The relationship between sentence processing and cognitive demand has received a lot of attention in the past decades. In valency theory, some elements of the sentence are determined by the verbs either in terms of their form or by their presence (Herbst & Schüller 2008). It has to be said that little attention has been paid to the processing of such fundamental categories in the theory of syntax. On the one hand, this is remarkable since given the amount of research, we still do not know whether this distinction is psychologically real, or whether it only serves a lexicographic and pedagogical purpose. On the other hand, there is a consensus among linguists about the problematic character of the distinction itself even on a more theoretical level (Dowty 2000; Herbst & Schüller 2008). Therefore, this study attempts to explore whether complements and adjuncts are associated with different kinds of processing. To answer the research questions, an experiment consisting in a mouse-controlled reading task has been designed. To the best of our knowledge, this is a new method in psycholinguistic research. The paper presents the results of a pilot study.