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- 1. . . . .. . . Tensor Analysis for EEG data Tatsuya Yokota Tokyo Institute of Technology February 2, 2012 February 2, 2012 1/26
- 2. Outline . ..1 Introduction . ..2 Principal Component Analysis . ..3 Experiments . ..4 Summary February 2, 2012 2/26
- 3. Brain Computer Interface A brain-computer interface(BCI) is a direct communication pathway between the brain and an external device. BCIs are often aimed at assisting, augmenting, or repairing human cognitive or sensory-motor functions. BCIs can be separated into three approaches as follow: Invasive BCIs Partially-invasive BCIs (ECoG) Non-invasive BCIs (EEG, MEG, MRI, fMRI) Invasive and partially-invasive BCIs are accurate. However there are risks of the infection and the damage. Furthermore, it requires the operation to set the electrodes in the head. On the other hand, non-invasive BCIs are inferior than invasive BCIs in accuracy, but costs and risks are very low. Especially, EEG approach is the most studied potential non-invasive interface, mainly due to its ﬁne temporal resolution, ease of use, portability and low set-up cost. February 2, 2012 3/26
- 4. Electroencephalogram:EEG EEG is the recording of electrical activity along the scalp. EEG measures voltage ﬂuctuations resulting from ionic current ﬂows within the neurons of the brain. (a) Electrodes(32 channels) (from ’wikipedia’) (b) EEG data (from ’wikipedia’) In this research, we analyze the EEG signals to extract the important features of February 2, 2012 4/26
- 5. Overview of EEG Analysis There are some steps in EEG analysis. Here, we consider following three steps: To begin with, we record EEG signals from electrodes. Next, EEG signals are transformed into the sparse representation. In this step, data becomes tensor. After that we apply the tensor decomposition technique to extract important features. February 2, 2012 5/26
- 6. Wavelet Transform for Sparse Representation [Goupillaud et al., 1984] In the ﬁrst, we introduce the wavelet transform (WT) as one of the approaches for sparse representation. The wavelet transform is given by W(b, a) = 1 √ a ∞ −∞ f(t)ψ t − b a dt, (1) where f(t) is a signal, ψ(t) is a wavelet function. There are many kind of wavelets such as Haar wavelet, Meyer wavelet, Mexican Hat wavelet and Morlet wavelet. In this research, we use the Complex MORlet wavelet (CMOR) which is given by ψfb,fc (t) = 1 √ πfb ei2πfct−(t2 /fb) . (2) February 2, 2012 6/26
- 7. What’s Tensor Tensor is a general name of multi-way array data. For example, 1d-tensor is a vector, 2d-tensor is a matrix and 3d-tensor is a cube. We can image 4d-tensor as a vector of cubes. In similar way, 5d-tensor is a matrix of cubes, and 6d-tensor is a cube of cubes. February 2, 2012 7/26
- 8. Tensor Calculation We introduce some important calculations for tensor algebra. A tensor is described as Y ∈ RI1×I2×···×IN . (3) And each element of Y is described as yi1,i2,...,iN . . mode-n tensor matrix product .. . . .. . . Y = G ×n A, (4) yi1,...,j,...,iN = In in=1 gi1,...,in,...,iN ain,j, (5) where Y ∈ RI1×···×J×···×IN , G ∈ RI1×···×IN , and A ∈ RIn×J . We have following calculation rules: (G ×n A) ×m B = (G ×m B) ×n A = G ×n A ×m B, (6) (G ×n A) ×n B = G ×n (BA). (7) February 2, 2012 8/26
