Local motion detectors in fruit flies sense complex patterns generated
by their own motion
Direction-selective neuron subtypes detect complex motion patterns and
not uniform directions of motion

Date:
April 5, 2022
Source:
Johannes Gutenberg Universitaet Mainz
Summary:
Scientists have gained new insights into how the eye of Drosophila
processes motion patterns that are generated by self-motion through
space. They have discovered that direction-selective cells can
distinguish six types of global motion patterns.



FULL STORY ========================================================================== Simple behaviors such as walking or driving require the human eye to
process complex visual cues to allow proper navigation. A fly eye needs
to accomplish even more to guide appropriate behavioral responses during flight. The fruit fly Drosophila melanogaster relies on rapid detection
and processing of information from its eyes to its nervous system to
adjust its behavior to an ever-changing environment. Scientists at
Johannes Gutenberg University Mainz (JGU) have gained new insights
into how the eye of Drosophila processes motion patterns that are
generated by self-motion through space. They have discovered that direction-selective cells can distinguish six types of global motion
patterns. "We thought that the visual system of Drosophila first detects
the four cardinal directions of motion, i.e.,front-to-back, back-to-front, upward, and downward," said Professor Marion Silies, the leader of the
study. "However, the computation of the motion patterns we have now
discovered matches Drosophila's actual behavior much closer."

==========================================================================
Each T4/T5 subtype can recognize one specific motion pattern A
fruit fly's compound eye consists of 800 visual units organized in a
hexagonal array. Each individual eye, in turn, is equipped with multiple photoreceptors, which pick up light stimuli from the environment. From
here, the information is then processed in the visual system and
transmitted to the central nervous system.

On the way from the photoreceptors to the brain, various neurons are
involved in processing image and motion information. Among these are
T4 and T5 cells, which act as local motion detectors. T4/T5 cells are
the first direction- selective cells in the eye, just a few cell layers
behind the photoreceptors.

They occur together and respond to moving bright contrasts in the case
of T4 cells and to moving dark contrasts in the case of T5 cells. If
fruit flies lack these cells, they cannot react to motion stimuli from
the environment and are "motion-blind." Previously, it was assumed
that there are four subtypes of T4/ T5 neurons and that each of the
800 individual units presents one of four directions by four T4 and
four T5 cells corresponding to local motion from specific regions in
visual space. This implied that all cells of a single subtype would
react to uniform motion -- either front-to-back, back-to-front, upward,
or downward -- and pass on the corresponding information.

Neurons represent the fly's actual behavior "The process is complicated
and it has been unclear how the flies could create a complex pattern
from these four directions of motion," said Dr. Miriam Henning from
Professor Marion Silies' group. The researchers employed two- photon
imaging to monitor the population activity of more than 3,500 of these
local T4 and T5 motion detectors. They revealed that the process involves
not just four, but six subtypes, which contribute to correctly sensing
and relaying the flies' movement through space. The findings have been published now in Science Advances. Henning, the lead author of the study,
has just received the Bernstein SmartSteps Award for her work from
the Bernstein Network Computational Neuroscience, a research network established in 2004 as part a funding initiative of the German Federal
Ministry of Education and Research (BMBF).

"The individual subtypes do not encode uniform directions of motion,
as we previously thought. Instead, each subtype consists of a group of direction- selective neurons that directly represent a complex global
motion pattern composed of many different local motion cues," explained
Dr. Miriam Henning.

"This matches the fly's real behavioral pattern much more closely,
the way it actually moves in space." In doing so, the subtypes all work together at the same time, but they are activated differently.

Previous work on mice demonstrated that the direction-selective neurons
in the mouse eye -- in this case, retinal ganglion cells -- likewise
represent the animal's self-motion as a complex pattern. Interestingly, however, only four subtypes exist in mice, while there are six subtypes in flies. Global motion computation of this kind may, therefore, have arisen independently twice during evolution. The authors of the study suggest
that the different number of subtypes may correspond to the different
patterns of self-motion: flying animals have to cover a three-dimensional space, while running animals mostly move in two dimensions.

Paradigm shift in neurobiology Neuronal processing of motion information
in Drosophila melanogaster has been studied for about 60 years and it
has been known since 2013 that T4 and T5 cells function as the local
motion detectors in the fruit fly eye. "The new findings are a paradigm
shift in our field, the neurobiology of vision," emphasized Dr. Marion
Silies, relating to their latest findings. "It seems to make more sense
to capture complex motion patterns directly rather than capturing four
uniform directions and then transform them into global patterns related
to self-motion in subsequent visual processing. In addition, the six
T4/T5 cell subtypes better match the hexagonal structure of the fly eye." However, many questions remain unanswered. The researchers still don't
know, for example, how the direction-selective subtypes map to different behaviors in species with different running or flying behaviors and how
they themselves control these behaviors. "We would like to explore this
in the future," said Silies.


========================================================================== Story Source: Materials provided by
Johannes_Gutenberg_Universitaet_Mainz. Note: Content may be edited for
style and length.


========================================================================== Journal Reference:
1. Miriam Henning, Giordano Ramos-Traslosheros, Burak Gu"r, Marion
Silies.

Populations of local direction-selective cells encode global motion
patterns generated by self-motion. Science Advances, 2022; 8 (3)
DOI: 10.1126/sciadv.abi7112 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2022/04/220405102857.htm

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