Behavioral signatures of a developing neural code

Background:

Animal behavior is probably specified early during development, however, knowledge about the contribution of neuronal circuits to the development and establishment of behavior is sparse. One of the major reasons for this is the challenge of measuring behavior in young mammals, owing to long gestational periods, the need for significant nurturing and absence of measurable behavioral outputs. This limitation can be overcome using developing zebrafish, since zebrafish larvae show robust measurable behavioral outputs. In addition, they are transparent, which makes them amenable to high-resolution brain imaging. In this preprint, Avitan et al use these two advantages to connect early larval hunting behavior with neural coding.

Key findings:

Zebrafish larvae sustain on the nutrients present in the yolk until 5 days post fertilization (dpf), following which they start hunting for sustenance. At this stage, larvae reared in the laboratory can be fed with powdered feed and/or live feed. The authors establish, using recordings from a high-speed camera, that larval zebrafish hunt Paramecia from 5 dpf onwards. Upon quantification of the recordings, the hunting bouts are observed to increase over development, but become shorter, more efficient and accurate as the larvae mature (measured from 5 dpf to 15 dpf). Hence, hunting performance of zebrafish larvae improves with age.

Following the characterization of hunting behavior and the pattern it follows with development, the authors focus on understanding the neural basis of this behavior and its age-dependent refinement. The authors perform 2-photon calcium imaging of the optic tectum (a region of the brain essential for hunting behavior), and measure the amplitude of each neuron’s response to a prey-like stimulus presented at different positions/angles ranging from 45⁰ to 165⁰ of the visual field (in increments of 15⁰; the body axis is 0⁰). The authors repeat this for larvae of different ages (from 5 dpf to 15 dpf). From this heatmap of amplitude of activity for about 200 neurons, at different angles of stimulus presentation compared across development, the authors find that, while rear sensory response does not undergo significant change, response to frontal stimuli improves during development. Hence, there are substantial functional changes at the single neuron and population level from 5-15 dpf as the larvae grow, especially in the frontal stimulus response.

In the next step, the authors use a linear decoder to correlate the original stimulus and the decoded stimulus position. From this, they find that decoding performance is higher as the fish grow, primarily due to improvement of response to frontal stimuli. Further, a population of young larvae shows more variability in decoding performance (correlated with hunting performance), which evens out by 15 dpf, by which time the larvae are expert hunters. Hence, the ability of zebrafish larvae to accurately spatially localize their prey reflects on their hunting success, and both these attributes improve as larvae mature.

Neural coding and hunting behaviour: Hunting accuracy improves with age (middle panel). This is due to better decoding performance especially in frontal sensory response (right panel-younger larvae are clearly less efficient in the frontal visual response, but there is not much difference in rear visual response).

To summarize, these results show that zebrafish larvae become more efficient hunters as they grow. Their hunting efficiency and coding efficiency, which are correlated, improve and are refined as the larvae grow, with a special emphasis on frontal visual response.

Why do I like this work?

As the authors have pointed out in the manuscript, the connection between the establishment of behavior and neural coding is not well understood. The study addresses this caveat in the field, taking hunting behavior as an example. Specifically, I liked the overlap between their findings in the time-dependent refinement of hunting behavior and neural coding.

Future directions/questions for the authors:

1. The brain is very plastic during development. How perfect is the resolution of the imaging that the authors can be sure that they are imaging the same neuron at different ages?
2. In the analysis of neural coding, the authors present prey-like spots and image the optic tectum. Is prey identification solely based on visual cues in larvae? Do other factors like smell play a role?
3. The authors provide correlative evidence that certain neurons respond to cues placed in different angles/positions. Is there any direct proof from related studies that these neurons are indeed involved in visual sensory response?

 

 

doi: https://doi.org/10.1242/prelights.16220

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