- 9. Outer product and Kronecker product The outer product of vectors is given by A = a ◦ b = abT ∈ RI×J , (8) Z = a ◦ b ◦ c ∈ RI×J×K , (9) Y = a(1) ◦ · · · ◦ a(N) ∈ RI1×···×IN . (10) The Kronecker product of two matrices A ∈ RI×J and B ∈ RT ×R is a matrix denoted as A ⊗ B ∈ RIT ×JR (11) and deﬁned as A ⊗ B = a11B a12B · · · a1J B a21B a22B · · · a2J B ... ... ... ... aI1B aI2B · · · aIJ B . (12) February 2, 2012 9/26
- 10. Unfolding Tensor (Matricization) Unfolding is a very important technique in tensor analysis. Y(n) denotes the mode-n unfolded matrix of Y. . Unfolding .. . . .. . . Let Y ∈ RI1×I2×···×IN is a Nd-tensor, the unfolded matrix is follows: Y(n) ∈ RIn×(I1···In−1In+1···IN ) . (13) Figure: Unfolding Image of 4d-tensor February 2, 2012 10/26
- 11. Tucker3 model We introduce the Tucker3 model is given by Z = C ×1 G ×2 H ×3 E, (14) = R r=1 S s=1 T t=1 crstgr ◦ hs ◦ et. (15) Using unfolding, it also can be described as Z(1) = GC(1)(ET ⊗ HT ), (16) Z(2) = HC(2)(GT ⊗ ET ), (17) Z(3) = EC(3)(HT ⊗ GT ). (18) February 2, 2012 11/26
- 12. Tucker Decomposition (general formula) Tucher Model is a very famous and general model of tensor decomposition. Given tensor Y is decomposed into a set of matrices {A(n) }N n=1 and one small core tensor G. . Tucker Model .. . . .. . . Y = G ×1 A(1) ×2 A(2) · · · ×N A(N) (19) = J1 j1=1 · · · JN jN =1 gj1,...,jN a (1) j1 ◦ · · · ◦ a (N) jN (20) Furthermore, it can be described as follow by using unfolding. . Unfolded Tucker Model .. . . .. . . Y(n) = A(n) G(n)(A(N) ⊗ · · · ⊗ A(n+1) ⊗ A(n−1) ⊗ · · · ⊗ A(1) )T (21) February 2, 2012 12/26
- 13. Kind of Tensor Decomposition [Cichocki et al., 2009] The degree of freedom of tensor decomposition is very large. So there are many methods of tensor decomposition. The kind of tensor decomposition is depend on the constraint. For example, if we constrain the matrices {A(n) }N n=1 and the core tensor G as non-negative matrices and tensor, then this method is the non-negative tensor factorization (NTF). And if we consider the in-dependency constraint, then this method is the independent component analysis (ICA). And if we consider the sparsity constraints, then it is the sparse component analysis (SCA). And if we consider the orthogonal constraints, then it is the principal component analysis (PCA). February 2, 2012 13/26
- 14. Principal Component Analysis [Kroonenberg and de Leeuw, 1980] [Henrion, 1994] Principal Component Analysis (PCA) is very typical method for signal analysis. In this slide, we explain PCA in case of 3d-tensor decomposition. The tensor decomposition model is given by Z = C ×1 G ×2 H ×3 E. (22) And the criterion of PCA is given by . Criterion for PCA .. . . .. . . minimize ||Z − C ×1 G ×2 H ×3 E||2 F (23) subject to GT G = I, HT H = I, ET E = I. (24) The goal of this criterion is to minimize the error of decomposed model, subject to the matrices {G, H, E} are orthogonal. And (23) also can be described as follow by using unfolding: min ||Z(1) − GC(1)(E ⊗ H)T ||2 F . (25) February 2, 2012 14/26
