The central nervous system is plastic, which means that its organization and function can change.Such "neural plasticity" is a general property of the brain and the spinal cord; it can be activated by external signals as well as internal signals.Activation of neural plasticity can be beneficial by making it possible to adapt to changing demands but plastic changes can also be harmful in that such changes can cause symptoms of disease.Learning basic skills such as bicycling is an example of how activation of neural plasticity can be beneficial to an individual; the vestibular ocular reflex that normally ensures a stable image on the retina during head movements can adapt to changes of spectacles; it is even possible to adapt to wearing reversing prisms.