The functional organization of the human brain—for instance that the temporal lobe processes auditory and occipital cortex visual information—is largely not hard-wired, unlike in some other species. Rather, the functional organization of the brain that we see in neuroimaging studies, or that which is reflected in the effects of focal brain damage, reflects the way that the brain has adapted to the environmental demands and tasks of the individual that for the most part stay fairly constant throughout the life. Indeed, it can be argued that functional organization of the brain is the end result of an adaptive process, reflecting all of the skills that one has learned to best cope with one’s environment. Consequently, as long as the environment and the tasks and goals of the individual stay the same also the functional organization of the brain remains stable.
Upon changes in the environment, however, such as when moving to another country with entirely different culture and language, changes in the sensory inputs to the brain (that occurs when one of the senses is damaged such as blindness or deafness), or when the functional neuronal networks are themselves altered by for example stroke or some other type of brain damage, there is a sudden need for the brain to re-adapt. As the result of this, there are changes in the functional organization of the brain, with some of these changes occurring surprisingly quickly, in a matter of minutes, and some changes taking place over more extended periods of days, weeks, and months. In some cases these changes are extensive, and in some other cases they are rather subtle. These adaptive changes in functional brain organization are called plasticity. There are multiple forms of plasticity that occur in the brain
There are different forms of plasticity
Different forms of plasticity can be defined based on the reason why re-adaptation is required and by the developmental stage—whether it is a developing brain or an adult brain that is learning or re-learning. Indeed, the developing brain is far more plastic than the adult brain. Developmental plasticity refers to the rapid functional organization that occurs when the developing nervous system starts to adapt to it’s environment, for instance, there are specific periods, called sensitive periods, during which acquiring certain skills such as language is easier than during other times.
Another class of plasticity studies have focused on characterizing robust reorganization in the functional architecture of the brain upon loosing sensory input due to, for instance, blindness, deafness, or loosing of a limb. This is also called activity-dependent plasticity, as the stimulus-related activity changes drastically. A third type of plasticity studies were introduced– perceptual or sensory learning, such as when learning to discriminate the phonemes of a foreign language, is made possible by very fine functional reorganization that takes place in the sensory cortices.
There is also a vast body of literature describing plasticity that follows damage to the neural networks themselves. Following brain damage, the challenge faced by the nervous system is to re-adapt with the remaining neural tissue to coping in the environment after previously learned skills and processing capacity are suddenly lost. In addition to these different types of long-lasting or even permanent plastic changes, rapid (or short-term) plasticity refers to functional re-organization that occurs very quickly, in a matter of seconds or minutes. As was already described in Chapter 6, short-term plasticity supports attentional filtering of task-relevant stimuli. What makes short-term plasticity especially interesting is that some of the rapid changes in neuronal tuning seem to linger, thus making perceptual/sensory learning possible. In the following, these different types of plasticity are described in more detail.