant stimuli from the background and connect them to downstream effectors. Sometimes, competing representations of the outside world arise in the brain, and these must be resolved to ensure adaptive behaviour. Through the study of an animal’s behaviour, we can learn about its inner world: which stimuli it cares about; the desires these stimuli engender within it; and how its movements enact and extinguish those desires, allowing new stimuli to emerge that reorchestrate the inner world and refresh the cycle. Here, I present three studies that investigate the emergence of this world in the neural circuits of zebrafish larvae. In the first study, I mapped the behavioural sequences of zebrafish larvae as they pursued and consumed prey. Manipulating their vision with genetic mutants, virtual reality, and lesion studies revealed the dynamic features of stimuli that drive switches in the behaviour. I showed that, by chaining kinematically varied swim types into regular sequences, larvae bring prey to a binocular zone in the near visual field. Here, the fused representation of the stimulus across hemispheres releases stereotyped strike manoeuvres, tuned to the distance to the prey. In the second study, I helped investigate how visual circuits build representations of prey and predator stimuli. Measuring the responses of neurons to visual stimuli revealed how feature selectivity arises from the integration of upstream inputs. Features are unevenly represented across space, matching predicted changes in prey percepts as animals progress through their hunting sequences. When neurons tuned to specific features were ablated, I showed that the detection of prey was altered, no longer eliciting the usual hunting responses from animals. In the third study, I contributed to the discovery of a circuit in the brain that coordinates behavioural responses to competing stimuli. When confronted with multiple threats, animals either ignore one and escape from the other, or average their locations and escape in an intermediate direction. I showed that these two strategies are mediated by distinct swims types. Inhibiting specific neurons in the brain reduced directional escapes, but not intermediate ones, revealing a circuit that contributes to a bottom-up attention mechanism. Together, these three studies reveal the organisation of behaviour within neural circuits of the larval zebrafish brain. Finally, I consider the broader networks in the brain that might implement and modulate responses to salient visual stimuli, and how these circuits could serve as a substrate for behavioural evolution.
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Mearns, Duncan S.
2021
English
Universitätsbibliothek der Ludwig-Maximilians-Universität München
Mearns, Duncan S. (2021): Neural organisation of innate behaviour in zebrafish larvae. Dissertation, LMU München: Graduate School of Systemic Neurosciences (GSN)
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Abstract

Animals’ inner worlds are a hazy imitation of reality, shaped by evolution. Of the infinitude of stimuli that can arise in their natural environment, only a few will bear significance for an animal’s survival and reproductive success. Thus, neural circuits have evolved to extract only these relevant stimuli from the background and connect them to downstream effectors. Sometimes, competing representations of the outside world arise in the brain, and these must be resolved to ensure adaptive behaviour. Through the study of an animal’s behaviour, we can learn about its inner world: which stimuli it cares about; the desires these stimuli engender within it; and how its movements enact and extinguish those desires, allowing new stimuli to emerge that reorchestrate the inner world and refresh the cycle. Here, I present three studies that investigate the emergence of this world in the neural circuits of zebrafish larvae. In the first study, I mapped the behavioural sequences of zebrafish larvae as they pursued and consumed prey. Manipulating their vision with genetic mutants, virtual reality, and lesion studies revealed the dynamic features of stimuli that drive switches in the behaviour. I showed that, by chaining kinematically varied swim types into regular sequences, larvae bring prey to a binocular zone in the near visual field. Here, the fused representation of the stimulus across hemispheres releases stereotyped strike manoeuvres, tuned to the distance to the prey. In the second study, I helped investigate how visual circuits build representations of prey and predator stimuli. Measuring the responses of neurons to visual stimuli revealed how feature selectivity arises from the integration of upstream inputs. Features are unevenly represented across space, matching predicted changes in prey percepts as animals progress through their hunting sequences. When neurons tuned to specific features were ablated, I showed that the detection of prey was altered, no longer eliciting the usual hunting responses from animals. In the third study, I contributed to the discovery of a circuit in the brain that coordinates behavioural responses to competing stimuli. When confronted with multiple threats, animals either ignore one and escape from the other, or average their locations and escape in an intermediate direction. I showed that these two strategies are mediated by distinct swims types. Inhibiting specific neurons in the brain reduced directional escapes, but not intermediate ones, revealing a circuit that contributes to a bottom-up attention mechanism. Together, these three studies reveal the organisation of behaviour within neural circuits of the larval zebrafish brain. Finally, I consider the broader networks in the brain that might implement and modulate responses to salient visual stimuli, and how these circuits could serve as a substrate for behavioural evolution.