Differential experience or learning as reflected in dendritic branching in the cortex has first been shown to result from an enriched environment in experiments using rats. The process was initiated to meet the sensory and perceptual demands such a stimulatory environment presents. The increase in the number of dendritic branches stretching toward neighboring neurons provided more access points of the presynaptic neurons to the outreaching postsynaptic neuron which ultimately allowed for a more frequently successful stimulation or action potential generation in the postsynaptic neurons, as the cumulative in-coming action potential signals amount to a proportionally higher and (with ever-increasing branching, almost guaranteed) depolarizing signal. Further studies later indicated such neural plasticity in humans as well – most notably in the initial forming of the brain and its connections in infancy. It was only relatively recently, and with much surprise and, understandably, excitement of scientists and the informed public alike that research showed neural plasticity to exist throughout our lives.
Previously, it was believed that the brain was moving one-directionally, showing greater or lesser degree of degeneration over time, but that such degeneration was to be expected and that there was no prevention, compensation or reversal for it. Today we know that, while certain mental abilities such as processing speed, learning and memory do show an individually variable rate of decline, others, such as the greater synthesis of knowledge and experience and analytical ability recognized as “wisdom” of older adults, tend to improve. The adult hippocampus and other brain areas (but apparently not the cortical regions) are also capable of actually making entirely new neurons if called for specifically by stimulatory environment and hence they can also compensate for loss or injury.
Neural plasticity in adulthood, much like in infancy, consists of making additional or stronger connections as well as pruning those no longer relevant, in response to a higher or lower use and activation rate of neighboring neurons, respectively. With continually enriched environment and related cognitive exercise such action can stave off and even prevent the learning and memory decline often seen in the elderly. This kind of neural plasticity in response to stimuli is what allows us to adapt to our environment: widely and frequently used and thus important neural connections are strengthened and preserved while those no longer beneficial are pruned and reorganized to support the former.
The more efficient our adaptive mechanisms the greater our chances to not only survive but thrive, and this is especially evident in the history of human evolution – humans may not have had Tiger’s claws or teeth but they possessed a brain just as sharp that was also flexible enough to observe, analyze and then translate its conclusions into strategy and tools to use to gain advantage over both predator and prey. This ability to learn and sometimes unlearn – neural plasticity at its core – ensured the continuation of the carrier’s gene pool and ultimately species.
Biology and Human Behavior: The Neurological Origins of Individuality, 2nd edition, Robert Sapolsky, Stanford University, 2005 (DVD)
PBS series: The Secret Life of the Brain, 2001 (DVD)