The better solution to the problem of monitoring inefficiency in the design of complex systems might be to take advantage of psychophysical factors, which are known to enhance vigilance performance and to lower operator workload. As Raja Parasuraman said, the responsibility for target detection in highly automated systems may be allotted to instruments and controls, but human operators are needed when systems malfunction or unusual events occur. The systematic study of vigilance began in Second World War when the Royal Air Force commissioned Norman J. F. Mackworth to investigate an unexpected and perilous finding. Most vigilance tasks make use of dynamic displays in which critical signals appear within an ensemble of recurrent no signal events. Together with variations in signal salience and event rate, the difficulty of vigilance tasks can also be influenced by variations in the monitor's spatial uncertainty as to the location of the events to be observed.