Figure 1: Plasticity in the adult visual cortex. In the presence of specific stimuli, such as performing a perceptual task, playing action video games, or pharmacological treatment, several functional alterations take place (image no. 214 from the Cell Image Library, neuron-neuron synaptic transmission). These include a decrease in inhibition/excitation ratio, epigenetic remodeling of chromatin structure, mitochondrial redistribution, activation of transcription factors, and protein synthesis. Structural plasticity includes modifications in neuronal morphology (axons, dendrites, and dendritic spines), suppression and creation of synapses, and genesis of new neurons and neuritis (image adapted from http://www.biomedcentral.com/1471-2202/6/24). The interplay of these mechanisms leads to adult neuronal plasticity, as revealed by the increased perception of a trained stimulus, improvement of visual function in amblyopia, and long-term adaptation to changes in the subject (such as cataract surgery) or in the environment. Plasticity is under the top-down influence of attention, as attention acts upon sensory signals at many levels to construct a selective representation of visual space.