To determine when pets could tell the time, scientists analysed the brain's medial entorhinal cortex - the part of the brain responsible for memory, navigation and the perception of time.
Researchers found a set of neurons that switched on in the brain when animals are waiting to be fed. These neurons act as an internal clock.
“Does your dog know that it took you twice as long to get its food as it took yesterday? There wasn’t a good answer for that before,” said Daniel Dombeck, lead author of the study.
“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.”
“Every memory is a bit different,” adds James Heys, a postdoctoral fellow in Dombeck’s laboratory.
“But there are two central features to all episodic memories: space and time. They always happen in a particular environment and are always structured in time.”
In their experiment, Dombeck and Heys recruited a mouse to run on a physical treadmill in a virtual reality environment. Halfway through the track, there is a door at the end of the hallway. The door opens up after 6 seconds allowing the mouse to continue and receive a reward.
The mouse repeated the exercise several times before researchers made the door invisible in the virtual reality scenario. Interestingly, despite removing the door, the mouse still recognised where it was located and as a result, still waited for six seconds at the very same spot before continuing on to its reward.
By using a virtual reality setup, the team of authors could control influencing factors such as sound.
“The important point here is that the mouse doesn’t know when the door is open or closed because it’s invisible,” continues Heys.
“The only way he can solve this task efficiently is by using his brain’s internal sense of time.”
“We wouldn’t be able to make the door completely invisible in a real environment,” Dombeck explains.
“The animal could touch it, hear it, smell it or sense it in some way. They wouldn’t have to judge time; they would just sense when the door opened. In virtual reality, we can take away all sensory cues.”
While conducting the experiment, researchers also ran imaging of the mice's brain activity. Dombeck and Heys noticed the mice's neurons activate after the exercise through high-resolution imaging.
“As the animals run along the track and get to the invisible door, we see the cells firing that control spatial encoding,” says Dombeck.
“Then, 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.
“Not only are the cells active during rest, but they actually encode how much time the animal has been resting.”
While the findings are interesting, the two suggest this could aid research on neurodegenerative diseases.
“Patients with Alzheimer’s disease notably forget when things happened in time. 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,” explains Heys.
“So this could lead to new early-detection tests for Alzheimer’s. We could start asking people to judge how much time has elapsed or ask them to navigate a virtual reality environment — essentially having a human do a ‘door stop’ task,” comments Dombeck.
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