Deconstructing Hunting Behavior Reveals a Tightly Coupled Stimulus-Response Loop.

Animal behavior often forms sequences, built from simple stereotyped actions and shaped by environmental cues. A comprehensive characterization of the interplay between an animal's movements and its environment is necessary to understand the sensorimotor transformations performed by the brain. Here, we use unsupervised methods to study behavioral sequences in zebrafish larvae. We generate a map of swim bouts, revealing that fish modulate their tail movements along a continuum. During prey capture, larvae produce stereotyped sequences using a subset of bouts from a broader behavioral repertoire. These sequences exhibit low-order transition dynamics and immediately respond to changes in visual cues. Chaining of prey capture bouts is disrupted in visually impaired (lakritz and blumenkohl) mutants, and removing the prey stimulus during ongoing behavior in closed-loop virtual reality causes larvae to immediately abort the hunting sequence. These results suggest that the continuous integration of sensory information is necessary to structure the behavior. This stimulus-response loop serves to bring prey into the anterior dorsal visual field of the larvae. Fish then release a capture strike maneuver comprising a stereotyped jaw movement and tail movements fine-tuned to the distance of the prey. Fish with only one intact eye fail to correctly position the prey in the strike zone, but are able to produce the strike itself. Our analysis shows that short-term integration of binocular visual cues shapes the behavioral dynamics of hunting, thus uncovering the temporal organization of a goal-directed behavior in a vertebrate.

A Visual Pathway for Looming-Evoked Escape in Larval Zebrafish.

Avoiding the strike of an approaching predator requires rapid visual detection of a looming object, followed by a directed escape maneuver. While looming-sensitive neurons have been discovered in various animal species, the relative importance of stimulus features that are extracted by the visual system is still unclear. Furthermore, the neural mechanisms that compute object approach are largely unknown. We found that a virtual looming stimulus, i.e., a dark expanding disk on a bright background, reliably evoked rapid escape movements. Related stimuli, such as dimming, receding, or bright looming objects, were substantially less effective, and angular size was a critical determinant of escape initiation. Two-photon calcium imaging in retinal ganglion cell (RGC) axons revealed three retinorecipient areas that responded robustly to looming stimuli. One of these areas, the optic tectum, is innervated by a subset of RGC axons that respond selectively to looming stimuli. Laser-induced lesions of the tectal neuropil impaired the behavior. Our findings demonstrate a visually mediated escape behavior in zebrafish larvae exposed to objects approaching on a collision course. This response is sensitive to spatiotemporal parameters of the looming stimulus. Our data indicate that a subset of RGC axons within the tectum responds selectively to features of looming stimuli and that this input is necessary for visually evoked escape.

A dedicated visual pathway for prey detection in larval zebrafish.

Zebrafish larvae show characteristic prey capture behavior in response to small moving objects. The neural mechanism used to recognize objects as prey remains largely unknown. We devised a machine learning behavior classification system to quantify hunting kinematics in semi-restrained animals exposed to a range of virtual stimuli. Two-photon calcium imaging revealed a small visual area, AF7, that was activated specifically by the optimal prey stimulus. This pretectal region is innervated by two types of retinal ganglion cells, which also send collaterals to the optic tectum. Laser ablation of AF7 markedly reduced prey capture behavior. We identified neurons with arbors in AF7 and found that they projected to multiple sensory and premotor areas: the optic tectum, the nucleus of the medial longitudinal fasciculus (nMLF) and the hindbrain. These findings indicate that computations in the retina give rise to a visual stream which transforms sensory information into a directed prey capture response.

Select Drosophila glomeruli mediate innate olfactory attraction and aversion.

Fruitflies show robust attraction to food odours, which usually excite several glomeruli. To understand how the representation of such odours leads to behaviour, we used genetic tools to dissect the contribution of each activated glomerulus. Apple cider vinegar triggers robust innate attraction at a relatively low concentration, which activates six glomeruli. By silencing individual glomeruli, here we show that the absence of activity in two glomeruli, DM1 and VA2, markedly reduces attraction. Conversely, when each of these two glomeruli was selectively activated, flies showed as robust an attraction to vinegar as wild-type flies. Notably, a higher concentration of vinegar excites an additional glomerulus and is less attractive to flies. We show that activation of the extra glomerulus is necessary and sufficient to mediate the behavioural switch. Together, these results indicate that individual glomeruli, rather than the entire pattern of active glomeruli, mediate innate behavioural output.

Propagation of olfactory information in Drosophila.

Investigating how information propagates between layers in the olfactory system is an important step toward understanding the olfactory code. Each glomerular output projection neuron (PN) receives two sources of input: the olfactory receptor neurons (ORNs) of the same glomerulus and interneurons that innervate many glomeruli. We therefore asked how these inputs interact to produce PN output. We used receptor gene mutations to silence all of the ORNs innervating a specific glomerulus and recorded PN activity with two-photon calcium imaging and electrophysiology. We found evidence for balanced excitatory and inhibitory synaptic inputs but saw little or no response in the absence of direct ORN input. We next asked whether any transformation of activity occurs at successive layers of the antennal lobe. We found a strong link between PN firing and dendritic calcium elevation, the latter of which is tightly correlated with calcium activity in ORN axons, supporting the idea of glomerular propagation of olfactory information. Finally, we showed that odors are represented by a sparse population of PNs. Together, these results are consistent with the idea that direct receptor input provides the main excitatory drive to PNs, whereas interneurons modulate PN output. Balanced excitatory and inhibitory interneuron input may provide a mechanism to adjust PN sensitivity.