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Activity Dependent Competition
Hebbian theory guides the idea of activity-dependent competition: if two neurons both have the potential to make a connection with a cell, the neuron that fires more will make the connection.

Ocular Dominance
This phenomenon of activity-dependent competition is especially seen in the formation of ocular dominance columns within the visual system. Early in development, most of the visual cortex is binocular, meaning it receives roughly equal input from both eyes. Normally, as development progresses, the visual cortex will segregate into monocular columns that receive input from only one eye. However, if one eye is patched, or otherwise prevented from receiving sensory input, the visual cortex will shift to favor representation of the uncovered eye. This demonstrates activity-dependent competition and Hebbian theory because inputs from the uncovered eye make and retain more connections than the patched eye.

Axon Growth
Axon formation and growth is another key part of plasticity and activity-dependent competition. Axon growth and branching has been shown to be inhibited when the neurons electrical activity is suppressed below the level of an active neighbor. This shows that axonal growth dynamics are not independent but rather depend on the local circuits within which they are active (i.e. the activity of the other neurons neurons competing for connections).

Excitatory/Inhibitory Balance
Another critical component of neuronal plasticity is the balance of excitatory and inhibitory inputs. Early in development, GABA, the major inhibitory neurotransmitter in the adult brain, exhibits an excitatory effect on it’s target neurons. However, due to changes in internal chloride levels due to the up-regulation of potassium chloride pumps, GABA then switches to inhibitory synaptic transmission. . The maturation of the GABAergic inhibitory system helps to trigger the onset of critical periods. Strengthened GABAergic systems can induce an early critical period, while weaker GABAergic inputs can delay or even prevent plasticity. Inhibition also guides plasticity once the critical period has begun. For example, lateral inhibition is especially important in guiding columnar formation in the visual cortex. Hebbian theory provides insight on the importance of inhibition within neural networks: without inhibition, there would be more synchronous firing and therefore more connections, but with inhibition, fewer excitatory signals get through, allowing only the more salient connections to mature.