The average adult human brain weighs approximately three pounds, is composed of about 65 percent water and contains roughly 86 billion neurons.

Human memory is a complex, brainwide process that is essential to who we are. Experts say once you've learned to ride a bicycle or drive a car or swim, you never forget. But surprisingly, new research suggests that while learning, the brain is actively trying to forget.

According to Cornelius Gross, who led the work at EMBL and University Pablo Olavide in Sevilla, Spain, a pathway in the brain has been linked to forgetting — actively erasing memories. Working with mice, Gross and colleagues studied the hippocampus, a region of the brain that's long been known to help form memories.

Information enters this part of the brain through three different routes. As memories are cemented, connections between neurons along the "main" route become stronger.

When the researchers blocked this main route, they found the mice were no longer capable of learning a Pavlovian response — associating a sound to a consequence and anticipating that consequence. But if the mice had learned that association before the scientists stopped information flow in that main route, they could still retrieve that memory.

This confirmed the main route is involved in forming memories but isn't essential for recalling those memories.

Unexpectedly, however, blocking that main route weakened the connections, suggesting the memory was being erased, a consequence that puzzled the researchers. As they investigated further, they discovered that activity in one of the other pathways was driving this weakening, and the active push for forgetting only happened in learning situations.

When the scientists blocked the main route into the hippocampus under other circumstances, the strength of its connections remained unaltered. The three routes into the hippocampus seem to be linked to different aspects of learning: forming memories, recalling them and forgetting.

The researchers concluded that because there is limited space in the brain, when we learn, some connections are weakened to make room for other connections. In other words, when we learn new things, we need to forget things we've learned before to create space.

Although findings were made using genetically engineered mice, the researchers demonstrated it is possible to produce a drug that activates this "forgetting" route in the brain without the need for genetic engineering, which might contribute to developing ways to help people forget traumatic experiences.