As we age, it just may be the ability to filter and eliminate old information -- rather than take in the new stuff -- that makes it harder to learn, scientists report.
"When you are young, your brain is able to strengthen certain connections and weaken certain connections to make new memories," said Dr. Joe Z. Tsien, neuroscientist at the Medical College of Georgia at Georgia Regents University and Co-Director of the GRU Brain & Behavior Discovery Institute. It's that critical weakening that appears hampered in the older brain.
The NMDA receptor in the brain's hippocampus is like a switch for regulating learning and memory, working through subunits called NR2A and NR2B. NR2B is expressed in higher percentages in children, enabling neurons to talk a fraction of a second longer; make stronger bonds, called synapses; and optimize learning and memory. This formation of strong bonds is called long-term potentiation. The ratio shifts after puberty, so there is more NR2A and slightly reduced communication time between neurons.
When Tsien and his colleagues genetically modified mice thatmimic the adult ratio -- more NR2A, less NR2B -- they were surprised to find the rodents were still good at making strong connections and short-term memories but had an impaired ability to weaken existing connections, called long-term depression, and to make new long-term memories as a result. It's called information sculpting and adult ratios of NMDA receptor subunits don't appear to be very good at it.