2023-07-14 18:30:52
fruit flies (‘drosophila melanogaster’) can discriminate between sets containing different numbers of objects and spontaneously show a preference for larger numbers. This numerical judgment requires the activation of specific neurons located in the optic lobe, which also intervene in a social context, allowing the fly to adapt its behavior in response to a threat.
The ability to “count” friends and enemies may have played a role in the evolution of these insects, researchers explain in a new study published in the journal Cell Reports.
Estimate the number of mates in a competing group before conflict, the amount of food available in a place that is difficult to access, or the number of possible sexual partners in a new territory is essential for survival and reproduction. For example, some species of ants orient themselves in the desert by calculating the number of steps required to reach a goal.
“Numerical sensitivity, that is, the ability to perceive information related to quantities, exists in many vertebrates and invertebrates. It has been documented in primates, birds, amphibians, fish, and bees,” explains Mercedes Bengochea, a postdoctoral researcher in Bassem Hassan’s team. at the Brain Institute in Paris (France)–, but we didn’t know which neural circuits were involved in this ability.”
To investigate this question, researchers must recording the brain activity of an animal during a numerical task and then turn specific neural cells on or off to determine which areas of the brain are involved. These operations are difficult to perform in vertebrates, but the right tools already exist with fruit flies.
“The ‘Drosophila melanogaster’ is a model of choice for studying cognition,” he continues. “These insects adjust their behavior in the face of a threat based on the number of congeners that could help them,” adds the researcher. In case of imminent danger, how much the smaller their group size, the more likely they are to freeze to keep themselves safe.”
To determine whether fruit flies can accurately assess numbers and assign values to perceived quantities, Mercedes Bengochea and her colleagues used an experimental setting that has already shown its relevance.
they placed flies in arenas called “Buridan arenas”, where they were exposed to visual stimuli: in that case, two sets of objects. Next, the researchers determined which stimulus the insects preferred by measuring the time they spent inspecting one set or the other.
Their results indicate that fruit flies stayed longer near the set containing three objects than the set containing only one, regardless of the size of the objects or the total volume occupied by the set. This taste for larger quantities was maintained when the insects had to choose between groups of 2 4 objects and 2 3 objects.
“The flies, however, were unable to distinguish between sets of 3 and 4 objects respectively,” explains Mercedes Bengochea. “It seems that the relationship between these two numbers is not enough for them to perceive a difference. Instead, they can compare very easily a group of 4 and another of 8 objects, that is to say, a ratio of simple to double”. Therefore, fruit flies are not limited to counting to 3: the relationship between the evaluated quantities must be clear enough to be perceived.
Evaluating the relationship between two quantities is a simple visual task common to animals. It is also useful for humans, as it allows us to gauge at a glance the size of a group that contains too many elements to be counted one by one: a crowd at a concert, for example.
It remains to be determined which neural circuits are involved in this numerical discrimination system in ‘Drosophila’. To do this, the researchers “disconnected” successively different areas of the brain of the insects, preventing the transmission of nerve signals in the synapses.
After several tests, they observed that the activity of a column of neurons located in the optic lobe, the LC11 neurons (for lobar columnar neurons 11), it was necessary for flies to distinguish different sets of objects.
“In a second experiment we taught the insects to go against their natural inclination for large numbers, using a simple conditioning method,” he says: an appetizing dose of sugar was placed next to the sets of smaller objects, adds the investigator”.
“Momentarily, thanks to the attractiveness of the food, we got them to prefer small numbers. But once the LC11 were inactivated, the insects stopped showing a preference… for both large and small numbers,” he continues. “This confirms that these neurons are essential for comparing quantities, regardless of the value that fruit flies assign to them.”
LC11s are also involved in the social behavior of fruit flies.: they are activated when the insects have to adapt their defense strategy based on the number of congeners that fly nearby. “We believe that the ability to assess quantities has been decisive in the evolution of invertebrates,” explains Bassem Hassan, head of the ‘Brain Development’ team.
“The cognitive solutions that insects use to ‘count’ are very simple,” he continues. “Several studies have shown that, in a computational model, a few artificial neurons are enough to perform a numerical task.”
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