A survey of stimulation methods used in SSVEP-based BCIs

Abstract

Brain-computer interface (BCI) systems based on the steady-state visual evoked potential (SSVEP) provide higher information throughput and require shorter training than BCI systems using other brain signals. To elicit an SSVEP, a repetitive visual stimulus (RVS) has to be presented to the user. The RVS can be rendered on a computer screen by alternating graphical patterns, or with external light sources able to emit modulated light. The properties of an RVS (e.g., frequency, color) depend on the rendering device and influence the SSVEP characteristics. This affects the BCI information throughput and the levels of user safety and comfort. Literature on SSVEP-based BCIs does not generally provide reasons for the selection of the used rendering devices or RVS properties. In this paper, we review the literature on SSVEP-based BCIs and comprehensively report on the different RVS choices in terms of rendering devices, properties, and their potential influence on BCI performance, user safety and comfort.

Figure 1

Functional model of an SSVEP-based BCI.

Figure 2

Typical waveform of an EEG signal (Oz-Cz) acquired during visual light stimulation with a frequency of 15 Hz and its frequency spectrum. (a) SSVEP waveform resulting from the time-locked average of 10 realizations. A transient VEP can be observed at the moment where the stimulation began and a clear oscillation (the steady state VEP) can be seen afterwards; (b) Frequency content of the signal in (a). The SSVEP manifests itself in oscillations at 15 Hz and higher harmonics.

Figure 3

(a) In single graphic stimuli the graphical object alternately appears and disappears in the background. (b) In pattern reversal stimuli at least two patterns are alternated at a specified frequency.

Figure 4

Literature search strategy and the number of papers retrieved at each step. “K” indicates “keyword” and “?” refers to arbitrary characters (e.g., e or es).