  The steady-state visual evoked potential (SSVEP) is one of the most widely
used modalities in brain-computer interfaces (BCIs) due to its many advantages.
However, the existence of harmonics and the limited range of responsive
frequencies in SSVEP make it challenging to further expand the number of
targets without sacrificing other aspects of the interface or putting
additional constraints on the system. This paper introduces a novel
multi-frequency stimulation method for SSVEP and investigates its potential to
effectively and efficiently increase the number of targets presented. The
proposed stimulation method, obtained by the superposition of the stimulation
signals at different frequencies, is size-efficient, allows single-step target
identification, puts no strict constraints on the usable frequency range, can
be suited to self-paced BCIs, and does not require specific light sources. In
addition to the stimulus frequencies and their harmonics, the evoked SSVEP
waveforms include frequencies that are integer linear combinations of the
stimulus frequencies. Results of decoding SSVEPs collected from nine subjects
using canonical correlation analysis (CCA) with only the frequencies and
harmonics as reference, also demonstrate the potential of using such a
stimulation paradigm in SSVEP-based BCIs.
