as polysialic acid (PSA). PSA acts, in part, by preventing Otx2 interaction with PV cells. Soon after the opening of the critical period, PSA levels decrease, allowing PV cell maturation by activating inhibitory GABAa receptors that facilitate inhibitory circuit remodeling. Artificially removing PSA, or experimentally manipulating inhibitory transmission can result in early opening of the critical period. While the timing of these molecular events seems to be partially explained by clock genes, experience is crucial as sensory deprivation experiments have been shown to interfere with the proper timing of critical periods. 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 neuron's 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 competing for connections). Microglia Microglia inherently play a role in synaptic pruning during adolescence. As resident immune cells of the central nervous system, microglia’s main role is phagocytosis and engulfment. Studies have found that during critical periods in the visual cortex, neural synapses become the target of microglial phagocytosis. Neurons who received less frequent input from retinal ganglion cells during early postnatal periods were more prone to be engulfed and pruned by microglia, as per monocular deprivation experiments. Similar results were found when manipulating G-coupled purinergic receptors on microglial processes. Blocking these receptors or performing a knockout experiment significantly lowered microglial interactions and