Superposition Model For Steady State Visually Evoked Potentials

Steady State Visually Evoked Potentials (SSVEP) are signals produced in the occipital part of the brain when someone gaze a light flickering at a fixed frequency. These signals have been used for Brain Machine Interfacing (BMI), where one or more stimuli are presented and the system has to detect what is the stimulus the user is attending to. It has been proposed that the SSVEP signal is produced by superposition of Visually Evoked Potentials (VEP) but there is not a model that shows that. We propose a model for a SSVEP signal that is a superposition of the response due to the rising and falling edges of the stimulus and that can be calculated for different frequencies.We fixed the model for 4 subjects that gazed stimuli in the frequencies of 9Hz, 11Hz, 13Hz and 15Hz, and duty-cycles of 20%, 35%, 50%, 65%, and 80%. Since the phases of SSVEP signals are stable over the time, these were used to fix the model, without the amplitude; however, signals of scattered phases were discarded. The model parameters were found using the Oz electrode signals and a genetic algorithm.The mean absolute error (MAE) between the measured phase and the obtained one was calculated for each subject (named S1, S2, S3, and S4). The model was fixed for the subjects in the fundamental frequencies, just two of them in the second harmonic, and one in the third harmonic. We obtained a maximum MAE for 3 subjects (S1, S2, and S4) in the fundamental frequencies at 0.30 rad and one of them (S2) with 0.21 rad in the second harmonic. The last one (S3) signals show poor results with a MAE between 0.46 rad and 1.79 rad by including fundamental frequencies, and second and third harmonics. The results show similarities among the different model parameters such that it suggests that a general model could be obtained.