Neurons in a visual brain area of zebrafish are arranged as a map for
catching prey

Date:
October 19, 2020
Source:
Max-Planck-Gesellschaft
Summary:
Spotting, pursuing and catching prey - for many animals this is
an essential task for survival. Scientists now show in zebrafish
that the localization of neurons in the midbrain is adapted to a
successful hunting sequence.



FULL STORY ========================================================================== Spotting, pursuing and catching prey -- for many animals this is an
essential task for survival. Scientists at the Max Planck Institute of Neurobiology now show in zebrafish that the localization of neurons in
the midbrain is adapted to a successful hunting sequence.


==========================================================================
Far away, in the periphery of its visual field, a tiny zebrafish larva
detects a small dot moving sideways. Is it prey or is it a threat,
for instance, a distant predator sneaking up on it? Within the shortest possible time, the fish decides that it must be potential prey. The larva
turns toward the object, approaches it, until it is right in front, and
snaps shut -- one of its daily hunting routines is successfully finished.

What might sound straightforward, is actually a highly
complex process. Many different visual stimuli are detected
simultaneously, transferred from the eye to the brain, and further
processed. Interestingly, the stimuli don't reach the brain at random locations: every position on the retina is transmitted to a very specific location in the tectum of the midbrain, the processing hub for visual
stimuli. However, apart from that, there is not much knowledge of how
the neurons are wired and organized, or which signals they specifically
react to.

Dominique Fo"rster and a team from Herwig Baier's laboratory analyzed
how retinal ganglion cells transfer visual information from the eye to
the tectum and how this input is further processed. To do so, zebrafish
larvae were presented in a virtual reality area with different visual
stimuli, ranging from small and big prey-like objects to approaching
threats similar to predatory fish. Using a special microscopy technique,
the researchers not only analyzed the activity of hundreds of neurons
in parallel, but also the location of their cellular projections.

Response to optical stimuli Analyzing this pool of data showed that
retinal ganglion cells as well as neurons in the tectum respond to optical stimuli in a highly specialized manner: While some cells are activated by
small objects, others react to bigger objects, or even threats. Some cells
are interested in the direction of motion, others only in whether the environment gets darker or brighter. Interestingly, the special-purpose
neurons do not distribute randomly in the tectum. Retinal ganglion cells reacting to threats send their projections into deep tectal layers, where
they meet the receiving processes, or dendrites, of tectal neurons. In contrast, cells responding to prey-like objects make synaptic connections
in more superficial layers. This specialization of different tectal
layers most likely allows zebrafish to quickly distinguish between prey
and threat -- an essential skill for survival.

The researchers then discovered that the prey-specific cells of the
upper layers are arranged in a way that is advantageous for the hunting sequence: prey usually appears first at a distance in the peripheral
visual field. Such images are represented in the back part of the
tectum. Strikingly, this region of the tectum is enriched with cells
reacting to small moving objects. When the fish turns toward the prey and approaches it, the prey's image moves to the front part of the tectum
and gets bigger, until it appears directly in front of the fish. There
it is held steady by the fish's own movement, until the prey can be
captured by a vigorous strike. Neurons tuned to visual input with these characteristics, that is, large and steady, sit in the frontal region
of the tectum -- exactly where this kind of information reaches the brain.

This study shows that the arrangement and connectivity of neurons in
the tectal brain map is adapted to the demands of hunting. Specialized
cells localize to brain regions where their function is best suited for an efficient catch. By using the hunting behavior of zebrafish as an example,
the researchers were able to demonstrate the impact of natural selection
on the layout of relevant brain regions. These results remind us that the
way animals (including us) perceive the world is shaped by evolution. The
way the brain is wired has worked best to ensure survival in the past.


========================================================================== Story Source: Materials provided by Max-Planck-Gesellschaft. Note:
Content may be edited for style and length.


========================================================================== Journal Reference:
1. Dominique Fo"rster, Thomas O Helmbrecht, Duncan S Mearns, Linda
Jordan,
Nouwar Mokayes, Herwig Baier. Retinotectal circuitry of larval
zebrafish is adapted to detection and pursuit of prey. eLife,
2020; 9 DOI: 10.7554/ eLife.58596 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2020/10/201019103504.htm

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