Scientists have found that the brain can tell different scents apart by integrating multiple signals to identify one unique smell.

The smell of an orange, a lemon, and a grapefruit each has strong acidic notes mixed with sweetness, yet each scent is distinguishable from its relatives. These fruits smell similar because they share many chemical compounds.

It has been a puzzle how the brain tells them apart or how it remembers a complex and often overlapping chemical signature as a particular scent.

Researchers at Cold Spring Harbor Laboratory (CSHL) in U.S. found that a group of neurons in the fruit fly brain recognise multiple individual chemicals in combination in order to define, or remember, a single scent.

The olfactory system of a fruit fly begins at the equivalent of our nose, where a series of neurons sense and respond to very specific chemicals. These neurons pass their signal on to a group of cells called projection neurons.

Then the signal undergoes a transformation as it is passed to a body of neurons in the fly brain called Kenyon cells.

Kenyon cells have multiple, extremely large protrusions that grasp the projection neurons with a claw-like structure.

Each Kenyon cell claw is wrapped tightly around only one projection neuron, meaning that it receives a signal from just one type of input.

In addition to their unique structure, Kenyon cells are also remarkable for their selectivity. Because they’re selective, they aren’t often activated. Yet little is known about what in fact makes them decide to fire a signal.

Study leader associate professor Glenn Turner and colleague Eyal Gruntman, lead author on their new paper, used cutting-edge microscopy to explore the chemical response profile for multiple claws on one Kenyon cell.

They found that each claw, even on a single Kenyon cell, responded to different odour molecules.

Additional experiments using light to stimulate individual neurons (a technique called optogenetics) revealed that single Kenyon cells were only activated when several of their claws were simultaneously stimulated, explaining why they so rarely fire.

Taken together, this work explains how individual Kenyon cells can integrate multiple signals in the brain to ‘remember’ the particular chemical mixture as a single, distinct odour.

Professor Turner will next try to determine “what controls which claws are connected, and how strong those connections are.” This will provide insight into how the brain learns to assign a specific mix of chemicals as defining a particular scent.

The study was published in journal Nature Neuroscience.