Fig 1.
Interface used in the experiment.
Each of the four squares has a unique combination of frequency and phase.
Fig 2.
Concept behind the classification of a single epoch.
Note that the actual analysis is done on a spatiotemporal epoch, involving multiple electrodes.
Table 1.
Segment lengths (in samples) for different frequencies.
T corresponds to one period of a given frequency. Segments are input to the beamformers.
Fig 3.
Activation patterns for subject 6.
Each row corresponds to a different target. The left column shows the spatiotemporal activation patterns Ast[i] that are used for building spatiotemporal beamformers. The middle and right columns show the spatial (asp[i]) and temporal activation patterns (at[i]) that are employed to build the chained beamformer.
Fig 4.
Temporal activation patterns for all subjects for target 2.
Each trace corresponds to the temporal activation pattern of one subject.
Fig 5.
Influence of channel set on beamformer performance.
(A) chained beamformer, (B) spatiotemporal beamformer. Stars indicate significance levels based on Wilcoxon Rank Sum Test: *(p < 0.05), **(p < 0.01), ***(p < 0.001).
Fig 6.
Influence of downsampling on beamformer-based classification performance.
(A) chained beamformer, (B) spatiotemporal beamformer, both for channel set Chenv.
Fig 7.
Performance of different types of classifiers.
Channel set Chenv and no downsampling was used. Stars indicate significance levels, based on a Wilcoxon Rank Sum Test: *(p < 0.05), **(p < 0.01), ***(p < 0.001).
Fig 8.
Confusion matrices of the stBF from Fig 7.
The numbers on the diagonal indicate the predictive accuracy (in %) of the stBF-classifier for each target. The values displayed are the average of the confusion matrices of all subjects. (A) For an epoch length of 0.5 seconds. (B) For an epoch length of 1 second. Targets 1 and 4 flicker at 12 Hz but with opposite phase, targets 2 and 3 flicker at 15 Hz but with opposite phase.
Fig 9.
EEG frequency spectrum, averaged over all subjects, revealing co-amplification effect: the peripheral target flickering at a different frequency (∼4.4° separating angle) adds a small amplitude to the SSVEP signal of the attended target. (A) For targets 1 and 2, flickering at 12 and 15 Hz, respectively. (B) For targets 3 and 4, flickering at 15 and 12 Hz, respectively.
Fig 10.
Frequency resolution of the time-domain analysis.
A signal from channel Oz in response to a 15 Hz stimulus, and its segments with lengths corresponding to one period (T) of 12, 13, 14 and 15 Hz. Signal length are (A) 250 ms, (B) 500 ms, (C) 750 ms and (D) 1000 ms.