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Título: A novel stimulation paradigm for a brain-computer interface based on SSVEP
Autor(es): Atencio, Anibal Cotrina
Orientador: Bastos Filho,Teodiano Freire
Coorientador: Ferreira, André
Data do documento: 30-Nov-2015
Editor: Universidade Federal do Espírito Santo
Resumo: Brain-computer interfaces (BCIs) are systems that provide a direct connection between users’ brain signals and a computer, generating an alternative channel of communication that does not involve the traditional way as muscles and nerves. They help to restore or replace useful functions of people with paralysis. For instance, BCI systems based on SSVEP (SSVEP-BCI) present a set of stimuli flickering at different frequencies to the users, detect which stimulus is being gazed and associate this information to commands that can be used to control a robotic wheelchair, an exoskeleton, or a speller. Nowadays, SSVEP-BCIs are being widely used due to the high SNR of their response that is achieved when the target stimulus is brought to the center of the user’s field of view. Paradoxically, it is their main disadvantage because bringing a stimulus to the center of the field of view demands muscular activity making them not suitably for paralyzed people who cannot control their head, neck and/or eyeball movements for redirecting their gaze. In optical systems, the range of distance near the point of focus where objects are perceived sharp is referred as Depth-of-field; objects outside this region are defocused and blurred. The mechanism of the visual system that adjusts the eye focal length for focusing does not demand neck, head and/or eyeball movements. Furthermore, ophthalmology studies state that the amplitude and the latency of visual evoked potentials are affected by defocusing. In this context, this Thesis proposes a novel SSVEP-BCI paradigm, in which two stimuli are presented together in the center of the user’s field of view but at different distances from him, ensuring that if one stimulus is focused on, the other one is non-focused, and vice versa; with the aim of providing an alternative way of sending commands through a stimuli selection by a focusing mechanism. In this sense, first, a model of VEP generation for two stimuli is introduced and the hypothesis that distinguishable SSVEP can be elicited by the focused stimulus, regardless of the non-focused stimulus is also present, is tested by employing the SFT-based ORD. Next, a demodulation method based in LPF and HPF is proposed for the case in that two stimuli are present in the field of view; and spatial-temporal retinal response for flickering stimuli is described by using PSF and NPSF functions. Finally, CCA, PSDA, LASSO with EEG signals re-referenced at Pz and CAR are employed to detect the SSVEP pattern. Accuracy rate, Kappa coefficient, AUC, and ITR are used to evaluate the detection performance. As a result of the hypothesis test, the absence of evoked potentials due to a focused stimulus, when a non-focused stimulus is also present was rejected in most of the cases. In SSVEP detection experiments, an average accuracy rate of 0.93 was achieved for a time window of 7s and for eight healthy subjects who were asked to focus on just one stimulus at a time.
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