1 EFFECTS OF P300-BASED BCI USE ON REPORTED PRESENCE IN A VIRTUAL ENVIRONMENT Christoph Groenegress 1 , Clemens Holzner 2 , Christoph Guger 2 , Mel Slater 1,3 1 EVENT Lab, Facultat de Psicologia, Universitat de Barcelona, Spain 2 g.tec OEG, Schiedelberg, Austria 3 ICREA - Institució Catalana de Recerca i Estudis Avançats, Barcelona, Spain Email address of corresponding author: cgroenegress@gmail.com Abstract Brain-computer interfaces (BCIs) are becoming more and more popular as an input device for virtual worlds and computer games. Depending on their function a major drawback is the mental workload associated with their use and there is significant effort and training required to effectively control them. In this paper we discuss two studies assessing how mental workload of a P300-based BCI affects the reported presence of participants in a virtual environment (VE). In the first study we employed a BCI exploiting the P300 event-related potential (ERP) that allows control of over 200 items in a virtual apartment. In a second study the BCI is replaced by a gaze- based selection method coupled with wand navigation. In both studies overall performance is measured and individual presence scores were assessed by means of a short questionnaire. Results suggest that the use of the P300 degrades reported presence in relation to the gaze-based approach. We argue that this is because P300 method employed introduces breaks in presence, does not require participants to understand the space in which the objects they are manipulated are located, and does not mobilize motor action, or even thoughts of motor action, that are relevant to the semantics of the task. We conclude with remarks about the obvious efficiency of the