Flies have more advanced cognitive abilities than previously believed, according to a new study that assessed the insects using a custom-built immersive virtual reality environment and real-time brain-activity imaging.
Researchers at the University of California San Diego’s Kavli Institute for Brain and Mind (KIBM) have found attention, working memory, and capabilities similar to conscious awareness in fruit flies – traits that have typically only been tested in mammals.
The study, published Wednesday in the journal Nature, threw light on the formation, distractibility, and eventual fading of a memory trace in the tiny brains of fruit flies (Drosophila melanogaster).
“Despite a lack of obvious anatomical similarity, this research speaks to our everyday cognitive functioning – what we pay attention to and how we do it,” Ralph Greenspan, study senior author and associate director of KIBM, explained in a statement.
“Since all brains evolved from a common ancestor, we can draw correspondences between fly and mammalian brain regions based on molecular characteristics and how we store our memories,” Dr Greenspan added.
In humans and higher-order mammals, researchers have extensively studied two forms of associative learning: delay conditioning and trace conditioning.
In delay conditioning, an unconditioned stimulus – such as an electric shock or any stimulus that can naturally and automatically trigger a response without prior learning or practice – is introduced in the final moments of a conditioned stimulus like a tone with both ending at the same time.
And in trace conditioning, a “trace” interval separates the conditioned stimulus and the unconditioned stimulus, requiring organisms to have some form of memory or neural representation of the initial stimulus.
In the new study, scientists created an immersive virtual reality environment to test the fly’s behaviour via visual stimulation and coupled the displayed imagery with an infrared laser as an averse heat stimulus.
The virtual arena provided the Drosophila a nearly 360-degree panoramic space to flap their wings freely while remaining tethered, and as the VR constantly updated based on their wing movement (analysed in real-time using high-speed machine-vision cameras), it gave the flies the illusion of flying freely in the world, scientists said.
Researchers created a virtual reality arena, which coupled with in vivo fluorescence brain activity imaging, helped observe brain structures implicated in learning and memory formation during conditioning
(Dhruv Grover, UC San Diego KIBM)
Using the setup, scientists could train and test Drosophila for conditioning tasks by allowing the flies to orient away from an image associated with the negative heat stimulus, and towards a second image not associated with heat.
They used the two types of conditioning: One in which the flies were given visual stimulation overlapping in time with the heat (delay conditioning), both ending together, and in the second – trace conditioning – by waiting 5 to 20 seconds to deliver the heat after showing and removing the visual stimulation.
Here, the intervening time, researchers said, is considered the “trace” interval.
During this intervening time, they said, the fly retains a “trace” of the visual stimulus in its brain – a feature indicative of traits like attention, working memory and conscious awareness in mammals.
Scientists also imaged the flies’ brains to track calcium activity in real-time using a fluorescent molecule they genetically engineered into their brain cells.
This setup allowed researchers to record the formation and duration of the fly’s living memory since they saw the trace blinking on and off while being held in the fly’s short-term memory.
The study also found that a distraction introduced during training in the form of a gentle puff of air made the visual memory fade more quickly in the flies.
“This work demonstrates not only that flies are capable of this higher form of trace conditioning, and that the learning is distractible just like in mammals and humans, but the neural activity underlying these attentional and working memory processes in the fly show remarkable similarity to those in mammals,” said Dhruv Grover, a UC San Diego KIBM research faculty member and lead author of the new study.
“This work demonstrates that fruit flies could serve as a powerful model for the study of higher cognitive functions. Simply put, the fly continues to amaze in how smart it really is,” Dr Grover added.
Researchers also found the area of the fly’s brain where the memory formed and faded, known as the ellipsoid body of the fly’s central complex.
This brain region in the fly, scientists say, corresponds to the outer layer in the human brain called the cerebral cortex.
The study also found that the brain chemical dopamine is required for the fly’s learning and higher cognitive functions, and the neurochemical’s reactions increasingly occurred earlier in the learning process, eventually anticipating the coming heat stimulus.
Scientists are currently studying details of how attention is physiologically encoded in the brain.
They believe the findings from the fruit fly model system can directly inform our understanding of human cognition and neural disorders that disrupt them.
Researchers say the study can also contribute to new engineering approaches for better artificial intelligence designs.
