Usually software is maintained by people different from those who developed it. In this context the maintenance activities are dominated by the comprehension effort. The study of the cognitive processes behind comprehension usually involves intrusive approaches. In this paper we present a non-intrusive approach to study the cognitive process that is based on the observation of how a maintainer uses the documentation. A simplified observational model is proposed to link the records of the documentation usage to the cognitive process. We focused on object-oriented programs written in Java, and used design patterns to make them well structured. The standard HTML based Javadoc documentation format allowed to record the use of the documentation easily. We conducted an experiment to validate the observational model and to investigate if the type of documentation affects the cognitive process. The proposed cognitive model was able to weakly discriminate among two simple cognitive processes. In addition it revealed that the type of documentation affected the cognitive process.