A fruit fly (centre) tethered on a spherical treadmill (red and black ball) with a green LED screen flashing in view. Photo credit: Matthew Van De Poll.
A fruit fly (centre) tethered on a spherical treadmill (red and black ball) with a green LED screen flashing in view. Photo credit: Matthew Van De Poll.
It's pretty easy to tell when most creatures doze off, but slumbering flies don't have the luxury of shutting their eyes. Instead, the insects stop moving and when they sleep deeply, they repeatedly extend their proboscises, as if sticking out their tongues. But how do they stop the visual world from intruding when they can't shut it out? ‘A key feature of the sleeping brain in animals is a decreased ability to register changes in the outside world’, says Matthew Van De Poll, from The University of Queensland (UQ), Australia. But it wasn't clear whether flies reign in their vision when slumbering, altering how alert they are to changes in their view, and how that is affected by how deeply they are asleep. Having previously figured out how to record brainwaves in flies while the insects are asleep, Van De Poll and Principal Investigator Bruno van Swinderen (UQ) decided to show a flashing green screen to slumbering fruit flies to find out how the sleeping insects’ brains deal with changes in their view.
Van De Poll secured individual fruit flies to a tiny spherical treadmill that the insects were happy to stroll on during daylight, filming each insect to keep track of when it was active and when it was slumbering before it descended into an even deeper sleep. Then, Van De Poll gently inserted a fine needle-like probe into the insect's brain to record brainwaves in different brain regions: from the deepest central complex – which handles decision making and perception – to the outermost peripheral brain, which processes visual information from the insect's eyes. Once the flies were settled, Van De Poll switched on a flashing green LED screen, placed slightly off to the left, recording thousands of brainwaves per fly overnight in response to the LED flashes. ‘There were so many pieces of data, it would have been easy to lose track... Matt lost a bit of sleep over that’, says van Swinderen, recalling the colossal number of readings that Van De Poll collected.
Impressively, the flies were able to sleep when the screen was flashing in their eyes, although the researchers say that wasn't entirely astonishing, because the flies are able to slumber even when being poked. However, when the duo compared the brainwaves of the conscious and the sleeping insects, they were almost indistinguishable. The sleeping fruit flies’ brains were as active as those of the flies that were awake. ‘The fly brain doesn't block visual stimuli out’, says van Swinderen, which he admits was unexpected.
Then, van Swinderen and Van De Poll tried surprising the flies, sometimes replacing every fifth green flash with a blue flash, other times making the infrequent blue flash appear irregularly (appearing after the third, fourth or fifth green flash) and even switching the green and blue flashes, so the blue predominated with the odd green flash thrown in. From painstaking analysis of the flies’ brainwaves, it was clear that the flies that were awake reacted to the surprise of a different coloured flash turning up unexpectedly. However, when the flies were asleep, the decision-making central brain was less responsive to the sudden surprise flashes, and when the flies slept most deeply, the brainwaves in the central brain were even smaller.
It seems that the brains of sleeping flies pay attention to regular events, even though the insects are immobile and unconscious; the region of the fly's brain that processes vision is still active when they are asleep. But the flies’ brains switch off when visual surprises occur – which require more thought – suggesting that the fly's decision-making central brain holds the key to a good night's sleep for fruit files.
