New research suggests that just as cluttered and fragmented files in the hard drive slow an aging computer, a comparable process seems to take place in the human brain, making it harder to learn new information.
“When you are young, your brain is able to strengthen certain connections and weaken certain connections to make new memories,” said neuroscientists Joe Z. Tsien, Ph.D., of the Brain & Behavior Discovery Institute at the Medical College of Georgia.
Researchers say that this critical weakening hampers the older brain.
In the hippocampus, a brain structure critical to memory, the NMDA receptor acts 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.
Neuroscientists say 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 that mimic 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.
However, the rodents had an impaired ability to weaken existing connections and to make new long-term memories as a result. This process is called information sculpting — something an adult does not appear to be very good at it.
“If you only make synapses stronger and never get rid of the noise or less useful information, then it’s a problem,” said Tsien, the study’s corresponding author.
While each neuron averages 3,000 synapses, the relentless onslaught of information and experiences necessitates some selective whittling.
Insufficient sculpting, at least in the mouse, meant a reduced ability to remember things short-term — like the ticket number at a fast-food restaurant — and long-term — like remembering a favorite menu item at that restaurant.
Both are impacted in Alzheimer’s and age-related dementia.
All connections were not lost in the mice, rather just response to the specific electrical stimulation levels that should induce weakening of the synapse.
Tsien expected to find the opposite: that long-term potentiation was weak and so was the ability to learn and make new memories. “What is abnormal is the ability to weaken existing connectivity.”
Acknowledging the leap, this impaired ability could also help explain why adults can’t learn a new language without their old accent and why older people tend to be more stuck in their ways, Tsien said.
“We know we lose the ability to perfectly speak a foreign language if we learn than language after the onset of sexual maturity. I can learn English but my Chinese accent is very difficult to get rid of. The question is why,” Tsien said.
The current study is found in the journal Scientific Reports.