Do you remember your first day of school? Perhaps the smell of a new backpack reminds you of that day? But if you are like me, your long-term memory is fuzzy at best. Fortunately, a quick internet search reveals many ways to enhance memory: sleep better, eat a balanced diet and exercise regularly. But just being around a group of fruit flies can improve a fly's memory recollection, according to a study by Aurélie Muria and colleagues from the University of Toulouse, France, and the University of British Columbia, Canada.

How is that possible? Fruit flies release a tiny quantity of carbon dioxide when they are stressed and as they breathe out. Muria and colleagues thought that fruit flies might improve their ability to recall information from their long-term memory after smelling the carbon dioxide released by other flies in the group, resulting in a form of socially facilitated memory, in addition to their existing individual memory.

To understand whether carbon dioxide can help with the retrieval of fly memories, the researchers trained the insects to associate one smell with a mild electric shock and another smell with no electric shock. The flies form an unpleasant memory of the smell associated with the shock. One day after training, the researchers tested this memory in individual flies and in a group of flies in a maze with two compartments, checking whether the flies avoid the compartment with the smell that they associated with the unpleasant shock. The flies that had been shocked into disliking an odour experienced stress and released carbon dioxide while their recollection of the distressing memory was being tested in the maze. The researchers were thrilled to find that flies tested in a group recalled their shocking experience better than those tested individually by choosing the compartment with the scent that was not associated with an unpleasant surprise. The fly's ability to remember improved as the concentration of carbon dioxide in their surroundings increased.

The researchers then wondered whether the carbon dioxide-evoked social memory and individual memory are processed differently within the insect brain. Carbon dioxide activates neurons called bilateral ventral projection neurons which connect to mushroom bodies, the memory center of the insect brain. The researchers blocked these neurons and found that group-tested flies, but not individually tested flies, had reduced recollection of the smell that was associated with an unpleasant jolt, indicating that bilateral ventral projection neurons are necessary for carbon dioxide-evoked memory. Mushroom bodies are composed of distinct types of neurons called Kenyon cells, so Muria and colleagues silenced the outputs of different types of Kenyon cells one by one to show that individual memory and carbon dioxide-evoked social memory involve distinct Kenyon cells in the mushroom body. So, individual memory and social memory engage different neural pathways within the fly brain and are processed independently.

The presence of other flies can improve the retrieval of a fly's memories via carbon dioxide-mediated social memory. Carbon dioxide released during a stress event may enhance the fly's attention and enable the recollection of memories. This study highlights the unexpected role of carbon dioxide in insect intelligence. I wish carbon dioxide could improve my memory!

Muria
,
A.
,
Musso
,
P.-Y.
,
Durrieu
,
M.
,
Portugal
,
F. R.
,
Ronsin
,
B.
,
Gordon
,
M. D.
,
Jeanson
,
R.
and
Isabel
,
G.
(
2021
).
Social facilitation of long-lasting memory is mediated by CO2 in Drosophila
.
Current Biology
31
,
2065
-
2074
.