Do you take the time to feed your pet? Fluffy and Fido are on to you — and they can tell when you're kidding. A new study from Northwestern University has found some of the clearest evidence that animals can judge time. By examining the medial entorhinal cortex of the brain, the researchers discovered a previously unknown set of neurons that turn on like a clock when an animal waits.
“Does your dog know that it took you twice as long as yesterday for you to feed him? There was no good answer for that,” said Daniel Dombeck, who led the study. “This is one of the most compelling experiments to show that animals really do have an explicit representation of time in their brains when challenged to measure a time interval.”
In planning the study, Dombeck's team focused on the medial entorhinal cortex, an area in the brain's temporal lobe associated with memory and navigation. Because that part of the brain encodes spatial information in episodic memories, Dombeck suggested that the area could also be responsible for encoding time.
"Every memory is a little different," said James Heys, a postdoctoral fellow in Dombeck's lab. “But there are two central features of all episodic memories:space and time:they always happen in a certain environment and are always structured in time.”
To test their hypothesis, Dombeck and Heys set up an experiment called the virtual "doorstop" task. In the experiment, a mouse runs on a physical treadmill in a virtual reality environment. The mouse learns to walk in a hallway to a door that is about halfway down the track. After six seconds, the door will open, allowing the mouse to pass through the hallway to receive its reward.
After conducting several training sessions, researchers made the door invisible in the virtual reality scene. In the new scenario, the mouse still knew where the now-invisible "door" was based on the changing textures of the floor. And it still waits six seconds at the “door” before abruptly rushing off the track to collect its reward.
"The important point here is that the mouse doesn't know when the door is open or closed because it's invisible," said Heys, the study's lead author. “The only way he can efficiently solve this task is to use his brain's internal sense of time.”
Using virtual reality, Dombeck and his team can neatly control possible influencing factors, such as doorway noise. “We couldn't make the door completely invisible in a real environment,” Dombeck said. “The animal can touch, hear, smell or somehow perceive it. They don't have to judge the time, they would just feel when the door opens.” In virtual reality we can remove all sensory clues.”
But Dombeck and his team did more than just watch the mice complete the task of stopping the door over and over again. They took the experiment a step further by imaging the brain activity of the mice. Using two-photon microscopy, which enables advanced high-resolution imaging of the brain, Dombeck and Heys looked at the neurons of the mice.
“As the animals run along the track and get to the invisible door, we see the cells firing the spatial coding of the controls,” Dombeck said. “Then, when the animal stops at the door, we see those cells turned off and a new set of cells turned on. This was a great surprise and a new discovery.”
Dombeck noted that these "timing cells" didn't fall during active running — only during rest. “Not only are the cells active during rest,” he said, “but they actually encode how much time the animal has been standing still.”
