A study from Northwestern University (NU) has found some of the clearest evidence that animals can judge time. By examining the brain's medial entorhinal cortex, the researchers discovered a previously unknown set of neurons that turn on like a clock when an animal is waiting.
NU researchers set up an experiment called the virtual "door stop" task. In the experiment, a mouse runs on a physical treadmill in a virtual reality environment. The mouse learns to run down a hallway to a door that is located about halfway down the track. After six seconds, the door opens, allowing the mouse to continue down the hallway to receive its reward.
After running 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 located based on the floor's changing textures. And it still waited six seconds at the "door" before abruptly racing down 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 James Heys, a postdoctoral fellow at NU and the study's first author. "The only way he can solve this task efficiently is by using his brain's internal sense of time."
NU researchers took the experiment one step further by imaging the mice's brain activity. Using two-photon microscopy, which allows advanced, high-resolution imaging of the brain, they watched the mice's neurons fire.
"As the animals run along the track and get to the invisible door, we see the cells firing that control spatial encoding," said Daniel Dombeck, an associate professor of neurobiology in NU's Weinberg College of Arts and Sciences. "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," he said, "but they actually encode how much time the animal has been resting."
The researchers have found these new time-encoding neurons, now they can study how neurodegenerative diseases might affect this set of cells.
"Patients with Alzheimer's disease notably forget when things happened in time," Heys said. "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."
"So this could lead to new early-detection tests for Alzheimer's," Dombeck added. "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."
The study has been published online in the journal Nature Neuroscience.