Higher-Order Partial Least Squares (HOPLS): A Generalized Multi-Linear
  Regression Method
  A new generalized multilinear regression model, termed the Higher-Order
Partial Least Squares (HOPLS), is introduced with the aim to predict a tensor
(multiway array) $\tensor{Y}$ from a tensor $\tensor{X}$ through projecting the
data onto the latent space and performing regression on the corresponding
latent variables. HOPLS differs substantially from other regression models in
that it explains the data by a sum of orthogonal Tucker tensors, while the
number of orthogonal loadings serves as a parameter to control model complexity
and prevent overfitting. The low dimensional latent space is optimized
sequentially via a deflation operation, yielding the best joint subspace
approximation for both $\tensor{X}$ and $\tensor{Y}$. Instead of decomposing
$\tensor{X}$ and $\tensor{Y}$ individually, higher order singular value
decomposition on a newly defined generalized cross-covariance tensor is
employed to optimize the orthogonal loadings. A systematic comparison on both
synthetic data and real-world decoding of 3D movement trajectories from
electrocorticogram (ECoG) signals demonstrate the advantages of HOPLS over the
existing methods in terms of better predictive ability, suitability to handle
small sample sizes, and robustness to noise.
