Multiset CCA for SSVEP Recognition
mensions of EEG tensor data for SSVEP frequency
recognition. The MwayCCA method has shown im-
proved recognition performance of SSVEP frequency
compared to the CCA method. On the other hand,
a pha se constrained CC A method (PCCA)
28
has
also been proposed for SSVEP frequency recogni-
tion. The PCCA method achieved significant ac-
curacy improvement in comparison with the CCA
method, by embedding the phas e information esti-
mated from training data into the reference signals.
Howe ver, the procedures of reference signal optimiza-
tion in both the MwayCCA and the PCCA meth-
ods are not completely based on training data but
still need to resort to the pre-constructed sine-cosine
waves.
In the present study, we introduce a multi-
set canonical correlation analy sis (MsetCCA)-based
method to reference signal optimizatio n for SSVEP
frequency recognition. MsetCCA was developed as
an extension of CCA to find multiple linear trans-
forms that maximize the overall correlation among
canonical variates from multiple sets of random vari-
ables
29
. Recently, MsetCCA has been successfully
applied to the joint blind source separation of multi-
subject fMRI data
30,31
, the functional connectiv-
ity analys is of fMRI data
32
, and also the fusion of
concurrent single trial ERP and fMRI data
33
. In
this study, the proposed MsetCCA method imple-
ments reference signal optimization for SSVEP fre-
quency recognition through extracting SSVEP com-
mon features from the joint spa tial filtering of multi-
ple sets of EEG data recorded at the same stimulus
frequency. Different from the MwayCCA and the
PCCA methods, the procedure of reference signa l
optimization in the MsetCCA method is completely
based on training data. EEG data recorded from ten
healthy subjects are used to validate the Ms etCCA
method in comparison with the CCA, the Mway-
CCA and the PCCA methods. Experimental results
indicate that the proposed MsetCCA method out-
performs the three c omp eting methods for SSVEP
recognition, especially for a small number of chan-
nels and a short time window length.
2. Materials and Methods
2.1. Experiments and EEG recordings
The experiments were performed by ten healthy sub-
jects (S1-S10, aged from 21 to 27, all males) who re-
ceived their remuneration. All of them had normal
or corrected to normal vision. In the experiments,
the subjects were seated in a comfor table chair 60
cm from a standard 17 inch CRT monitor (85 Hz re-
fresh rate, 1024 × 768 screen resolution) in a shielded
room. Four red squares were presented on the screen
as stimuli (see Fig. 1 (a)) and flickered at different
four frequencies 6 Hz, 8 Hz, 9 Hz and 10 Hz, respec-
tively. There were 20 experimental runs completed
by each subject. In each run, the subject was asked
to focus attention on each of the four stimuli once for
4 s, respectively, preceded by each target cue dur a-
tion of 2 s. A total of 80 trials (4 trials in each run)
were therefore performed by each subject.
EEG signals were recorded by using the Nuamps
amplifier (NuAmp, Neurosca n, Inc .) at 250 Hz sam-
pling rate with high-pass and low-pass filters of 0.1
and 70 Hz from 30 channels arranged ac cording to
standard positions of the 10-20 international system
(see Fig. 1 (b)). The average of two mas toid elec-
trodes (A1, A2) was used as reference and the elec-
trode on the forehead (GND) as ground. With a
sixth-order Butterworth filter, a band-pass filtering
from 4 to 45 Hz was implemented on the recorde d
EEG signals before further analysis.
2.2. CCA for SSVEP Recognition
As a multivariate statistical method, canonical
correlation analysis (CCA)
34
explores the underly-
ing cor relation between two se ts of data. Given two
sets of random variables X ∈ R
I
1
×J
and Y ∈ R
I
2
×J
,
which are norma lized to have zero mean and unit
variance, CC A is to seek a pair of linear transforms
w
x
∈ R
I
1
and w
y
∈ R
I
2
such that the corr e lation be-
tween linear combinations ˜x = w
T
x
X and ˜y = w
T
y
Y
is maximized as
max
w
x
,w
y
ρ =
E
˜x˜y
T
p
E [˜x˜x
T
] E [˜y˜y
T
]
=
w
T
x
XY
T
w
y
q
w
T
x
XX
T
w
x
w
T
y
YY
T
w
y
. (1)
The maximum of correlation coefficient ρ with re-
spect to w
x
and w
y
is the maximum canonica l cor-
relation.
A CCA-based frequency recognition method
21
was first introduced by Lin et al. to SSVEP-based