- 15. Criterion for 3-way PCA Criterion for 3-way PCA is given by minimize f := ||Z(1) − GC(1)(ET ⊗ HT )||2 F (26) subject to GT G = I, ET E = I, HT H = I. (27) From (27), C(1) = GT Z(1)(E ⊗ H). (28) Substituting (28) into f, f = ||Z(1) − GGT Z(1)(E ⊗ H)(ET ⊗ HT )||2 F (29) = tr(Z(1)ZT (1)) − tr(GT Z(1)(EET ⊗ HHT )ZT (1)G). (30) tr(Z(1)ZT (1)) is constant, then the criterion is rewritten by maximize tr(GT Z(1)(EET ⊗ HHT )ZT (1)G) (31) subject to GT G = I, ET E = I, HT H = I. (32) February 2, 2012 15/26
- 16. Solution Algorithm Note that p(G, H, E) :=tr(GT Z(1)(EET ⊗ HHT )ZT (1)G) (33) =tr(HT Z(2)(GGT ⊗ EET )ZT (2)H) (34) =tr(ET Z(3)(HHT ⊗ GGT )ZT (3)E). (35) The image of solution algorithm is described as follows: Figure: Alternating Least Square(ALS) Algorithm February 2, 2012 16/26
- 17. Experiments:Data sets [Blankertz, 2005] . BCI Competition III : IVa .. . . .. . . EEG Motor Imagery Classiﬁcation data set (right, foot) There are 5 subjects(aa,al,av,aw,ay) One imagery for 3.5s 118 EEG channels Table: Number of Samples #train #test aa 168 112 al 224 56 av 84 196 aw 56 224 ay 28 252 -1 -0.5 0 0.5 1 -1 -0.5 0 0.5 1 1 2 3 4 5 6 7 8 9 10 11 12 1314 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 3031 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 7778 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 9697 98 99 100 101 102103 104105106107108 109110 111 112 113 114 115 116 117 118 February 2, 2012 17/26
- 18. Experiments:Procedure ...1 Transformation into CMOR domain (time × frequency × channels × samples) CMOR 6-1 (case 1, case 2, case 3) ...2 Applying Dimensionality Reduction Unused PCA (6-6-6,4-4-4,2-2-2,1-1-1) ...3 Classiﬁcation K-Nearest Neighbor method Least Squares Regression (Kernel regression) time frame frequency channels samples # of elements case 1 35(0:0.1:3.5) 23(8:30) 118 280 26597200 case 2 350(0:0.01:3.5) 23(8:30) 7(51:57) 280 15778000 case 3 35(0:0.1:3.5) 23(8:30) 7(51:57) 280 1577800 February 2, 2012 18/26
- 19. Experiments:Results I Table: case 1, kNN-3 Unused (6-6-6) (4-4-4) (2-2-2) (1-1-1) aa 50.00 49.10 44.64 50.00 58.92 al 62.50 62.50 62.50 58.92 46.42 av 54.08 51.53 53.06 53.06 55.10 aw 54.91 50.00 53.12 49.10 54.01 ay 51.98 47.22 44.84 41.66 44.84 Ave. 54.69 52.07 51.63 50.55 51.86 Table: case 1, LSR Unused (6-6-6) (4-4-4) (2-2-2) (1-1-1) aa 58.92 60.71 59.82 55.35 58.92 al 76.78 71.42 76.78 75.00 57.14 av 58.16 57.65 54.08 55.61 56.12 aw 63.83 64.28 56.69 62.50 62.50 ay 48.81 51.19 50.79 48.41 48.80 Ave. 61.31 61.05 59.63 59.37 56.69 February 2, 2012 19/26
- 20. Experiments:Results II Table: case 2, kNN-3 Unused (6-6-6) (4-4-4) (2-2-2) (1-1-1) aa 56.25 44.64 52.67 47.32 48.21 al 78.57 80.35 78.57 80.35 57.14 av 60.20 52.04 57.65 55.61 46.42 aw 58.48 57.58 54.46 57.58 52.67 ay 57.93 59.92 59.92 58.33 44.84 Ave. 62.28 58.90 60.65 59.83 47.65 Table: case 2, LSR Unused (6-6-6) (4-4-4) (2-2-2) (1-1-1) aa 62.50 58.92 58.03 58.92 53.57 al 87.50 87.50 85.71 83.92 64.28 av 61.22 56.12 56.12 52.04 55.61 aw 66.07 64.73 61.60 62.94 60.71 ay 57.53 62.69 70.23 73.80 48.41 Ave. 66.96 65.99 66.33 66.32 56.51 February 2, 2012 20/26
