INTRODUCTION Human information processing is an integral part of the interaction among the human, the machine, and their operating environment, and it influences human-machine system performance. The hysteresis effect is an aspect of human information processing that should be considered when designing equipment, procedures, and training. A system is said to exhibit hysteresis when it responds differently to identical inputs depending on the direction in which the system is being driven. In the context of human information processing, the term hysteresis effect refers to the shape of task performance curves plotted against the task demand. Figure 1 shows the results of a forced-pace serial reaction task, otherwise called a number shadowing task, in which participants were asked to respond to a linearly increasing then decreasing task demand (Goldberg & Stewart, 1980). The response reached a maximum point and then began to fall for an increasing demand of 0.5 to 4.0 characters/s over 36 s. As the demand decreased symmetrically, performance remained at a degraded level until the demand was quite low. In essence, participants performed better during increasing than during decreasing demand, thus exhibiting a form of hysteresis. One of the earliest anecdotal evidences of hysteresis in a complex system was a study of pedestrian accidents near zebra crossings, conducted by the Australian Road Research Board (Chamberlain, 1968). Observations showed that more crossing accidents occurred on the departure side of intersections as task demand decreased with distance from the intersection than on the approach side as the demand increased symmetrically. Two distinct curves were produced when the number of accidents was plotted against the absolute distance from the intersection (i.e., hysteresis). Drivers' performance remained low, even though demand was reduced when leaving the intersection. An investigation of air traffic control (ATC) operational errors showed that a high proportion of near misses occurred after a period of sustained high workload, suggesting that the hysteresis effect may have been a strong contributor (East, 1993; Smolensky, 1990). ATC incidents continue to be reported, and it is mainly the ATC scenario that motivates this research. In general, a system may be susceptible to the hysteresis effect once the changing demand pushes the human beyond his or her capability to process information in a timely fashion. Cumming and Croft (1973) studied the hysteresis effect under laboratory conditions using a number shadowing task. Participants responded to a series of numbers presented aurally by pressing the appropriate key. The presentation rate increased and then decreased linearly. The experiment showed that the hysteresis effect was a repeatable psychological phenomenon for simple, controlled tasks. The Short-Term Memory (STM) hypothesis was the proposed mechanism for the hysteresis effect in cases in which storage load gradually increased at higher demand rates as information entered STM faster than it was transmitted. This resulted in a performance decrement. However, a capacity-limited model of human processing suggests that STM load would reach a maximum and yield a performance plateau, as shown by the theoretical limits in Figure 1. An expectancy hypothesis was proposed, according to which hysteresis is caused by a participant's expectation of a high input rate coupled with an inability to recognize when the demand rate was decreasing. Alcohol was used to lower the participant's level of expectancy, and the results showed a significant difference in the hysteresis curve between the control and experimental runs when alcohol was administered. Subsequently, Goldberg and Stewart (1980) provided a direct test of the expectancy hypothesis. An expectancy cue was used in which characters scrolled from left to right across the screen during increasing demand and from right to left during decreasing demand. …
['Philip S. E. Farrell']
THE HYSTERESIS EFFECT