Abstract: The purpose of this study was to find out how human performance and mental workload changes when people face multiple task demands. In an experiment,the subjects were asked to do four information processing tasks simultaneously.The four tasks were tracking,choice reaction time, monitoring and mental arithmetic.The task demands were changed by increasing the speed of task presentation on computers.The experimental results showed that with the increase of task demands,the subjects'performance deteriorated, but the deterioration was task specific.When facing multiple tasks simultaneously,the human subjects seemed to be like a single channel mechanism,concentrating on one task and then switching their attention to another task.The reasons and the implications were discussed.

Key words: human performance, mental workload, attention