Patients may also be asked to undergo an optional procedure called a lumbar puncture (spinal tap) to examine the relationship between cognitive impairment and the amount of antibodies in the cerebrospinal fluid (CSF)- fluid that circulates around the brain and spinal cord. For this procedure a small area of skin on the lower back is numbed with a local anesthetic. A needle is then inserted in the space between the bones in the lower back, and about 2 tablespoons of CSF is withdrawn through the needle.