How do people and other organisms evolve into individuals that are distinguished from others by their own personal brain structure and behavior?

Why do identical twins not resemble each other perfectly even when they grew up together?

To shed light on these questions, the scientists observed 40 genetically identical mice that were kept in an enclosure that offered a rich shared environment with a large variety of activity and exploration options.

They showed that individual experiences influence the development of new neurons in mice, leading to measurable changes in the brain.

“The animals were not only genetically identical, they were also living in the same environment,” explained principal investigator Gerd Kempermann, Professor for Genomics of Regeneration, CRTD, and Site Speaker of the DZNE in Dresden. “However, this environment was so rich that each mouse gathered its own individual experiences in it. Over time, the animals therefore increasingly differed in their realm of experience and behavior.”

New neurons for individualized brains

Each of the mice was equipped with a special microchip emitting electromagnetic signals. This allowed the scientists to construct the mice movement profiles and quantify their exploratory behavior.

The result: despite a common environment and identical genes, the mice showed highly individualized behavioral patterns. In the course of the three-month experiment, these differences increased in size.

“These differences were associated with differences in the generation of new neurons in the hippocampus, a region of the brain that supports learning and memory,” said Kempermann “Animals that explored the environment to a greater degree also grew more new neurons than animals that were more passive.”

Adult neurogenesis [generation of new neurons] in the hippocampus allows the brain to react to new information flexibly. With this study, the authors show for the first time that personal experiences and ensuing behavior contribute to the “individualization of the brain.” The individualization they observed cannot be reduced to differences in environment or genetic makeup.

“Adult neurogenesis also occurs in the hippocampus of humans,” said Kempermann. “Hence we assume that we have tracked down a neurobiological foundation for individuality that also applies to humans.”

Implications

“The finding that behavior and experience contribute to differences between individuals has implications for debates in psychology, education, biology, and medicine,” said Ulman Lindenberger, Director of the Center for Lifespan Psychology at the Max Planck Institute for Human Development (MPIB) in Berlin.

“Our findings show that development itself contributes to differences in adult behavior. This is what many have assumed, but now there is direct neurobiological evidence in support of this claim. Our results suggest that experience influences the aging of the human mind.”

In the study, a control group of animals housed in a relatively unattractive enclosure was also examined; on average, neurogenesis in these animals was lower than in the experimental mice. “When viewed from educational and psychological perspectives, the results of our experiment suggest that an enriched environment fosters the development of individuality,” said Lindenberger.