Involving users in the design process of new products and services is generally disputed as a prerequisite for fulfilling users' needs and requirements. The importance of user involvement has been argued also regarding the development of new medical technology. Collaboration between users and developers/designers is however not without problems due to differences in, e.g. background, training, perspective, and vocabulary. In order to address these differences, the need for different 'mediating tools' has been emphasized. One type of mediating tools is product representations (PRs). Earlier studies have most often focused on the type of PR that should be used in different phases of the development process in order to get input on different designs. This paper describes instead how and in what situations different PRs mediated communication and collaboration between professional users (medical experts) and designers in an innovation project targeting a solution for long-term monitoring of brain activity based on electroencephalographic (EEG) signals.