In the spared areas of the brain, brain damage increases neuronal plasticity. This permits the neurons in these locations to take up the sensory or motor duties that the injured parts had been doing. This remapping of function (which is analogous to creating a new map) is crucial in function recovery. However, excessive remodeling can lead to inappropriate connections being made, which could cause additional problems. After a stroke or other type of brain injury, some neurons die immediately while others may survive for several months or even years. The survival of these damaged cells is a major factor in determining how much functional loss will be experienced by the patient. If most or all of the neurons are lost, there will be no recovery from the injury. It is estimated that about 20 percent of neurons in the cerebral cortex and about 50 percent of those in the hippocampus die after an episode of severe acute stress such as trauma or stroke. The remaining neurons may remain dormant for some time after the insult, but eventually they, too, will die. In fact, studies have shown that almost all neurons are replaced by their collateral connections within five years; however, many of these connections are likely to be inappropriate. Thus, even with extensive rehabilitation, many patients are left with symptoms due to lack of stimulation of these dead neurons. Neuronal replacement therapy aims to stimulate this endogenous neurogenesis after brain injury or disease.