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Steady state visual evoked potential (SSVEP) is the brain's natural electrical potential response for visual stimuli at specific frequencies. Using a visual stimulus flashing at some given frequency will entrain the SSVEP at the same frequency, thereby allowing determination of the subject's visual focus. The faster an SSVEP is identified, the higher information transmission rate the system achieves. Thus, an effective stimulus, defined as one with high success rate of eliciting SSVEP and high signal-noise ratio, is desired. Also, researchers observed that harmonic frequencies often appear in the SSVEP at a reduced magnitude. Are the harmonics in the SSVEP elicited by the fundamental stimulating frequency or by the artifacts of the stimuli? In this paper, we compare the SSVEP responses of three periodic stimuli: square wave (with different duty cycles), triangle wave, and sine wave to find an effective stimulus. We also demonstrate the connection between the strength of the harmonics in SSVEP and the type of stimulus.

The sum of sine waves was used to evoke Steady State Visual Evoked Potentials (SSVEP). The SSVEP responses were elicited by visual stimuli in the form of a flickering light emitting diode (LED). We recorded EEG responses to these stimuli and analyzed the observed spectral differences due to square waves, triangle waves and sine waves. We showed that the resolution of the dual stimuli, which is defined as the difference between two frequencies, is essential to SSVEP experiments. Reliable dual SSVEP spikes from EEG signals can only be acquired if the dual stimuli are at least 4Hz apart. We also tested tri-stimuli and showed that consistent EEG responses are not achievable, even though the stimuli were 4Hz apart. The essence of these experiments is to demonstrate the resolutions under which dual sine waves can generate consistent SSVEP responses.