Towards a SSVEP-BCI Based on Depth of Field Anibal Cotrina 1 , Teodiano Bastos 1 , Andre Ferreira 1 , Alessandro Benevides 1 , Javier Castillo-Garcia 1,2 , David Rojas 3 and Alessander Benevides 4 1 Post-Graduate Program of Electrical Engineering, Federal University of Espirito Santo, Brazil anibal.atencio@ufes.br 2 Post-Graduate Program of Electronics Engineering, Del Valle University, Cali, Colombia 3 Computer Vision Center, Autonomous University of Barcelona, Barcelona, Spain 4 ICT Doctoral School, University of Trento, Trento, Italy Abstract It has been shown that the visual evoked potential amplitude reduces as the stimulus becomes increasingly defocused. Based on depth-of-field theory, which states that subject distance is the range of distance in which an object appears sharp at the retinal image, the present work attempts to verify an alternative SSVEP BCI setup, where the user gaze simultaneously two SSVEP stimuli flickering with different frequencies and located at different distances. This setup relies on the assumption that the focused stimulus is able to elicit distinguishable SSVEP pattern regardless the non-focused stimulus that is also present. Three subjects and two stimuli were considered. Clear SSVEP pattern was elicited when they were asked to focus either the nearest or the farthest stimulus. 1 Introduction Visual evoked response (VEP) is an event related potential that occurs involuntarily in response to a visual stimulus. It has been shown that the defocusing of the retinal image has a greater effect on the latency of this potential [8]. An object is defocused when it is located out of the eye’s Depth of Field (DOF), that is defined as the range of distance in which an object appears sharp at the retinal image [6]. The focusing of a target is performed by an accommodation mechanism that is achieved when a neural signal is sent to the ciliary muscle changing the shape of the crystalline lens. It modifies the angle of refraction minimizing automatically the blurriness in the retinal image [3]. Then, the performance of brain-computer interfaces (BCI) based on steady-state visual evoked potentials (SSVEP) could be affected by this optical phenomenon, specifically when the flickering stimuli go out of focus of the subject’s eye. As the amplitude of the VEP pattern can be reduced as the retinal image is increasingly defocused, an arising hypothesis is: if a BCI user is gazing simultaneously two stimuli flickering with different frequencies and located at different distances (enough to get only one into the subject’s DOF), the focused stimulus will be able to elicit distinguishable SSVEP pattern regardless the non-focused stimulus is also present. Due to the user choose the target stimulus by shifting the focus instead of gaze movements or attention, this work becomes an alternative method for presenting SSVEP stimuli either to traditional SSVEP paradigm or covert attention based SSVEP paradigms. Results showed that clear SSVEP pattern can be elicited when the subjects are asked to focus either the nearest or the farthest stimulus. 2 Methods Three healthy subject without any experience with BCI were considered in this work. The experiments were undertaken with the understanding and written consent of them. This study Proceedings of the 6th International Brain-Computer Interface Conference 2014 DOI:10.3217/978-3-85125-378-8-97 Published by Graz University of Technology Publishing House Article ID 097-1