Kavya NarayananOct 24, 2018 17:58:57 IST
New research from Northwestern University has found distinct evidence that the animals can tell time. Certain cells in a part of the brain called the entorhinal cortex turned on like clockwork as animals began to wait in anticipation of something happening.
There hasn't been a convincing answer to whether a dog knew how long you took to get food on a certain day compared to the previous day, Dr Daniel Dombeck, lead author of the study told university press.
"This is one of the most convincing experiments to show that animals really do have an explicit representation of time in their brains when they are challenged to measure a time interval.
The region where these newly discovered memory cells were found — the entorhinal cortex — is involved in memory and navigation in animals.
Since episodic memories, which record events that we experience over time, are also recorded in this region, the researchers decided to look for neurons in this particular region of the brain for traces of time.
They used an experiment called the virtual reality (VR) “doorstop”, which is a widely used test for mice in tasks involving some or the other aspect of 3-dimensional space.
In the study, mice would run on a physical treadmill in a VR environment — a hallway with a door located halfway down the track. The mouse would have to wait six seconds in front of the door after which the door would open and lead the mouse to a reward.
After many such sessions, the “door” was made invisible. The mice, knowing where the door was located, waited for the six stipulated seconds in front of the invisible “door” before racing down for the reward.
“The important point here is that the mouse doesn't know when the door is open or closed because it's invisible," James Heys, the paper's first author, told university press.
"The only way he can solve this task efficiently is by using his brain's internal sense of time."
Dombeck adds that the choice of VR made it possible to tune out all the other sensory inputs — touch, sound and smell — so the mice could judge time and not the door opening using any other sensory means.
Throughout the experiment, the brain activity of the mice was imaged using a high-resolution scan that uses two-photon microscopy. The resolution provided by this kind of imaging can even pick up the firing of individual neurons.
"As the animals run along the track and get to the invisible door, we see the cells firing that control spatial encoding," Dombeck told university press.
"When the animal stops at the door, we see those cells turned off and a new set of cells turn on… this was a big surprise and a new discovery."
The ‘timing cells’ that the study has found only came on during periods of rest — they were biological traces that encoded how much time the animal had been at rest, Dombeck said.
This finding could find its way into Alzheimers’ research, where tracing a faulty neuron which encodes time could mean early detection of the neurodegenerative disease.
"Perhaps this is because they are losing some of the basic functions of the entorhinal cortex, which is one of the first brain regions affected by the disease," Heys added.
The findings from the study were published in the journal Nature Neuroscience on 22 October.