Stimulation of brain regions during neurosurgery is another common method to determine brain function. Chapman and his associates stimulated the hippocampus of several epileptic patients and found that bilateral stimulations produced retrograde amnesia that persisted for a few hours and reached back about two weeks. Immediate memory and other long term memories were intact. However there are problems with the interpretation of these observations. In a series of experiments, Rasmussen and colleagues discovered that there were significant alterations of electrical activity in the temporal cortex following the hippocampal stimulation. Thus the disturbances in memory might be due to the disruption of activity elsewhere in the brain.
Sodium amytal testing is another clinical tool that is used to determine hemispheric function. Discovered by Wada and Rasmussen, this technique involves injection of sodium amytal into the carotid artery to produce a brief period of anesthesia of a single hemisphere. This technique is typically used to determine the speech hemisphere before surgery. The study by Jones-Gotman showed that the technique can also be used to determine the extent of unilateral lesions. Injection of sodium amytal to the contralateral hemisphere leaves the patient with the damaged side, and allows for the measurement of memory deficit.