Plasticity

Measuring Plasticity

[Page 1668]Changes in the brain can be shown at many levels of analysis from behavior to molecules:

• behavioral analysis,
• functional organization (e.g., sensory and motor maps),
• brain structure (e.g., neuron size, capillary growth),
• synaptic structure (e.g., size, shape),
• physiological changes (e.g., brain activity, neuronal activity),
• mitotic activity (neurogenesis or gliogenesis), and
• molecular structure (e.g., gene expression).

For example, by using various forms of noninvasive in vivo imaging such as functional magnetic resonance imaging, it is possible to see that cerebral networks change with experience. But changes observed at the global level of functional magnetic resonance imaging are driven by changes at lower levels such as modifications of synapses (i.e., shedding or adding synapses), and this process results from more molecular changes that are ultimately tied to changes in gene expression. No level of analysis is correct, but rather the level must be suited to the question(s) being asked. Analysis of molecular changes in the brain will not help advance understanding of the nature of second-language acquisition, but it will help advance understanding of how one becomes addicted to psychoactive drugs. Nonetheless, both language acquisition and addiction reflect the processes of brain plasticity.

Types of Brain Plasticity

There are three general types of neuroplasticity. Experience-independent plasticity refers to changes in neuronal organization in utero as the brain begins with a rough organization that is modified through spontaneous neural activity. As the brain develops, there is experience-expectant plasticity as the brain is expecting certain experiences, such as light or sound to mold the developing neural networks. In their absence, development is disrupted, sometimes irreversibly. Experience-dependent plasticity is the process of changing existing networks of neurons. This plasticity occurs throughout the life span. For example, when one learns new information (e.g., school) or new motor skills (e.g., playing a musical instrument), brain circuits change, allowing retention of information or playing music. But not all plastic changes are beneficial. Stress, psychoactive drugs, and traumatic experiences can also change the brain, leading to pathological plasticity. Other examples include epilepsy, schizophrenia, pathological pain, phantom limb pain, and dementia.

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