Science News

Rat race for chocolate!

Subhra Priyadarshini

doi:10.1038/nindia.2012.54 Published online 18 April 2012

The experiment

© Khan et al.

They trained rats to run after a chocolate trail, tracked their sniffing behaviour and fed some interesting observations into a computer model. With this chocolaty experiment, researchers at the National Centre for Biological Sciences (NCBS) in Bangalore have taken the first steps towards understanding how rats detect odour in the wild.

In technical words, they have made the first quantitative study of near-natural olfactory behaviour in animals1.

"Despite an abundance of studies of rat and mouse olfaction, there's lack of knowledge about how olfaction is actually used in the wild," says lead researcher Adil Ghani Khan.

Tracking odour trails is pretty crucial for many animals to trace food, mates or danger. It is an excellent example of what is called 'active sampling', where the animal takes control of how to sense its environment. Such studies were pioneered by the Nobel laureate trio of Karl von Frisch, Konrad Lorenz and Nikolaas Tinbergen.

"However, combining such studies with direct recordings from the brain is far more challenging. The difficulty lies in allowing the animals to move unrestrained and yet carrying out stable electrophysiological recordings", Khan says.

To study this, Khan and colleagues Manaswini Sarangi and Upinder Singh Bhalla, chose the rodent olfactory system, which has provided many important and fundamental insights into the basic principles of brain function.

They created a chocolate trail on a paper and spooled it around a treadmill. Then they trained the rats to run on the treadmill. As a reward for the rodents, they placed small chocolate pieces on the trail from time to time. The rats soon realised that if they tracked the odour trail, they would get food as a reward.

"What emerged was a spontaneous rapid zigzagging behaviour in which the rats kept extremely close to the trail, within about a centimeter, even at high speeds and at sharp turns and bifurcations," Khan says.

The scientists recorded impulses generating from the brain and nose of the rats using miniature implants and found that the rats could track the chocolate trail using an extremely rapid and efficient strategy, comparing only between consecutive sniffs to know precisely where the trail was.

The team reached a conclusion that the rats used a multi-layered strategy — they employed fall-back options on losing the trail as also assumptions from their previous experience — to keep to the track. The rats compared odour signals across their nostrils (stereo olfaction) to do this task. Khan and colleagues repeated the experiment with other odour trails with similar results.

The study used a computer model to predict that while a rat was tracking its trail, it had an explicit representation in its brain of not only the position of the trail but also an estimation of the amount of error.

One of the ultimate goals of neuroscience, Khan says, is to understand how the brain interprets the complex natural world and produces elaborate and complicated behaviours. "It is clearly important to study the unrestrained behaviour of animals in their natural environment."


  1. Khan, A. G. et al. Rats track odour trails accurately using a multi-layered strategy with near-optimal sampling. Nature Comm. 3 doi: 10.1038/ncomms1712 (2012)