A stroke may occur when there is an insufficient supply of blood to a brain region, or when damage to the vasculature in a specific brain region causes bleeding into the nervous tissue. In general, there are three types of strokes that occur. They are classified based on the type of vascular defect that causes them. Cerebral hemorrhaging occurs when a vessel bursts, causing bleeding in a region of the cerebral cortex. An embolic stroke occurs when small clots that formed on other parts of the circulatory system occlude cerebral arteries and cut off the blood supply to a part of the cerebral cortex. An ischemic stroke occurs when a narrowing or blockage of cerebral vasculature results in oxygen deprivation in a specific part of the cerebral cortex. The specific functional deficits observed in someone who has suffered a stroke can be correlated with the functions of the damaged cortical region. In a healthy brain, the nerve impulses from the cerebral cortex travel through the brain and cross over to the opposite (contralateral) side of the body. When a stroke occurs, damage to the cortex is usually unilateral since information to and from the cortex crosses over the midline, the effects of a stroke on the left side of the brain occur on the right side of the body. A person who has suffered a stroke may experience paralysis that may involve the face, weakness or paralysis of limbs, often limited to one side of the body, loss of sensation that parallels paralysis, difficulty with speech, double vision, loss of vision, dizziness, or vomiting, severe headache, and loss of consciousness.