In a way, fruit flies are like us. They have eyes, legs, a nervous system and they love fruit. Unlike us, however, they only have a few thousand neurons in their brains, meaning scientists can map every cell but every connection between them – producing for the first time a complete digital “connectome” of a living creature that is, when you think about it, basically a human.
I may be exaggerating our similarities to fruit flies, commonly referred to by their scientific name, Drosophila (melanogaster, although that part isn’t usually needed), but there’s a reason we use them in many biological experiments. You might think you don’t look much like one of these creatures, but you certainly look more like a fruit fly than bacteria or a dinoflagellate. Understanding even a relatively simple animal like Drosophila tells us a lot about animals and life in general.
Despite being, along with yeast, perhaps the best understood organisms, a single Drosophila is still orders of magnitude too complex to simulate all aspects. Hell, we’re having trouble simulating a single cell properly. However, if you consider a creature not as a gestalt but as a set of interrelated systems, you can start biting the elephant.
The most recent bite, from a team led by Cambridge University biologists, is a “synapse-by-synapse map” of a Drosophila larval brain. With 3,016 neurons and 548,000 synapses, that’s ten times the complexity of the last organism whose brain was mapped, a congressman. (In fact, it was one of the worst kinds of worms, an annelid. Humans have about 86 billion neurons and almost countless synapses.)
The fruit fly larva is of course not a fly, but it is already a sophisticated creature, with adaptive behaviors, brain-like structures of adult flies, short- and long-term memory, and other expected brain functions. Plus, they’re easier to catch. More importantly, he has “a compact brain with several thousand neurons that can be imaged at the nanometer scale with electron microscopy (EM) and its circuitry reconstructed in a reasonable amount of time,” as the paper published today puts it. today in Science. In other words, it’s the right size and not too weird.
The brain was sliced into impossibly thin layers and imaged by EM, and the resulting slices were carefully examined to determine how neurons, axons, and other cellular structures continued together. “We developed an algorithm to track signal propagation throughout the brain through polysynaptic pathways and analyzed feedforward (sensory to output) and feedback pathways, multisensory integration, and interactions between hemispheres. “, they write.
The result is the model you see, looking like a slug wearing a clown wig (I needn’t add that’s not what it looks like live).
Of course, there are a lot of interesting observations about how the brain is organized, nested recurrent loops, multisensory integration, interactions between hemispheres, and all that good stuff. But having a full connectome of a complex living creature is fundamentally exciting for anyone in this space – there’s a lot you can do when you have a decent simulation of a brain. While previous studies have replicated individual subsystems or smaller brains, this is the largest and most comprehensive characterization to date and, as a 3D digital resource, will almost certainly be used and cited throughout the discipline.
Some of these things are even found in artificial neural networks; studying how such complex behavior is produced by such a sparsely populated brain could “perhaps inspire new machine learning architectures.”
Interestingly enough, we already have a detailed mechanical model of the body and movements of the adult fly, and while the question is obvious, the answer is no: we can’t put this brain in this body and say we’ve simulated the whole thing. But maybe next year.