Study in mice reveals brain process that measures distance travelled, helps navigate

New Delhi, Dec 18 (PTI) A study in mice has revealed how cells in the brain's hippocampus, important for both memory and navigation, help one track how far they have travelled -- the findings may help in understanding of the initial stages of Alzheimer's disease, where patients have trouble tracking distance and time.

Researchers from the US' Max Planck Florida Institute for Neuroscience trained mice to run a specific distance in a gray virtual reality environment with no visual landmarks in exchange for a reward. Since the environment lacked external cues, monitoring own movements was the only way the mice could judge how far they had travelled.

The team observed the electrical activity of neurons in the hippocampus as the mice performed the task.

"In this study, we removed as many sensory cues as possible to mimic situations such as moving in the dark. In these simplified conditions, we found that only a small number of hippocampal cells signalled a specific place or a specific time," Yingxue Wang, research group leader and senior author of the study published in the journal Nature Communications, said.

The observation made the team think what the rest of the neurons were doing, Wang said.

Most neurons in the hippocampus were seen to follow one of two opposing patterns of activity -- in one group, activity sharply increased as soon the animal started moving, seemingly triggering the "distance-counting" process, while in another, activity decreased when a mouse started moving, and steadily increased as the mouse travelled further.

The two phases of distinct activity act as a neural code for distance, the researchers said.

Both short and long distances could be tracked in the brain by using neurons with different ramping speeds, they added.

"We have discovered that the brain encodes the elapsed distance or time needed to solve this task using neurons that show ramping activity patterns," lead researcher Raphael Heldman, a PhD student, said.

"This is the first time distance has been shown to be encoded in a way that differs from the well-known place-based coding in the hippocampus. These findings expand our understanding that the hippocampus is using multiple strategies -- ramping patterns in addition to the place-based coding -- to encode elapsed time and distance," Heldman said.

Disrupting the brain's working by manipulating the circuits involved in measuring distance resulted in the mice facing difficulty performing the task accurately, often searching for the reward in a wrong location, the researchers found.

"These findings reveal parallel hippocampal circuits that initiate and maintain time or distance encoding," the authors wrote.

The results "provide insights into the neural computations supporting goal-directed navigation," they said.

Wang said, "Understanding how time and distance are encoded in the brain during path integration is especially important because this ability is one of the earliest to degrade in Alzheimer's disease. Patients report early symptoms of getting spatially disoriented in familiar surroundings or not knowing how they got to a particular place." PTI KRS KRS MG MG