How to smell a smell that is actually not there

How can we experience a sensation that is not real? How to mimic a sensory stimulus in its absence? How to smell a smell when there is none there? By activating certain neurons, it seems.

A group of researchers at New York University’s Grossman School of Medicine have recently published an article in Science ¹, where they report on their results inducing neuronal activity in olfactory regions in mice so as if they were activating by a certain smell.

Activating neurons recreates smell

To recreate smells in mice’s brains, researchers used optogenetics, a technique uses light to stimulate genetically engineered neurons and lead them to fire a neural signal. By targeting neurons in the olfactory bulb, they were able to recreate the sensation of the smell in the mice’s brain. However, how did they check if the mice effectively smelled the smell? By coupling the smell detection to a behavioural response. Mice were instructed to lick a certain spot when experiencing the smell, when the stimulation was random, they did not perform the task, showing the association between the two.

It being an artificial stimulus, the researchers could play around with its variables, like sequence, speed…what they found was that the first steps of stimulation were key and changes to the sequence there led to misinterpretation of the stimulus. It would be as if we took one smell for another because the initial sensation differed.

Now there are two questions remaining: are these properties applicable to other senses, like vision or hearing? And more importantly, are this findings transferable to what happens to real odours?