Does anesthesia cause loss of consciousness?

Although about 30 million operations carried out under general anesthetic are routinely performed each year in the USA alone, it is not possible to determine reliably whether or not a given anesthetized patient is conscious during surgery. As a result, some patients may be either partially aware during the operation or may be able to recall some aspects of it afterwards. It is therefore crucial to develop some experimental means to evaluate the state of consciousness of the anesthetized patient. Recent developments suggest that 40 Hz neuronal oscillations might help to solve this problem.