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FMRI scans show “hot spots” of human brain activity as subjects perform memory tasks. The brain contains 100 billion nerve cells, or neurons, with 100 trillion connections, or synapses.  (Photo courtesy of the Walter Reed National Military Medical Center) FMRI scans show “hot spots” of human brain activity as subjects perform memory tasks. The brain contains 100 billion nerve cells, or neurons, with 100 trillion connections, or synapses.