Working memory is a mechanism for short-term active storage of information as well as for processing stored information. Although evidence for neuronal mechanisms of temporary storage of information has accumulated for the prefrontal cortex, little is known about neuronal mechanisms for processing information. To understand how information is processed by prefrontal neurons, we first need to know what information is represented by single-neuron activity, and then examine how information represented by single-neuron activity or a population of activities changes along the temporal sequence of the trial. By examining task-related single-neuron activities while monkeys performed various working memory tasks, delay-period activity observed in the prefrontal cortex is considered to be a neuronal correlate of the mechanism for temporary active storage of information. Delay-period activity represents a variety of information including the spatial position and the physical feature of the stimulus, the forthcoming behavioral response, the quality of reward that the subject would receive, the difference of the task, or the rule of the task. Although delay-period activity could represent this variety of information, the information represented by delay-period activity is only the information relevant for task performance. In addition, using complex conditional tasks, delay-period activity has been shown to represent several kinds of information simultaneously. Based on these results, we examined how information represented by a population of prefrontal activities changes along the temporal sequence of the trial. Using two kinds of oculomotor delayed-response tasks, we first identified what information each task-related activity represents. Then, using population vector analysis, we could not only visualize information represented by a population of prefrontal activities, but also demonstrate the temporal change of information represented by a population of prefrontal activities. These attempts are important to understand information processes for working memory.