Day 3 on Navigation: Barbara Webb, Julien Serres and Pavan Ramdya

The day started with an introduction from Florian Engert who posed the question "What problems are animal solving or evolved to solve?". Then he posed three specific questions that are interesting to ask:

First: Identify what is the problem the animal is solving.

 

Second: how does it solve it? What is the nature of the input? What is the dynamics and what is the precise nature of the output? What is the algorithm through which the animal solve it?

 

Third: How is it implemented?

 

He mentioned that the day reovolves around these questions in the context of navigation. Naviagation in terms of Today is insect-heavy. 

 

Baraba mentioned that you can never be insect heavy (she loves insect brain) :)

 

Although today's discussion is mostly about insects, Florian mentioned that he believes the same principles apply to all animals. 

 

Barbara started by asking only students (and not faculty and navigation experts) to say what comes to their minds when they think about the navigation. These are the words that were mentioned:

 

Hippocampus

Landmarks

Optical flow

Destination

Mapping

Path integration

Sequence prediction

Polarization

SLAM

Food gradients

Magnetic field

Exploration

Cognitive map

Flocking

Obstacle avoidance

Place preference

Whiskers

World model

Planning

Foraging

Random walk

 

 

Great!

 

Three things that insects do:

 

Ants do path integration to go from nest to food. Wasps also do it. Maximum range is about 10km that they go. For ants the maximum is 1km. They try to go on a straight line, but sometimes there is something on the way so they have to deviate. 

 

Florian: this is phenomenal. They keep two things in mind: angle to the nest and distance. 

Ants use the sun as the compass. They can also sense the magnetic field. They have an actual compass. Some insects also use the moon. Even when they want to do height climbing they do the path integration. 

There was a lot of engagement from the audience with many questions.

Someone asked if insects try to minimize energy usage? They are trying to come back to the nest as soon as they can. 

 

If the ant wants to go from nest (N) to food (F) and there are some bushes on the way, they have to go around. They use visual memory to find their way. Path Integration (PI) and Visual memory. If they go on a routh a couple of times, if we pick them up from Nest and put them on food, they have the visual memory to follow the same route to go back to the nest. 

 After Barbara, Julien Serres talked about other mechansims by which insects find their way. One is the magnetic compass and the other is the polarization. 

Julien is showed his direction polarization device. Direction polarization is what also helps for navigation. Florian mentions that the problem is to find the direction of the sun. If the sun is not in the sky it helps to find the direction of the sun. If it is in the sky it is not a hard problem. Especially if the sun is below the horizon but you have a very good polarization direction. 

Julien further elaborated that the wind doesn’t affect the sky in terms of information, but some insects do use wind as a semi-reliable cue for navigation. However, it doesn’t change the way polarization sets in the sky. Dung beetles are good examples of using wind and polarization to move their so-called “dung”.Optic flow is the angular velocity of the contrasted features moving in your visual field. There are many properties inside the optic flow but the major useful information is the optical velocity of the ground. When you are moving along the ground, you can see an apparent movement of the ground which depends on the ratio of your speed and your eye above the ground. This is useful for ants to integrate this information in order to compute the distance and it’s also one of the major pieces of information for bees to control their altitude.Julien designed an experiment where using a pair of mirrors, honeybees would crash on the ground without the optic flow information to set their altitude.In colloquial terms, it’s what you “see as you move”. You use it to set your speed and it’s confounded by your distance. Bees keep relatively steady heights by using such optical flow.Florian then talked about visualizing an airplane and taking off. When you start, the ground moves very quickly as you’re taking off. As you’re lifting off, the relative speed of the ground changes. Same thing as in a train or a car - the farther the objects are, the slower objects seem to move. If you have a sense of how fast you’re going, you can use the relative speed to judge distance. This is a fundamental trick animals use to judge distance. This is called “motion parallax.” However, for optical flow, you need to know the distance to judge the speed. Motion parallax seems to be orthogonal to optic flow because it assumes you don’t know the distance to begin with.But what’s REALLY cool is that dung-beetles use the Milky Way to navigate - crazy, right??? Keep in mind, however, light pollution does play a role. If dung-beetles are close to the light of a city, then they’ll most likely use the city lights to navigate rather than the Milky Way.

After the coffee break, we first started by describing what modeled organisms are. Some examples are: zebra fish, mice, c. Elegans, and fruit flies. The distinguishing factors are: critical mass, genetic tools (meaning, how do you access neurons over and over again and manipulate them). Some kinds of manipulations are: silencing neurons, activate neurons, selectively labeling and tracing the circuits, and recording their activity using optical tools.

What seems to limit the experimentation is availability - what is around us? Mice seem to be everywhere which is the main limitation. The second limitation in experimentation is robustness, which is what relates most of the modeled organisms.

Fruit flies have incredible behaviors: they have appendages for reaching, moving, etc. The history of the fly, however, is more interesting. Their genetics are studied very well and fruit flies became the organisms to study the genome. As a result, scientists created lots of genetically varied fruit flies.

So how do we interact and target neurons? We use transgenesis, CRISPR, and more broadly, the UAS Gal4 system. Basically, there is a protein that targets a specific portion of the DNA that triggers another reaction that allows you to look at features of neurons.

Another interesting tool is optogenetics. This is where we can take a protein  channel that’s light sensitive and use light to cause neurons to turn on or off with light.

An important side-note is that the work done on activating neurons is the “tip of the iceberg” that seems to be biased by high-profile journals.

Pavan went on to talk about the neurophysiology of a fruit fly by illustrating the limb-topology. Interesting and fun affect of activating the “moonwalking” neurons is moonwalking! - or walking backwards.

We finished the morning sessions by talking about the mushroom bodies in flies that are associated with learning. They are sparsely connected networks that have few output neurons (called mushroom body output neurons).