As a product of interactions with environment, the learning takes place at the cellular levels, and is governed by two fundamental principles:
1. Selective Stabilisation and Elimination of Synapse
Throughout our lives, but especially during infancy, our synaptic connections are shaped by our sensory experience. It is through the individual’s sensorimotor experience that the initial circuitry will be tested and the fine structure of the neural networks will be adjusted. Research into the formation process of neural circuits has found that there is a selective process regulated by the electrical activity of the brain, which shapes the baby’s brain. The selection process of synapses depends on the activity of the neurons. Those synapses whose activity is rarely correlated with other neurons will gradually weaken until they disappear completely. Only those synaptic connections that are incorporated into a functional neural circuit will survive. The selection of synapses serves to make the pattern of innervations more precise and efficient.
Mark F. Bear, Professor of Neuroscience at Massachusetts Institute of Technology, and colleagues at Johns Hopkins University Medical School also explains in Nature Neuroscience, how temporarily depriving one eye of vision soon after birth induces a long-lasting loss of synapses that causes blindness. Professor Bear, who studies how early experience modifies synapses in the brain, says the weakening and elimination of synapses serving the deprived eye occurs because activity in the closed eye no longer correlates with responses from the other eye. Again, this confirms that it is the activities of neural circuits that control the numbers of synaptic contacts, while the absence of stimulation leads to the elimination of synapses.
2. Hebbian Learning
Learning depends on the plasticity of the circuits in the brain – the ability of the neurons to make lasting changes in the efficiency of their synaptic transmission. Neurons can increase the efficiency of these connections through the process of long-term potentiation (LTP), or they can decrease it through the process of long-term depression (LTD). Both of these processes contribute to the fine-tuning of our neuronal connections, but long-term depression plays an especially important role in the selective elimination of synapses that characterizes certain critical periods of human development.
These rules governing the networking of neurons are based on the groundbreaking work done by Donald Hebb over 60 years ago. Hebbian theory explains the adaptation of neurons in the brain during the learning process. It describes a basic mechanism for synaptic plasticity wherein the correlated activity of two neurons causes a synapse to be strengthened. The theory is often summarized as “Cells that fire together, wire together”, in which simultaneous activation of cells, such as reading aloud, leads to pronounced increases in synaptic strength between those cells. Such learning is known as Hebbian learning.