A paper published in Nature Communications on August 3 may have just unlocked some of the mystery behind one of our sensory experiences ― smell. The study looked at the brains of locusts, a species of grasshopper, in an attempt to understand physical processes in the brain that are associated with our experience of smell.
Previously, sensory experiences were thought to be the result of neurons firing in the same particular sequence, however, the former hypothesis does not take into account the variability of the real world. In other words, what happens in dynamic environments presenting more than one scent? According to a team of scientists led by Prof Barani Raman of the School of Engineering & Applied Science at Washington University in St. Louis, USA, the ability of locusts to still recognise a particular scent among other scents is down to something called flexible interpretation. So how did they come up with this conclusion?
The researchers trained locusts to recognize a “target” scent, more specifically, hexanol. They did this by first exposing hungry locusts to a waft of the scent followed by a food reward of grass. After about six rounds, the locusts began anticipating the reward by immediately open their palps ― two long appendages found in the mouths of insects that are involved in feeding ― upon detecting the scent of hexanol. The experiment was then made more interesting, by adding another scent, referred to as the “distractor” odour. While the locusts continued to respond the same way each time they were exposed to the target scent on its own, when the distractor scent was introduced, the combination of neurons fired was altered.
Smells evoke spikes in the activity patterns of neurons, which is hypothesized to encode information about the identity of a particular odour. This is why every time the target odour was introduced on its own, the locust’s neural activity was the same. But when the locusts were exposed to a distractor smell first, different combinations of neurons were fired.
According to the paper, the patterns follow an OR-of-ANDs logical operation ― based on computational neurobiology, a branch of neuroscience that uses mathematical logic to model brain function― which compensates for variability and allows for flexible decoding. Predictions based on the logical classifier matched the results of behavioural experiments. When only one smell is detected, the locusts show a fixed brain activity pattern, whereas if the locusts smell the target after first smelling the distractor, the pattern is a combination of only a subset of those neurons.
In other words, the response of the locusts to certain odours depends on their history of exposure to other smells. Moreover, when a distractor smell is presented, the subset of activated neurons associated with the target will depend on what the distractor is, for example, the pattern will be different if the distractor smell is coffee compared to the scent of a rose.
In the words of William Shakespeare, “A rose by any other name would smell as sweet,” and the neurons it activates would be the same. Unless of course there is a coffee shop nearby.
(1) Nizampatnam S. et al. Dynamic contrast enhancement and flexible odor codes. Nature Communications (2018). DOI: 10.1038/s41467-018-05533-6
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