Abstract
BackgroundIn a visual oddball paradigm, attention to an event usually modulates the event-related potential (ERP). An ERP-based brain-computer interface (BCI) exploits this neural mechanism for communication. Hitherto, it was unclear to what extent the accuracy of such a BCI requires eye movements (overt attention) or whether it is also feasible for targets in the visual periphery (covert attention). Also unclear was how the visual design of the BCI can be improved to meet peculiarities of peripheral vision such as low spatial acuity and crowding.MethodHealthy participants (N = 13) performed a copy-spelling task wherein they had to count target intensifications. EEG and eye movements were recorded concurrently. First, (c)overt attention was investigated by way of a target fixation condition and a central fixation condition. In the latter, participants had to fixate a dot in the center of the screen and allocate their attention to a target in the visual periphery. Second, the effect of visual speller layout was investigated by comparing the symbol Matrix to an ERP-based Hex-o-Spell, a two-levels speller consisting of six discs arranged on an invisible hexagon.ResultsWe assessed counting errors, ERP amplitudes, and offline classification performance. There is an advantage (i.e., less errors, larger ERP amplitude modulation, better classification) of overt attention over covert attention, and there is also an advantage of the Hex-o-Spell over the Matrix. Using overt attention, P1, N1, P2, N2, and P3 components are enhanced by attention. Using covert attention, only N2 and P3 are enhanced for both spellers, and N1 and P2 are modulated when using the Hex-o-Spell but not when using the Matrix. Consequently, classifiers rely mainly on early evoked potentials in overt attention and on later cognitive components in covert attention.ConclusionsBoth overt and covert attention can be used to drive an ERP-based BCI, but performance is markedly lower for covert attention. The Hex-o-Spell outperforms the Matrix, especially when eye movements are not permitted, illustrating that performance can be increased if one accounts for peculiarities of peripheral vision.
Highlights
In a visual oddball paradigm, attention to an event usually modulates the event-related potential (ERP)
Classifiers rely mainly on early evoked potentials in overt attention and on later cognitive components in covert attention. Both overt and covert attention can be used to drive an ERP-based brain-computer interface (BCI), but performance is markedly lower for covert attention
The present study addresses the question whether an ERP-based BCI is dependent on eye movements, or whether it can detect attention deployed in the visual periphery
Summary
Counting accuracy Due to technical difficulties, counting data was not available for one of the participants. For each component, this yielded a fourfactor design, Speller [Matrix, Hex-o-Spell] × Attention [overt, covert] × Status [target, nontarget] × Electrode, for analysis with a 4-way repeated measures ANOVA. For the N2 and P3 components, amplitude modulations in the overt attention condition were not significantly different for the two spellers (p-values .6438 and .516, respectively), but they were stronger for the Hex-o-Spell than for the Matrix in the covert attention condition (F = 28.67, p < .001, and F = 5.23, p < .05, respectively). Using overt attention, spelling success is mainly based on visually evoked potentials (VEPs) measured at occipital and parieto-occipital sites This confirms earlier conjectures [38,49] and it is in accordance with [50] who showed that classifying on posterior electrodes in addition to the classical P3 sites improves classification performance. The vocabulary is necessarily small with overlapping targets, which illustrates how difficult it is to reconcile independence from eye movements with high information throughput
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