- 21. Experiments:Results III Table: case 3, kNN-3 Unused (6-6-6) (4-4-4) (2-2-2) (1-1-1) aa 55.35 50.89 50.00 46.42 44.64 al 78.57 78.57 78.57 78.57 57.14 av 60.20 53.57 55.61 51.02 50.00 aw 57.14 55.35 54.01 57.58 51.78 ay 58.33 58.33 59.52 56.34 44.64 Ave. 61.91 59.34 59.54 57.98 49.64 Table: case 3, LSR Unused (6-6-6) (4-4-4) (2-2-2) (1-1-1) aa 57.14 60.71 55.35 56.25 51.78 al 87.50 85.71 85.71 83.92 60.71 av 59.18 55.61 54.08 54.08 53.06 aw 66.07 63.39 58.03 62.50 60.26 ay 57.53 57.93 61.11 63.88 48.41 Ave. 65.48 64.67 62.85 64.12 54.84 February 2, 2012 21/26
- 22. BCI Competition: IVa Ranking contributor ave. aa al av aw ay 1 Yijun Wang 94.74 95.5 100 80.6 100 97.6 2 Yuanqing Li 87.40 89.3 98.2 76.5 92.4 80.6 3 Liu Yang 84.54 82.1 94.6 70.4 87.5 88.1 4 Zhou Zongtan 77.24 83.9 100 63.3 50.9 88.1 5 Michael Bensch 74.14 73.2 96.4 70.4 79.9 50.8 6 Codric Simon 73.28 83.0 91.1 50.0 87.9 54.4 7 Elly Gysels 72.36 69.6 96.4 64.3 69.6 61.9 8 Carmen Viduarre 69.62 66.1 100 63.3 64.3 54.4 9 Le Song 69.00 66.1 92.9 67.3 68.3 50.4 10 Ehsan Arbabi 68.26 70.5 94.6 56.1 63.8 56.3 11 Cyrus Shahabi 61.98 57.1 76.8 57.7 64.3 54.0 12 Kiyoung Yang 59.02 52.7 85.7 61.2 51.8 43.7 13 Hyunjin Yoon 53.76 50.0 67.9 52.6 52.7 45.6 14 Wang Feng 52.26 50.9 53.6 54.6 56.2 46.0 My best Result is 66.96% average. February 2, 2012 22/26
- 23. Summary N-way PCA is very eﬃcient for dimensionality reduction. High dimensional data can be reduced in easily. In this results, the accuracies almost kept. However, the accuracies didn’t become good. EEG classiﬁcation is very diﬃcult problem. It is considered that the feature extraction and preprocessing are important. Especially, channel selection might be very important. February 2, 2012 23/26
- 24. Bibliography I [Blankertz, 2005] Blankertz, B. (2005). Bci competition iii. http://www.bbci.de/competition/iii/. [Cichocki et al., 2009] Cichocki, A., Zdunek, R., Phan, A. H., and Amari, S. (2009). Nonnegative Matrix and Tensor Factorizations: Applications to Exploratory Multi-way Data Analysis. Wiley. [Goupillaud et al., 1984] Goupillaud, P., Grossmann, A., and Morlet, J. (1984). Cycle-octave and related transforms in seismic signal analysis. Geoexploration, 23(1):85 – 102. [Henrion, 1994] Henrion, R. (1994). N-way principal component analysis theory, algorithms and applications. Chemometrics and Intelligent Laboratory Systems, 25:1–23. [Hyv¨arinen et al., 2001] Hyv¨arinen, A., Karhunen, J., and Oja, E. (2001). Independent Component Analysis. Wiley. February 2, 2012 24/26
- 25. Bibliography II [Kroonenberg and de Leeuw, 1980] Kroonenberg, P. and de Leeuw, J. (1980). Principal component analysis of three-mode data by means of alternating least squares algorithms. Psychometrika, 45:69–97. February 2, 2012 25/26
- 26. Thank you for listening February 2, 2012 26/26

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