If you’re building an automobile, redundancy is a key design concern. When the brakes fail, it’s nice to have a backup. But what about when building a brain? While neuroscientists know many things about how neurons propagate signals, they do not know how much brain circuitry carries duplicate signals and, when it does, they do not always know why.
New work from the lab of Rachel Wilson, HMS associate professor of neurobiology, provides new insight into the mechanisms and functions of redundancy in the brain. The work, accomplished using novel and meticulously precise techniques (see sidebar), suggests a new twist to a longstanding theory that the brain tends to minimize redundancy in the way information is processed and stored. “Biological systems are not so different from systems designed by humans,” said Wilson. “Some redundancy is useful. It’s a tradeoff between efficiency and robustness.”
Wilson and first author research fellow Hokto Kazama recorded neural signals between pairs of neurons in the olfactory system of the fruit fly Drosophila. Because the fly brain contains only 100,000 neurons (the human brain contains 100 billion), it is a useful test bed for investigating neural circuits. Moreover, unique genetic tools available in the fly olfactory system allow researchers to label certain types of neurons with fluorescent markers so they can specifically record from those cells. Recording multiple neurons at once allowed Wilson and Kazama to measure whether the electrical signals in these brain cells are independent or redundant.
They found, surprisingly, that many pairs of neurons carry highly redundant signals about the olfactory environment. In addition, neurons carrying redundant signals send this information to two distinct higher brain regions. In one of these regions, the redundant signals converge onto neurons that may exploit the replication for error checking. For instance, if the same faint signal comes in on multiple inputs, it is likely to reflect a real odor, not merely electrical noise.
Redundancy might be a useful safety factor,” said Wilson. “We think this brain region is very good at detecting weak signals in an ambiguous environment.”
In a second brain region, the redundant signals appear to be distributed to distinct destinations. “Instead of error correction, this may help propagate the signal to a bunch of different places,” said Wilson.
These findings, described in the September Nature Neuroscience, are reminiscent of results seen in the retina, where signals are initially received and integrated for vision, suggesting that redundancy might be a fundamental design principle for the earliest stages of sensory processing.
Students may contact Rachel Wilson at rachel_wilson@hms.harvard.edu for more information.
Conflict Disclosure: The authors declare no conflicts.
Funding Sources: The National Institutes of Health, a Pew Scholar Award, a McKnight Scholar Award, a Sloan Foundation Research Fellowship, and a Beckman Young Investigator Award.
